I NlST Measurement Services: 

Spectroradiometric Detector Measurements: 

Part I-Ultraviolet Detectors and 

Part II-Visible to Near-Infrared Detectors 

IJIST 

Special 

Publicatio 

250-41 

Thomas C. Larason, Sally S. Bruce, and Albert C. Parr 

U.S. Department of Commerce 

Technology Administration 

National Institute of Standards and Technology 
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NlST Special Publication 250-41 

NlST MEASUREMENT SERVICES: 

Spectroradiometric Detector Measurements: 

Part I-Ultraviolet Detectors and Part II-Visible to Near-Infrared Detectors 

Thomas C. Larason, Sally S. Bruce, and Albert C. Parr 

Optical Technology Division 

Physics Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899-0001 

Supersedes NBS Special Publication 250-1 7 

February 1998 

U.S. DEPARTMENT OF COMMERCE 

William M. Daley, Secretary 

Technology Administration 

Gary R. Bachula, Acting Under Secretary for Technology 

National Institute of Standards and Technology Raymond G. Kammer, Director 
National Institute of Standards and Technology Special Publication 250-41 

Natl. Inst. Stand. Technol. Spec. Publ. 25041, 118 pages (Feb. 1998) 

CODEN: NSPUEP 

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON: 1998 

For sale by the Superintendent of Documents, US. Government Printing Office, Washington, DC 20402-9325 
Preface 

The calibration and related measurement services of the National Institute of Standards and 

Technology (NIST) are intended to assist the makers and users of precision measuring 

instruments in achieving the highest possible levels of accuracy, quality, and productivity. NIST 

offers over 300 different calibrations, special tests, and measurement assurance services. These 

services allow customers to directly link their measurement systems to measurement systems and 

standards maintained by NIST. NIST offers these services to the public and private 

organizations alike. They are described in NIST Special Publication (SP) 250, NIST Calibration 

Services Users Guide. 

The Users Guide is supplemented by a number of Special Publications (designated as the “SP250 

Series”) that provide detailed descriptions of the important features of specific NIST calibration 

services. These documents provide a description of the: (1) specifications for the services; (2) 

design philosophy and theory; (3) NIST measurement system; (4) NIST operational procedures; 

(5) assessment of the measurement uncertainty including random and systematic errors and an 

error budget; and (6) internal quality control procedures used by NIST. These documents will 

present more detail than can be given in NIST calibration reports, or than is generally allowed in 

articles in scientific journals. In the past, NIST has published such information in a variety of 

ways. This series will make this type of information more readily available to the user. 

This document, SP250-4 1 (1 998), NIST Measurement Services: Spectroradiometric Detector 

Measurements, is a revision of SP250-17 (1988). It covers the calibration of standards and 

special tests of photodetector absolute spectral responsivity from 200 nm to 1800 nm (Service ID 

numbers 39071s - 390818 in SP250, NIST Calibration Services Users Guide). Inquiries 

concerning the technical content of this document or the specifications for these services should 

be directed to the authors or to one of the technical contacts cited. 

NIST welcomes suggestions on how publications such as this might be made more useful. 

Suggestions are also welcome concerning the need for new calibration services, special tests, and 

measurement assurance programs. 

Peter L. M. Heydemann (Acting) 

Director Director 

Measurement Services Physics Laboratory 

Katharine B. Gebbie 

... 

111 
Abstract and Key Words 

The National Institute of Standards and Technology supplies calibrated photodiode standards and 

special tests of photodetectors for absolute spectral responsivity from 200 nm to 1800 nm. (This 

service will soon be expanded to 20 pm in the infrared.) The scale of absolute spectral 

responsivity is based solely on detector measurements traceable to the High Accuracy Cryogenic 

Radiometer maintained by the National Institute of Standards and Technology. Silicon 

photodiode light-trapping detectors are used to transfer the optical power unit from this 

cryogenic radiometer to monochromator-based facilities where routine measurements are 

performed. The transfer also involves modeling the quantum efficiency of the silicon photodiode 

light-trapping detectors. A description of current measurement services is given along with the 

procedures, equipment, and techniques used to perform these calibrations. Detailed estimates 

and procedures for determining uncertainties of the reported values are also presented. 

Key Words: absolute spectral responsivity ; calibration; cryogenic radiometer; light -trapping 

detectors; optical measurement; optical power; photodetector; photodiode; quantum efficiency; 

quality system; radiometry; responsivity; scale; standards; silicon photodiode 

iv 
TABLE OF CONTENTS 

1 . INTRODUCTION ...................................................................................................................... 1 

2.NIST SPECTRORADIOMETRIC DETECTOR MEASUREMENT SERVICE ............... 3 

2.1 Description of Measurement Services ....................................................................................... 3 

2.2 Measurement Limitations .......................................................................................................... 6 

2.3 How To Order Photodiodes Or Special Tests ............................................................................ 6 

2.4 Technical Contacts ..................................................................................................................... 7 

3.1 Measurement Equation .............................................................................................................. 8 

3.1.1 Approximations ................................................................................................................. 11 

3.2 Substitution Method ................................................................................................................. 13 

3.2.1 General Substitution Method ............................................................................................ 13 

3.2.2 Photodetector Substitution ................................................................................................ 14 

3.2.3 Substitution Method with Monitor .................................................................................... 14 

3.2.4 Measurement Equation Applied to the SCFs .................................................................... 15 

4.1 Visible to Near-Infrared (Vis/NIR) Comparator Description .................................................. 17 

4.1.3 Vis/NIR Optics .................................................................................................................. 18 

4.1.4 Vis/NIR Translation Stages - Detector Positioning .......................................................... 19 

4.1.6 Beam Splitter and Monitor Detector ................................................................................. 19 

4.1.7 Alignment Lasers .............................................................................................................. 19 

4.1.8 Enclosure ........................................................................................................................... 20 

4.2 Ultraviolet (UV) Comparator Description ............................................................................... 20 

4.2.1 UV Source ......................................................................................................................... 21 

4.2.2 UV Monochromator .......................................................................................................... 21 

4.2.3 UV Optics ......................................................................................................................... 21 

4.2.4 UV Translation Stages - Detector Positioning .................................................................. 22 

4.2.5 UV Working Standards ..................................................................................................... 22 

4.2.7 Alignment Lasers .............................................................................................................. 22 

4.2.8 Enclosure ........................................................................................................................... 22 

4.3 Electronics ................................................................................................................................ 22 

4.3.1 Electronics - Signal Measurement .................................................................................... 22 

4.3.2 Electronics - Auxiliary Equipment ................................................................................... 25 

3 . MEASUREMENT THEORY ................................................................................................... 8 

4 . EQUIPMENT DESCRIPTION .............................................................................................. 16 

4.1.1 Vis/NIR Source ................................................................................................................. 17 

4.1.2 Vis/NIR Monochromator .................................................................................................. 18 

4.1.5 Vis/NIR Working Standards ............................................................................................. 19 

4.2.6 Beam Splitter and Monitor Detector ................................................................................. 22 

5 . ABSOLUTE SPECTRAL RESPONSIVITY SCALE REALIZATION ............................. 25 

5.1 Transfer from HACR to Traps (405 nm to 920 nm) ................................................................ 25 

5.2 Traps to Visible Silicon Working Standards ............................................................................ 28 

5.3 Extension to Ultraviolet (200 nm) and Near-Infrared (1 800 nm) ............................................ 29 

V 
5.3.1 Pyroelectric Detector ......................................................................................................... 29 

5.4 Germanium (NIR) Working Standards .................................................................................... 30 

5.3.2 Responsivity Measurements ............................................................................................. 30 

5.5 UV Silicon Working Standards ................................................................................................ 31 

5.6 Extension of Visible Silicon Working Standards .................................................................... 31 

6 . CALIBRATION PROCEDURES AND COMPUTER AUTOMATION ........................... 33 

6.1 Spectral Responsivity ............................................................................................................... 33 

6.1.1 Calibration Procedures ...................................................................................................... 33 

6.1.2 Quantum Efficiency .......................................................................................................... 35 

6.2 Spatial Uniformity .................................................................................................................... 36 

6.2.1 Measurement Method and Calibration Procedure ............................................................ 37 

6.2.2 Limitations ........................................................................................................................ 37 

6.3 Computer Automation .............................................................................................................. 40 

6.3.1 Computer Automated Equipment ..................................................................................... 40 

6.3.2 Computer Calibration Programs ....................................................................................... 43 

7 . UNCERTAINTY ASSESSMENT .......................................................................................... 46 

7.1 Uncertainty Components .......................................................................................................... 46 

7.1.1 “Indirect” Uncertainty Components .................................................................................. 49 

7.1.2 Uncertainty Components due to Assumptions and Approximations ................................ 51 

7.1.3 Other Factors Considered and Neglected .......................................................................... 54 

7.2 Transfer from Traps (HACR) to Working Standards .............................................................. 55 

7.2.1 Visible Silicon Working Standards ................................................................................... 55 

7.2.2 Germanium (NIR) Working Standards ............................................................................. 57 

7.2.3 UV Silicon Working Standards ......................................................................................... 58 

7.3 Transfer to Test (Customer) Detectors ..................................................................................... 60 

7.3.1 UV Silicon Transfer .......................................................................................................... 61 

7.3.2 Visible Silicon Transfer .................................................................................................... 61 

7.3.3 NIR Transfer ..................................................................................................................... 62 

7.3.4 Filtered Detector Transfer ................................................................................................. 64 

7.4 Spatial Uniformity Measurement Uncertainty ......................................................................... 64 

8 . QUALITY SYSTEM ................................................................................................................ 65 

8.1 Control Charts .......................................................................................................................... 65 

8.2 Comparison to Other Laboratories ........................................................................................... 66 

9 . CHARACTERISTICS OF PHOTODIODES AVAILABLE FROM NIST ....................... 67 

9.1 Hamamatsu S1337-1010BQ ..................................................................................................... 67 

9.2 Hamamatsu S2281 ................................................................................................................... 68 

9.3 UDT Sensors UV 100 ............................................................................................................... 68 

9.4 Detector Apertures ................................................................................................................... 70 

9.5 Detector Fixture Mechanical Drawings ................................................................................... 70 

10 . FUTURE WORK ................................................................................................................... 74 

11 . ACKNOWLEDGMENTS ..................................................................................................... 74 

vi 
12 . REFERENCES ....................................................................................................................... 75 

13 . BIBLIOGRAPHY .................................................................................................................. 82 

APPENDIX . SAMPLE TEST REPORTS 

3907 1 S . UV Silicon Photodiode Spectral Responsivity Test Report Sample ............................ A.1 

390733 . Visible to Near IR Silicon Photodiode Spectral Responsivity Test Report Sample .... A-8 

390758 . Near IR Photodiode Spectral Responsivity Test Report Sample ............................... A-15 

39081 S . Responsivity Spatial Uniformity Test Report Sample ............................................... A-21 

LIST OF FIGURES 

Figure 2.1. NIST UV. visible. and near-IR spectral responsivity measurement uncertainties ........ 4 

Figure 3.1 . The geometry for detector spectral responsivity measurements .................................. 9 

Figure 3.2. Block diagram of photodetector substitution method ................................................. 14 

Figure 3.3. Block diagram of photodetector substitution method with monitor ........................... 14 

Figure 4.1. Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR SCF) ..................... 17 

Figure 4.2. Spectral output flux of the UV and visible to near-IR monochromators .................... 18 

Figure 4.3. Ultraviolet Spectral Comparator Facility (UV SCF) .................................................. 20 

Figure 4.4. Vis/NIR detector, amplifier, DVM, and computer control block diagram ................. 23 

Figure 4.5. NIST SCF precision transimpedance (IN) amplifier circuit ...................................... 24 

Figure 4.6. Vis/NIR detector, amplifier, lock-in, DVM, and computer control block diagram .... 24 

Figure 5.1. NIST High Accuracy Cryogenic Radiometer (HACR) .............................................. 26 

Figure 5.2. Trap detector arrangement of photodiodes minimizes light lost to reflections .......... 27 

Figure 5.3. Scale transfer by substitution method with the HACR ............................................... 27 

Figure 5.4. NIST spectral power scale propagation chain ............................................................ 28 

Figure 5.5. NIST detector spectral responsivity scale realization ................................................. 32 

Figure 6.1 . Typical signals from Hamamatsu S 13 37 and monitor photodiodes ........................... 34 

Figure 6.2. Spectral responsivities of typical Si, InGaAs, and Ge photodiodes ........................... 35 

Figure 6.3. Quantum efficiencies of typical Si, InGaAs, and Ge photodiodes ............................. 36 

Figure 6.4. Spatial uniformities of typical Hamamatsu S 1337 and S2281 photodiodes .............. 38 

Figure 6.6. Vis/NIR SCF computer control block diagram .......................................................... 41 

Figure 6.7. UV SCF computer control block diagram .................................................................. 42 

Figure 6.5. Spatial uniformities of typical UDT Sensors UVl 00 and EG&G Judson Ge 

photodiodes .................................................................................................................. 39 

vii 
Figure 6.8. Measurement (computer) program flowchart for one detector scan .......................... 44 

Figure 7.1. Visible Working Standard (Vis WS) relative combined standard uncertainty ........... 57 

Figure 7.2. Germanium (NIR) Working Standard (Ge WS) relative combined standard 

uncertainty .................................................................................................................... 57 

Figure 7.3. Ultraviolet Working Standard (UV WS) relative combined standard uncertainty ..... 59 

Figure 7.4. Transfer to test (customer) detectors relative combined standard uncertainty ........... 61 

Figure 8.1. Control chart example for a NIST Visible Working Standard (Vis WS) ................... 66 

Figure 9.1. Temperature coefficient of silicon Hamamatsu S1226 and S1337 photodiodes ........ 68 

Figure 9.3. Linearity of UDT Sensors UV 100 at 442 nm ............................................................. 69 

Figure 9.2. Linearity of Hamamatsu S1337-1010BQ at 633 nm .................................................. 68 

Figure 9.4. Responsivity dependence on bias voltage of UDT Sensors UVlOO at 442 nm ......... 69 

Figure 9.5. Mechanical diagram of Hamamatsu S1337-1010BQ fixture body ............................ 71 

Figure 9.6. Mounting pieces for the Hamamatsu S 1337-1 0 1 OBQ photodiode and BNC connector 

...................................................................................................................................... 71 

Figure 9.7. Exploded view diagram of Hamamatsu S 1337 - 10 1 OBQ fixture ................................ 72 

Figure 9.8. Mechanical diagram of UDT Sensors UVlOO and Hamamatsu S2281 fixture .......... 72 

Figure 9.9. Mechanical diagram of pre - 1993 aperture plate for detector fixtures ........................ 73 

Figure 9.10. Mechanical diagram of present aperture plate for detector fixtures ......................... 73 

LIST OF TABLES 

Table 2.1. NIST Spectroradiometric Detector Measurement Services ........................................... 3 

Table 2.2. Detector Measurement Services Uncertainties .............................................................. 4 

Table 7.1. Visible Working Standard Uncertainty ........................................................................ 56 

Table 7.2. Germanium Working Standard Uncertainty ................................................................ 58 

Table 7.3. UV Working Standard Uncertainty ............................................................................. 59 

Table 7.5. Transfer Uncertainty to Test (Customer) S1337 and S2281 Silicon Photodiodes ....... 62 

Table 7.4. Transfer Uncertainty to Test (Customer) UV 100 Silicon Photodiodes ....................... 61 

Table 7.6. Transfer Uncertainty to Test (Customer) TE Cooled Germanium Photodiodes .......... 63 

Table 7.7. Transfer Uncertainty to Test (Customer) InGaAs Photodiodes ................................... 64 

Table 7.8. Photodetector Spatial Uniformity Measurement Repeatablity Uncertainty ................ 65 

... Vlll 
1. Introduction 

This document describes the National Institute of Standards and Technology (NIST) 

Measurement Service for providing calibrated photodiodes for absolute spectral responsivity in 

the ultraviolet (UV), visible, and near-infrared (NIR) spectral regions (200 nm to 1100 nm) and 

the special service of measuring photodetectors sent to NIST in the 200 nm to 1800 nm spectral 

region. This document supersedes NBS Special Publication 250-17 (1988), “The NBS 

Photodetector Spectral Response Calibration Transfer Program.” 

The theory, measurement system, operation, and transfer standards of the Spectroradiometric 

Detector Measurement Service are described in this publication. The traceability of the absolute 

spectral power responsivity scale to the High Accuracy Cryogenic Radiometer (HACR) and 

detailed uncertainty estimates are discussed. Also presented are the recently expanded list of 

absolute spectral power responsivity measurement services provided by NIST and the 

enhancements to its quality system to comply with ANSI/NCSL 2540-1-1994 [ 11. 

The material presented in this document describes the equipment and procedures for the 

Spectroradiometric Detector Measurement Service as they exist at the time of publication. 

Improvements in the equipment, procedures, and services offered are continuous. The 

discussions in this document will be primarily directed at the procedures developed to measure 

the absolute responsivity of photodiodes supplied by NIST to customers. The procedures for 

characterizing customer-supplied detectors is a straightforward extension of the underlying 

procedures developed for the NIST furnished devices. 

Note: This document follows the NISTpolicy of using the International System of 

Units (So. Only units of the SI and those units recognized for use with the SI are 

used Equivalent values in other units may be given in parentheses following the 

SI values. The mechanical drawings in section 9.5 were originally prepared in 

English units and are presented without converting the values shown to SI units. 

Background 

Many radiometric, photometric, and colorimetric applications require the determination of the 

absolute spectral power responsivity of photodetectors. The absolute spectral power responsivity 

is the ratio of the photodetector’s signal (amperes or volts) to the spectral radiant flux (watts) 

incident on the photodetector. The absolute spectral power responsivity is also referred to 

simply as the absolute spectral responsivity. Accurate measurement of absolute spectral power 

responsivity of photodetectors has been a service provided by the Optical Technology Division 

and its predecessors for over 20 years. 

Various techniques have been employed to determine photodetector absolute spectral power 

responsivity [2, 31. In the late 1970’s a room-temperature electrical substitution radiometer 

(ESR), also known as an electrically calibrated radiometer (ECR), was used in conjunction with 

1 
lasers [4, 51 as the detector scale base. Relative uncertainties were reported on the order of 1.5 % 

to 5 % (3 standard deviation estimate’) over the spectral range from 390 nm to 1100 nm [6]. 

Note: This document follows, to the extent possible, the IS0 Guide to the 

Expression of Uncertainty in Measurement (International Organization for 

Standardization, Geneva, Switzerland, 1993). Since 1994, the NIST policy has 

‘ been to conform to the Guide in reporting its activities, using an expanded 

uncertainty coverage factor (as defined in the Guide) of k = 2. See Ref. r/] for a 

detailed explanation of the NIST policy. Unless otherwise noted all uncertainties 

will be stated as k= 2. 

A major advance came in the early 1980’s with the silicon photodiode self-calibration techniques 

[8, 91 and the subsequent introduction of 100 % quantum efficient (QE) photodiodes [ 101 and 

their use as the basis for detector calibrations. These photodiodes were used in the development 

of the United Detector Technologies (UDT) QED-2002 trap detector [ll], which is a 

light-trapping device constructed of multiple, windowless, 100 % QE silicon photodiodes. The 

QED-200 had a limited spectral range (usually 400 nm to 750 nm) where it operated with 100 % 

QE, and suffered from limited dynamic range due to the relatively high bias currents used. The 

absolute spectral power responsivity was transferred to customers at this time via the Detector 

Response Transfer and Intercomparison Program (DRTIP). Customers would rent a radiometer 

from NIST and transfer the detector scale to their working standard(s). The scale relative 

uncertainty was typically 0.8 % to 6.0 % (3 standard deviation estimate) over the spectral range 

from 250 nm to 1100 nm [12]. 

A second generation trap detector (very similar to the commercially available Graseby Optronics 

QED-150) was later used as the basis for the NIST detector scale [13]. Both second generation 

trap detectors are constructed with a different type of silicon photodiode (Hamamatsu 

S1337-1010). They do not have 100 % QE, but the QE could be measured with the QED-200, 

and extrapolated with high accuracy over a large spectral range from 400 nm to 900 nm [ 141. 

Relative uncertainties were reported on the order of 0.33 % to 1.0 % (3 standard deviation 

estimate) over the spectral range from 250 nm to 1100 nm (between 200 nm and 250 nm the 

relative uncertainty was reported as 5.0 %). The next advance came in the late 1980’s when 

cryogenic ESRs were reported with improved uncertainties over the 100 ‘YO QE detector-based 

measurements [15, 16, 171. The current state of cryogenic ESRs and future direction of 

detector -based radiometry are discussed in Refs. [ 18 and 191. 

NIST has recently made changes to improve and expand the absolute spectral responsivity 

measurements it provides to its customers. The most significant change is to base the 

‘The term “3 standard deviation estimate” or “30” in the context of this paper is the estimated standard deviation 

multiplied by 3. Prior work at NIST was generally reported with “3 0’’ uncertainties. 

2Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such 

identification does not imply recommendation or endorsement by the National Institute of Standards and 

Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the 

purpose. 

2 
measurements on the NIST High Accuracy Cryogenic Radiometer [20]. The HACR is the U.S. 

primary standard for optical power. The scale of absolute spectral responsivity [21] is 

transferred by silicon photodiode trap detectors to working standards used with the NIST Visible 

to Near-Infrared [22] and Ultraviolet Spectral Comparator Facilities (hereafter referred to as the 

VisNIR SCF and UV SCF respectively in this document) where the Spectroradiometric Detector 

measurements are performed. 

2. NIST Spectroradiometric Detector Measurement Service 

This section describes the photodetector calibrations and measurements offered by the Optical 

Technology Division. A complete listing of the calibration services across all the measurement 

parameters offered by NIST can be found in the NIST Calibration Services Users Guide [ 231. 

Current fees can be found in the Fee Schedule [24] which is updated annually and on the internet 

at the URL address: http://ts.nist.gov/ts/htdocs/230/233/233 .htm. 

2.1 Description of Measurement Services 

There are two types of measurement services provided by NIST: Fixed Services and Special 

Tests. Fixed services (Service ID numbers ending in the letter C) have fixed measurement 

conditions and NIST issues a calibration report to the customer. Special tests (Service ID 

numbers ending in the letter S) have no fixed measurement conditions; are services that are 

under development; and/or are for unique customer-supplied test items. Service ID numbers 

390718 through 39075s are currently in the process of being converted from Special Tests to 

Fixed Services (i.e., calibrations). 

The present spectral range for photodetector absolute spectral responsivity measurements are 

from 200 nm to 1800 nm. Table 2.1 lists the services offered with typical measurement ranges 

and typical uncertainties. All services listed are provided routinely. The relative expanded 

uncertainties of the Spectroradiometric Detector Measurement Service are shown in figure 2.1 

and table 2.2. See section 7 for a detailed explanation of the uncertainties. 

Table 2.1. NIST Spectroradiometric Detector Measurement Services 

Service ID Item of test Range Relative expanded 

number uncertainty (k = 2) 

390718 Ultraviolet Silicon Photodiodes (UDT UVlOO) 200 MI to 500 nm 0.4 %to 13 Yo 

39072s 200 nm to 500 nm 

390733 350 nm to 1100 nm 

Can be extended to 200 nm 

390743 Retest of Visible to Near-Infrared Silicon Photodiodes 350 nm to 1100 nm 

Can be extended to 200 nm 

390758 700 nm to 1800 nm 

390808 200 nm to 1800 nm 

39081s 200 nm to 1800 nm 

Retest of Ultraviolet Silicon Photodiodes (UDT UVIOO) 

Visible to Near -Infrared Silicon Photodiodes 

(Hamamatsu S2281) 

(Hamamatsu S1337-1010BQ or S2281) 

Special Tests of Near -Infrared Photodiodes 

Special Tests of Radiometric Detectors 

Special Tests of Photodetector Responsivity Spatial Uniformity 

'Depends on photodetector and signal level. 

0.4 Yo to 13 Yo 

0.2 %to 4 % 

0.2 Yo to 13 Yo 

0.2 Yo to 4 Yo 

0.2 Yo to I3 % 

0.5 %to 7 %' 

0.2 %to 13 %' 

0.0024 %to 0.05 %' 

3 
Wavelength [nm] 

(Note: Relative expanded uncertainty at 200 nm is 13 %.) 

Figure 2.1. NIST UV, visible, and near -1R spectral responsivity measurement uncertainties. 

The three curves are the relative expanded uncertainty (k = 2) for measurements with the 

three different working standard types. 

Table 2.2. Detector Measurement Services Uncertainties 

Relative expanded uncertainty (k = 2) [%I 

UVlOO (UV) SI337 (Visible) Ge (NIR) InGaAs (NIR) lnml 

250 1.36 

300 2.06 

350 1.68 2.96 

400 1.46 1.56 

450 0.38 0.24 

500 0.38 0.22 

550 0.20 

600 0.20 

650 0.20 

700 0.20 1.78 1.80 

750 0.22 1.42 1.46 

800 0.22 I .32 1.38 

850 0.22 1.12 1.16 

900 0.22 0.92 0.96 

950 2.58 1.80 1.82 

1000 1.72 1.50 1.52 

1100 4.16 1.56 1.56 

1150 1.92 1.92 

1200 2.26 2.26 

1250 2.60 2.60 

1300 2.56 2.56 

1350 3.46 3.46 

1400 4.80 4.80 

1450 4.64 4.64 

1500 5.66 5.66 

1550 4.94 4.96 

1600 4.30 4.34 

1650 5.04 5.08 

1700 4.92 5.04 

1750 5.24 5.38 

1800 5.02 5.36 

1050 2.66 1.74 1.74 

4 
Descriptions of each Service ID number are provided below. 

3907 1 S - UV Silicon Photodiodes 

NIST will supply customers with a UDT Sensors, Inc. model UVlOO windowed silicon 

photodiode characterized in the ultraviolet (UV) spectral region. The UV silicon photodiode 

includes the measured spectral responsivity [A-W-*]3 from 200 nm to 500 nm in 5 nm steps. The 

1 cm2 photosensitive area of the photodiodes is underfilled for the measurements with a beam of 

diameter 1.5 mm. The spectral responsivity is measured at radiant power levels of less than 20 

pW. The bandpass of the measurement is 4 nm. The relative expanded uncertainty ranges from 

0.4 YO to 13 %, depending on the wavelength. The spatial uniformity of responsivity over the 

photosensitive area is also measured at 350 nm. 

39072s - Retest of UV Silicon Photodiodes 

Special tests of UV silicon photodiodes previously supplied by NIST (under 3907 

performed by measuring spectral responsivity from 200 nm to 500 nm. 

39073s - Visible to Near-Infrared Silicon Photodiodes 

NIST will supply customers with a Hamamatsu model S2281 (previously a Ham 

S) are 

matsu 

S1337-101 OBQ) windowed silicon photodiode characterized in the visible to near-IR spectral 

region. The spectral responsivity of the photodiode is measured from 350 nm to 1100 nm in 

5 nm steps. The 1 cm2 photosensitive area of the photodiodes is underfilled for the 

measurements with a beam of diameter 1.1 mm. The spectral responsivity is measured at radiant 

power levels of less than 1 ,pW. The bandpass of the measurement is 4 nm. The relative 

expanded uncertainty ranges from 0.2 YO to 4 YO, depending on the wavelength. The spectral 

range can be extended to 200 nm with a relative expanded uncertainty from 0.2 % to 13 % for an 

additional fee. The spatial uniformity of responsivity over the photosensitive area is also 

measured at 500 nm. 

390748 - Retest of Visible to Near-Infrared Silicon Photodiodes 

Special tests of visible to near-infrared silicon photodiodes previously supplied by NIST (under __ 

390738) are performed by measuring spectral responsivity from 350 nm to 1100 nm. The 

spectral range can be extended to 200 nm for an additional fee. 

390758 - Special Tests of Near-Idrared Photodiodes 

Special tests of customer-supplied near-infrared photodiodes are performed by measuring 

spectral responsivity from 700 nm to 1800 nm. A beam of diameter 1.1 mm is centered on and 

underfills the photosensitive area. The spectral responsivity is measured at radiant power levels 

of less than 1 pW. The bandpass of the measurement is 4 nm. The relative expanded 

uncertainty ranges from 0.5 % to 7 % or greater, depending on the wavelength and the individual 

item measured. Customers should communicate with one of the technical contacts listed in 

section 2.4 to discuss details before submitting a formal request. 

For clarity, the coherent SI unit is given in brackets for many quantities used in this publication. Of course, SI 

multiples and submultiples of these units may also be used. 

5 
390808 - Special Tests of Radiometric Detectors 

Special tests of radiometric detectors in the ultraviolet, visible, and near-infrared regions of the 

spectrum can be performed. Detector characteristics that can be determined in this special test 

include spectral responsivity and quantum efficiency (electrons per photon). For example 

detector responsivity can be measured between 200 nm and 1800 nm at power levels less than 

4.0 pW. The relative expanded uncertainty ranges from 0.2 % to 13 % or greater, depending on 

the wavelength and the individual item measured. Since special tests of this type are unique, 

details of the tests should be discussed with one of the technical contacts listed in section 2.4 

before submitting a formal request. 

3908 1 S - Special Tests of Photodetector Responsivity Spatial Uniformity 

Special tests consisting of measuring the relative changes in responsivity across the 

photosensitive area (spatial uniformity) can be performed for customer-supplied photodetectors. 

The uniformity is typically measured at a single wavelength in 0.5 mm spatial increments with a 

beam diameter of 1.5 mm in the 200 nm to 400 nm spectral region at power levels less than 

20 pW, and a beam of diameter 1.1 mm in the 400 nm to 1800 nm spectral region at power 

levels less than 1 pW. The relative expanded uncertainty ranges from 0.0024 % to 0.05 % or 

greater, depending on the wavelength and the individual item measured. Customers should 

communicate with one of the technical contacts listed in section 2.4 to discuss details before 

submitting a formal request. 

2.2 Measurement Limitations 

There are a few limitations on the types of photodetectors that can be measured. Because of the 

beam size of the comparators, the detector’s active area must be greater than 3 mm in diameter. 

Due to the monochromator flux level an amplifier gain of 10 to 10’ is typically required, thus 

the photodiode dynamic impedance (shunt resistance) must be greater than 10 162. 

Physical size and weight are limited by the translation stages used in the UV SCF and VisNIR 

SCF. Detector packages submitted for testing are limited in size to approximately 20 cm by 

20 cm by 20 cm and 2 kg. 

Special tests with conditions greatly differing fiom those listed above may be accepted, but will 

take longer to complete. The photodetector signal (either voltage or current) must be provided 

via a BNC connector. Computer communication to a customer’s detector package is not possible 

at this time. 

23 How To Order Photodiodes Or Special Tests 

1) Reference the Service ID number(s1 on the purchase order. 

Details of Service ID numbers 390758, 390808, and 390818 should be discussed with one of 

the technical contacts listed in section 2.4 prior to submitting a formal request. 

6 
2) The purchase order should be sent to: 

National Institute of Standards and Technology 

Calibration Program 

Building 820, Room 232 

Gaithersburg, MD USA 20899-0001 

Phone number: (301) 975 -2002 

FAX Number: (301) 869-3548 

E-mail: calibrations@nist.gov 

Internet URL: http://ts.nist.gov/ts/htdocs/230/233/23 3. htm 

3) The purchase order must include the following: 

A) Bill to address. 

B) 

C) 

D) 

E) Service ID number(s). 

Ship to address (for the test detector(s) return delivery). 

Method of return shipment (the costs below do 

If nothing is stated, NIST will return by common carrier, collect, and uninsured. 

User’s name, address (for test report), and phone number. 

include shipping costs). 

4) The cost for special tests is based on the actual labor and material costs involved and 

customers are responsible for all shipping costs. 

2.4 Technical Contacts 

For technical information or questions contact: 

Thomas Larason (301) 975 -2334 email: thomas.larason@nist.gov 

Sally Bruce (301) 975 -2323 email: sally.bruce@nist.gov 

or fax (301) 869-5700 

Technical information can also be found at the following Internet URL: 

http://phy sics .nist. gov/Divisions/Div844/facilities/phdet/phdet. html 

All test detector(s) should be shipped to the following address: 

Sally Bruce 

NIST 

Building 221 / Room B208 

Gaithersburg, MD USA 20899-0001 

7 
3. Measurement Theory 

This section describes the theory and the mathematical basis for the measurement methods used 

for the services outlined in this publication. The measurement equation is developed and the 

generalizing assumptions are discussed. The measurement equation is then used in estimating 

the uncertainties in section 7. The calibration method using detector substitution is described 

first in general, and then, in detail for the measurement services. 

3.1 Measurement Equation 

Developing the measurement equation is fundamental to understanding the physics and optics 

involved with the measurement. This derivation provides an analytical foundation for the 

measurement process and the assumptions and approximations used in the measurement process. 

The measurement equation is also fundamental to the analysis of the uncertainty of the 

measurement process. In essence, a detector comparator is a spectroradiometer that measures the 

response of different detectors instead of different sources. Thus, the measurement equation 

developed for a general spectroradiometer can be adapted to detector spectral responsivity 

measurements. 

The measurement equation presented here for spectral responsivity is developed following the 

general procedure described in chapter 5 of the Self-study Manual on Optical Radiation 

Measurements [25] and is similar to the development of eq (7.18) in Ref. [26] for spectral 

irradiance when using a monochromator -based spectroradiometer. The measurement equation 

for spectral responsivity is 

where V(A, AA, A,) is the output signal (in U units, typically volts or amperes); E, (x, y, Ao, A) is 

the spectral irradiance function in A of the comparator system at detector position x,y for a 

wavelength setting of Ao; S, (x, y, A) is the spectral (radiant flux) responsivity of the detector; A 

is the area of the radiant flux beam at the detector (See fig. 3.1 .); and A?,, is the wavelength 

interval for which the value of Ea is not zero (i.e., the full-width bandpass). This equation is 

equivalent to eq (7.1) [26], with some minor changes in the notation, where A and Ail are left 

symbolically in V to indicate that Y depends on how these elements are chosen. 

To simplify the analysis, the responsivity S, is assumed uniform throughout A, so, 

8 
Thus, the responsivity S, is no longer dependent on position (x,y) and can be removed from the 

area integral 

(3.3) 

Figure 3.1. The geometry for detector spectral responsivity measurements. 

Evaluating the area integral gives the spectral radiant flux function of the comparator system at 

the detector 

where 

optics (and atmosphere) between the monochromator and the detector (See fig. 3.2.). 

(A,, A) is the output flux fiom the monochromator and T( A) is the transmittance of the 

Considering only the spectral dependence of the signal for any given A, the measurement 

equation can be written 

Equation (3 -5) is the flux equivalent to eq (7.1 b) [ 261. Introducing the slit-scattering function 

z(A, -A) to the measurement equation allows the spectral radiant flux function of the 

monochromator @*(A,, A) to be written as the product of two functions (with appropriate 

normalization) 

where the slit -scattering function z(A, - A) is dependent only on the difference between the 

wavelength setting of the monochromator and the wavelength of the flux and the “spectral flux” 

factor @f,n is dependent only on the wavelength of the flux. The factor @f,I is the spectral 

9 
radiant flux at Ao, @I(Ao), and is equivalent to the responsivity factor rf introduced in 

eq (7.12) [26]. Both the slit-scattering function z(Ao -A) and the factor can be determined 

experimentally, although the latter requires deconvolution from the measured output flux of the 

monochromator. 

Thus the output signal Vcan now be written 

Equation (3.7) is the flux equivalent to eq (7.13) [ 261 and is in a form that can be used for finding 

the spectral responsivity of a detector. Thus a measurement equation has been developed for a 

spectrally selective source instead of a spectrally selective detector (spectroradiometer). If the 

product S, .z. is approximately constant (or linear) over the spectral range for which 

z(A, - A) is significant (Le., within AA) and z is approximately symmetrical with respect to &, 

eq (3.7) can be written 

Equation (3.8) is the equivalent flux version of eq (7.18) [26]. Implicit in this derivation is the 

assumption that the slit-scattering function z(Ao - A) does not change with the wavelength 

setting A0 of the monochromator (that is, the dispersion is the same for all wavelengths) and thus 

z(Ao - A) need only be measured once. Optical aberrations, scattering, and diffraction are also 

assumed to be the same whether a monochromatic beam is varied in A over the monochromator 

bandpass, AA, or Lo of the monochromator is varied over a beam of fixed wavelength A. 

Equation (3.7) is the measurement equation for an ideal monochromator. A real monochromator 

system has spectrally scattered light (also known as stray light or out-of-band radiation and 

hereafter will be referred to as simply stray light) due to imperfections in the monochromator 

(and other optics). This is light from outside the spectral region AA which is scattered into AA 

and which contributes to the measured signal. Adding a stray light term, V,, (An, A,), to eq (3.8) 

gives 

The V,, term in eq (3.9) is typically small for radiometric measurement systems and is normally 

ignored in the “routine” measurement equation but is included in the uncertainty estimate 

calculations in section 7.1.2. Equation (3.9) separates the measurement equation into two parts, 

the first term represents the in-band signal and the second term represents the out-of-band signal 

as indicated by the limits on each integral. 

10 
The integral remaining in the first term can be evaluated and combined with the “spectral flux” 

factor Of,* to give an expression for the flux leaving the monochromator ai, 

(3.1 1) 

Using eq (3.1 1) the measurement equation (eq (3.9)) can now be written in a form that is easily 

applied to the situation of measuring detector spectral responsivity 

(3.12) 

Before developing a “routine” measurement equation, it is important to reiterate the simplifying 

assumptions that were made to develop eq (3.12). 

3.1.1 Approximations 

The assumptions made in the development of the measurement equation (eq (3.12)) are 

summarized and discussed below. The development of the measurement equation followed the 

procedure described in chapter 5 of the Self -Study Manual on Optical Radiation Measurements 

[25] and does not include parameters for time, polarization, or incident angle; nor does it include 

environmental parameters such as ambient temperature or humidity, corrections for diffraction 

effects (departure from geometrical (ray) optics), and nonlinear responsivity . 

A key requirement to detector-based radiometry is that detector responsivities are stable over 

time. In general the light from any monochromator system can be polarized, thus the effect of 

polarization on the responsivity of the detectors needs to be evaluated. Generally, this is not a 

problem for most photodetectors since the detectors are measured at normal incidence to the 

optical axis (and the detector surfaces are isotropic). The effect of the converging beam angle on 

the reflectance (and transmittance) from the detector surface (and window) is small compared to 

the variance of repeated measurements and is typically neglected. However this is not the case 

when filters are used with the detectors (e.g., photometers), especially interference filters, where 

the transmission is a strong fbnction of the angle of incidence. Also, the detector area must be 

larger than the optical beam so that all of the optical radiation is collected by the detector (Le., 

the detectors are underfilled). This also requires that the detector Field-of-View (FOV) be larger 

than the optical beam from the comparator system. The detector size and FOV of the typical 

1 cm2 photodiodes NIST provides meet these requirements. 

Humidity can affect the measurement by changing the transmittance z of the system and will be 

taken into account in the measurement equation developed below. Humidity can also affect the 

detector itself (or its window). For example, with windowless silicon photodiodes the absorption 

of water by the SiOz surface passivation layer changes the photodiode reflectivity which changes 

the responsivity [27]. Typically, for a windowed detector in the laboratory, effects due to water 

absorption (onto the detector or window) is not observed. 

11 
The temperature variation of the laboratory is small (typically < 1 "C) over the measurement 

time; therefore, the responsivity temperature dependence is neglected over most of the spectral 

region. When of concern, it can be applied as an additional uncertainty term. 

Diffraction effects can be esiimated from [ 281 

a gd = 2.44- [rad], d (3.13) 

where Od is the diffraction angle (for the first Airy disk), A is the longest wavelength of the 

system, and d is the diameter of the aperture at the monochromator exit slit (the smallest aperture 

in the system). For the Vis/NIR SCF A= 2 pm and d = 1 mm which gives 

2 Pm O~is,,,=2.44----=4.88x10-3 =5mrad. 

1 mm 

(3.14) 

The Vis/NIR SCF has a focal ratio (f-number or f /#) of = 9 giving a beam angle of = 1 10 mrad 

which is over 20 times greater than the diffraction limit. For the UV SCF (A = 0.5 pm, 

d = 1.5 mm, and f /# = 5) the diffraction effects are even smaller due to the shorter wavelengths. 

In this case, the beam angle is = 200 times greater than the diffraction limit. Thus diffraction 

effects can be ignored for most UV, visible, and near-IR systems of the type described in this 

publication. Likewise, coherence effects are negligible [29] and are ignored in the analysis of 

these comparator systems. 

The reiponsivity S, is assumed uniform throughout the area A of the incident beam on the 

detector. In practice, this is accomplished by reducing A (both mechanically with apertures and 

optically via imaging optics) so that S, is uniform over A. This limits the amount of flux 

(power) that can be delivered to the detector but this does not hinder the responsivity 

measurements for typical photodiodes. Deviations from this approximation are considered as 

uncertainty terms. 

Certain assumptions were made about the system [ 261 (primarily the monochromator). One 

assumption is that the dispersion remains the same for all wavelengths and that the slit-scattering 

function z(& -A) does not change with the wavelength setting A. of the monochromator. 

These approximations are not included in the uncertainties because their effect was determined 

to be negligible. This allows z(Ao - A) to be measured only once for an instrument and used 

over the entire spectral range. Another assumption is that optical aberrations, scattering, and 

diffraction are the same whether the A of a monochromatic source is varied over the 

monochromator bandpass AA or whether the monochromator wavelength setting Lo is varied over 

a monochromatic source of wavelength A. Also z(A, - A) is assumed approximately 

symmetrical with respect to A0 which is the case for the monochromators described in this 

publication. 

12 
The product S, . Z. @f,A is assumed to be constant (or linear) over AA. In practice, this is affected 

by reducing AA so that S, *@f,n is constant (or linear) over AA. This also limits the amount of 

flux (power) that can be delivered to the detector, but this is not a hindrance for typical 

measurements. Most measurements are made on broadband detectors using a broadband source 

to provide a wide range of measurement wavelengths. However, this assumption may not be 

valid with some arc and discharge sources (primarily used in the UV) that can have strong 

spectral variations in output intensity. Typically, they are small and considered as uncertainty 

terms. 

The spectrally scattered light is typically negligible for double monochromator systems but not 

for many single monochromator systems. Because the spectrally scattered light is small in 

magnitude it will not be used as a correction term in the measurement equation, but it will be 

evaluated and considered as an uncertainty term in section 7. 

3.2 Substitution Method 

The substitution method uses a standard detector to transfer its responsivity (output divided by 

input) to an unknown (test) detector. This method has a number of advantages that will be 

discussed in general and applied specifically to photodetectors. The measurement equation will 

then be developed and the general assumptions and their consequences discussed. Finally the 

measurement equation as applied to the systems described in this publication is developed. 

3.2.1 General Substitution Method 

The output (signal) Y from a detector (or system) with a linear response in U units, typically volts 

or amperes, can be written in general 

where Y, is the signal from the detector, S, is the responsivity of the detector, and X, is the input 

to the detector. The signal from a standard detector with a known responsivity S, is given by 

Y, =ss.x, VI. 

Dividing eq (3.15) by eq (3.16) we have 

If the input X to each system is assumed to be constant 

method), solving for the detector responsivity S, gives 

(3.16) 

(3.17) 

(this is the basis of the substitution 

YX S, = -.S, [Uainput units-']. 

YS 

(3.18) 

13 
Thus, using the substitution method, the standard detector responsivity S, is scaled by the ratio of 

the outputs. The transfer is accomplished in essence by the standard detector measuring the 

input Xto the test detector. 

u Source 

3.2.2 Photodetector Substitution 

Test Monochromator Focusing 

(A. selection) Optics OD,,=T* 0, 

Photodetector responsivity measurements by detector substitution can be made using eq (3.18) 

and the following equipment: a broadband source, monochromator, focusing optics, and 

standard detector. (See fig. 3.2.) The radiant flux (power) @ is the output flux from the source 

that enters the monochromator. The spectral radiant flux @a is the output flux from the 

monochromator and z is the transmittance of any optics (and the atmosphere) between the 

monochromator and the detector. The spectral radiant flux received by the detectors is 

@D,n =Z.@,. 

I I I 1 I I 

Using eq (3.18) assumes that the source is stable over the comparison time. This is generally 

true for incandescent sources (Le., quartz-halogen (QTH) lamps), but less true for arc sources 

which are primarily used in the UV. 

3.2.3 Substitution Method with Monitor 

To eliminate the effect of power fluctuations, a beamsplitter and monitor detector are used. (See 

fig. 3.3 .) Here Qi,, = z, is the flux received by the detectors and OM 1 = pbs z - @a is the 

flux received by the monitor. The transmittance and reflectance of the beam splitter are Zbs and 

Pbs, respectively. The monitor detector and beamsplitter are assumed to be stable (constant) over 

the measurement time. Thus the monitor records the source power fluctuations and the ratio of 

detector to monitor signals will be constant. 

Beam 

Splitter . z * @A 

Monochromator 

(A selection) 

Monitor 

Figure 3.3. Block diagram of photodetector substitution method with monitor. 

14 
3.2.4 Measurement Equation Applied to the SCFs 

The actual measurement equation is now developed. Dropping the A notation and changing the 

sub- and superscripts for clarity from eq (3.12), it can easily be shown that the signal from a 

detector x is 

where S, is the spectral responsivity in A.W-’, G, is an explicit gain term for a transimpedance 

amplifier in V-A-’, z, is the transmittance of any optics (and the atmosphere) between the 

monochromator and the detector, @, is the output flux from the monochromator in W, and V,,, is 

the dark output (sometimes called the “background signal”) in V, i.e., the signal produced when 

no flux is incident on the detector. (For photodiodes this is caused by their dark current.) In 

practice Vd,, is found by measuring the signal from the detector with the shutter closed at the exit 

slit of the monochromator. The net signal AV, is the signal due to the radiant (optical) flux 

received by the detector 

AV, =Vx -Vdx =Sx .Gx ’7, .Ox [VI. (3.20) 

This is the “routine” measurement equation for the substitution method depicted in figure 3.2. 

The measurement equation for the “substitution method with monitor” (fig. 3.3) is written by 

explicitly including the transmittance Tbs and reflectance pbs of the beam splitter assembly (which 

also contains the reflectance of the monitor turning mirror). The systems described in this 

publication simultaneously sample both the detector and monitor. The signals from the test 

detector x and monitor detector mx are 

The ratio of these two signals is 

AVx - S, *G, “T, *Ox R, =- - 

“mx ‘mx *Gmx ‘Pbsx “x ’Ox ’ 

(3.23) 

wherez,,, ‘7, a@, is the fraction of light received by the test detector, and pbsx ST, .cDX is the 

fraction of light received by the monitor. 

There is a similar ratio for the two signals from the working standard detector and monitor, 

(3.24) 

15 
Dividing eq (3.23) by eq (3.24) (taking the ratio of the ratios) gives 

Looking at eq (3.25), it is now seen that variations or drifts in the source flux or system 

transmittance during the time between the measurement of the test detector and the working 

standard detector are canceled by the monitor detector. If the beamsplitter and monitor detector 

responsivity and amplifier gain are stable (constant) over the comparison time, other terms 

cancel, leaving 

(3.26) 

Solving eq (3.26) for the test detector spectral responsivity S, and substituting the signal 

measurements into the ratios we have 

sx =-.-. Rx Gs ss = ‘mx - ’d, mx *-ass Gs [AeW-’]. 

Rs Gx ’s -‘d,s G~ 

(3.27) 

‘ms - ‘d, ms 

This is the working form of the measurement equation. Note that it still has the general form 

S, = - YX .s, [U-input units“ I. 

YS 

(3.18) 

Depending on how the monochromator wavelength scale is calibrated, a correction term may be 

needed when the responsivity curves of the test and working standard detectors have different 

slopes. If the centroid wavelength of the bandpass is used to calibrate the monochromator 

wavelength scale, then there is no correction term. But if the peak wavelength of the bandpass is 

used, then eq (3.27) requires a correction term. Thus the measurement equation becomes 

sx =-.-. Rx Gs Ss+C, [AeW-’], (3.28) 

Rs Gx 

where Cbw is a correction term due to the bandpass of the monochromator and is referred to as 

the “bandwidth-effect.” Like the stray light term, the bandwidth-effect is small for this system 

and is ignored in the “routine” measurement equation. The bandwidth-effect is analyzed in 

section 7.1.2 as an uncertainty term and included in the uncertainty estimate. 

4. Equipment Description 

In this section, the Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR SCF) and 

Ultraviolet Spectral Comparator Facility (UV SCF) components are described along with the 

associated electronics. 

16 
4.1 Visible to Near-Infrared (Vis/NIR) Comparator Description 

The Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR SCF) is a 

monochromator -based system that typically measures the uniformity and absolute spectral 

responsivity of photodiodes in the 350 nm to 1800 nm spectral region. The Vis/NIR SCF 

operates from 350 nm to 1100 nm using silicon photodiodes as working standards and from 

700 nm to 1800 nm using germanium photodiodes as working standards. 

The Vis/NIR SCF uses the direct substitution method and automated translation stages to 

position the photodetectors for measurement. The test detectors as well as the working standards 

are fixed onto optical mounts that rotate and tilt for accurate alignment. A variety of sources can 

be selected. Typically a 100 W quartz-halogen lamp is used as the source in the Vis/NIR SCF. 

A shutter is located just after the monochromator exit slit. A monitor detector located after the 

monochromator measures source fluctuations. The detectors and the exit optics are enclosed in a 

light tight box. A diagram of the Vis/NIR SCF is shown in figure 4.1. Each of the primary 

components is now described in greater detail. 

Light Tight Enclosure 

Splitter f Beam Monitor & Detector 

Shutter 

Baffle 

Sources 

0 

Figure 4.1. Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR SCF). 

4.1.1 Vis/NIR Source 

Typically the Vis/NIR SCF uses a 100 W (12 V) quartz-halogen lamp that is constant-voltage 

controlled for stability. The lamp is used over the spectral range of 350 nm to 1800 nm. An FEL 

lamp, tungsten strip lamp, xenon arc lamp, argon arc, or alignment laser can be used as a source 

in the Vis/NIR comparator as well. The spectral output flux (at the detector) of the 100 W 

quartz-halogen lamp with the Vis/NIR SCF is shown in figure 4.2. 

17 
Safety considerations 

Quartz-halogen lamps, lasers, and arc sources are potential eye hazards. Care is taken to never 

look directly into any of the sources or the laser beam. Shields mounted on the side of the 

optical table inhibit direct visual contact with the sources used with the comparator. The lasers 

used for alignment are Laser Safety Class 11. Protective eyeware is worn when working in 

proximity to these sources. 

10 

1 

0.1 

0.01 

nm ~ I 

- -. 

- 

. 

- 

......... 

- - 

L 

- 

- 

- 

Ill Ill Ill Ill Ill Ill Ill Ill 

200 400 600 800 1000 1200 1400 1600 1800 

Wavelength (nm) 

Figure 4.2. Spectral output flux of the UV and visible to near-IR monochromators. 

The argon mini-arc data are from the UV monochromator and 100 W 

quartz-halogen lamp data are from the Vis/NIR monochromator. 

4.1.2 Vis/NIR Monochromator 

The visible comparator uses a prism-grating monochromator, a NIST modified Cary - 14, which 

employs a 30" fused silica prism in series with a 600 lines per mm echelette grating. The 

monochromator's spectral range is 186 nm to 2.65 pm. The spectral range used in the detector 

characterization facility is 350 nm to 1800 nm. In the typical measurement configuration, the 

monochromator slits are set to 1.1 mm with a bandpass of 4 nm. A circular aperture of 1.1 mm 

diameter just past the exit slit determines the beam size. The exit beam is fl9. The 

monochromator has a stray light rejection of lo-*. Greater than 99 % of the beam flux lies within 

an area of 1.6 mm diameter around the optical axis. 

4.1.3 Vis/NIR Optics 

A 46 mm x 61 mm flat mirror rotates on an automated stage for source selection. The source is 

imaged by a stationary 15.24 cm diameter spherical mirror onto the entrance slit of the 

monochromator. A shutter, with a 25 mm diameter aperture, is placed inside the enclosure past 

the monochromator exit slit and aperture. 

18 
Two 15.24 cm diameter spherical mirrors and two 7.62 cm diameter flat mirrors image the exit 

aperture of the monochromator with 1:l magnification onto the detector. The two flat mirrors 

used to optically fold the system are not shown in figure 4.1. Mirrors are used to prevent 

chromatic aberrations, and the astigmatism of the spherical mirrors is corrected (to first order) by 

tilting the second spherical mirror perpendicular to the plane of the first spherical mirror [ 301. 

4.1.4 Vis/NIR Translation Stages - Detector Positioning 

The Vis/NIR comparator uses two orthogonal linear positioning stages to translate the test 

detectors and working standard detectors. The horizontal stage travel range is 400 mm with a 

manufacturer specified resolution of 0.1 pm and an accuracy of 0.2 pm per 100 mm. The 

vertical stage travel range is 50 mm with a manufacturer specified resolution of 0.1 pm and an 

accuracy of 0.25 pm per 25 mm. 

Each detector can be translated along the optical axis for focusing. A gimbal mount allows the 

rotation and tilt of each detector to be adjusted perpendicular to the optical axis. 

4.1.5 Vis/NIR Working Standards 

The visible working standards (Vis WS) are four Hamamatsu S 1337-1010BQ silicon 

photodiodes with fused quartz windows. This is a p-n photodiode with a 1 cm2 active area. The 

Hamamatsu S1337 diode is a popular diode for radiometric standards, and the linearity, 

uniformity, and stability of the Hamamatsu S1337 diode has been documented in many places 

[31, 321. For 

near-infrared measurements (700 nm to 1800 nm), four EG&G Judson J16TE2 -8A6 -ROSM-SC 

temperature-controlled germanium photodiodes are used as working standards (Ge WS). This is 

a p-n photodiode with a 5 mm diameter active area and a wedged sapphire window. Typically, 

two working standards are used for each spectral comparison measurement. The second working 

standard is used as a “check” standard. 

The Vis WS are normally used for 350 nm to 1100 nm measurements. 

4.1.6 Beam Splitter and Monitor Detector 

Variations in the source intensity are corrected by using a beam splitter and monitor detector as 

discussed in section 3.2.3. The beam splitter is a 50.8 mm x 50.8 mm flat fused quartz plate. 

Hamamatsu S1337-1010BQ and EG&G Judson J16TE2-8A6-ROSM-SC photodiodes are used as 

monitor detectors. The monitor detector is typically the same type as the working standard used 

for the measurement. A 35 mm x 46 mm flat oval mirror is used to optically fold the monitor 

beam to a convenient physical location for the monitor. 

4.1.7 Alignment Lasers 

Two HeNe lasers are aligned to the optical axis of the Vis/NIR comparator defined by the 

monochromator. One laser is located in one of the source positions and is typically used to align 

the detectors. The other is located inside the enclosure and is pointed “backwards” through the 

comparator to align the sources. It is not shown in figure 4.1. 

19 
4.1.8 Enclosure 

Each comparator has a light tight enclosure to eliminate the background light from the room. 

This is essential for the routine dc (optically unmodulated) measurements. The enclosure is a 

box that sits on the optical table. Several doors allow easy access to the equipment and 

detectors. The enclosure houses the test detectors, working standards, monitor detector, and 

associated electronics, including the amplifiers. The horizontal and vertical translation stages and 

the exit optics are housed inside the enclosure as well. 

The enclosure also reduces the amount of dust settling on the detectors and exit optics. An 

exhaust fan is used to regulate the environment inside of the enclosure, and the temperature 

inside the enclosure is monitored. 

4.2 Ultraviolet (UV) Comparator Description 

The Ultraviolet Spectral Comparator Facility (UV SCF) is a monochromator -based system that 

measures the uniformity and absolute spectral responsivity of photodetectors in the 200 nm to 

500 nm spectral region. The UV SCF is very similar in configuration and operation to the 

Vis/NIR SCF. Only the differences between the two will be described. A diagram of the UV 

SCF is shown in figure 4.3. 

Light Tight Enclosure 

Detector uv ws 

k- Order 

Sources 

Beam Splitter Sorting I 

-___---- -A li'ili 

Figure 4.3. Ultraviolet Spectral Comparator Facility (UV SCF). 

UV enhanced silicon photodiodes serve as the working standards for the UV SCF. A rotary 

stage is used in the UV SCF and currently only one test detector at a time can be measured. The 
test and working standard detectors are fixed onto optical mounts that rotate and tilt. But only 

the test detector can automatically be positioned in the horizontal and vertical planes. The 

working standards are positioned manually. Typically an argon arc is used as the source in the 

UV SCF. 

4.2.1 UV Source 

The UV SCF primarily uses a NIST-designed argon mini-arc as its source over the 200 nm to 

500 nm spectral range. The argon mini-arc was developed at NIST as a secondary spectral 

radiance standard and has been well characterized [33]. The argon mini-arc is an intense, 

uniform uv source, with argon gas flowing through the arc structure at = 35 kPa. The arc can be 

operated from 30 A to 100 A, but is typically operated at = 40 A (and 60 V). The arc is cooled 

using the normal facility 5.6 "C (42 OF) chilled water. The water runs through plastic 

nonconducting tubing. An FEL lamp, tungsten strip lamp, xenon arc lamp, lOOW 

quartz-halogen lamp, or alignment laser can be used as a source in the UV comparator as well. 

The spectral output flux (at the detector) of the argon mini-arc with the UV SCF is shown in 

figure 4.2. 

Safety considerations 

UV filtering safety glasses are worn whenever the argon mini -arc source is lit. Even with safety 

glasses on, the argon mini-arc is never viewed directly. Shields mounted on the side of the 

optical table inhibit direct visual contact with the sources. Since very high currents are used, 

extra precautions are taken when using the arc. Care is taken to avoid accidental contact with the 

arc electrical contacts and to insulate the arc from the optical table (and surroundings). Also, to 

avoid overheating the arc and electrical shorts, attention is paid that the cooling water is flowing 

before the arc is turned on and that no water leaks exist. Nonconducting plumbing is always 

used. The alignment lasers are Laser Safety Class 11. 

4.2.2 UV Monochromator 

The UV comparator uses a Spex 1680, 114 m, double grating monochromator utilizing 2840 lines 

per mm gratings. The monochromator's spectral range is 180 nm to 1000 nm. The spectral 

range used in the detector comparator facility is 200 nm to 500 nm. In the typical measurement 

configuration, the entrance and exit slits are circular 1.5 mm diameter apertures with a bandpass 

of 4 nm. The exit beam is f /5. Greater than 99 % of the beam flux lies within an oval area of 

diameters 2.0 mm and 2.5 mm around the optical axis. 

4.2.3 UV Optics 

A 10.16 cm diameter flat mirror on a rotary stage is used for source selection. The source is 

imaged by a stationary 15.24 cm diameter spherical mirror onto the entrance slit of the 

monochromator. A shutter, with a 25 mm diameter aperture, is placed after the monochromator 

exit slit inside the enclosure. 

21 
One 15.24 cm diameter spherical mirror and one 7.62 cm (3 in) diameter flat mirror images the 

exit aperture of the monochromator with 1:l magnification. Mirrors are used to prevent 

chromatic aberrations. 

4.2.4 UV Translation Stages - Detector Positioning 

The UV comparator has optical mounts for one test detector and two working standard detectors 

on a rotary positioning stage. The maximum travel of the rotary stage is 360" with a 

manufacturer specified resolution of 0.001" and an accuracy of 0.0014". The test detector is 

mounted on two automated orthogonal linear translation stages. The stages have a travel range 

of 50 mm with a manufacturer specified resolution of 0.1 pm and an accuracy of 0.25 pm per 

25 mm. 

The working standard detectors are mounted on two manual orthogonal linear translation stages. 

Each detector is mounted to allow its position to be adjusted along the optical axis (focus). A 

gimbal mount allows the rotation and tilt of each detector to be adjusted perpendicular to the 

optical axis. 

4.2.5 UV Working Standards 

The ultraviolet working standards (UV WS) are four UDT Sensors UVlOO silicon photodiodes 

with quartz windows and 1 cm circular active areas. The UV 100 is an inverted channel diode 

with enhanced resistance to UV radiation damage. Typically two working standards are used for 

each spectral comparison measurement. 

4.2.6 Beam Splitter and Monitor Detector 

The beam splitter is a 50.8 mm diameter flat quartz plate. A UDT Sensors UVlOO photodiode is 

the monitor detector. 

4.2.7 Alignment Lasers 

Similar to the Vis/NIR SCF, two HeNe lasers are used to align the optical path of the UV 

comparator. The second laser is not shown in figure 4.3. 

4.2.8 Enclosure 

The W SCF enclosure is similar in design and construction to that of the Vis/NIR SCF. 

4.3 Electronics 

4.3.1 Electronics - Signal Measurement 

This section describes the electronics used with the UV SCF and Vis/NIR SCF for detecting and 

amplifying the signals from the photodetectors. The electronics for both comparators are 

identical with some equipment shared between the two facilities. Only the Vis/NIR SCF will be 

described and the differences between the comparators will be noted. 

22 
Figure 4.4 shows a block diagram of the typical setup for measurements in the Vis/NIR SCF. 

The design of the electronics and control of the UV SCF is very similar to the Vis/NIR SCF. 

Four NIST built and characterized amplifiers are housed in one module for convenience and a 

separate single amplifier is used with the monitor detector. The digital voltmeters (DVMs) are 

computer controlled via an IEEE-488 bus. The two DVMs simultaneously measure the signals 

from the monitor and one of the four detectors that can be moved into the SCF beam. One DVM 

has a multiplexed input for selecting any of four amplifier channels or temperature monitoring 

inputs. (Some detectors have temperature monitoring circuitry that produces a voltage signal 

proportional to their temperature.) 

Instrument Control E 

and Data Stroage tr 

k Computer 

c= 

m 

E 

I 

Data Analysis 

Computer 

I 

(--- Monitoring 

Input 

/Germanium 

Working Standard 

7 

.. .. 

TE .y /Germanium 

Temperature Working Standard 

Controller 

- 

c-- 

I DVM with Test Reports 

Computer Multiplexer I Input 

F Silicon 

Working Standard 

Silicon 

Working Standard 

F 

Test Detector 

e=- 

Test Detector 

4 

onitor 

4 

w Germanium Monitor Instruments 

Figure 4.4. Vis/NIR detector, amplifier, DVM, and computer control block diagram. 

The amplifiers are identical in design and operate with gains of lo4 V-A-' to lo9 VeA-'. 

References [34 and 351 and several titles in the bibliography describe similar transimpedance 

amplifiers and their operation. The schematic for the precision transimpedance (IN) amplifiers 

used in both the UV and Vis/NIR SCFs is shown in figure 4.5. 

Figure 4.6 shows a block diagram for chopped (ac) measurements. Four additional pieces of 

equipment are required: two lock-in amplifiers, an optical chopper, and a signal multiplexer. In 

this configuration, the outputs of the four amplifiers from the detectors are fed through the 

multiplexer and then to a lock-in amplifier. The monitor signal is fed straight from the amplifier 

to the lock-in. Both lock-ins are run in a mode where the phase (which remains constant) is set 

to zero, thus the only output signal is the signal magnitude. This signal is sent to the appropriate 

23 
DVM. The DVM input is multiplexed as before with the temperature monitoring inputs. A 

discussion of the frequency dependent gain characteristics for similar photodiodes and amplifiers 

is found in Ref. [36]. 

Input High 

hpvt Low 

O"lpu1 

27AI 

0"lp"L 

C""" 

Figure 4.5. NIST SCF precision transimpedance (IN) amplifier circuit. 

-- 

Test Reports 

Computer 

I 

- 

I 

a 

Computer k 

z 

Instrument Control 

and Data Stroage m 

00 

Data Analysis 

Computer 

Working Standard 

Working Standard 

Working Standard 

Working Standard 

Test Detector 

Test Detector 

Figure 4.6. Vis/NIR detector, amplifier, lock-in, DVM, and computer control block diagram. 
4.3.2 Electronics - Auxiliary Equipment 

In both configurations, temperature-controlled germanium photodiodes can be used which 

require the use of the thermoelectric (TE) temperature controllers shown in figures 4.4 and 4.6. 

The following electronics not previously mentioned are also used routinely: a 4-channel TE 

temperature controller for the germanium working standards; a TE temperature controller for the 

germanium monitor; a digital I/O control unit for the optical shutter; and laboratory 

environmental monitoring system. The environmental monitoring system measures the 

temperature in each comparator enclosure, the laboratory temperature, relative humidity, 

barometric pressure, and the electrical power line voltage and frequency. 

5. Absolute Spectral Responsivity Scale Realization 

This section describes how the NIST absolute spectral responsivity scale and its traceability to 

the HACR are determined. First, the operation of the HACR and the scale transfer to the trap 

detectors are briefly reviewed. Then the transfer of the scale from the trap detectors to the 

visible working standards is described. Finally, the extension of the scale to the UV and near-IR 

working standards using a pyroelectric detector is explained. The scale realizations are planned 

annually. 

5.1 Transfer from HACR to Traps (405 nm to 920 nm) 

The HACR was constructed to improve the accuracy and spectral range of optical power (flux) 

measurements and is the U.S. primary standard for optical power (fig. 5.1). Cryogenic 

radiometers are currently used as the primary standard for optical power at other national 

laboratories [31, 37, 381. The HACR will be briefly reviewed here. A full description can be 

found in Ref. [20]. The HACR is an electrical substitution radiometer (ESR) that operates by 

comparing the temperature rise induced by optical power absorbed in a cavity to the electrical 

power needed to cause the same temperature rise by resistive (ohmic) heating. Thus the 

measurement of optical power is determined in terms of the electrical watt in the form of voltage 

and resistance standards maintained by NIST [39]. Several advantages are realized by operating 

at cryogenic temperatures (= 5 K) instead of room temperature. The heat capacity of copper is 

reduced by a factor of 1000, thus allowing the use of a relatively large cavity. Also the thermal 

radiation emitted by the cavity or absorbed from the surroundings is reduced by a factor of = lo7, 

which eliminates radiative effects on the equilibrium temperature of the cavity. Finally, the 

cryogenic temperature allows the use of superconducting wires to the heater, thereby removing 

the non-equivalence of optical and electrical heating resulting from heat dissipated in the wires. 

The relative combined standard uncertainty of the NIST HACR measurements is 0.021 YO [21] at 

= 1 mW. The largest components of the uncertainty are those due to the systematic correction 

for the Brewster angle window transmission and the random error associated with the cavity 

temperature measurement. 

There are drawbacks for making routine measurements of photodetectors with the HACR. 

Because of its design, the only source that can be used with the HACR is a laser. Equipment 

setup and measurements with the HACR are very time consuming, typically taking several days 

for each wavelength. The measurement wavelengths are limited to available laser wavelengths. 

The power levels used for the highest accuracy measurements with the HACR are = 0.8 mW 

25 
which is higher than those desired for typical radiometric applications. Therefore a practical 

means of disseminating the optical power scale to customers is to transfer the scale to other NIST 

facilities specifically designed for transferring the optical power scale to customers (e.g., UV and 

Vis/NIR SCFs). Trap detectors make excellent transfer standards since they are stable, have 

uniform responsivity, good linearity, and low noise [21, 15, 311. Figure 5.2 shows the 

arrangement of the photodiodes in the trap detectors used for HACR transfer standard detectors. 

Various detector arrangements acting as “light traps” for the light reflected from the first 

photodiode have been generically called “trap detectors.” The light beam reflection into 

orthogonal planes reduces effects of polarization on the responsivity. 

Germanium Resistance 

Thermometer 5 K Reference Block 

50 K Radiation Shie 

77 K Radiation Shield 

Radiation Trap (4.2 K) 

Absorbing Cavity 

(specular black paint) 

0 - 100” 

Figure 5.1. NIST High Accuracy Cryogenic Radiometer (HACR). 

As a step in transferring the detector spectral power scale, the HACR was used to determine the 

external quantum efficiency (EQE) of two trap detectors in the visible spectral region (see 

fig. 5.3a). Figure 5.3b shows a diagram of the setup used to transfer the scale from the HACR to 

26 
a trap detector using the substitution method. These trap detectors were calibrated at nine laser 

wavelengths between 406 nm and 920 nm. The reflectance of each trap detector was measured 

in this spectral range, allowing the internal quantum efficiency (IQE) to be determined. The IQE 

was modeled and the EQE for the entire spectral range was determined with a relative combined 

standard uncertainty of 0.03 %. Figure 5.3a shows the measured and modeled EQE of a trap 

detector. The trap detectors used in the scale transfer showed a maximum relative difference of 

0.04 % for orthogonal linear polarizations [21]. Because the output from both of the SCFs is a 

mixture of polarizations (as with most monochromators) and different from the linear 

polarization of the lasers used with the HACR, this value is combined by the root-sum-ofsquares method (explained in sec. 7.) with the relative combined standard uncertainty. The 

calibration relative standard uncertainty for the trap detectors when used with the SCFs is 

0.05 Yo. 

---- 

0 1- 0-1 0 0I 0 0I 

I I 

I 0 I-/ - -1- - -I 

Figure 5.2. Trap detector arrangement of photodiodes minimizes light lost to reflections. 

(a) Measured (cirda) and modelled (solid line) nternal quantum eBciency 

ora trap detector. Error bars are the uncertaintia given in Ref. 21. (b) Measurement setup. 

".,... 4w sw 600 7w (100 900 loll0 

Wavelength [nml 

Substitution 

HeCd, Ar, Nd:YAG, Electro-optic wedged ' / Turning 

Bumsplitter Aperture Mirror HeNc, Ti SAF Laser Stabilizer 

Figure 5.3. Scale transfer by substitution method with the HACR. 

27 
Figure 5.4 shows the entire spectral power scale propagation schematically. The HACR is the 

primary standard for the scale and trap detectors are used as transfer standards as described 

above. The trap detectors can then be used in a variety of measurements. First, the trap 

detectors could calibrate a Vis WS. The trap detectors could also calibrate the UV WS or 

Ge WS over the part of their spectral ranges that overlap. The Vis WS could do this calibration 

of the UV WS or Ge WS. The pyroelectric detector is used to extend the spectral power scale 

beyond the trap detectors calibrated spectral range. The calibrated UV WS and Ge WS are used 

to calibrate the Vis WS in the spectral region beyond the trap detectors. These transfer steps are 

described below. 

Detector 

Relative 

Transfer 

L Absolute 

Transfer 

Absolute Absolute 

Transfer/ \ Transfer 

Detector 

Relative 

Transfer 

I 

Absolute 

Transfer 

Figure 5.4. NIST spectral power scale propagation chain. 

5.2 Traps to Visible Silicon Working Standards 

Trap detectors are not used for routine spectral power (responsivity) measurements in the SCFs 

because the positioning of the trap detectors is time consuming and requires critical alignment to 

ensure the beam from the Vis/NIR SCF fl9 optical system does not overfill any of the trap 

detector’s photodiodes. Also trap detectors have not been available for the UV and near-IR 

spectral regions, requiring that the spectral power scale be transferred to other detectors. The Vis 

WS (Hamamatsu S1337-1010BQ) are used for routine measurements since they are easier than 

the trap detectors to align in the focal plane. Trap detectors can be used to provide higher 

accuracy measurements when required (over the modeled EQE spectral region). 

The four Vis WS are measured against two trap detectors, using the typical measurement setup 

and routine described later. An automated computer routine is used to determine the center of 

the active area for each detector. The measurements are then taken at that position. The Vis WS 

and traps are operated unbiased (the photovoltaic or short-circuit mode) and the signals are 

measured with calibrated transimpedance amplifiers and DVMs. The amplifier gain for both the 

working standards and trap detectors is lo6. 

The spectral responsivity is found using eq (3.27) where SS = S,, the modeled EQE from the 

HACR. The spectral responsivity of each Vis WS is the weighted mean of the measurement 
with both trap detectors [40] (see fig. 5.5a.). Figure 4.2 shows that the optical power used for 

these measurements is typically less than 1 pW. 

5.3 Extension to Ultraviolet (200 nm) and Near-Infrared (1800 nm) 

Expanding responsivity measurements beyond the modeled EQE spectral region of the trap 

detectors requires different calibrated detectors. Detectors are required that cover the spectral 

regions of measurement interest and also overlap with the trap modeled EQE spectral region. If 

the relative responsivity of the detector were known, comparisons with the trap detectors would 

then transfer the spectral responsivity scale to the detector. This detector could then be used as a 

working standard. 

One method to extend the spectral range of the responsivity measurements is to use a spectrally 

flat detector. The spectrally flat detector could be calibrated by comparison with the trap 

detectors and used as a working standard. A second method is to use a spectrally flat detector to 

measure the relative responsivity of another detector. This detector could then be calibrated by 

comparison with the trap detectors and used as a working standard. The latter method was used 

to extend the spectral range of the responsivity measurements. The transfer method described 

here uses one broadband, spectrally flat, windowed, pyroelectric detector. The “flatness” of the 

responsivity is determined by measuring the reflectance from the pyroelectric’s surface 

(absorber) and the window transmission. 

The method chosen not only depends on the transfer uncertainty, but also upon the operation of 

the detectors. Depending on the situation, some detectors, especially spectrally flat detectors, 

have drawbacks to their use. Cryogenically cooled detectors are often large and heavy, making 

automated movement of the detectors difficult. And, there is the added complexity of using 

cryogens. Pyroelectric detectors require a chopped beam (thus additional equipment) and some 

models have a poor signal-to-noise ratio (SNR) at the power levels typically found with 

monochromator-based systems. These characteristics may make a detector cumbersome and 

impractical to use routinely as a working standard. 

5.3.1 Pyroelectric Detector 

A pyroelectric detector only responds to a change in radiant flux. The operation of pyroelectric 

detectors thus requires modulated or chopped (ac) optical radiation and lock-in detection. The 

pyroelectric material is coated with an optically black (flat) material, in this case a gold black. 

Details of the characteristics and operation of pyroelectric detectors can be found in the 

literature [4 11. 

The operation of a pyroelectric detector depends upon the spectral absorptance of the gold black 

material. Using the law of conservation of energy the absorptance ap(A) is given as 

a,@) = 1 - p,(A) [unitless], (54 

where p&A) is the reflectance and the transmittance %(A) is assumed to be equal to zero. The 

predicted spectral responsivity Sp( A) of the pyroelectric detector is 

29 
where z,(A) is the pyroelectric detector window transmittance and CF, is a calibration factor 

which scales the output signal (in U units) to the optical power received by the pyroelectric 

detector. The calibration factor could be determined by measurements with the HACR or 

comparison to the trap detectors. If the calibration factor is set to an arbitrary value, then the 

relative responsivity s,(A) is determined. Note that the lower case “s” is used to designate 

relative responsivity [ 21. 

The reflectance of the pyroelectric detector pp(A) is measured with a modified Perkin-Elmer 

Lambda-1 9 spectrophotometer [42]. The window transmittance %(A) is measured using the 

NIST transmittance measurement service facility [43]. The window transmittance can also be 

found by determining the ratio of the signals from two measurements using the SCF, one with 

and the second without the window in front of a detector. 

5.3.2 Responsivity Measurements 

From eq (3.27), it can be shown that the relative responsivity sx(A) of a test detector x can be 

written 

where ,,(A) is the relative responsivity of the pyroelectric detector. Equation (5.3) can now be 

used to determine the relative responsivity of a test detector using the pyroelectric detector as a 

standard by the same substitution method (measurement equation) described earlier. It should be 

noted that the pyroelectric detector described here had a poor SNR adding significantly to the 

measurement uncertainties. 

The test detector is then calibrated using the trap or Vis WS detectors over the part of their 

spectral ranges that overlap. This data is used to scale the relative responsivity of the test 

detector. Only one point is needed in principal; but, in practice, about 50 different spectral 

points are used to obtain an average scaling factor to apply to the entire relative responsivity 

curve. 

5.4 Germanium VIR) Working Standards 

The Ge WS relative spectral responsivities are determined using the Vis/NIR SCF and the 

pyroelectric detector. The weighted average of several measurements are taken, due to the SNR 

of the pyroelectric detector. The Ge WS responsivity is then measured using the trap detectors 

over the spectral region from 700 nm to 920 nm. The trap comparison data is the average of 

three scans during one measurement. This data is used to scale the relative responsivity. Two 

additional Ge WS, identical to the first two, were measured using the first two Ge WS 

(comparison using the pyroelectric was not done because of time constraints). See fig. 5.5b 

and c. 

30 
5.5 UV Silicon Working Standards 

The relative spectral responsivity of the four UV WS is measured using the UV SCF and the 

pyroelectric detector. Measurements have been taken with and without the pyroelectric’s 

window. The data without the window is noisier and is not used in determining the UV WS 

relative spectral responsivity. Even with the window, getting a good SNR requires that many 

measurements be taken and averaged. 

The responsivity of all four UV WS is also measured using all four Vis WS in the Vis/NIR SCF 

over the spectral region of 405 nm to 500 nm. The average of three scans with each Vis WS was 

used as the responsivity for each UV WS. This data was used to scale the relative responsivity 

of the UV WS. See fig. 5.5d and e. The trap detectors were not available, so the Vis WS were 

used for the scale transfer increasing the relative standard uncertainties from 405 nm to 500 nm 

by 0.02 %. 

5.6 Extension of Visible Silicon Working Standards 

The spectral responsivities of the four Vis WS is measured in the Vis/NIR SCF from 925 nm to 

1100 nm with the first two Ge WS. The average of three scans with each Ge WS is assigned as 

the responsivity for each Vis WS in the 925 nm to 1100 nm spectral region. The Vis WS are 

also measured from 350 nm to 405 nm with the UV WS in the Vis/NIR SCF. The average 

responsivity of three scans with all four UV WS is assigned to each Vis WS. Figure 5.5f shows 

the Vis WS scale extension measurements. The uncertainties are significantly higher in these 

regions, compared to the trap spectral region, due to the pyroelectric detector SNR and the 

additional transfer measurements. *I 

31 
0.6 

0.5 

f 0.4 

2. 

$ 03 

:E 

1 0.2 

P 

a SI 

4 

0.1 

0.0 

I .o 

08 f 

0 '5 0.6 

'3 

H 5 0.4 

3 

SI 

0.2 

0.0 

(a) Absolute responsivity of Vis WS using trap detector. (b) Relative responsivity of Ge WS using pyroelectric detector. 

400 500 600 700 800 900 920 

Wavelength (nm) 

(c) Scale Ge WS relative responsivity using Vis WS 

from 700 nm to 900 nm. 

.... ....... ......... : ......... 9 ......... ..... .... .... ... ... ... ... ... 

.... .... 

... Scaled Relative Responsivity . Absolute Responsivity 

700 800 900 1000 1100 I200 1300 1400 I500 1600 1700 1800 

Wavelength (nm) 

(d) Relative responsivity of UV WS using pyroelectric detector. 

700 800 900 1000 1100 1200 1300 1400 I500 1600 1700 1800 200 

Wavelength (nm) 

e 

e..** 

300 400 

Wavelength (nm) 

500 

0.30 

0.25 

0.20 .$ 

'a 

0.15 

e?: 

Y 4 0.10 

D < 

0 05 

0.00 

(e) Scale UV WS relative responsivity using Vis WS 

from 400 nm to 500 nm. 

(0 Absolute Responsivity of Vis WS from 350 nm to 400 nm using UV WS and from 925 nm to 1100 nm using Ge WS. 

06 

05 

0.4 

.$- 

0.3 

P 

f 0.2 

a 

4 

0. I 

0.0 

200 300 500 300 400 500 600 700 800 900 915 1000 1100 

Wavelength (nm) Wavelength (nm) 

Figure 5.5. NIST detector spectral responsivity scale realization. Shaded areas 

indicate regions of spectral overlap for calibration transfer. 
6. Calibration Procedures and Computer Automation 

6.1 Spectral Responsivity 

This section describes the spectral responsivity measurement procedures for both the UV SCF 

and Vis/NIR SCF. These correspond to Service ID numbers 39071s through 390758. The 

primary difference between the operation of the two SCFs is that the UV SCF can accommodate 

only one test detector per responsivity measurement run, and the Vis/NIR SCF is set up for two 

test detectors. The spatial uniformity of the responsivity for new photodiodes, which is included 

in 39071s and 39073S, is also measured as described in section 6.2. 

6.1.1 Calibration Procedures 

New photodiodes are visually inspected for defects and placed in one of the specially designed 

fixtures described later in section 9.5. Each photodiode has a serial number engraved on the 

fixture. The photodiode dynamic impedance (shunt resistance) is measured with an HP 4145A 

I-V plotter to confirm the manufacturer’s specification. The shunt resistance of the photodiode 

needs to be 1000 times greater than the input impedance of the transimpedance amplifier for it 

not to deviate from linear operation. Typically, an amplifier gain of lo6 is used, requiring that 

the photodiode shunt resistance must be greater than 10 kR. If needed, the diode window is 

cleaned with lens tissue and spectral-grade acetone before any optical measurements are made. 

Ethanol has also been used by others to clean photodiodes and optical windows [44]. 

The spectral responsivity is measured by direct comparison to two working standards using the 

“substitution method with monitor’’ described in section 3.2. The two working standards are 

selected from a randomly generated weekly schedule. The test detector(s) and working standard 

detectors are aligned perpendicular to the optical axis by using the He-Ne laser as the 

monochromator source and retroreflecting the He-Ne beam back onto itself. The appropriate 

broadband source used for the measurement is chosen and the detector positioned at the focal 

plane of the SCF exit optics. An automated computer routine centers the active area of each 

detector on the optical axis. 

The typical comparison measurement consists of scanning the monochromator through the 

desired spectral range at wavelength intervals of 5 nm for each detector. This spectral scanning 

process is repeated three times. The test detector(s) and Vis WS are operated unbiased (the 

photovoltaic or short-circuit mode) and the signals are measured with calibrated transimpedance 

amplifiers and DVMs. Figure 4.2 shows that the optical power used for these measurements is 

typically less than 1 pW. The test to monitor (eq (3.23)) and working standard to monitor 

(eq (3.24)) ratio data are stored on the computer for later analysis to determine the spectral 

responsivity. The standard deviation of the test (or working standard) and monitor detector 

ratios is less than either individual signal standard deviation. This is the result of simultaneously 

sampling both the test (or working standard) and monitor detector. Examples of the typical 

signals from a Hamamatsu S1337 and the monitor photodiode are shown in figure 6.1. Also, 

shown are the signal ratios and the relative standard deviations of each signal and ratio. 

Figure 6.1 shows the ratio relative standard deviation is lower than either individual signal 

standard deviation. 

33 
10' 

10" 

IO" 

1 o2 

1 o-2 

300 400 500 600 700 800 900 1000 1100 

Wavelength [nm] 

Figure 6.1. Typical signals from Hamamatsu S1337 and monitor photodiodes. 

The S1337 and monitor signals are shown as closed squares and triangles, 

respectively. Also shown are the signal ratios as closed circles. All curves are 

the means of 10 samples. The relative standard deviations of the S1337 and 

monitor signals are shown respectively as open squares and triangles in the 

shaded area. The relative standard deviations of the ratios are shown as open 

circles and for measurements above 400 nm are lower than the individual 

signal standard deviations. Note: The S 13 37 and monitor signal standard 

deviation curves are nearly indistinguishable because the source noise rather 

than detector noise dominates the measurement. 

The laboratory environment (temperature, humidity, etc.) is monitored and recorded at 1 ie start 

of each scan although this data is not used to correct the measurement results. The temperature 

of specially designed detectors that have temperature sensors built into their housings can also be 

recorded. The average temperature during the measurements is then reported. 

The spectral responsivity is determined by using eq (3.27) for each test and working standard 

detector spectral scan combination. The reported spectral responsivity of the test detector is the 

weighted mean [40] of all the scans with both working standard detectors. Examples of typical 

photodiode spectral responsivities are shown in figure 6.2. 
1.2 

1 .o 

0.8 

0.6 

0.4 

0.2 

0.0 

200 400 600 800 1000 1200 1400 1600 1800 

Wavelength [nm] 

Figure 6.2. Spectral responsivities of typical Si, InGaAs, and Ge photodiodes. 

6.1.2 Quantum Efficiency 

The quantum efficiency is the photon-to-electron conversion efficiency of a photoelectric 

detector. The quantum efficiency of a detector is often required for particular applications. 

There is a simple calculation to convert spectral responsivity [A-W-'1 to external quantum 

efliciency. The external quantum efficiency EQE is given by [ 101 

where I is the photocurrent (output current minus the dark output), h is Planck's constant, c is the 

velocity of light, @ is the input radiant flux (power), n is the index of refraction of air, e is the 

elementary electronic charge, and A is the spectral wavelength. Substituting for the constants h, 

cy n, and e gives 

I(') = 1239.85.- W) , 

a EQE(A) = 1239.85. @(a) a 

where S(A) = I(A)/@(A) is the spectral responsivity [A-W-'], and for convenience, A is in nm. 

Examples of typical photodiode EQEs are shown in figure 6.3. 

35 
n 

s 

.d 8 

8 

w 

100 

80 

60 

40 

20 

0 

200 400 600 800 1000 1200 1400 1600 1800 

Wavelength [nm] 

Figure 6.3. Quantum efficiencies of typical Si, InGaAs, and Ge photodiodes. 

6.2 Spatial Uniformity 

This section describes the procedures used in NIST Service ID numbers 39071S, 39073S, and 

3908 1 S to determine the responsivity spatial uniformity of detectors. The spatial uniformity 

measurement procedures are the same for both the UV SCF and Vis/NIR SCF. The major 

difference between the two is the beam size. The UV SCF beam diameter is 1.5 mm and the 

Vis/NIR SCF beam diameter is 1.1 mm. Also, only one detector can be measured at a time in the 

UV SCF while up to four detectors can be measured in the Vis/NIR SCF. 

The spatial uniformity is measured at 350 nm for the UDT Sensors UVlOO photodiodes 

(39071s). The spatial uniformity for the Hamamatsu S1337-1010BQ and, more recently, the 

Hamamatsu S228 1 photodiodes is measured at 500 nm (39073 S). UV 100 photodiodes are not 

issued if the responsivity nonuniformity (i.e., slope) is greater than 1 'YO over the active area or 

greater than 0.5 'YO within the center 50 % of the active area or if a discontinuity (a peak or 

valley) in the responsivity greater than 0.5 % is found within the center 90 % of the active area. 

Hamamatsu S 13 3 7- 10 1 OBQ and S228 1 photodiodes are not issued if the nonuniformity is greater 

than 0.5 % over the active area or greater than 0.25 % within the center 50 'YO of the active area 

or if a discontinuity greater than 0.25 % is found within the center 90 % of the active area. 

In the early 1990's, it was found that many silicon photodiodes have a significant change in the 

uniformity as a function of wavelength, particularly as the wavelength approached the bandgap 

(1 100 nm). Nonuniformity is due to inhomogeneity in the photodiode material - typically 

inhomogeneity in surface recombination centers at shorter wavelengths [45, 461 and bulk 

recombination centers at longer wavelengths [47]. The nonuniformity near the bandgap has also 

been related to the (n0n)uniformit.y of the bonding material's reflectance [48]. Because the 

semiconductor is almost transparent near the bandgap, changes in the spatial reflectivity of the 

bonding material affect the amount of light reflected and therefore the responsivity of the 

photodiode. 

36 
Since 1993, the uniformity of the S1337-1010BQ and the S2281 diodes are also measured at 

1000 nm [49]. Photodiodes are not issued if the nonuniformity is greater than 1 YO over the 

active area or greater than 0.5 % within the center 50 % of the active area. Also, if a 

discontinuity greater than 0.5 YO is found within the center 90 YO of the active area. 

S1337-1010BQ photodiodes with a significant change in uniformity between 500 nm and 

1000 nm are seen less frequently in recently manufactured diodes. Diodes issued by NIST prior 

to the discovery of this effect in 1993 were not measured at 1000 nm. When these diodes are 

resubmitted for measurement (39074S), the uniformity is checked at 1000 nm; and the customer 

is notified if a significant nonuniformity is found. 

6.2.1 Measurement Method and Calibration Procedure 

The detectors are aligned as described above in section 6.1. The typical measurement consists of 

setting the monochromator to the desired wavelength and, for a 1 cm2 test detector, scanning a 

12 mm x 12 mm area in 0.5 mm steps. The test detector is operated unbiased (the photovoltaic 

or short-circuit mode); and the signal is measured with a calibrated transimpedance amplifier and 

a DVM. The typical amplifier gain for the test detector is lo6. Figure 4.2 shows the optical 

power used for these measurements is typically less than 1 pW. The test to monitor (eq (3.23)) 

ratio data is stored on the computer for later analysis. 

The scans are always in the same horizontal direction; and the vertical direction is reversed in a 

“raster” scan, starting in the upper left corner moving to the lower right corner of the photodiode. 

The scan is large enough for the beam to move completely off of the active area (or onto an 

aperture). Scanning horizontally in only one direction puts the stage drive against the same side 

of the drive screw. For the vertical, gravity keeps the stage always against the same side of the 

screw. This reduces hysteresis in the movement of the stages. 

The laboratory environment (temperature, humidity, etc.) is monitored and recorded at the 

beginning and end of each scan, although this data is not used to correct the measurement results. 

The temperature of specially designed detectors that have temperature sensors built into their 

housings can also be recorded. The average temperature during the measurements is then 

reported. 

The test to monitor detector ratios are normalized to the mean of the center ratios. The reported 

spatial uniformity figure is constructed from these normalized ratios. Figure 6.4 shows the 

spatial uniformities of the central portion of typical Hamamatsu S 1337 - 10 1 OBQ and S228 1 

photodiodes at 500 nm and 1000 nm. Similar spatial uniformity scans are shown in figure 6.5a 

for UVlOO and figure 6.5b, c, and d for Judson EG&G thermoelectrically cooled Ge 

photodiodes. 

6.2.2 Limitations 

The size of the active area to be measured has to be significantly larger than the beam size since 

the beam is “clipped” (or vignetted) at the edges of the active area or aperture in front of the 

detector. Also the spatial shape of the optical beam is assumed constant during the 

measurement. 

37 
6 I I I I I I 

-6 - 

-4 - 

-2 - 

0- 

2- 

4- 

6- 

- 

- 

- 

- 

- 

- 

-6 -4 -2 0 2 4 6 

mm 

(a) Hamamatsu S1337 at 500 nm. 

0 

2 :! 4 

-6 -4 -2 0 2 4 6 

mm 

(c) Hamamatsu S2281 at 500 nm. 

-6 -4 -2 0 2 4 6 

mm 

(b) Hamamatsu S1337 at 1000 nm. 

-6 

2 "' 4 

6 I l~l~l~l~l 

-6 -4 -2 0 2 4 

mm 

(d) Hamamatsu S228 1 at 1000 nm. 

Figure 6.4. Spatial uniformities of typical Hamamatsu S 1337 and S228 1 photodiodes. 

The responsivities are normalized to the center values. 
- 

-4 - 

-2 - 

0- 

2- 

4- 

6- 

- 

- 

- 

- 

- 

-3 3 

I I I I I I 

-3 -2 1 

21-T" 3 

-3 -2 -1 0 1 2 3 

(c) EG&G Judson TE Ge at 1500 nm. 

mm 

-3 -2 -1 0 1 2 3 

(b) EG&G Judson TE Ge at 1000 nm. 

mm 

-3 

-2 

1 4 

1 

fi -I 2 

-3 -2 -1 0 1 2 3 

mm 

(d) EG&G Judson TE Ge at 1600 nm. 

Figure 6.5. Spatial uniformities of typical UDT Sensors UV 100 and EG&G Judson 

Ge photodiodes. The responsivities are normalized to the center values. 

39 
6.3 Computer Automation 

This section describes the computer automation of the UV and Vis/NIR comparator facilities. 

The automated equipment and computer software used for typical measurements are briefly 

described. Automating the equipment (which sometimes requires modification) and writing, 

testing, and documenting the software is a major undertaking. The operation of the facilities 

would not be practical without this high degree of computer automation. 

6.3.1 Computer Automated Equipment 

Block diagrams that include current and future computer controlled equipment for the Vis/NIR 

SCF and UV SCF are shown in figures 6.6 and 6.7, respectively. All of the equipment is 

controlled by one computer via an IEEE-488 (GPIB) bus. This computer stores all of the data 

and the analyzed responsivity files. A Local Area Network (LAN) connects this computer with 

several others which are used for further data comparisons and writing test reports; allowing the 

SCF control computer to be dedicated to taking measurements. Weekly backups of the data files 

are sent to a second computer. 

Several key components of the SCFs are controlled by commercial servo motor controllers: the 

horizontal and vertical (x,y) translation stages, source section, and the rotary stage in the UV 

SCF. The wavelength drive of the Cary-14 monochromator used with the Vis/NIR SCF was 

modified and is also driven by a computer controlled servo motor. 

All of the DVMs, lock-in amplifiers, and multiplexers are computer controlled. A digital I/O 

module addressed over the IEEE-488 bus signals the shutter controllers to open or close. A 

commercial laboratory environmental monitor records the laboratory and enclosure temperatures, 

the humidity and barometric pressure, and the electrical power line voltage and frequency. Some 

detectors have temperature monitoring circuitry that produces a voltage signal proportional to 

their temperature. These signals can also be multiplexed via computer control to the DVM. 

In the future the TE temperature controllers and transimpedance amplifiers will also be 

controlled via IEEE488 bus and computer. The Vis/NIR SCF wavelength encoder and display 

will be upgraded, allowing communication with the controlling computer. A new 

monochromator will soon be incorporated in UV SCF which will have computer selectable order 

sorting filters. 

40 
Future 

L 

c 

Test Reports 

Computer 

Germanium 

Standards 

4 

II . Monitoring 

Instrument Control 

i;j 

m 

F 

and Data Stroage 

P 

00 

00 

Computer 

a 

uI 

I 

Data Analysis 

and File Backup 

Computer 

W 

E 

Future Addition 

Multiplexer 

Working Standard 

& Test Detectors 

Temperature 

I . Inputs 

'-A - DVM 

Monitor Silicon Monitor 

e 

Germanium Monitor 

x-Y 

- Translation 

Stages 

Wavelength 

Drive 

Source 

- Selection 

Drive 

Laboratory 

Temperature 

Sensor 

Servo - 

Motor 

Controller - 

- & Humidity 

Enviromnent 

Monitor 

Temperature 

Shutter 

Figure 6.6. Vis/NIR SCF computer control block diagram. 

41 
F-1 ~~ 

Test Reports 

Computer 

Future orking 

4 

I3 

Computer P 

Instrument Control 

and Data Stroage F 

00 

Qo 

c 

W 

v, 

Data Analysis 

and File Backup 

Computer 

Future Addition 

I I Working 5 Standard 

/Multiplex; I & Test Detectors 

1 . . 

Temperature 

Monitoring 

Inputs 

---- 

I 

DVM 4?- 

Monitor Silicon Monitor 

Amplifier I 

I , 

x-Y 

- Translation 

Stages 

Rotary 

Stage 

Servo - 

Motor 

Controller 1 -4=l Selection 

I Drive I 

Temperature 

Enviromnent 

L Enclosure 

Temperature 

I 

Shutter 

Figure 6.7. UV SCF computer control block diagram. 
6.3.2 Computer Calibration Programs 

SCF Setup Program 

This program performs three functions: initialize the SCF instruments, align the test and working 

standard detectors, and verify the signal levels from the detectors before starting measurements. 

The operator uses this program interactively (via the front panel controls and translation stage 

joystick) to align a new detector placed in the SCF. This program updates a file that contains the 

detector names and translation stage (x,y) positions for use later by other computer programs. 

Spectral Scanning Program 

This program takes data from one to four detectors (test and working standard detectors), stores 

the data on hard disk, and prints the results. The analysis program can be set to automatically 

run when the measurements finish. The operator sets several detector parameters such as the 

time constant and amplifier gain, whether to use a detector (position), and whether the detector is 

a working standard. Other operator inputs are the spectral range start and stop wavelengths, 

spectral step size, number of samples at each wavelength, number of (repeat) scans for the 

measurement, and which SCF source to use. The operator can also input specific comments 

about the measurement. 

After the parameters have been entered, the program positions the first detector into the beam 

and starts taking data (see fig. 6.8). The program determines the test (measurement) number and 

creates unique filenames for each detector using this number and the detector’s name entered in 

the SCF setup program. The program updates the computer screen with the current scan number, 

the detector name and position for the detector currently being measured, filename, and the 

pathname to the data stored on the hard disk. The program also graphs the test detector to 

monitor ratio (eq (3.23)), the working standard detector to monitor ratio (eq (3.24)), and the 

percent standard deviation of the ratios. The program saves the “raw” ratio data files on the hard 

disk after each detector spectral scan. After completion of the measurement, a record is printed 

of the operator inputs, graph of the detector to monitor ratios, and graph of the percent standard 

deviation of the ratios. 

The “raw” ratio data files are two dimensional ASCII text arrays that are tab delimited and 

consist of “header” lines and data lines. The “header” lines contain information about the 

measurement parameters, laboratory environment (e.g., temperature), and detector temperature 

(if provided) or auxiliary thermistor reading. The data columns are: (1) wavelength, (2) mean 

ratio of the (test or working standard) detector signal to the monitor signal, (3) the percent 

standard deviation of those ratios, (4) mean detector signal, (5) detector percent standard 

deviation, (6) mean monitor signal, and (7) monitor signal percent standard deviation. (See 

fig. 6.1 .) If more than one scan is taken then the environmental and detector temperature 

information and additional data are added at the end of the data file after each scan. If selected 

by the operator, the spectral responsivity calculation program is automatically executed after all 

the spectral scans have completed. 

43 
Time Constant (ms) 

# of Samples 

Wavelength (A) Range: (Start, End, Step) 

Which SCF (Vis/NIR or UV) ? 

i Mult. Background Samples (yh) 

Close Shutter; 

Move Monochrometer to R20 

I Wait 6 TC; Move to 1st A I 

Autorange DVMs (Dark); 

Read Ranges; 

Open Shutter 

-4 Set Wavelength (A) I 

Read Signal (using single background or 

multiple background sample subroutine); 

Send Wavelength (A), Signal and % SD 

to Computer Screen Plot 

Check Global "Abort" 

This is a For N Loop where 

N = # of data "points" (A steps) 

N=Endl-Startl + 1 

Step I 

I i 

Exit; Send data 2D array 

(7 columns) 

+ 

I11 121 I31 I41 151 [61 I71 

Gut put: 

Wavelength (A), Test Sig, %SD of Ratio, Test Sig, Test %SD, Mon Sig, Mon %SD 

Mon Sig I 

Figure 6.8. Measurement (computer) program flowchart for one detector scan. 

44 
Spectral Responsivity Calculation Program 

This program calculates the spectral responsivity for the test detector(s), stores the results on the 

hard disk, and prints the (weighted) mean responsivity and relative measurement uncertainty 

(graphs), detector filenames, and control settings. The inputs to the program are the test and 

working standard detector “raw” ratio data filenames, which working standard reference 

(calibration) data to use, and whether a NIST transimpedance amplifier or other calibrated 

amplifier was used with the test detector. 

The gain range and detector position in the SCF are extracted from the data files for the test and 

working standard detectors. This is used to select the corrected (calibrated) gain factor for the 

appropriate NIST amplifier. For detectors with internal amplifiers the gain range is multiplied 

by the customer supplied amplifier gain correction factor to determine the corrected gain factor. 

The program next reads and formats the ratio data files and the working standard reference data 

file. The program determines the responsivity for a test detector by using eq (3.27) for each test 

and working standard detector spectral scan combination. Then if more than one spectral scan 

was taken a weighted average of the mean is calculated [ 401 using the standard deviations from 

each scan as the weighting factor. The weighted average uncertainty is also calculated. 

If the responsivity data is negative (because of reversed polarities on the photodiodes) the 

responsivity data is inverted, i.e., -S(A). Then the spectral responsivity and relative uncertainty 

are graphed on the computer monitor. The detector temperature monitor (or auxiliary thermistor) 

data for all the scans are averaged for each test detector. The output filename is unique and 

constructed very similar to the “raw” data filename using the test number and test detector name. 

The responsivity output file is created as a tab delimited ASCII text array. The file consists of 

“header” lines and data lines. The “header” lines contain information about the test number, 

detector (name), test date, working standard(s) and reference data, internal amplifier gain and 

correction factor (if applicable), average detector temperature, and column headings. The data 

columns are: (1) wavelength, (2) weighted average spectral responsivity, (3) relative uncertainty. 

After the measurement is complete a record of the operator inputs and graphs of the weighted 

average spectral responsivity and relative uncertainty are printed. 

Spatial Scanning Program 

This program spatially scans the SCF beam across the active area of the detector by using the x,y 

translation stages to translate the detector. The data is stored on hard disk, and the measurement 

results are printed when the program finishes execution. One detector in the UV SCF and up to 

four detectors in the Vis/NIR SCF can be measured at one time. The measurements can be 

programmed to repeat at different wavelengths. 

The program inputs are the scan wavelength, number of samples at each data point, detector 

parameters such as the time constant and amplifier gain, whether to use a detector (position), and 

operator comments. Also input are the spatial scan horizontal (x) and vertical (y) dimensions and 

step sizes. The program determines the test number and creates a unique test filename with the 

test number, detector name, and wavelength. 

45 
The program moves the detector to the first x and y position and takes the detector to monitor 

signal ratio measurement (eq (3.23)). The program scans the detector from top to bottom and left 

to right. That is, the x,y movement is from the top left to the bottom right (or top right depending 

on the number of columns measured). The program displays a quasi three dimensional graph of 

the signal ratios. The absolute value of the ratio data is plotted in case the polarity of the diode is 

reversed. This graph is updated after each data point measurement. Also displayed are the 

current horizontal and vertical positions. The ratio and percent standard deviation data from 

each x,y position are combined into arrays and added to the output file saved on the hard disk. 

After the spatial scan is complete a record of the operator inputs and a spatial graph of the signal 

ratios is printed. 

The responsivity spatial uniformity output data file is an ASCII text array that is tab delimited. 

The file consists of “header” lines and data lines. The “header” lines contain information about 

the measurement parameters, operator comments, laboratory temperature, humidity, etc., and 

detector temperature (if provided). The data lines consist of y rows and x columns of the test 

detector to monitor detector signal ratios. The first data point (array cell) corresponds to the 

upper left x,y scan position. That is, the array corresponds to looking at the detector where the 

left side of the array is the left side of the detector, the top of the array is the top of the detector, 

etc, Following the ratio data array is a similar array containing the corresponding relative 

standard deviations. 

7. Uncertainty Assessment 

The assessment of the uncertainties for the detector responsivity measurement is explained in 

this section. First the uncertainty is evaluated for the measurement equation developed earlier in 

this document. In addition to the uncertainty terms that come directly from the measurement 

equation there are indirect terms due to the wavelength uncertainty of the monochromator, 

long-term stability of the working standards, and the assumptions and approximations made 

during the development of the measurement equation. Second, the uncertainty for each group of 

working standards and the transfer to customer detectors is given in detail. The uncertainty 

analysis follows the method outlined in Ref. [7]. A general discussion of the sources of error in 

radiometry can be found in Refs. [50 and 511. A detailed explanation of the evaluation and 

expression of measurement uncertainty is given in Ref. [ 521. 

7.1 Uncertainty Components 

In general, a measurement result y can be expressed as a functional relationship fof N input 

quantities xi given by, 

The combined standard uncertainty u,(y) is given by the law of propagation of uncertainty as 

the following sum, 

46 
where the partial derivatives af/ax, are the sensitivity coefficients and #(xi) are the standard 

uncertainties of each input x, which are assumed to be uncorrelated. This method is also called 

the root -sm-of-squares or "RSS" method. 

When the functional relationship involves products and quotients, as the measurement equation 

described in this document, the relative 

convenient, and is given by 

combined standard uncertainty u,(y)/y is more 

where (l/y) (af/axi) is the relative sensitivity coefficient. The expanded uncertainty U is 

obtained by multiplying uc(y) by a coverage factor k, 

where k is chosen on the basis of the level of confidence desired. Replacing uc(y) with the 

relative combined standard uncertainty uc, , (y) gives the relative expanded uncertainty 

U, = U/y. The coverage factor k = 2 was used in this document and it is assumed that the 

possible estimated values of spectral responsivity are approximately normally distributed with 

approximate standard deviation u,, thus, the interval defined by U has a level of confidence of 

approximately 95 %. 

To help clarify the following discussion of uncertainties, an arbitrary uncertainty u,(S,,) in the 

working standard spectral responsivity is defined. The uncertainty u,(S ws) can be derived 

directly from the measurement equation in eq (3.27), repeated below, by using the propagation of 

standard uncertainty relationship in eq (7.2). The measurement equation is 

sws=-.-. Rws Gs S, [A*W'']. 

Rs Gw, (7.5) 

The uncertainty u,(S,,) is 

47 
The relative uncertainty uo( S ,,)/S ws is 

where u(R,,) is the standard deviation of the mean of i ratios of the working standard and the 

monitor, u(R,) is the standard deviation of the mean of i ratios of the calibration standard and 

the monitor, u(G,) is the calibration standard amplifier gain uncertainty, u(G,) is the working 

standard amplifier gain uncertainty, u(S,) is the calibration standard uncertainty, and i is the 

number of ratio measurements taken. 

The calibration standard uncertainty, u(S,) , is determined previously by the HACR transfer 

measurements. The amplifier gains, G, and G,, are calibrated using a precision current source 

and DVM. The amplifier gain uncertainties, u(G,) and u(G,), are the RSS of the uncertainties 

from the precision current source and DVM. 

The first two terms contribute uncertainty due to the measurement statistics (Type A 

uncertainties). The remaining terms are Type B uncertainties. If more than one scan is taken 

during a measurement (typically three scans are taken), then a weighted average of the mean, 

R,, of the ratio R = Rws/R, is calculated [40] using the RSS of u(Rw,) and u(R,) from each 

scan as weighting factors. An uncertainty term u(Rw ) is calculated for the weighted average and 

is an indication of the measurement repeatability. 

There is also a statistical uncertainty term u(R,) reflecting the reproducibility of the 

measurements (e.g., setting up the measurement again the next day). The relative uncertainty 

term, u(R,)/R, is combined by RSS with u(R,)/R, to get the relative measurement 

uncertainty, u(R, )/I?,,, . The relative measurement uncertainty contains the temperature and 

uniformity variations since it is the result of repeated measurements at different temperatures and 

alignments and is an indication of the measurement reproducibility. 

By calculating the ratios of the test to monitor and standard to monitor signals, we take into 

consideration the correlation of the signals due to source fluctuations, thus the covariance is not 

included in the R uncertainty term. The standard deviation of the ratio of the detector to monitor 

signals is less than the standard deviation of either by themselves, as shown in figure 6.1 . 

The combined uncertainty u,(S,,) in the spectral responsivity of the working standard is 

determined by calculating the RSS of the uncertainty terms thus far discussed and the uncertainty 

terms from the assumptions and approximations made in the measurement equation and so -called 

“indirect” effects that are not explicitly shown in the measurement equation as expressed in 

eq (7.5) such as, the monochromator wavelength uncertainty, DVM uncertainty, and long-term 

stability of the working standards. 
The relative combined uncertainty uc (S ,,)/S ws is 

where u(S, )/Sa is the monochromator wavelength relative uncertainty, u(V)/V is the DVM 

relative uncertainty, u(S,, )/S,, is the relative uncertainty due to the long-term stability of the 

working standards, u( y )/y is the scaling factor relative uncertainty for the relative responsivity 

measurements, u(R,, )/Rsl is the relative uncertainty due to stray light, and u(S,,, )/Sbw is the 

relative uncertainty due to the bandwidth-effect. These uncertainty terms are explained below. 

7.1.1 “Indirect” Uncertainty Components 

Several uncertainty components are not explicitly contained in the measurement equation as 

expressed in eq (7.5) and thus far have not been discussed as part of the uncertainty analysis. 

These are the wavelength uncertainty in the monochromator setting, DVM uncertainty, 

long-term stability, and the scaling factor for the relative responsivity measurements. Each of 

these is discussed below and is evaluated in sections 7.2 and 7.3. 

Wavelength Uncertainty 

The wavelength uncertainty is the uncertainty in the measured spectral responsivity due to the 

uncertainty of the set wavelength of the monochromator. The repeatability of that wavelength 

setting is negligible compared to the wavelength uncertainty and is considered part of the relative 

measurement uncertainty. The wavelength uncertainty of the monochromator u(A) introduces 

an uncertainty proportional to the slope of the responsivity dSJdA [ 531 

u(S,)=-. .#(A) [U-W-’],where dil (7-9) 

(7.10) 

and G = G,/G, is the ratio of the amplifier gains, which is a constant with respect to 

wavelength. Carrying out the differentiation gives 

(7.1 1) 

49 
The wavelength uncertainty of the monochromator u(A) is the standard deviation of the residuals 

to a fit of the wavelength calibration. 

The relative uncertainty can be shown to be 

Typically the gain ratio G equals unity since the same gain range is used for the same detector 

type (model). For the same type of detector R, = R, and G = 1, eq (7.12) reduces to 

(7.13) 

While the wavelength uncertainty typically is not a large contribution to the measurement 

uncertainty, it can be significant if the responsivity slope is large. This is especially noticeable 

with filter and detector packages where the responsivity curve can be very steep in the cut-on and 

cut-off regions of the filter. A good example is a photometer. 

DVM Uncertainty 

The values Rw and R, are the ratio of the difference of four signals which were all taken by 

DVMs. It follows that the measurement uncertainty of the DVMs are to be included. An 

uncertainty is calculated for each of the eight signal measurements taken with a DVM (Type B 

uncertainties) and combined by RSS (ignoring the covariances due to using two DVMs) to give 

the overly conservative DVM relative uncertainty, u( V)/V . 

Long-term Stability 

The long-term stability is a component of the uncertainty analysis. The relative uncertainty 

u(Slt )/Sit is the relative difference of the measured responsivity over approximately a one year 

time period. The long-term stability numbers presented in the uncertainty tables could be 

influenced by temperature changes. For most of the spectral region the temperature effect is less 

than the stated long-term uncertainty. In the near-IR spectral region (> 960 nm for the 

Hamamatsu S1337) the temperature effect may be larger than the stated long-term uncertainty. 

A more thorough investigation could increase these numbers. However, using recently acquired 

temperature-controlled fixtures for the silicon working standards is expected to reduce the 

temperature effect significantly, and could reduce the uncertainty component for long -term 

stability in this spectral region. 

The long-term changes in the germanium working standards are not understood, but could be due 

to reflections from the wedged window reflecting off of the baffle or incident on the monitor 

detector. This is being studied. 
Scaling factor 

The scaling factor y is the ratio of absolute responsivity and relative responsivity measurements. 

The scaling factor enters the measurement equation with the relative responsivity sp of the 

pyroelectric detector. The scaling factor relative uncertainty u( y )/ y is the relative standard 

deviation of the mean of the y;’s calculated in the spectral region of i wavelengths where the 

measured responsivities overlap. 

7.1.2 Uncertainty Components due to Assumptions and Approximations 

Stray Light 

Stray light does not add error to measurements when the test and standard detectors have 

identical responsivity curves. But when the test and standard detectors have different 

responsivity curves then the error introduced by stray light must be taken into account. This is 

often very difficult to do, since the slit-scattering function is needed and is very difficult to 

measure practically; thus it has to be estimated or measured indirectly [54] over the entire 

spectral range of the monochromator or the responsivity spectral range of the detectors used in 

operation. 

In principle, the stray light is very small when using a double monochromator. Typically this is 

verified by measurements with various cut-off filters, and the stray light is considered to be a 

negligible source of error. For this uncertainty analysis, an estimation of the stray light was 

calculated; and its contribution to the measurement uncertainty was shown to be negligible for 

almost all of the measurements. 

The error due to stray light can be estimated by letting Vx(Ao) be the signal (in U units) from a 

detector x when the monochromator is set to wavelength A0 

where Kbw (A,) is the signal due to light within the full-width bandpass AA and Vx,, (Ao) is the 

signal due to stray light. Using eqs (3.3) and (3.4), the measurement equation for the detector 

signal due to the light within the bandpass AA is 

Similarly, the signal due to stray light is 

(7.15) 

51 
where the integral spans the detector responsivity spectral range but is not inclusive of the 

bandpass AA. The incident flux @DA(AyAo) on the detector from the monochromator can be 

expressed as a product with the slit -scattering function 

where z(Ao - A) is the same slit-scattering function introduced in eq (3.6), and cDfA(A) is the 

spectral radiant flux on the detector at A0. (Note: the term @f,d(A), in this situation, implicitly 

includes the transmittance of the optics and atmosphere unlike in sec. 3.1 .) The measurement 

equation for the detector at wavelength A. is thus 

Similarly, the measurement equation for a working standard detector s is 

For estimating the stray light contribution to the measured signal, the slit -scattering function is 

assumed to have the computationally convenient form 

1 for ao-Aasasao+Aa 

1 0-7 otherwise z(ao - a) = (7.20) 

The error due to stray light is estimated by converting the integrals to summations and taking the 

difference between the ratio of the measured signals from the test and working standard 

detectors, RT, and the ratio Rbw of the signals due to light within the bandpass AA. The relative 

error due to stray light when the monochromator is set to A. is estimated as 

(7.21) 

(7.22aYb) 

The relative error was calculated over the measurement spectral range at 5 nm intervals of A0 and 

will be shown below. Because these errors are small, a correction is not applied, but instead the 

component of relative standard uncertainty is assumed equal to the relative error. It can be seen 

from tables 7.1 to 7.7 that this component makes a negligible contribution to the combined 

standard uncertainty. 

52 
Bandwidth effect 

As mentioned in section 3.2.4, the bandwidth of the monochromator output flux causes an error 

in the measurement when the monochromator wavelength scale is calibrated using the peak 

wavelength of the bandpass and the responsivity curves of the test and standard detectors do not 

have the same slope. This error can be approximated for the uncertainty analysis. Consider the 

case of a pyroelectric detector used as a standard. The responsivity for the standard is assumed 

constant (flat) and the responsivities for the test and monitor detector are assumed linear over the 

bandpass. This is the situation when measuring the relative responsivity of working standard 

photodiodes. Following the development of eq (17) in Ref. [53] it can be shown that the 

measurement equation for spectral responsivity, taking into consideration the bandwidth of the 

monochromat or, 

(7.23) 

where the ratio of signals is now replaced by the ratio of test to monitor and standard to monitor 

signal ratios. Note the change in responsivity and bandwidth notation from Ref. [53]. The other 

terms are defined as 2p = AA = full-width bandpass ( p = AA/2 =FWHM for triangular 

bandpass) 

S d@D b= [m-'I, where s = - [W-m-'1, ajD -a, .s dA (7.24aYb) 

and daD is the change in flux reaching the detector (i.e., s = slope of the output flux function). 

This assumes a triangular bandpass for the monochromator and that the flux varies linearly over 

the bandpass. 

When the responsivity of the standard also changes spectrally, it can be shown that the 

measurement equation is similar to eq (1 8) in Ref. [ 531, 

(7.25) 

where all the terms and conditions are the same as before. This is the general test situation. The 

bandwidth-effect error component, using eq (7.23), is 

[U-W-']. bop2 dSx 6s =c =--.- 

6 CIA bw bw 

The relative error is 

(7.26) 

(7.27) 

53 
and 

%-A - 6s .-=-.- 4 -b*’2 4 ax [unitless]. 

‘bw S, Is, 6.S, Is, dA (7.28) 

Similarly, for eq (7.25) the relative error is eq (7.27) where the bandwidth -effect uncertainty 

component is 

and 

(7.29) 

(7.30) 

As stated in Ref. [53] and in section 3.2.4, the effect of the bandwidth could be treated as a 

correction term Cbw, but since it is small for the measurements described here it is applied as an 

uncertainty component. Numerical results are presented in tables 7.1 to 7.7 and it can be seen 

that this component makes a negligible contribution to the combined standard uncertainty. 

7.1.3 Other Factors Considered and Neglected 

Several other factors that could contribute uncertainty components were mentioned in 

section 3.1.1 and are discussed here. Stable detector responsivities are fundamental for 

detector-based radiometry. Detector responsivity stability has been the subject of several past 

and present studies reported [55]; and, for this application, short-term instability has not been 

observed. Long-term stability has already been discussed. Effects of polarization have been 

studied and found to be negligible for the typical situation, where the detectors are measured at 

normal incidence to the optical axis (and the detector surfaces are isotropic). The effect of the 

converging beam angle on the reflectance (and transmittance) from the detector surface (and 

window) is small compared to the variance of repeated measurements and is typically neglected. 

Water condensation (onto the detector or window) and the effects of water absorption have not 

been observed and are neglected. 

The responsivity temperature coefficient can be significant, especially for wavelengths near the 

bandgap of a photodiode. Typically, the temperature variation is small over the measurement 

time and is accounted for in the relative measurement uncertainty since the measurement consists 

of multiple scans. Temperature variation was considered in the long -term stability estimation. 

Therefore no explicit term is included in the uncertainty estimate for the responsivity 

temperature dependence. As mentioned earlier, diffraction and coherence effects are negligible 

for these comparator systems. 

The responsivity spatial uniformity is typically measured but not explicitly used in the 

uncertainty analysis. Instead, like the temperature variations, it is considered part of the relative 

measurement uncertainty and long-term stability uncertainty components since the spatial 

54 
uniformity directly affects these measurements (Le., the reproducibility). The same holds true 

for the monochromator beam profile and shape. 

Geometrically scattered (stray) light is light scattered out of the nominal monochromator beam 

and is thought to be primarily due to imperfections in the optics. The geometrically scattered 

light cancels when the test and standard detectors are large enough to collect all of the radiation 

from the monochromator over an area of uniform responsivity. This is the typical measurement 

condition and thus the geometrically scattered light is neglected. Background radiation 

(sometimes called diffuse stray light) has been measured and found to be negligible. 

The reflected beam from the test detector(s) and working standard detectors travels “backwards” 

along the optical axis and assumed to be scattered inside the monochromator. Any of this 

reflected beam that returns to the detector is considered as part of the geometrically scattered 

light discussed above. The reflected beam from the monitor is not returned along the same path 

and is diffusely scattered inside the enclosure on the opposite side of the baffle from the test 

detector and working standard detectors. It contributes to the background radiation previously 

mentioned. 

Effects of detector nonlinearity have been discussed in the literature [32, 34, 561. The detector 

linearity has been measured for the Hamamatsu S 1337- 101 OBQ and the UDT Sensors UV 100 

silicon photodiodes and is discussed in section9. At the power levels used for routine 

measurements, nonlinearity is not a consideration. 

The frequency response of the detector and amplifier is important to consider when comparing 

absolute measurements between chopped (optically modulated or ac) and dc measurements. But 

since the chopped (pyroelectric comparison) measurements are relative measurements, 

corrections due to frequency response effects are not considered. 

7.2 Transfer from Traps (HACR) to Working Standards 

In this section, a detailed listing of the uncertainty components for each type of working standard 

is provided along with a description of how each component was obtained. The data was 

calculated at 5 nm intervals as shown in the figures; abbreviated numerical tables are provided. 

The relative measurement uncertainty was determined by calculating the RSS of the weighted 

uncertainties from multiple measurements. The amplifier calibration uncertainties are all 

identical because identical amplifiers are used and they are calibrated using the same equipment 

and procedure. Stray light, bandwidth-effect, and wavelength calibration uncertainties were all 

calculated as described above using typical data. All components were considered independent 

and combined by RSS to get the relative combined standard uncertainty shown. 

7.2.1 Visible Silicon Working Standards 

Table 7.1 lists the uncertainty components for the three calibrations used for the visible silicon 

working standards. The UV and Ge working standard calibration uncertainties are the combined 

standard uncertainties from tables 7.3 and 7.2, respectively. The trap uncertainties are from the 

HACR transfer [21]. The long-term stability is the typical data for a visible working standard 

55 
over a one year period. All the components were combined by RSS to get the relative combined 

standard uncertainty shown at 5 nm intervals in figure 7.1. 

Table 7.1. Visible Working Standard Uncertainty 

Transfer from Traps, UV WS, and Ge WS 

Relative UV WS Visible WS Wavelength Visible WS combined 

standard 

stability uncertainty 

Relative 

Dm "' ws amplifier amplifier calibration Stray light Bandwidth- long-term effect Source.of measurement uncertainty uncertainty uncertainty calibration gain gain (zkO.1 nm) 

Type A B B B B B B B A I"/.) 

&(&" Y sv. 

- 

Root-sumof-squares 

350 0.87 0.56 0.83 0.04 0.04 0.04 0.000 -0.001 0.06 1.33 

Relative 

Bandwidth- ~~~~

combined 

effect standard 

uncertainty 

Relative Trap Trap Visible WS Wavelength 

uncertainty uncertainty uncertainty gain (zkO.1 nm) 

Of measurement Dm detector detector amplifier calibration stray light amplifier 

gain 111 -~~-- -~ X-IIIX 

Type A B B B B B B B A ["/I 

R~IYR~I U(&J&w U(&YSr MsviYS~m 

- 

Root-sumof-squares ~~ 

405 0.03 0.05 0.05 0.04 0.04 -0.05 0.001 -0.001 0.05 0.12 

450 0.03 0.02 0.05 0.04 0.04 -0.03 0.000 -0.001 0.01 0.09 

500 0.03 0.01 0.05 0.04 0.04 -0.02 0.000 0.000 0.01 0.08 

550 0.03 0.01 0.05 0.04 0.04 -0.02 0.000 0.000 0.01 0.08 

600 0.02 0.01 0.05 0.04 0.04 -0.02 0.000 0.000 0.01 0.08 

650 0.03 0.01 0.05 0.04 0.04 -0.02 0.000 0.000 0.01 0.08 

700 0.03 0.01 0.05 0.04 0.04 -0.01 0.000 0.000 0.01 0.08 

750 0.03 0.01 0.05 0.04 0.04 -0.01 0.000 0.000 0.02 0.08 

800 0.02 0.02 0.05 0.04 0.04 -0.01 0.000 0.000 0.03 0.09 

850 0.02 0.02 0.05 0.04 0.04 -0.01 0.000 0.000 0.04 0.09 

900 0.02 0.01 0.05 0.04 0.04 -0.01 0.000 0.000 0.04 0.09 

920 0.02 0.01 0.05 0.04 0.04 -0.01 0.000 0.000 0.05 0.10 

Relative 

Bandwidth- ~~~!~~ combined 

ws amplifier amplifier calibration Stray light effect standard 

stability uncertainty 

Relative Dm Ge WS Visible WS Wavelength 

Source Of measurement 

uncertainty uncertainty uncertainty calibration gain gain (* 0.1 nm) - Tv~e A B B B B B B B A 1%1 

Root-sumof-squares 

Wavelength Estimated value [%] 

950 0.90 0.02 0.90 0.04 0.04 -0.02 0.002 0.001 0.22 1.29 [nml 

1000 0.39 0.01 0.75 0.04 0.04 0.03 0.001 0.002 0.13 0.86 

1050 0.43 0.01 0.87 0.04 0.04 0.17 0.000 0.003 0.87 131 

1100 0.40 0.02 0.78 0.04 0.04 0.18 -0.002 0.001 I .86 2.06 
2.5 

2.0 

1.5 

1 .O 

0.5 

0.0 

300 400 500 600 700 800 900 1000 1100 

Wavelength [nm] 

Figure 7.1. Visible Working Standard (Vis WS) relative combined standard uncertainty. 

The discontinuities in the curve are the result of calibrations with different standards. 

7.2.2 Germanium (NIR) Working Standards 

Table 7.2 lists the uncertainty components for the relative responsivity measurements using the 

pyroelectric detector and the absolute responsivity transfer measurement uncertainty components 

using the HACR-calibrated trap detectors. The absolute measurements are used to scale the 

relative data of the germanium (NIR) working standards. The low SNR when using the 

pyroelectric gives rise to larger relative measurement uncertainty values. Also, the long-term 

stability adds significantly to the germanium working standard uncertainty. The relative 

combined standard uncertainty was determined by the RSS of all the components. Figure 7.2 

shows the relative combined standard uncertainty at 5 nm intervals. 

4.0 

3.5 

3.0 

2.5 

2.0 

1.5 

1 .O 

0.5 

0.0 

- 

- - .......... ............... .............. .............. .............. .............. .............. .......... .............. .......... - - 

- - - - :A 

, Ai n; 

.......... ............. .............. .............. .............. ............. .............. ......... ........... ...................... - 

j : i j j < 1 A ~ < 2 

.......... ............. .............. .............. ............. .......... ......... 

i i 1 j ....$A i - 

- - .......... ....... 

i _i ,... 

............................ ........................ 

............. t.1 

............................. ............. .......... 

- 

- 

.............. j .............. j ......................... 

E 

- 

- - I I I I700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 

Wavelength [nm] 

Figure 7.2. Germanium (NIR) Working Standard (Ge WS) relative combined 

standard uncertainty. 

57 
Table 7.2. Germanium Working Standard Uncertainty 

Transfer from pyroelectric (relative) and scaling with trap (absolute) 

Trap Relative 

ws combined 

standard 

stability uncertainty 

u(RmYRm 4VYV u(Sr)/s~ ~GTYGT ~(GQYGQ u(s,Ys. u(RdYRd 4&Y& 4slYsl Ic(SdSe 

Relative DvM Trap detector Ge WS Wavelength 

gain (ItO.1 nm) detector amplifier of measurement amplifier calibration Stray light long-term 

uncertainty uncertainty uncertainty calibration 

Type A B B B B B B B A [”/I 

Relative 

..~~... 

Root-sum- Estimated value [%] [nm] of-squares 

700 0.05 0.03 0.05 0.04 0.04 -0.09 0.008 0.002 0.87 0.88 

Wavelength 

750 0.05 0.03 0.05 0.04 0.04 -0.07 0.008 0.002 0.69 0.70 

800 0.04 0.04 0.05 0.04 0.04 -0.06 0.008 0.001 0.65 0.66 

850 0.03 0.04 0.05 0.04 0.04 -0.05 0.008 0.001 0.54 0.55 

900 0.03 0.02 0.05 0.04 0.04 -0.04 0.003 0.001 0.44 0.45 

920 0.03 0.02 0.05 0.04 0.04 -0.04 0.003 0.001 0.23 0.25 

Relative 

Relative DvM Pyroelectric Wavelength Ge ws combined 

calibration Stray light Bm:zp- long-term standard 

(It 0.1 nm) stability uncertainty 

of measurement relative Scaling uncertainty uncertainty uncertainty calibration factor Y 

TvDe A B B B B B B A 1%1 

Root-sum- Estimated value rh] [nm] of-squares 

950 0.68 0.02 0.52 0.14 -0.02 0 000 0.001 0.26 0.90 

Wavelength 

1000 0.39 0.01 0.52 0.14 -0.06 0.000 0.000 0.35 0.75 

1050 0.63 0.01 0.52 0.14 -0.01 -0.001 0.000 0.25 0.87 

1100 0.56 0.01 0.52 0.14 0.00 -0.001 0.000 0.09 0.78 

1150 0.78 0.01 0.52 0.14 0.00 -0.001 -0.001 0.15 0.96 

1200 0.98 0.01 0.52 0.14 0.00 -0.001 0.000 0.18 1.13 

1250 1.17 0.01 0.52 0.14 -0.03 -0.001 0.000 0.11 130 

1300 1.14 0.01 0.52 0.14 -0.01 -0.001 0.000 0.21 1.28 

1350 1.64 0.01 0.52 0.14 -0.05 -0.002 0.000 -0.05 1.73 

1400 2.34 0.02 0.52 0.14 0.02 -0.003 0.000 -0.04 2.40 

1450 2.25 0.01 0.52 0.14 0.07 -0.003 0.000 -0.11 231 

1500 2.77 0.01 0.52 0.14 0.06 -0.003 0.000 -0.11 2.83 

1550 2.34 0.02 0.52 0.14 0.08 -0.003 -0.001 0.57 2.46 

1600 1.47 0.02 0.52 0.14 -0.02 -0.002 0.000 1.47 2.14 

1650 1.75 0.03 0.52 0.14 0.08 0.000 0.000 1.72 2.52 

1700 1.69 0.05 0.52 0.14 0.04 0.003 -0.001 1.70 2.46 

1750 2.10 0.09 0.52 0.14 0.18 0.016 -0.001 1.45 2.61 

1800 2.12 0.26 0.52 0.14 0.06 0.064 -0.001 1.15 2.49 

7.2.3 UV Silicon Working Standards 

Table 7.3 lists the uncertainty components for the relative responsivity measurements using the 

pyroelectric detector and the absolute responsivity transfer measurement uncertainty components 

from the visible working standards which are traceable to the HACR. The absolute 

measurements are used to scale the relative data of the UV silicon working standards. The 

pyroelectric relative measurement uncertainty was affected by a low SNR below 250 nm. The 

long-term stability is also a large component in the uncertainty. As with the previous working 

standards, the relative combined standard uncertainty was determined by the RSS of each 

component. Figure 7.3 shows the relative combined standard uncertainty at 5 nm intervals. 

58 
1111 IIII Ill1 I I I I I I 111- 

- 

- 

- ..................... A .......................... L ................................................................................ 1 ..................... .- - - - - - - 

- - - ...................... > .......................... )--- ....................... ~ .......................... L ................................................. - 

- - - - - - - - ...................... : ........................... : ........................... ) ..................................................... i ...................... - - - - 

- - - - - ...................... : ........................... >- ...................................................................................................... - 

5 > - 

- - / Pyroelectric Detector - - ..................... ; ......................... ;..---- .................... i .......................... : : .......................... ...................... 

Trap Detector +wa**.i"i".~Y+.i.e ......... ; .......................... ; ................................................. 

..a ?.........* 

200 250 

Figure 7.3. Ultraviolet 

uncertainty. 

300 

Working 

350 

Wavelength [nm] 

Standard (UV 

400 450 500 

WS) relative combined standard 

Table 7.3. UV Working Standard Uncertainty 

Transfer from Pyroelectric (relative) and Scaling with Visible WS (absolute) 

----__I-- ~ ---~ ~ - Ix~ --___^ 

4RmYRm 4vYV 4spYsp U(YY U(SAYSA U(RslY& U(sbW)/sbw u(sr Ysk k(swYSw Relative 

uncertainty ll."----- - 1111 ~ I I - 

Root-sumEstimated value [%] of-squa 

0 1s -0 9s 0 160 0 002 0 72 6.03 

250 0 34 0 01 0 52 0 IS -0 05 0 001 0 001 -0 19 0.67 

300 0 38 0.01 0 52 0 18 -0 03 0 000 0 001 0 17 1.02 

350 0 44 0 01 0 52 0 1s 0 03 0 001 0 000 0 44 0.83 

400 0 42 0 01 0 52 0 18 -0 06 0 000 0 000 0 17 0.71 

Iyliy~---w"I-yy -- - ly~ -_ ~ xIIv y__II_* -Ivy59 
7.3 Transfer to Test (Customer) Detectors 

This section details the uncertainty components when transferring the spectral responsivity scale 

from the working standards to test (customer) detectors. The tables and figures are calculated for 

specific photodiode models and would, in general, be different for other detectors. These tables 

and figures serve as a starting point to estimate the minimum uncertainty value possible from this 

measurement service using the current configuration. 

The uncertainty in the spectral responsivity transferred to a test detector is determined similarly 

to the working standards using eq (7.8). Since, in general, it is not possible to know the details 

of how a test detector is used, NIST policy [ 71 is to not include estimates of the uncertainties 

introduced by transporting the detector or its use by the customer as a reference standard. 

Examples are uncertainties due to the passage of time (long-term stability) and differences in 

laboratory environmental conditions. 

The stray light and bandwidth-effect are negligible and effects due to wavelength calibration 

uncertainty are minimized when the working standard and test detector have the same response 

curve. In the case of an InGaAs photodiode calibrated with the germanium working standards, 

stray light and bandwidth-effect are almost completely negligible except near the ends of the 

responsivity spectral range. 

As with the transfer to the working standards, abbreviated numerical tables are provided even 

though the data was calculated at 5 nm intervals; full data sets are shown in the figures. The 

relative measurement uncertainty was determined by the RSS of the weighted uncertainties from 

multiple measurements except for the InGaAs example where only one measurement was made 

and the weighted uncertainties were used. The amplifier calibration uncertainties are all identical 

because the amplifiers used are identical and calibrated in the same way. Bandwidth-effects, 

stray light, and wavelength calibration uncertainties were all calculated using typical data as 

described above. All uncertainty components were considered independent and were combined 

by RSS to get the relative combined standard uncertainty shown. 

Figure 7.4 shows the relative combined standard uncertainty at 5 nm intervals for three types of 

transfer measurements routinely provided to customers. Note that the NIST policy is to report to 

customers the uncertainty using a expansion factor of k = 2. The expanded uncertainties are 

shown in figure 2.1. 
n 4.0 

B 

5 2 3.0 b .- 

E$ 35 

0 c 2.0 .d >1 

0.0 

200 400 600 800 1000 1200 1400 1600 1800 

Wavelength [nm] 

(Note: Relative standard uncertainty at 200 nm is 6.5 %.) 

Figure 7.4. Transfer to test (customer) detectors relative combined standard 

uncertainty. 

7.3.1 UV Silicon Transfer 

Table 7.4 lists the uncertainty components for the transfer comparison measurements for test UV 

silicon photodiodes using the UV working standards. The UV working standard calibration 

uncertainties are the combined standard uncertainties from table 7.3. The transfer relative 

combined standard uncertainty was determined by the RSS of the components. 

Table 7.4. Transfer Uncertainty to Test (Customer) UV100 Silicon Photodiodes 

~~~~,,~,~~~6 

Relative 

Bandwidth- combined 

standard 

uncertainty 

UV WS Test detector Wavelength "" ws amplifier amplifier calibration stray light' DVM Re1 ative 

Source Of measurement 

uncertainty uncertainty uncertainty calibration gain gain (+O.l nm) 

B _LI 

Type A B B B B ,-""~--- 

V u(sw)/suv 4GwYGuv ~(GT~YGT~s u(s,JSA 

.,---. -."ll' _*.I- ~ ,**,*--,-.%--,-**7p 

Root-sumEstimated value ["h] of-s- [nm] __,,.-~----II___^^_ ~--".*-,"IxII" .---*..- ."~,~- 

200 2.4 1 0.38 6.03 0.04 0.04 -0.05 0.00 0.00 6.51 

250 0.09 0.01 0.67 0.04 0.04 -0.05 0.00 0.00 0.68 

300 0.10 0.01 1.02 0.04 0.04 -0.03 0.00 0.00 1.03 

350 0.09 0.01 0.83 0.04 0.04 0.03 0.00 0.00 0.84 

400 0.15 0.01 0.71 0.04 0.04 -0.06 0.00 0.00 0.73 

450 0.04 0.01 0.17 0.04 0.04 -0.05 0.00 0.00 0.19 

'These terms are zero for identical responsivities. 

7.3.2 Visible Silicon Transfer 

Table 7.5 lists the uncertainty components for the transfer comparison measurements for test 

Hamamatsu S1337 and S228 1 visible silicon photodiodes using the visible working standards. 

The visible working standard calibration uncertainties are the combined standard uncertainties 

from table 7.1. The transfer relative combined standard uncertainty was determined by the RSS 

of the components. 

61 
Table 7.5. Transfer Uncertainty to Test (Customer) S 1337 and S228 1 Silicon Photodiodes 

Relative 

Bandwidth- combined 

eff& standard 

Relative Visible WS Test detector Wavelength 

amplifier calibration Stray lightt DVM Visible WS Source Of measurement uncertainty uncertainty uncertainty calibration 

uncertainty gain gain (*O.l nm) 

u(&Y% u(J",Yv u(SvsYsa u(GmYGm ~(GTMYGTM u(s~Ys. u(RdYRd .(&JSb, UXST~ST.S 

Type A B B B B B B B ["/I 

Relative 

uncertainty 

Wavelength Root-sumof-squares Estimated value [YO] [ml 

350 0.3 1 0.56 1.33 0.04 0.04 0.01 0.00 0.00 1.48 

400 0.05 0.12 0.76 0.04 0.04 -0.05 0.00 0.00 0.78 

450 0.02 0.04 0.09 0.04 0.04 -0.03 0.00 0.00 0.12 

500 0.01 0.02 0.08 0.04 0.04 -0.02 0.00 0.00 0.11 

550 0.00 0.02 0.08 0.04 0.04 -0.02 0.00 0.00 0.10 

600 0.01 0.01 0.08 0.04 0.04 -0.02 0.00 0.00 0.10 

650 0.01 0.01 0.08 0.04 0.04 -0.02 0.00 0.00 0.10 

700 0.01 0.02 0.08 0.04 0.04 -0.01 0.00 0.00 0.10 

750 0.01 0.02 0.08 0.04 0.04 -0.01 0.00 0.00 0.11 

800 0.02 0.03 0.09 0.04 0.04 -0.01 0.00 0.00 0.11 

850 0.02 0.03 0.09 0.04 0.04 -0.01 0.00 0.00 0.11 

900 0.01 0.02 0.09 0.04 0.04 -0.01 0.00 0.00 0.11 

950 0.01 0.01 1.29 0.04 0.04 -0.01 0.00 0.00 1.29 

1000 0.01 0.01 0.86 0.04 0.04 0.04 0.00 0.00 0.86 

1050 0.04 0.02 1.31 0.04 0.04 0.18 0.00 0.00 133 

1100 0.08 0.03 2.06 0.04 0.04 0.23 0.00 0.00 2.08 

'These terms are zero for identical responsivities. 

7.3.3 NIR Transfer 

The transfer comparison measurement uncertainties for example germanium and indium gallium 

arsenide (InGaAs) test photodiodes are presented in tables 7.6 and 7.7 respectively. Several of 

the data columns are detector -dependent, thus these tables are presented as references of typical 

uncertainties for these types of photodiodes. The reported uncertainties do not include estimates 

for several components that are unknown for these test photodiodes. The unknown uncertainty 

components are photodiode responsivity uniformity, polarization sensitivity, linearity, 

temperature coefficient, and long-term stability. These components could significantly add to 

the reported uncertainty. 

7.3.3.1 Germanium Transfer 

Table 7.6 lists the uncertainty components for the transfer comparison measurements for test 

thermoelectrically cooled germanium photodiodes using the germanium working standards. The 

germanium NIR working standard calibration uncertainties are the combined standard 

uncertainties from table 7.2. The transfer relative combined standard uncertainty was determined 

by the RSS of the components. 

The transfer relative combined standard uncertainty presented in table 7.6 is for a Judson 

EG&G J16TE2-8A6-ROSM-SC test photodiode. For different customer-supplied germanium 

photodiodes the uncertainty is reanalyzed for that device. 

62 
Table 7.6. Transfer Uncertainty to Test (Customer) TE Cooled Germanium Photodiodes 

Relative 

Bandwidth- combined 

effect' standard 

Relative DvM Ge WS Test detector Wavelength 

Source.of measurement ws amplifier amplifier calibration Stray light' uncertainty uncertainty uncertainty calibration 

gain (k 0.1 nm) uncertainty ~~-_IIx--~-xI 

gain -~_ 

Type A B B B B B B B I%] Il~~..~ox------~ 

u(RdYR4 u(sbrvYsbw &(sTexl)lsTnt 

---- 

Root-sumof-squares Estimated value ["h] 

700 0.01 0.05 0.88 0.04 0.04 -0.09 0.00 0.00 0.89 

750 0.01 0.05 0.70 0.04 0.04 -0.07 0.00 0.00 0.71 

800 0.03 0.06 0.66 0.04 0.04 -0.06 0.00 0.00 0.66 

850 0.03 0.06 0.55 0.04 0.04 -0.05 0.00 0.00 0.56 

900 0.02 0.03 0.45 0.04 0.04 -0.04 0.00 0.00 0.46 

950 0.02 0.02 0.90 0.04 0.04 -0.03 0.00 0.00 0.90 

I000 0.01 0.01 0.75 0.04 0.04 -0.03 0.00 0.00 0.75 

1050 0.01 0.01 0.87 0.04 0.04 -0.03 0.00 0.00 0.87 

1100 0.01 0.01 0.78 0.04 0.04 -0.02 0.00 0.00 0.78 

1150 0.01 0.01 0.96 0.04 0.04 -0.02 0.00 0.00 0.96 

1200 0.01 0.01 1.13 0.04 0.04 -0.02 0.00 0.00 1.13 

1250 0.01 0.01 1.30 0.04 0.04 -0.01 0.00 0.00 1.30 

1300 0.01 0.01 1.28 0.04 0.04 -0.01 0.00 0.00 1.28 

1350 0.02 0.01 1.73 0.04 0.04 -0.01 0.00 0.00 1.73 

1400 0.03 0.02 2.40 0.04 0.04 -0.01 0.00 0.00 2.40 

1450 0.02 0.01 2.31 0.04 0.04 -0.01 0.00 0.00 2.32 

1500 0.03 0.01 2.83 0.04 0.04 -0.01 0.00 0.00 2.83 

1550 0.03 0.02 2.46 0.04 0.04 0.11 0.00 0.00 2.47 

1600 0.03 0.02 2.14 0.04 0.04 0.06 0.00 0.00 2.15 

1650 0.03 0.03 2.52 0.04 0.04 0.06 0.00 0.00 2.52 

1700 0.03 0.05 2.46 0.04 0.04 0.10 0.00 0.00 2.46 

1750 0.03 0.09 2.61 0.04 0.04 0.18 0.00 0.00 2.62 

Wavelength 

[nml ~, ~-,," ,,..,.. I-,,x . ".II..~,,~~" 

'These terms are zero for identical responsivities 

7.3.3.2 InGaAs Transfer 

Table 7.7 lists the uncertainty components for the transfer comparison measurements for test 

InGaAs photodiodes using the germanium working standards. The weighted uncertainty of one 

measurement (of three scans) was used as the relative measurement uncertainty. The germanium 

(NIR) working standard calibration uncertainties are the combined standard uncertainties from 

table 7.2. The transfer relative combined standard uncertainty was determined by the RSS of the 

components. 

The transfer relative combined standard uncertainty presented in table 7.7 is for an example 

InGaAs test photodiode. The InGaAs transfer uncertainties are very similar to the germanium 

transfer uncertainties and were omitted from figure 7.4 for clarity. As with the germanium 

transfer uncertainty in table 7.6 several of the data columns are detector-dependent, thus this 

table is presented as a reference representing typical uncertainties for this type of photodiode. 

For customer-supplied photodiodes the uncertainty is reanalyzed for that device. 

63 
Table 7.7. Transfer Uncertainty to Test (Customer) InGaAs Photodiodes 

- -?I,%*- __(s __~~,~~~j(~(,l?~~_

Relative 

Bandwidth- combined 

effect standard 

Ge WS Test detector Wavelength 

gain (rt0.l nm) 

DVM Ge WS Relative 

uncertainty Source Of measurement uncertainty uncertainty amplifier amplifier calibration Stray light 

uncertainty --..--”,-”-.,-,””,” “x”x-~~x.^--_ 

gain ”~~,~~~-,~-,,”,~- ---- Type A B B B B B B ~I__~,, 

B I”/.] ,--”-_-”~ 

Relative 

Wavelength 

uncertain u(RmYRm u( VYV u(sceYs~, u(GaYGa u(G~estYG~est ~(SAYSA 4RdYR-i u(&w)/sbs &(ST~*I)~STW *xIw -.*-, xx-~-)y *I*xu--”yL, II.- 

of-squares Estimated value [“h] 

700 0.16 0.04 0.88 0.04 0.04 -0.06 0.01 0.00 0.90 

750 0.18 0.05 0.70 0.04 0.04 -0.06 0.01 0.00 0.73 

800 0.17 0.06 0.66 0.04 0.04 -0.06 0.01 0.00 0.69 

850 0.13 0.06 0.55 0.04 0.04 -0.11 0.01 0.00 0.58 

900 0.1 1 0.03 0.45 0.04 0.04 -0.1 1 0.00 0.00 0.48 

950 0.09 0.02 0.90 0.04 0.04 -0.08 0.00 0.00 0.91 

1000 0.08 0.01 0.75 0.04 0.04 -0.01 0.00 0.00 0.76 

1050 0.08 0.01 0.87 0.04 0.04 -0.02 0.00 0.00 0.87 

1100 0.06 0.01 0.78 0.04 0.04 -0.01 0.00 0.00 0.78 

1150 0.06 0.01 0.96 0.04 0.04 -0.01 0.00 0.00 0.96 

1200 0.05 0.01 1.13 0.04 0.04 -0.01 0.00 0.00 1.13 

1250 0.04 0.01 1.30 0.04 0.04 -0.01 0.00 0.00 1.30 

1300 0.04 0.01 1.28 0.04 0.04 -0.01 0.00 0.00 1.28 

1350 0.04 0.01 1.73 0.04 0.04 0.00 0.00 0.00 1.73 

1400 0.01 0.02 2.40 0.04 0.04 -0.01 0.00 0.00 2.40 

1450 0.02 0.01 2.31 0.04 0.04 -0.01 0.00 0.00 2.32 

1500 0.03 0.01 2.83 0.04 0.04 0.00 0.00 0.00 2.83 

1550 0.26 0.02 2.46 0.04 0.04 0.01 0.00 0.00 2.48 

1600 0.30 0.02 2.14 0.04 0.04 0.02 0.00 0.00 2.17 

1650 0.35 0.03 2.52 0.04 0.04 0.09 0.00 0.00 2.54 

1700 0.45 0.05 2.46 0.04 0.04 0.33 0.01 0.01 2.52 

1750 0.52 0.13 2.61 0.04 0.04 0.30 0.07 0.00 2.69 

A,..---- j.,-^_x,,,,-,~-*%*-,--I^_x__ ~“-xI~~~-~-~I. ~---” ,,,, .,,,,, ~,,-*,,~~, 

7.3.4 Filtered Detector Transfer 

Special tests of filtered detectors (such as photometers) can be made. It should be noted that the 

relative measurement uncertainty (measurement statistical uncertainty) will increase due to the 

decrease in signal in the “wings” of the response. As mentioned above, when the slope of the 

responsivity curve is steep, the uncertainties due to stray light, bandwidth, and wavelength 

calibration can increase by orders of magnitude relative to the uncertainties within the bandpass 

of the filtered detector. 

7.4 Spatial Uniformity Measurement Uncertainty 

The repeatability uncertainty components for the responsivity uniformity measurement of a 

typical photodiode are listed in table 7.8. This is the repeatability of the measured relative 

responsivity in the central portion of the active area during one measurement scan. The 

measurement noise is the average standard deviation of the mean of the measurements in the 

central portion of the active area of the detector. The relative combined standard uncertainty is 

the RSS of the measurement noise and the one-day DVM uncertainty specification. 

The measurement repeatability uncertainty depends on the SNR of the detector and noise due to 

the amplifier and DVM. The relative measurement noise varies spectrally, primarily due to the 

64 
change in monochromator flux magnitude with wavelength. Note that the variation in 

responsivity over the measured area is much larger than this uncertainty value. 

The reported uncertainty is not an indication of the uniformity measurement reproducibility. It 

does not consider other components which contribute to the reproducibility uncertainty, such as, 

the ability to reproduce the same irradiance geometry and detector alignment. Uniformity 

reproducibility results have been reported [ 491 for a Hamamatsu S 1337- 10 1 OBQ with a standard 

deviation of 0.033 % at 500 nm and 0.25 % at 1000 nm. 

The intended primary use of the reported uniformity results is qualitative. That is, to indicate if 

any large discontinuities are present in the responsivity uniformity which can lead to larger than 

expected uncertainties in absolute responsivity measurements. Quantitative application of the 

reported uniformity results requires examination of the irradiance geometry and equipment 

involved. The generalized application of the uniformity measurement results is currently being 

studied. 

Table 7.8. Photodetector Spatial Uniformity Measurement Repeatablity Uncertainty ,- $~<~**>7-?-w\*-w 

Relative 

Source of Relative DVM combined 

uncertainty standard uncertainty measurement 

noise' uncertaintv 

Root-sum-ofsquares 

0 0009 0 0007 0.0012 

Estimated value [%I xx--Ix" 

'Depends on photodetector and signal level 

8. Quality System 

The spectroradiometric detector measurements described in this publication are part of the NIST 

Optical Technology Division calibration services and are in compliance with ANSINCSL 

2540-1-1994 [ 57, 58, 591. Although quality procedures were previously in place, they varied 

from calibration service to calibration service within the Division. The quality control 

procedures were typically limited to the technical aspects of the measurements, such as the 

yearly calibration of voltmeters, the use of multiple working standards, and their routine rotation. 

The goal of the Optical Technology Division's ANSINCSL 2540-1-1 994 compliance project 

(which began in 1993) was to unify all the calibration services offered by the Division with 

standard formats and similar procedures. Balancing functionality and bureaucracy was a concern 

from the start. Efforts were directed toward developing a useful and practical quality system. 

Excessively sophisticated and complex procedures are avoided, along with redundant 

documentation. Tools such as checklists, forms, and flowcharts are used where applicable. 

8.1 Control Charts 

Control charts are a standard statistical tool used for tracking a process over time [60]. Two 

working standard detectors are used for each test (customer) detector measurement. Thus, one 

65 
working standard can be used to calculate the spectral responsivity of the second working 

standard. Since the working standards are randomly chosen each week, this allows all of the 

working standards to be compared to each other over time. The responsivity of a given working 

standard detector can then be tracked over time using control charts. In practice, only a few 

wavelengths (e.g., every 100 nm) need to be plotted on a control chart for each working standard. 

An example control chart for a visible working standard at 600 nm is shown in figure 8.1. The 

center line is the mean responsivity during a period when the measurement process is stable 

(i.e., “in control”). The upper control limit (UCL) and lower control limit (LCL) are respectively 

plus and minus three times the average standard deviation of the mean of the responsivity 

measurements. This should include almost all of the expected random measurement fluctuations. 

Figure 8.1 shows a trend in the responsivity measurements that does not appear to be due to 

random fluctuations 

: 

but is still within the control limits. This demonstrates the value of control 

UCL 

0.3 13 10 

0.3 1305 

1 0.31300 e e 

0 

21-Sep-1992 1-Dec-1992 19-Apr-1993 14-Sep-1995 6-Aug-1996 3-Oct-1996 

Time [days] 

Figure 8.1. Control chart example for a NIST Visible Working Standard (Vis WS). 

8.2 Comparison to Other Laboratories 

Comparisons have been made between the UV responsivity scales of the UV SCF and the NIST 

Synchrotron Ultraviolet Radiation Facility (SURF) I1 [ 6 1 621. The SURF I1 responsivity scale is 

derived from a different physical basis (synchrotron radiation) than the UV SCF’s scale (derived 

from the HACR). The comparisons have agreed within the stated uncertainties of both scales. 

Future UV intercomparisons are planned with SURF I11 [63] and a second absolute cryogenic 

radiometer being installed at SURF 111. 

There have been few international spectral responsivity intercomparisons. The latest carried out 

by the Bureau International des Poids et Measures (BIPM) was in 1992 [32,64]. NIST agreed 

with the BIPM responsivity scale to better than 0.01 % from 500 nm to 900 nm. The relative 

difference increased to = 0.4 % at 950 nm and 1000 nm and steadily increased from 0.03 YO at 

66 
450 nm to 3 % at 248 nm. The larger deviations below 400 nm were reported by several labs 

(including NIST) but were typically within the stated measurement uncertainties. 

Currently there is a spectral responsivity comparison underway between NIST and Centro 

Nacional De Metrologia (CENAM) in Mexico. It is hoped that with the establishment of the 

North American Calibration Cooperation (NACC) this will be an ongoing intercomparison and 

will eventually include National Research Council (NRC) in Canada as well. It is also expected 

that there will be additional intercomparisons among regional groups of Europe, Asia, North 

America, and South America. 

9. Characteristics of Photodiodes Available from NIST 

This section describes the characteristics of the silicon photodiodes provided by NIST under 

Service ID numbers 390713 and 39073s. The physical and electrical characteristics of the 

photodiodes are discussed and the results of linearity measurements are given. The precision 

apertures are described and the photodiode fixture mechanical diagrams are shown. 

9.1 Hamamatsu S1337-1010BQ 

Hamamatsu S1337-1 OlOBQ silicon photodiodes have been supplied by NIST as spectral (power) 

responsivity standards. NIST currently supplies the Hamamatsu S228 1 for this purpose (NIST 

Service ID number 39073s). As mentioned previously, the Hamamatsu S1337 series diode is a 

popular diode for radiometric standards and it has been extensively characterized [3 1, 321. 

Hamamatsu describes [ 651 the S 1337- 10 1 OBQ as a p-n diode with a 1 cm x 1 cm active area, a 

fused quartz window, and a ceramic case. The spectral response range is 190 nm to 1100 nm 

with a peak at 960 nm. The S1337-1010BQ also has a high shunt resistance (dynamic 

impedance), with a typical value of 200 MQ and a minimum value of 50 MQ. 

The typical measured spectral responsivity and quantum efficiency are shown in figures 6.2 and 

6.3, respectively. The typical spatial uniformities measured at 500 nm and 1000 nm are shown 

in figure 6.4a and b, respectively. The temperature coefficient of several and S1337 series 

photodiodes were measured using a temperature-controlled fixture. All of the measurements 

were made following the typical spectral responsivity procedures at temperatures around 25 "C. 

Figure 9.1 shows the average temperature coefficient of the Hamamatsu S1337 series 

photodiode. A second common silicon photodiode series, the S 1226, is shown for comparison in 

figure 9.1 along with the wavelength of peak responsivity for each photodiode. 

The linearity of the S1337-1010BQ at 633 nm is shown in figure 9.2 spanning irradiance levels 

from 0.5 mW/cm2 to 6.6 mW/cm2. Each data point represents the ratio of the photodiode 

responsivity at the indicated irradiance to the responsivity at low power. The linearity was 

measured by using a beamsplitter to irradiate two photodiodes at approximately a 10: 1 intensity 

ratio, with the diode aperture filled and uniformly irradiated [66]. The linearity is dependent on 

the irradiation geometry and will differ for spot sizes significantly smaller than the aperture size. 

67 
5 1.2 

g 1.0 

111,)1111 III ,(*I,,(,,, ,l,,,,l I II,J,,,,J 

.. ... ; .................................................................... ; ...................... 4 ...................... ................ + ................ !.- 

I .- 

- ........... *: - ................... ; ................ ...................... i ...................... ; ...................... i ...................... 4 ...................... < 

-..... 

O".... "- / S1337 Si (25 "C) j .*' .. - ................... ; ...................... i ............................................. 4 ...................... < ....... = 960 nm .....,.......... 1 

j Rrsppcsk !'I.: t .. -... ................ 4 ...................... ; ...................... : ................ Sl226 Si (25 OC) ............................. L ............................. 

L ....... ~~.~,.~..~..g.,....*.....g~.....~.g.~..~. b. ~.~~~.~~~.~~~~.~....*9.*-....~........~.~ 

:. ' .' = 720 nm Rap peak 

h ................... ...................... .................. ...................... ...... ............. /...... j .. ./ L I 1 ........................... i i \.+?.{ ......................... I 

..... .................... 

i,,,,i,,,,i,,,,i,,,,i,,,,i,,,,i,,,,- 

l""l""l""~""l""!""- 

_......... a ........... 4 ........... ........... & .......... .......... *: ......... '. ....................................... ; ......................... : ........................ - ai 

aj 

- ....................... i .......................... : .......................... ; ......................... ; ......................... < ............................... ...................- 

ia - .......................... .; 

- ....................... 2 ......................... ,. ................................................................................................................................. j. 

111111111111iIIIIiIIIIiIIIIIIIII 

300 400 500 600 700 800 900 1000 1100 

Wavelength [nm] 

Figure 9.1. Temperature coefficient of silicon Hamamatsu S 1226 and S 1337 photodiodes. 

1.005 

1 .ooo 

0.995 

0.990 

0.985 

0.980 

2 3 4 5 7 

Irradiance [mW/cm2] 

Figure 9.2. Linearity of Hamamatsu S1337-1010BQ at 633 nm. 

9.2 Hamamatsu S2281 

Hamamatsu S228 1 silicon photodiodes are currently provided by NIST as standards of spectral 

responsivity (NIST Service ID number 39073 S). The characteristics are essentially identical to 

the S 1337- 10 1 OBQ. Since the S228 1 is almost indistinguishable from the S 1337, the spectral 

responsivity and quantum efficiency are not shown in figures 6.2 and 6.3. The significant 

differences between the two types of diodes are the S2281 has a 1 cm2 active area that is circular 

instead of square and it is housed in a metal case with a BNC connector. The BNC case 

simplifies the photodiode fixture since no electrical wiring is required. 

9.3 UDT Sensors UVlOO 

UDT Sensors UVl 00 silicon photodiodes are provided as spectral responsivity standards in the 

UV (NIST Service ID number 39071s). UDT Sensors literature [67] describes the UVlOO as an 

inverted channel diode with enhanced resistance to damage from UV radiation. The UVlOO has 

a quartz window, a 1 cm2 circular active area, and is housed in a metal case with a BNC 
connector similar to the Hamamatsu S228 1. The spectral response range is from 200 nm to 

1100 nm with a peak around 760 nm. The typical shunt resistance value is 10 MR. 

1.1 

1.0 

% 0.9 

8 0.8 > 

(d 0.7 

0.6 1 i I i i i I I I i i i I I 

L. 

2 

l"'l"'l"'l"'I"~~ .I 

.- t ................. * ...... : .............. * ...... * .... + ......... .. ............... * .......................... * ............................ j ............ * \I a. i No Bias Voltage + w ~ 

1 ........................... L ............................. ; .............................. :..e ........................ Bias ................ 1 ........................... : 

0 

_... ....................................................... 

0 

0; 

2 

2 0 

_ ........................... i ............................................................. < .............................. ; ............................... ............................. 

.M c.l 

The linearity of the UV100, shown in figure 9.3 at 442 nm, spans irradiance levels from 

0.1 mW/cm2 to 1.1 mW/cm2 with and without a reverse bias voltage. Each data point represents 

the ratio of the photodiode responsivity at the indicated irradiance to the responsivity within the 

linear region. The linearity was measured by using a beamsplitter to irradiate two photodiodes at 

approximately a 10: 1 intensity ratio, with the diode aperture filled and uniformly irradiated [ 661. 

The linearity is dependent on the irradiation geometry and will differ for spot sizes significantly 

smaller than the aperture size. 

The change in the responsivity as a function of bias voltage for this type of photodiode at 442 nm 

is shown in figure 9.4. For wavelengths shorter than 450 nm, a 1 V bias can be used to improve 

the linearity of the photodiode without significantly changing the spectral responsivity. There 

will however be some leakage current which will limit the minimum usable signal. 

1.1 

1 .o 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

1""1'"'I'"'I""I""1""1"" 

.............. *+........*.....* ............. .................... .................... 4 ................... 

..... ....... ... ..,.......... .........!... ............................................................................................................. 0 o\ 

0 ................ 1 ..................... .,............. 

.......................................... j ...................... i ...................... { ..................... 

..........,........ ..................................... ! ...................... .................... 1mw/cm2 i \ I 

4mw/cm2 j ................. i ...................... i .................... - 

................... ; ............................................. i ...................... ; ...................... ; ...................... i ...................... i .................... j.i 

................... i .................. ...................... ............................................................................................................... , ____, ,. 

-In II1I ,I,, I,,, I,,, ,#,I IIII IIII 

0.0 0.5 1 .o 1.5 2.0 2.5 3.0 3.5 4.0 

Bias Voltage [VI 

Figure 9.4. Responsivity dependence on bias voltage of UDT Sensors UVlOO at 442 nm. 

69 
9.4 Detector Apertures 

The precision apertures provided with the photodiodes by NIST Service ID numbers 39071 S and 

39073s are Buckbee Mears part number SK#030483-1073. They are electroformed optical 

apertures that have a bi-metal construction (copper with a nickel finish on both sides). The 

aperture is a thin disc with an outside diameter of 13.92 mm (0.5480 in) and an inside diameter 

of 7.9789 mm (0.31413 in), the specified aperture area is nominally 0.5 cm’. The apertures are 

dimensionally measured at NIST in the Fabrication Technology Division in an environmentally 

controlled laboratory. The aperture diameter is typically measured twice along perpendicular 

axes. The reported aperture area is the average area calculated from the two (or more) diameters. 

The reported aperture area and distance from the aperture plate to the photodiode surface do not 

enter into the responsivity measurement results because the optical beam underfills the aperture. 

The uncertainty values reported with the aperture dimensions are expanded uncertainties. 

Recently an new method has been reported for measuring aperture areas [68]. This method 

determines the area by optical comparison and is faster than the mechanical diameter 

measurements. Some of the apertures provided have been measured in this manner. Soon, all of 

the apertures will be measured using this new method. 

9.5 Detector Fixture Mechanical Drawings 

Note: As stated in the Introduction, this document follows the NISTpolicy of 

using the International System of Units (So. The following mechanical drawings 

were originally prepared in English units and are presented without converting 

the values shown to SI units. 

The detector fixtures for the Hamamatsu S1337-1010BQ and S2281 and the UDT Sensors 

UV 100 silicon photodiodes are designed for convenient handling and use. The fixture housings 

are black anodized aluminum and the 5.08 cm diameter was chosen as a convenient size for use 

with common optical table fixtures. Most of the fixtures also have a 1/4-20 threaded hole (not 

shown in drawings) on the side of the fixture for a standard optical table post. Each fixture has 

an engraved serial number on the back. Black anodized aluminum covers (not shown) were 

added later to protect the photodiodes (and apertures) during storage and shipment. 

The Hamamatsu S 1337 - 10 1 OBQ fixture design includes space to couple the photodiode anode 

and cathode to a BNC connector. Figure 9.5 shows the mechanical diagram for the fixture body. 

Figure 9.6 shows additional pieces for holding the diode lead sockets and BNC connector. 

Figure 9.7 is an exploded view of the fixture including the photodiode, aperture, and aperture 

plate. 

The UDT Sensors UVlOO and Hamamatsu S2281 photodiodes are housed in metal BNC cases. 

This simplifies the fixture and reduces the construction time since no electrical wiring is 

required. The mechanical diagram for the UVlOO and S2281 diodes is shown in figure 9.8. 

The precision apertures are attached to a black anodized aluminum plate. Figure 9.9 shows the 

first design for the aperture plate. This was found to scatter light onto the diode when light 

70 
struck the countersunk area around the aperture. An improved design shown in figure 9.10 has 

been used since 1993. Later productions of the aperture plate are engraved with the detector 

fixture serial number facing the diode. 

3 x 2-56 clearance 

countersunk 

0.25" L H 

0.125 17- J 0.125" 

& 1" 0.75" 0.625" 

II0.125" 

4 x 2-56 

tapped 

, 1 ol 41- 

-+0.625"+ 

1.25" 

Material Aluminum 

Finish: BlackAnodized 

blindhole 

Figure 9.5. Mechanical diagram of Hamamatsu S 1337-1 01 OBQ fixture body. 

Material Aluminum 

Finish:BlackAnodized 

Material: Black DeIrinB m* 0.25" 

0.050" 0.25" 

0.05O"z 

dia. 

t 

0.125" 

3 x 2-56 tapped through 

Drill and Tap for 

supplied BNC connector 

Figure 9.6. Mounting pieces for the Hamamatsu S1337-101 OBQ photodiode and 

BNC connector. 

71 
Aperture Plate 

- 

r 

Photodiode 

Sockets 

- - 

Delrina Disk 

Precision 

Aperture 

1 - I BNC 

-I 

Figure 9.7. Exploded view diagram of Hamamatsu S 1337- 101 OBQ fxture. 

MateriakAluminum 

Finish: Black Anodized 

4-40 tapped 

blind hole 

Figure 9.8. Mechanical diagram of UDT Sensors UV 100 and Hamamatsu S228 1 fixture. 

72 
4 x 2-56 clearance 

countersunk 

\- 

t-r 

0.55" 0.4" 

0.006"f0.0005 

Shoulder 

0.125" 

Material: Aluminum 

Finish: Black Anodized 

Figure 9.9. Mechanical diagram of pre - 1993 aperture plate for detector fixtures. 

Material: Aluminum 

Finish: Black Anodized 

Shoulder 

0.55" 

?- 

I 1.25" 

i 0.0625" d 

r 

Figure 9.10. Mechanical diagram of present aperture plate for detector fixtures. 

73 
10. Future Work 

Previous sections mentioned future modifications and improvements to the UV and Vis/NIR 

SCFs. This section will discuss these changes and other future work. Several publications are 

planned to describe these modifications and improvements to the SCFs. This publication 

describing the measurement services offered by the UV and Vis/NIR SCFs will be revised every 

3 to 5 years. 

One of the first additions will be to temperature control the UV and Vis/NIR SCFs’ silicon 

working standards. A comparison of the UV SCF working standards to the SURF I11 [63] 

responsivity scale is planned in the near future. A long-term study is ongoing to find new UV 

detectors with better responsivity, uniformity, and stability. 

Several major modifications will be made to the UV SCF. A new, higher efficiency UV 

monochromator (with automated order sorting filters) and larger linear translation stages have 

been purchased and will be integrated into the UV SCF when the measurement schedule allows. 

The translation stages will replace the rotating stage and small linear stages that have limited the 

UV SCF to measuring only one test detector at a time. The new stages will be configured similar 

to the Vis/NIR SCF stages. Additional imaging mirrors for astigmatism correction will be added 

to the UV SCF similar to the Vis/NIR SCF. 

A higher accuracy, computer-addressable wavelength encoder will be added to the Vis/NIR SCF 

to improve the wavelength control. Additional modifications to improve the measurement 

service are high-accuracy aperture measurements [ 681, automated control of the amplifier gains, 

and a computer interface (via IEEE -488 bus) with customer -supplied test equipment. 

As seen in the uncertainty assessment, the measurement uncertainty was significantly affected by 

the poor response of the pyroelectric detector in the SCFs. A flat detector with lower noise 

equivalent power PEP) and higher SNR would be better suited for the power levels in the SCFs. 

There are efforts currently underway at NIST to develop better spectrally flat detectors. One is a 

silicon bolometer [69, 701 designed as a standard with the new IR SCF covering 2 pm to 20 pm 

[71, 721. Since the bolometer has a flat response, it can also, in principle, be used below 2 pm in 

the UV, visible, and near-IR. 

11. Acknowledgments 

The development, operation, and evolution of this measurement service has involved several 

NIST personnel. The major contributors have been: Bob Saunders and Jeanne Houston for 

constructing the Vis/NIR SCF; Ed Zalewski and Jeanne Houston for their work establishing and 

operating the Detector Response Transfer and Intercomparison Program (DRTIP); and Joel 

Fowler for the design of the DRTIP electronics. Chris Cromer added the UV SCF, enhanced the 

computer automation, and started the sales of silicon photodiodes as part of the measurement 

service. 

The authors would like to thank Tom Gentile, Jeanne Houston, Jonathan Hardis, Chris Cromer, 

and Chris Classon for their work with the HACR and the traps; Joel Fowler for providing the 

transimpedance amplifiers; and Bob Saunders for his informative discussions on 

74 
spectroradiometry and equipment. Thanks are also given to the staffs of the NIST Calibration 

Program and NVLAP for their help with the measurement service. In particular thanks go to 

Janet Land of the Precision Engineering Division and Carroll Brickenkamp of the Measurement 

Services office for their help with quality systems and documentation. 

12. 

VI 

PI 

References 

American National Standard for Calibration - Calibration Laboratories and Measuring 

and Test Equipment - General Requirements, ANSI/NCSL 2540-1 -1 994. (Currently 

available through the National Conference of Standards Laboratories, 1 800 30th Street, 

Suite 305B, Boulder, CO 80301.) 

Determination of the Spectral Responsivity of Optical Radiation Detectors, Publ. 64 

(Commission Internationale de L’Eclairage, Paris, 1984). (Currently available through 

the U. S. National Committee of the CIE, c/o Mr. Thomas M. Lemons, TLA-Lighting 

Consultants, Inc., 72 Loring Avenue, Salem, MA 01970.) 

W. Budde, Optical Radiation Measurements, Volume 4: Physical Detectors of Optical 

Radiation, Academic Press, Inc., Orlando, FL (1 983) pp. 38 -59. 

J. Geist, L. B. Schmidt, and W. E. Case, “Comparison of the Laser Power and Total 

Irradiance Scales Maintained by the National Bureau of Standards,” Appl. Opt. 12, 

2773-2776 (1 973). 

J. Geist, M. A. Lind, A. R. Schaefer, and E. F. Zalewski, “Spectral Radiometry: A New 

Approach Based on Electro-Optics,” Natl. Bur. Stand. (U.S.), Tech. Note 954 (1 977). 

Unpublished preprint of sequel to NBS Technical Note 950 describing the radiometric 

characteristics of the DRTIP radiometers, p. 9. 

B. N. Taylor and C. E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty 

of NIST Measurement Results, Natl. Inst. Stand. Technol., Tech. Note 1297 (1994 ed.). 

E. F. Zalewski and J. Geist, “Silicon Photodiode Absolute Spectral Response 

Self-Calibration,” Appl. Opt. 19, 1214-1216 (1980). 

J. Geist, E. F. Zalewski, and A. R. Schaefer, “Spectral Response Self-Calibration And 

Interpolation of Silicon Photodiodes,’’ Appl. Opt. 19,3795-3799 (1980). 

E. F. Zalewski and C. R. Duda, “Silicon Photodiode Device with 100 YO External 

Quantum Efficiency,” Appl. Opt. 22,2867-2873 (1983). 

The QED-200 is now manufactured by UDT Instruments (formerly Graseby Optronics), 

Orlando, FL. 

75 
E. F. Zalewski, The NBS Photodetector Spectral Response Calibration Transfer Program, 

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-17,p. 45 (1988). 

J. M. Houston and E. F. Zalewski, “Photodetector Spectral Response Based on 100 % 

Quantum Efficient Detectors,” Optical Radiation Measurements 11, James M. Palmer, 

Editor, Proc. SPIE 1 109, pp. 268 -277 (1 989). 

C. L. Cromer, “A New Spectral Response Calibration Method using a Silicon Photodiode 

Trap Detector,” presented at the 1991 Measurement Science Conference, Anaheim, CA, 

Jan. 3 1 -Feb. 1, 199 1 (unpublished). 

N. P. Fox, “Trap Detectors and Their Properties,” Metrologia 28, 197-202 (1991). 

J. Geist, “Current Status of, and Future Directions In, Silicon Photodiode 

Self-Calibration,” Optical Radiation Measurements 11, James M. Palmer, Editor, Proc. 

SPIE 1 109,246 -256 (1 989). 

E. F. Zalewski and C. C. Hoyt, “Comparison Between Cryogenic Radiometry and the 

Predicted Quantum Efficiency of pn Silicon Photodiode Light Traps,” Metrologia 28, 

203-206 (1991). 

N. P. Fox, “Radiometry with Cryogenic Radiometers and Semiconductor Photodiodes,” 

Metrologia 32,535-543 (199996). 

A. C. Parr, A National Measurement System for Radiometry, Photometry, and Pyrometry 

Based Upon Absolute Detectors, Natl. Inst. Stand. Technol., Tech. Note 142 1 (1 996). 

T. R. Gentile, J. M. Houston, J. E. Hardis, C. L. Cromer, and A. C. Parr, “National 

Institute of Standards and Technology High-Accuracy Cryogenic Radiometer,” Appl. 

Opt. 35, 1056-1068 (1996). 

T. R. Gentile, J. M. Houston, and C. L. Cromer, “Realization of a Scale of Absolute 

Spectral Response Using the National Institute of Standards and Technology 

High-Accuracy Cryogenic Radiometer,” Appl. Opt. 35,4392-4403 (1 996). 

T. C. Larason, S. S. Bruce, and C. L. Cromer, “The NIST High Accuracy Scale for 

Absolute Spectral Response from 406 nm to 920 nm,” J Res. Natl. Inst. Stand. Technol. 

101, 133-140, (1996). 

NIST Calibration Services Users Guide, Joe D. Simmons, Editor, Natl. Inst. Stand. 

Technol., Spec. Publ. 250 (1991). 

NIST Calibration Services Users Guide Fee Schedule, Natl. Inst. Stand. Technol., Spec. 

Publ. 250 Appendix (1 997). 

76 
1251 H. J. Kostkowski and F. E. Nicodemus, An Introduction to the Measurement Equation, 

Chapter 5 in Self-study Manual on Optical Radiation Measurements: Part I--Concepts, 

Chapters 4 and 5, F. E. Nicodemus, Editor, Natl. Bur. Stand. (U.S.), Tech. Note 910-2 

(1 978). 

H. J. Kostkowski, The Relative Spectral Responsivity and Slit-Scattering Function of a 

Spectroradiometer, Chapter 7 in Self-Study Manual on Optical Radiation Measurements: 

Part I--Concepts, Chapters 7, 8, and 9, F. E. Nicodemus, Editor, Natl. Bur. Stand. (U.S.), 

Tech. Note 91 0-4 (1 979). 

R. Kohler, R. Goebel, R. Pello, and J. Bonhoure, “Effects of Humidity and Cleaning on 

the Sensitivity of Si Photodiodes,” Metrologia 28,2 1 1-2 15 (1 991). 

A description of diffiaction and the Airy disk can be found in most basic optics texts, 

e.g., F. L. Pedrotti and L. S. Pedrotti, Introduction to Optics, 2nd edition, Prentice Hall, 

Englewood Cliffs, NJ (1 993) or E. Hecht, Optics, 2nd edition, Addison-Wesley 

Publishing Company, Reading, MA (1 987). 

J. B. Shumaker, Introduction to Coherence in Radiometry, Chapter 10 in Self-study 

Manual on Optical Radiation Measurements: Part I--Concepts, Chapter 10, F. E. 

Nicodemus, Editor, Natl. Bur. Stand. (U.S.), Tech. Note 910-6 (1983). 

J. L. Gardner, “Astigmatism Cancellation in Spectroradiometry,” Metrologia 28,25 1-254 

(1 991). 

F. Lei and J. Fischer, “Characterization of Photodiodes in the UV and Visible Spectral 

Region Based on Cryogenic Radiometry,” Metrologia 30,297-303 (1 993). 

R. Kohler, R. Goebel, and R. Pello, Report on the International Comparison of Spectral 

Responsivity of Silicon Detectors, Rapport BIPM-94/9, document CCPW94 -2, dated July 

27, 1994, Bureau International des Poids et Mesures, Pavillon de Breteuil, 93212 Sevres, 

Cedex, France. 

J. M. Bridges and W. R. Ott, “Vacuum Ultraviolet Radiometry. 3: The Argon Mini-Arc 

as a New Secondary Standard of Spectral Radiance,” Appl. Opt. 16,367-376 (1977). 

G. Eppeldauer and J. E. Hardis, “Fourteen-Decade Photocurrent Measurements with 

Large-Area Silicon Photodiodes at Room Temperature,” Appl. Opt. 30, 3091 -3099 

(1991). 

W. Budde, Optical Radiation Measurements, Volume 4: Physical Detectors of Optical 

Radiation, Academic Press, Inc., Orlando, FL (1983) pp. 265 -268. 

77 
1371 

[3 81 

G. Eppeldauer, “Electronic Characteristics of Ge and InGaAs Radiometers,” Infiared 

Technology and Applications XXII, Bjorn F. Andresen and Marija Strojnik, Editors, Proc. 

SPIE 3061, pp. 833-838 (1997). 

J. E. Martin, N. P. Fox, and P. J. Key, “A Cryogenic Radiometer for Absolute 

Radiometric Measurements,” Metrologia 21, 147-1 55 (1 985). 

At the 1994 CCPR meeting in Paris approximately 10 nations indicated that they had 

purchased a cryogenic radiometer or were in the process of purchasing one and would use 

it as the basis of the radiometric measurements in their respective nations. 

Electrical standards are maintained in the Electronics and Electrical Engineering 

Laboratory at NIST. 

R. C. Paule and J. Mandel, “Consensus Values and Weighting Factors, ” J. Res. Natl. Bur. 

Stand. (U.S.) 87, 5 (1982). 

There are several books on optical detectors and radiometry that describe pyroelectric 

detectors. Many of these books are listed in the bibliography along with the following. 

E. L. Dereniak and D. G. Crowe, Optical Radiation Detectors, Wiley, New York, NY 

(1 984). 

W. Budde, Optical Radiation Measurements, Volume 4: Physical Detectors of Optical 

Radiation, Academic Press, Inc., Orlando, FL (1 983). 

V. R. Weidner and J. J. Hsia, NIST Measurement Services: Spectral Reflectance, Natl. 

Bur. Stand. (U.S.), Spec. Publ. 250-8 (1987). 

P. Y. Barnes and E. A. Early, NIST Measurement Services: Spectral Reflectance, Natl. 

Inst. Stand. Technol., Spec. Publ. 250-8 Revised (in preparation). 

K. L. Eckerle, J. J. Hsia, K. D. Mielenz, and V. R. Weidner, NIST Measurement 

Services: Regular Spectral Transmittance, Natl. Bur. Stand. (U.S.), Spec. Publ. 2506 

(1987). 

See Ref. 27 and personal communication with one of the authors. Note: Cleaning a 

photodiode can change its responsivity. 

E. M. Gullikson, R. Korde, L. R. Canfield, R. E. Vest, “Stable Silicon Photodiodes for 

Absolute Intensity Measurements in the VUV and Soft X-Ray Regions,’’ J. Elect. Spect. 

Rel. Phenom. 80,313-316 (1996). 

K. D. Stock, “Internal Quantum Efficiency of Ge Photodiodes,” Appl. Opt. 27, 12-14 

(1 988). 

78 
A. R. Schaefer, E. F. Zalewski, and J. Geist, “Silicon Detector Nonlinearity and Related 

Effects,’’ Appl. Opt. 22, 1232-1236 (1983). 

Private communication with Richard Austin of Gamma Scientific, San Diego, CA. 

T. C. Larason and S. S. Bruce, “Spatial Uniformity of Responsivity for Silicon, Gallium 

nitride, Germanium, and Indium Gallium Arsenide Photodiodes,’’ Metrologia (to be 

published). 

E. F. Zalewski, Radiometry and Photometry, Chapter 24 in Handbook of Optics, Vol. 11, 

2nd edition, M. Bass, Editor-in-Chief, McGraw-Hill, New York, NY (1 995). 

C. L. Wyatt, Radiometric Calibration: Theory and Methods, Academic Press, San 

Diego, CA (1 978). 

American National Standard for Expressing Uncertainty - US Guide to the Expression of 

Uncertainty in Measurement, ANSINCSL 2540-2- 1997. (Currently available through 

the National Conference of Standards Laboratories, 1800 30th Street, Suite 305B, 

Boulder, CO 80301.) 

A. Corrons and E. F. Zalewski, Detector Spectral Response from 350 to 1200 nm Using a 

Monochromator Based Spectral Comparator, Natl. Bur. Stand. (U.S.), Tech. Note 988 

(1 978). 

R. D. Saunders and J. B. Shumaker, “Apparatus Function of a Prism-Grating Double 

Monochromator,” Appl. Opt. 25,3710-3714 (1986). 

N. M. Durant and N. P. Fox, “Evaluation of Solid-state Detectors for Ultraviolet 

Radiometric Applications,” Metrologia 32, 505-508 (1 995/96). 

R. Goebel, R. Kohler, and R. Pello, “Some Effects of Low-Power Ultraviolet Radiation 

on Silicon Photodiodes,” Metrologia 32, 5 15-5 18 (1 995/96). 

G. Eppeldauer, “Longterm Changes of Silicon Photodiodes and Their Use for 

Photometric Standardization,” Appl. Opt. 29,2289-2294 (1 990). 

L. R. Canfield, J. Kerner, and R. Korde, “Stability and Quantum Efficiency Performance 

of Silicon Photodiode Detectors in the Far Ultraviolet,” Appl. Opt. 28,3940-3943 (I 989). 

R. Korde and J. Geist, “Quantum Efficiency Stability of Silicon Photodiodes,” Appl. Opt. 

26, 5284-5290 (1987). 

K. D. Stock, “Spectral Aging Pattern of Carefully Handled Silicon Photodiodes,” 

Measurement 5, 141-144 (1987). 

79 
K. D. Stock and R. Heine, “On the Aging of Photovoltaic Cells,” Optik (Weimar) 71, 

137-142 (1985). 

[56] A. L. Migdall and C. Winnewisser, “Linearity of a Silicon Photodiode at 30 MHz And Its 

Effect on Heterodyne Measurements,” J. Res. Natl. Inst. Stand. Technol. 96, 143-1 46 

(1991). 

J. L. Gardner and F. J. Wilkinson, “Response Time and Linearity of Inversion Layer 

Silicon Photodiodes,” Appl. Opt. 24, 153 1-1 534 (1985). 

A. R. Schaefer, E. F. Zalewski, and J. Geist, “Silicon Detector Nonlinearity and Related 

Effects,” Appl. Opt. 22, 1232-1236 (1983). 

W. Budde, “Multidecade Linearity Measurements on Si Photodiodes,” Appl. Opt. 18, 

1 5 5 5 - 1 5 5 8 ( 1 979). 

T. C. Larason, “The Radiometric Physics Division’s Efforts at Building a Quality System 

Based on ISO/IEC Guide 25,” presented at the Asociacion Mexicana De Metrologia, A. 

C. 1994 Conference, Acapulco, Mexico May 10- 13, 1994 (unpublished). 

S. S. Bruce and T. C. Larason, “Building a Quality System Based on ANSINCSL 

2540- 1 - 1994 - An Effort by the Radiometric Physics Division at NIST, ” Proc. NCSL 

I995 Workshop and Symposium (National Conference of Standards Laboratories), Dallas, 

TX July 16-20, 1995. 

S. S. Bruce and T. C. Larason, Developing quality system documentation based on 

ANSINCSL 2540- 1-1 994 -- the optical technology division’s effort, Natl. Inst. Stand. 

Technol., Internal Report 5866 (1996). 

Control charts are discussed in many texts on quality control and statistics, e.g., W. 

Mendenhall and T. Sincich, Statistics for Engineering and the Sciences, 3rd edition, 

Dellen Publishing Company, San Francisco, CA (1 992). 

L. R. Canfield, “New Far UV Detector Calibration Facility at the National Bureau of 

Standards,” Appl. Opt. 26,3831-3837 (1987). 

L. R. Canfield and N. Swanson, Far Ultraviolet Detector Standards, Natl. Bur. Stand. 

(U.S.), Spec. Publ. 250-2 (1987). 

NIST is currently upgrading SURF 11, creating SURF 111. Improvements will be made in 

many areas, including beam current monitoring, magnetic structure uniformity, and 

electron energy. 

R. Kohler, R. Goebel, and R. Pello, “Results of an International Comparison of Spectral 

Responsivity of Silicon Photodetectors,” Metrologia 32,463-468 (1 995/96). 

80 
1691 

~701 

These values are from the manufacturer’s catalog, Photodiodes, Cat. No. KPD 0001E05, 

Aug. 1996 T, Hamamatsu Photonics K. K., Solid State Division, 1126-1, Ichino-cho, 

Hamamatsu City, 435 -9 1, Japan 

R. D. Saunders and J. B. Shumaker, “Automated Radiometric Linearity Tester,” Appl. 

Opt. 23, 3504-3506 (1984). 

These values are from the manufacturer’s catalog (obtained =: 1993), Optoelectronic 

Components Catalog, UDT Sensors, Inc., 12525 Chadron Ave., Hawthorne, CA, USA 

90250 

J. B. Fowler and G. Dezsi, “High Accuracy Measurement of Aperture Area Relative to a 

Standard Known Aperture,” J. Res. Natl. Inst. Stand. Technol. 100,277-283 (1 995). 

G. Eppeldauer, A. L. Migdall, and C. L. Cromer, “Characterization of a High Sensitivity 

Composite Silicon Bolometer,” Metrologia 30, 3 17-320 (1993). 

G. Eppeldauer, A. L. Migdall, and C. L. Cromer, “A Cryogenic Silicon Resistance 

Bolometer for Use as an Infrared Transfer Standard Detector,” Thermal Phenomena at 

Molecular and Microscales and in Cryogenic Infrared Detectors, edited by M. Kaviany 

et al., (ASME HTD-Vol. 277, New York, NY, 1994), pp. 63 -67. 

G. Eppeldauer, “Near Infrared Radiometer Standards,” Optical Radiation Measurements 

I.1, James M. Palmer, Editor, Proc. SPIE 2815, pp. 42-54 (1996). 

A, L. Migdall and G. Eppeldauer, NIST Measurement Services: Spectroradiometric 

Detector Measurements: Part I11 -- Infrared Detectors, Natl. Inst. Stand. Technol., Spec. 

Publ. 250-42 (in preparation). 

81 
13. Bibliography 

Optics 

E. Hecht, Optics, 2nd edition, Addison-Wesley Publishing Company, 1987. 

F. L. Pedrotti and L. S. Pedrotti, Introduction to Optics, 2nd edition, Prentice Hall, Englewood 

Cliffs, NJ, 1993. 

F. A. Jenkins and H. E. White, Fundamentals ofOptics, 4th edition, McGraw-Hill, New York, 

NY, 1976. 

M. Born and E. Wolf, Principles of Optics, 6th (corrected) edition, Pergamon Press, Oxford, 

England, 1993. 

Applied Optics and Optical Engineering, Vol. V, Optical Instruments, R. Kingslake, editor, 

Academic Press, San Diego, CA, 1969. 

D. C. O'Shea, Elements of Modern Optical Design, Wiley, New York, NY, 1985. 

W. J. Smith, Modern Optical Engineering, 2nd edition, McGraw-Hill, , New York, NY, 1990. 

Military Handbook 141, Optical Design, U. S. Department of Defense, Washington, D. C., 1962. 

Handbook of Optics, Vol. I and 11, 2nd edition, M. Bass, editor-in-chief, McGraw -Hill, New 

York, NY, 1995. 

The Optics Problem Solver, Research and Education Association, Piscataway, NJ, 1990 revision. 

E. Hecht, Schaum 's Outline of Theory and Problems of Optics, McGraw-Hill, New York, NY, 

1975. 

Detectors and Amplifiers 

Determination of the Spectral Responsivity of Optical Radiation Detectors, CIE Publication 64 

(Commission Internationale de l'Eclairage, Paris, 1984). Currently available through the U.S. 

National Committee of the CIE, c/o T. M. Lemons, TLA-Lighting Consultants, Inc., 72 Loring 

Ave., Salem, MA 01970. 

E. L. Dereniak and D. G. Crowe, Optical Radiation Detectors, Wiley, New York, NY, 1984. 

E. L. Dereniak and G. D. Boreman, Infrared Detectors and Systems, Wiley, New York, NY, 

1996. 

G. H. Rieke, Detection of Light: from the Ultraviolet to the Submillimeter, Cambridge 

University Press, New York, NY, 1994. 
J. D. Vincent, Fundamentals of Infrared Detector Operation and Testing, Wiley, New York, NY, 

1990. 

T. M. Frederiksen, Intuitive Operational AmpliJiers, McGraw-Hill, New York, NY, 1988. 

J. G. Graeme, Photodiode Amp1iJier.s: Op Amp Solutions, McGraw-Hill, New York, NY, 1996. 

D. L. Terrell, Op Amps: Design, Application, and Troubleshooting, Butterworth-Heinemann, 

Newton, MA, 1996. 

P. Horowitz and W. Hill, The Art of Electronics, 2nd edition, Cambridge University Press, New 

York, NY, 1989. 

S. M. Sze, Physics of Semiconductor Devices, 2nd edition, Wiley, New York, NY, 1981. 

S. M. Sze, Semiconductor Devices, Physics and Technology, Wiley, New York, NY, 1985. 

Radiometry 

Optical Radiation Measurements, F. Grum and C. J. Bartleson, editors, VoI. I: Radiometry, F. 

Grum and R. J. Becherer; Vol. 4: Physical Detectors of Optical Radiation, W. Budde. 

Academic Press, San Diego, CA, Vol. 1: Vol. 4: 1983. 

Absolute Radiometry: Electrically Calibrated Thermal Detectors of Optical Radiation, 

F. Hengstberger, editor, Academic Press, San Diego, CA, 1989. 

C. L. Wyatt, Radiometric Calibration: Theory and Methods, Academic Press, San Diego, CA, 

1978. 

C. L. Wyatt, Electro-Optical System Design: For Information Processing, McGraw -Hill, New 

York, NY, 1991. Expanded and revised version of the author’s Radiometric System Design. 

W. R. McCluney, Introduction to Radiometry and Photometry, Artech House, Norwood, MA, 

1994. 

R. W. Boyd, Radiometry and the Detection of Radiation, Wiley, New York, NY, 1983. 

R. H. Kingston, Optical Sources, Detectors, and Systems: Fundamentals and Applications, 

Academic Press, San Diego, CA, 1995. 

R. H. Kingston, Detection of Optical and Infrared Radiation, Springer-Verlag, New York, NY, 

1978. 

New Developments and Applications in Optical Radiometry 111, Metrologia 28, (1 99 1). 

New Developments and Applications in Optical Radiometry IV, Metrologia 30, (1 993). 

New Developments and Applications in Optical Radiometry V, Metrologia 32, (1 996). 

83 
Statistics/Error Analysis/Uncertainties 

P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, 

New York, NY, 1969. 

J. R. Taylor, An Introduction to Error Analysis: 

Measurements, University Science Books, Sausalito, CA, 1 982. 

The Study of Uncertainties in Physical 

ANSI/NCSL 2540-2-1 997, U.S. Guide to the Expression of Uncertainty in Measurement. 

Currently available through the National Conference of Standard Laboratories (NCSL) 

Secretariat, 1800 30th Street, Suite 305B, Boulder, CO 80301. 

B. N. Taylor and C. E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of 

NIST Measurement Results, Natl. Inst. Stand. Technol. (US), Tech. Note 1297 (1 994 ed.). 

W. Mendenhall and T. Sincich, Statistics for Engineering and the Sciences, 3rd edition, Dellen 

Publishing Company, San Francisco, CA, 1992. 

NBS Handbook 91, Experimental Statistics, Natl. Bur. Stand. (U.S.), NBS HDBK 91, 1963. 

Precision Measurement and Calibration, Harry Ku, editor, Natl. Bur. Stand. (U.S.), Spec. Publ. 

300, Vol. 1, 1969. 

Quality and Laboratory Accreditation 

NIST Handbook 150, National Voluntary Laboratory Accreditation Program Procedures and 

General Requirements, James L. Cigler and Vanda R. White, Editors, 1994. 

NIST Handbook 150-2, NVLAP Calibration Laboratories Technical Guide, 1994. 

ANSI/NCSL 2540-1 - 1994, Calibration Laboratories and Measuring and Test Equipment - 

General Requirements. Currently available through the National Conference of Standard 

Laboratories (NCSL) Secretariat, 1800 30th Street, Suite 305B, Boulder, CO 80301. 

International Organization for Standardization (ISO) / International Electrotechnical Commission 

(IEC), General Requirements for the Competence of Calibration and Testing Laboratories 

Guide 25 (1 990). Currently available through the American National Standards Institute (ANSI), 

1 1 West 42nd Street, 13th Floor, New York, NY 10036. 

84 
UNITED STATES DEPARTMENT OF COMMERCE 

National Institute of Standards and Technology 

Gaithersburg, Maryland 208990001 

REPORT OF TEST 

NIST Test # 3907 1 S - Spectral Responsivity 

for 

UDT Sensors UV 100 Silicon Photodiode, U1 xxx 

Submitted by: 

Any Company 

Mr. Daniel Doe 

123 Calibration Court 

Measurement City, MD OOOOO-OOOO 

(See your Purchase Order No. XXXX-XX, dated January 1, 1997) 

1. Description of Test Material 

The test photodiode, labeled U 1 xxx, is a UDT Sensors UV 100 inverted layer silicon photodiode in 

an anodized aluminum mount with a removable precision aperture and a BNC connector. The 

active area of the photodiode is -- 1 cm2. 

2. Description of Test 

The test photodiode was compared to two silicon photodiode working standards, U5xx and U~XX, 

using the NIST Ultraviolet (UV) monochromator-based comparator facility [I] from 200 nm to 

500 nm in 5 nm increments. The spectral comparisons between the test photodiode and working 

standard photodiodes were performed using a double monochromator and an argon arc as the 

tunable monochromatic source. 

The circular exit aperture of the UV monochromator was imaged (=.f/5) on the test photodiode, 

resulting in a beam diameter at the photodiode of 1.5 mm. The beam was centered on, and 

underfilled, the aperture. 

The wavelength scale of the monochromator was calibrated with several laser and emission lines 

and is accurate to f. 0.1 nm over the entire spectral range. The bandpass of the monochromator 

was 4 nm. The short-circuit photocurrent from the test photodiode and each working standard 

photodiode was measured with a calibrated transimpedance amplifier. The test photodiode and 

each working standard photodiode were measured with zero bias voltage. Beam power 

fluctuations were monitored with a beamsplitter and silicon photodiode. The absolute spectral 

responsivity scale is based on a high accuracy cryogenic radiometer, with a relative expanded 

uncertainty (k = 2) to absolute (SI) units of 0.2 %. 

The spatial uniformity of the responsivity across the test photodiode photosensitive area was 

measured at 350 nm using the described comparator facility. The uniformity was measured in 

0.5 mm increments using a 1.5 mm diameter beam. 

Laboratory Environment: 

Temperature: 23.x "C f. 0.8 "C 

Test Date: December 24, I997 

NIST Test No.: 844/xxxxxx-97/1 

A- 1 

Page 1 of 7 NET= 
REPORT OF TEST 

NIST Test # 3907 1s - Spectral Responsivity 

Any Company 

0.30 1111l1111~111111111~IIIIIIIII - - 

tL 41 

e0.25 .................................... i ................................................... i ...................................... c - 

- e e - - e 

......... e - - e - - 1 Absolute Responsivity [A/W] - - - - - * .................................. - .............................................. ................................................. \I ............. *e *** 0.20 * j > 

- - - ie 

ie - - :e - 

- 0 ej 

ej 

0.15 _.... ......................... ....... : ............................................. .... > .................................................. - 

- - e: - 

- 0 e~****.eeeeee *, i Relative Expanded - 

- *****eeee*. i *e* ~ Uncertainty [%I 1 

0.10 _ ....................................................................................................... ........................................ e - 

- 

- e*~~onOO~~OO~ ~~~oPooooooooooooo j 000000~ !I - 

~~%~ 00 po 0 0 on 7 0 7 m 0.05 1111111111111111111 

- 

-e 

Manufacturer: UDT Sensors 

Model #: UVlOO 

Serial #: Ulxxx 

14 

12 

10 

-8 

-6 

4 

-2 

! 0 

3. Results of Test 

The absolute spectral responsivity in amperes per watt of the test photodiode is presented as a 

function of wavelength in table 1 and plotted in figure 1. The relative expanded uncertainties in the 

NIST absolute scale are described in Ref. [ 11. The relative expanded uncertainty (k = 2) presented 

as a function of wavelength for this measurement is stated relative to absolute (SI) units and is 

listed in table 1 and is plotted in figure 1. 

Table 2 lists the dimensions of the precision aperture furnished with the test photodiode. The 

reported aperture area and distance from the aperture plate to the photodiode surface do not enter 

into the responsivity measurement results because the optical beam underfills the aperture. The 

uncertainty values reported with the aperture dimensions are expanded uncertainties. 

Figure 2a is a plot of the uniformity of the test photodiode, showing 0.2 % contours at 350 nm of 

the deviations from the responsivity at the photodiode center. Figure 2b is a 3-dimensional plot 

showing the responsivity relative to the center of the photodiode. Note that the response of the 

photodiode can vary by as much as a percent over the active area. This can lead to errors larger 

than the stated uncertainties if the irradiation geometry is significantly different from the test 

conditions described in section 2. 

200 300 400 

Wavelength [nm] 

500 

d w 

2. 

B 

Y 

n 

?? 

II 

t4 

W 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/1 

A- 2 

Page 2 of 7 
REPORT OF TEST 

NIST Test # 3907 IS - Spectral Responsivity 

Any Company 

Manufacturer: UDT Sensors 

Model #: UVlOO 

Serial #: Ulxxx 

Table 1 

Absolute Spectral Responsivity of Silicon Photodiode Ulxxx 

Relative Relative 

Absolute Expanded Absolute Expanded 

Wavelength Responsivity Uncertainty Wavelength Responsivity Uncertainty 

[nml [ml (k = 2) [%I [nml [WI (k = 2) [%] 

200 6.28E-2 13 350 I .24E- 1 1.7 

205 

210 

215 

220 

225 

230 

235 

240 

245 

250 

255 

260 

265 

270 

275 

280 

285 

290 

295 

300 

305 

310 

315 

320 

325 

330 

335 

340 

345 

6.68E-2 

6.95E-2 

7.34E-2 

7.78E-2 

8.42E-2 

9.45E-2 

1.06E- 1 

1.13E-1 

1.17E- 1 

1.19E-1 

1.21E-1 

1.20E- 1 

1.17E- 1 

1.13E- 1 

1.12E- 1 

1.13E- 1 

1.15E-1 

1.19E-1 

1.24E- 1 

1.27E- 1 

1.29E- 1 

1.29E- 1 

1.28E- 1 

1.27E- 1 

1.27E- 1 

I .26E- 1 

1.26E- 1 

1.25E- 1 

1.25E- 1 

Area: 

8.4 

5.1 

4.1 

2.8 

1.5 

1.7 

1.5 

1.5 

1.4 

1.4 

1.3 

1.3 

1.4 

1.5 

1.7 

2.0 

2.0 

1.9 

2.0 

2.1 

1.8 

1.8 

1.8 

1.8 

1.9 

1.7 

1.6 

1.6 

1.7 

355 

360 

365 

370 

375 

380 

385 

390 

395 

400 

405 

410 

415 

420 

425 

430 

435 

440 

445 

450 

455 

460 

465 

470 

475 

480 

485 

490 

495 

500 

1.22E- 1 

1.20E- 1 

1.18E- 1 

1.21E-1 

1.27E- 1 

1.35E-1 

1.43E- 1 

1.50E- 1 

1.57E- 1 

1.63E- 1 

1.689E- 1 

1.750E- 1 

1.8 16E- 1 

1.882E- 1 

1.935E- 1 

1.993E- 1 

2.049E- 1 

2.1 OOE- 1 

2.160E- 1 

2.220E- 1 

2.28 1E-1 

2.341E-1 

2.40 1E- 1 

2.462E- 1 

2.523E- 1 

2.583E-1 

2.643E-1 

2.704E- 1 

2.764E- 1 

2.824E- 1 

Table 2 

Aperture Dimensions 

0.5xxx 0.0005 cm2 

Distance from aperture plane 

to photodiode surface: 4.8 & 0.4 mm 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/1 

A- 3 

1.6 

1.6 

1.5 

1.5 

1.5 

1.4 

1.4 

1.4 

1.4 

1.5 

0.60 

0.56 

0.58 

0.76 

0.60 

0.54 

0.50 

0.42 

0.42 

0.38 

0.38 

0.40 

0.38 

0.42 

0.40 

0.38 

0.40 

0.38 

0.40 

0.38 

Page 3 of 7 
REPORT OF TEST 

NIST Test # 3907 1 S - Spectral Responsivity 

Any Company 

Manufacturer: UDT Sensors 

Model #: UV 100 

Serial #: Ulxxx 

Figure 2a 

Responsivity Uniformity of Silicon Photodiode Ulxxx 

0.2 % contours at 350 nm; 1.5 mm resolution; 0.5 "/Step 

-6 

-4 

-2 

0 

2 

4 

6 

-6 -4 -2 0 24 6 

mm 

Figure 2b 

Surface Plot of Responsivity Relative to 

Center of Photodiode for Silicon Photodiode Ulxxx 

at 350 nm; 0.5 "/Step 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/1 

A- 4 

' 1.010 

1.005 

1 .ooo 

0.995 

6 

6 -6 

Relative Responsivity 

Page 4 of 7 
REPORT OF TEST 

NIST Test # 39071s - Spectral Responsivity 

Any Company 

l"'l"'I"'I"'I"'- 

L ....... a ....... a .....; ............ * .......... ; .......... 

_ ........................ : ........................... : ................... * ..... 2. .......................... : ........................... : ............... 

............. ........ ..... * ....... 3 ......... ............... j .......... ............ L 

> 1 V Bias ~ 

'a, 

a: 

j No Bias Voltage+' 

L ................................................................................ :. ........ 9 ........ 

_ ................................................................................................................................................................ ;a - 

: .......................... .L ................ z 

-. ............................................................................................................................................................. - 

-. ......................................................................................................... ., .......................... , ........................ a; 

_ ........................ ; ........................... < ................................................... .....: ..................................................... 

l,,,l,,,l,,,l#I~ ,a 

Manufacturer: UDT Sensors 

Mode1 #: UV 100 

Serial #: U l xxx 

l'"'I""l'"'l""l""l""l"" 

_ ............ *..*.....* *..*..* .................... 1. ......."____.*...................,.... .............I.. ............... .............. 

............................................ ......... .................................................................................................. 

ah + - 1mw/cm2 ~ 

....................................................................................................................... 

i ' h4mW/cm* ~ 

_..... ............ < ..................... ;. ........................................................... 4. ........................................ 

........................................................................................................................................................ 

I.! - 

- ............................................................. L .......................................................... : .................. i 

rl I 1 I I I I I I I I I 1 I I I I 1 I I I I 1 I I I I I I I I I 1 I I I I4. General Information 

The linearity of this type of photodiode is shown in figure 3 at 442 nm at irradiance levels from 0.1 

to 1.1 mW/cm2 with and without a reverse bias voltage. Each data point represents the ratio of the 

photodiode responsivity at the indicated irradiance to the responsivity within the linear region. The 

linearity was measured by using a beamsplitter to irradiate two photodiodes at approximately a 10: 1 

intensity ratio, with the diode aperture filled and uniformly irradiated. The linearity is dependent on 

the irradiation geometry and will differ for spot sizes significantly smaller than the aperture size. 

The change in the responsivity for this type of photodiode at 442 nm is shown in figure 4 as a 

function of bias voltage. For wavelengths shorter than 450 nm, a 1 V bias can be used to improve 

the linearity of the photodiode without significantly changing the spectral responsivity. There will 

however be some leakage current which will limit the minimum usable signal. 

Figure 3 

Linearity of UDT Sensors UVlOO at 442 nm 

1 .os 

1 .oo 

0.95 

0.90 

0.85 

0.80 

0.75 

0.70 

0.65 

Figure 4 

Responsivity dependence to bias voltage of UDT Sensors UVlOO at 442 nm 

2 

% 

0 

2 

0 

> 

cd 

.- 

+I 

d 

2 

1.1 

1 .0 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

Test Date: December 24, I997 

NIST Test No.: 844/xxxxxx-97/1 

A- 5 

Page 5 of 7 
REPORT OF TEST 

NIST Test # 3907 1 S - Spectral Responsivity 

Any Company 

Manufacturer: UDT Sensors 

Model #: UVlOO 

Serial #: U 1 xxx 

A documentation appendix includes operating instructions for the test photodiode. The laboratory 

temperature is reported for information only. It is not intended that this data be used for 

corrections to the spectral responsivity data in this report. This report shall not be reproduced, 

except in full, without the written approval of NET. 

Prepared by: Reviewed by: 

Sally S. Bruce Thomas C. Larason 

Optical Technology Division Optical Technology Division 

Physics Laboratory Physics Laboratory 

(301) 975-2323 (301) 975-2334 

Approved by: 

Joseph L. Dehmer 

For the Director, 

National Institute of 

Standards and Technology 

(301) 975-2319 

Reference: 

[l] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services: 

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part I1 - 

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41 

(1998). 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/1 

A- 6 

Page 6 of 7 
REPORT OF TEST 

NIST Test # 3907 1 S - Spectral Responsivity 

Any Company 

Manufacturer: UDT Sensors 

Model #: UVloO 

Serial #: U 1 xxx 

APPENDIX: 

OPERATING INSTRUCTIONS 

FOR 

NIST PHOTODIODE 

The NIST characterized photodiode consists of a silicon photodiode with a removable precision 

aperture, a quartz window, and a BNC connector. 

A. The photodiode should be rigidly mounted on a dual-axis tilt mount such that the 

photodiode can be tilted about two orthogonal axis. The photodiode should be adjusted to 

be perpendicular to the incident radiation. 

The incident beam of radiation should be smaller than the aperture, and should be centered 

in the photodiode aperture. 

B . 

C. The photodiode should be connected with a BNC cable to an electrometer grade amplifier 

(transimpedance amplifier) which measures the current from the photodiode. 

The inside edge of the precision aperture is extremely delicate and should not be touched 

with fingers or any other object. 

The diode window can be cleaned with lens tissue and spectral grade solvent. The precision 

aperture should be removed before cleaning the photodiode window. 

D. 

E. 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/1 Page 7 of 7 

A- 7 
UNITED STATES DEPARTMENT OF COMMERCE 

National Institute of Standards and Technology 

Gaithersburg. Maryland 208990001 

REPORT OF TEST 

NIST Test # 39073s - Spectral Responsivity 

for 

Hamamatsu S228 1 Silicon Photodiode, Cxxx 

Submitted by : 

Any Company 

Mr. Daniel Doe 

123 Calibration Court 

Measurement City, MD 00000-0000 

(See your Purchase Order No. XXXX-XX, dated January 1 , 1993) 

1. Description of Test Material 

The test photodiode, labeled Cxxx, is a Hamamatsu S2281 silicon photodiode in an anodized 

aluminum mount with a removable precision aperture and a BNC connector. The active area of the 

photodiode is = 1 cm2. 

2. Description of Test 

The test photodiode was compared to two silicon photodiode working standards, H6xx and H6xx, 

using the NIST visible to near Infrared (Vis/NIR) monochromator-based comparator facility [ 1 ] 

from 350 nm to 1100 nm in 5 nm increments. The spectral comparisons between the test 

photodiode and working standard photodiodes were performed using a double monochromator and 

a quartz-halogen lamp as the tunable monochromatic source. 

The circular exit aperture of the Vis/NIR monochromator was imaged (=f/9) on the test 

photodiode, resulting in a beam diameter at the photodiode of 1.1 mm. The beam was centered on, 

and underfilled, the aperture. 

The wavelength scale of the monochromator was calibrated with several laser and emission lines 

and is accurate to k 0.1 nm over the entire spectral range. The bandpass of the monochromator 

was 4 nm. The short-circuit photocurrent from the test photodiode and each working standard 

photodiode was measured with a calibrated transimpedance amplifier. The test photodiode and 

each working standard photodiode were measured with zero bias voltage. Beam power 

fluctuations were monitored with a beamsplitter and silicon photodiode. The absolute spectral 

responsivity scale is based on a high accuracy cryogenic radiometer with a relative expanded 

uncertainty (k = 2) to absolute (SI) units of 0.2 %. 

The spatial uniformity of the responsivity across the test photodiode photosensitive area was 

measured at 500 nm using the described comparator facility. The uniformity was measured in 

0.5 mm increments using a 1.1 mm diameter beam. 

Laboratory Environment: 

Temperature: 23.x "C k 0.3 "C 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/2 

A- 8 
REPORT OF TEST 

NIST Test # 39073s - Spectral Responsivity 

Any Company 

3. Results of Test 

Manufacturer: Hamamatsu 

Model #: S2281 

Serial #: Cxxx 

The absolute spectral responsivity in amperes per watt of the test photodiode is presented as a 

function of wavelength in table 1 and is plotted in figure 1. The relative expanded uncertainties in 

the NIST absolute scale are described in Ref. [I]. The relative expanded uncertainty (k = 2) 

presented as a function of wavelength for this measurement is stated relative to absolute (SI) units 

and is listed in table 1 and plotted in figure 1. 

Table 2 lists the dimensions of the precision aperture furnished with the test photodiode. The 

reported aperture area and distance from the aperture plate to the photodiode surface do not enter 

into the responsivity measurement results because the optical beam underfills the aperture. The 

uncertainty values reported with the aperture dimensions are expanded uncertainties. 

Figure 2a is a plot of the uniformity of the test photodiode, showing 0.2 % contours at 500 nm of 

the deviations from the responsivity at the photodiode center. Figure 2b is a 3-dimensional plot 

showing the responsivity relative to the center of the photodiode. Errors larger than the stated 

uncertainties can occur if the irradiation geometry is significantly different from the test conditions 

described in section 2. 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

Figure 1 

Absolute Spectral Responsivity of Silicon Photodiode Cxxx 

300 400 500 600 700 800 900 1000 1100 

Wavelength [nm] 

c 

CD 

C 

Y 

8 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/2 

A- 9 

Page 2 of 7 
REPORT OF TEST Manufacturer: Hamamatsu 

NIST Test # 39073s - Spectral Responsivity Model # S2281 

Any Company Serial# Cxxx 

Table 1 

Absolute Spectral Responsivity of Silicon Photodiode Cxxx 

Relative Relative 

Absolute Expanded Absolute Expanded 

Wavelength Responsivity Uncertainty Wavelength Responsivity Uncertainty 

[nml [ml (k = 2) [%I [nml [m (k = 2) [%] 

350 1 SOE-1 3 .O 550 2.848E-1 0.20 

355 

360 

365 

370 

375 

380 

385 

390 

395 

400 

405 

410 

415 

420 

425 

430 

435 

440 

445 

450 

455 

460 

465 

470 

475 

480 

485 

490 

495 

500 

505 

510 

515 

520 

525 

530 

535 

540 

545 

1.49E- 1 

1.47E- 1 

1.45E- I 

1.47E- 1 

1.51E-1 

1.58E- 1 

I .65E- 1 

1.7 1E- 1 

I .77E- 1 

I .82E- 1 

I .860E- 1 

1.909E- 1 

1.959E- 1 

2.008E- 1 

2.04 I E- 1 

2.079E- 1 

2.1 13E-I 

2.14 1E- 1 

2.178E- 1 

2.2 14E- 1 

2.250E- 1 

2.284E- 1 

2.3 I9E- 1 

2.352E- 1 

2.3 85E- 1 

2.4 18E- 1 

2.450E- 1 

2.482E- 1 

2.5 13E- 1 

2.545E- 1 

2.576E- 1 

2.607E- 1 

2.638E- 1 

2.668E- 1 

2.699E- 1 

2.729E- I 

2.759E- 1 

2.788E- I 

2.8 18E- 1 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/2 

2.6 

2.6 

2.2 

2.1 

2.0 

1.8 

1.8 

1.6 

1.7 

1.6 

0.36 

0.34 

0.32 

0.30 

0.28 

0.26 

0.26 

0.24 

0.24 

0.24 

0.24 

0.24 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.20 

~~ 

555 

560 

565 

570 

575 

580 

585 

590 

595 

600 

605 

610 

615 

620 

625 

630 

635 

640 

645 

650 

655 

660 

665 

670 

675 

680 

685 

690 

695 

700 

705 

710 

715 

720 

725 

730 

735 

740 

745 

A- 10 

2.877E-1 

2.906E- 1 

2.935E- 1 

2.965E- 1 

2.994E- 1 

3.023E-1 

3.052E- 1 

3.08 1E- 1 

3.109E-1 

3.138E-1 

3.166E-1 

3.195E-1 

3.223E- 1 

3.252E- 1 

3.280E- 1 

3.308E-1 

3.337E- 1 

3.365E- 1 

3.393E-1 

3.42 1E- 1 

3.449E- 1 

3.477E- 1 

3.505E-1 

3.534E- 1 

3.562E-1 

3.589E- 1 

3.6 17E- 1 

3.645E-1 

3.673E-1 

3.70 1E- 1 

3.729E- 1 

3.756E- 1 

3.784E- I 

3.8 12E- 1 

3.840E- 1 

3.868E-1 

3.896E- 1 

3.923E- 1 

3.95 1E- 1 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.20 

0.22 

0.22 

0.20 

Page 3 of 7 
REPORT OF TEST 

NIST Test # 39073s - Spectral Responsivity 

Any Company 

Manufacturer: Hamamatsu 

Model #: S2281 

Serial #: Cxxx 

Table 1 (cont.) 

Absolute Spectral Responsivity of Silicon Photodiode Cxxx 

Re1 at i ve Relative 

Absolute Expanded Absolute Expanded 

Wavelength Responsivity Uncertainty Wavelength Responsivity Uncertainty 

[nml [MI (k = 2) [%I [nml [WI (k = 2) [%I 

750 3.978E- 1 0.22 925 4.93E- 1 2.8 

755 

760 

765 

770 

775 

780 

785 

790 

795 

800 

805 

810 

815 

820 

825 

830 

835 

840 

845 

850 

855 

860 

865 

870 

875 

880 

885 

890 

895 

900 

905 

910 

915 

920 

4.007E- I 

4.034E- 1 

4.06 1 E- 1 

4.088E- 1 

4.1 16E- 1 

4.144E- 1 

4.171E- 1 

4.199E- 1 

4.227E- I 

4.254E- 1 

4.282E- 1 

4.309E- 1 

4.336E- 1 

4.364E-1 

4.391E-1 

4.4 I8E- 1 

4.446E- 1 

4.473E- I 

4.502E- 1 

4.528E- 1 

4.556E- 1 

4.584E-1 

4.6 1 OE- 1 

4.638E- 1 

4.665E- 1 

4.693E- 1 

4.720E- 1 

4.747E- 1 

4.774E- 1 

4.802E- 1 

4.833E- 1 

4.861E-1 

4.876E- 1 

4.902E- 1 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/2 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.24 

0.22 

0.24 

0.24 

0.24 

0.22 

0.22 

0.22 

0.22 

0.22 

0.22 

0.24 

0.22 

0.22 

0.22 

0.22 

0.22 

0.24 

0.22 

0.22 

930 

935 

940 

945 

950 

955 

960 

965 

970 

975 

980 

985 

990 

995 

1000 

1005 

1010 

1015 

1020 

1025 

1030 

1035 

1040 

I045 

1050 

1055 

1060 

1065 

1070 

1075 

I080 

1085 

1090 

1095 

1100 

4.95E-1 

4.97E-1 

5 .OOE- 1 

5.02E- 1 

5.03E- I 

5.04E- 1 

5.06E-1 

5.05E- I 

5.05E- 1 

5.04E-1 

5.02E- 1 

4.98E- 1 

4.93E-1 

4.87E- 1 

4.79E- I 

4.69E- 1 

4.57E- 1 

4.42E- 1 

4.26E- 1 

4.08E- 1 

3.87E-1 

3.63E- 1 

3.38E-I 

3.13E2.86E2.60E2.33E2.09E1.91E1.75E- 1 

1.60E- 1 

1.46E- 1 

1.33E- 1 

1.20E- I 

1.09E- 1 

2.8 

2.8 

2.5 

2.4 

2.6 

2.4 

2.3 

2.2 

2. I 

1.9 

1.6 

1.6 

1.7 

1.9 

1.7 

1.8 

1.8 

1.8 

1.8 

2.2 

2.5 

2.1 

1.9 

2.6 

2.7 

2.5 

2.0 

2.4 

2.9 

3.7 

3.2 

3.2 

3.7 

4.4 

4.2 

Page 4 of 7 

A-11 
REPORT OF TEST 

NIST Test # 39073s - Spectral Responsivity 

Any Company 

Manufacturer: Hamamatsu 

Model #: S2281 

Serial #: Cxxx 

Figure 2a 

Responsivity Uniformity of Silicon Photodiode Cxxx 

0.2 % contours at 500 nm; 1.1 mm resolution; 0.5 “/Step 

-6 

-4 

-2 

0 

2 

4 

6 

-6 -4 -2 0 2 4 6 

mm 

Figure 2b 

Surface Plot of Responsivity Relative to 

Center of Photodiode for Silicon Photodiode Cxxx 

at 500 nm; 0.5 “/Step 

’ 1.010 

1.005 

1 .ooo 

0.995 

-6 6 

6 -6 

Relative Responsivity 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/2 A- 12 

Page 5 of 7 
REPORT OF TEST 

NIST Test # 39073s - Spectral Responsivity 

Any Company 

Manufacturer: Hamamatsu 

Model #: S2281 

Serial #: Cxxx 

Table 2 

Aperture Dimensions 

Area: 0.5xxx & 0.0005 cm2 

Distance from aperture plane 

to photodiode surface: 5.0 0.5 mm 

4. General Information 

A documentation appendix includes operating instructions for the test photodiode. The laboratory 

temperature is reported for information only. It is not intended that this data be used for 

corrections to the spectral responsivity data in this report. This report shall not be reproduced, 

except in full, without the written approval of NIST. 

Prepared by: Reviewed by: 

Sally S. Bruce Thomas C. Larason 

Optical Technology Division Optical Technology Division 

Physics Laboratory Physics Laboratory 

(301) 975-2323 (301) 975-2334 

Approved by: 

Joseph L. Dehmer 

For the Director, 

National Institute of 

Standards and Technology 

(301) 975-2319 

Reference: 

[l] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services: 

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part I1 - 

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41 

(1998). 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/2 Page 6 of 7 

A- 13 
REPORT OF TEST 

NIST Test # 39073s - Spectral Responsivity 

Any Company 

Manufacturer: Hamamatsu 

Model #: S2281 

Serial #: Cxxx 

APPENDIX: 

OPERATING INSTRUCTIONS 

FOR 

NIST PHOTODIODE 

The NIST characterized photodiode consists of a silicon photodiode with a removable precision 

aperture, a quartz window, and a BNC connector. 

A. The photodiode should be rigidly mounted on a dual-axis tilt mount such that the 

photodiode can be tilted about two orthogonal axis. The photodiode should be adjusted to 

be perpendicular to the incident radiation. 

The incident beam of radiation should be smaller than the aperture, and should be centered 

in the photodiode aperture. 

The photodiode should be connected with a BNC cable to an electrometer grade amplifier 

(transimpedance amplifier) which measures the current from the photodiode. 

B . 

C. 

D. The inside edge of the precision aperture is extremely delicate and should not be touched 

with fingers or any other object. 

E. The diode window can be cleaned with lens tissue and spectral grade solvent. The precision 

aperture should be removed before cleaning the photodiode window. 

Test Date: December 24, I997 

NIST Test No.: 844/xxxxxx-97/2 

A- 14 

Page 7 of 7 
<+€W*T OF co 

8 kif 3 

: * 

44 

i 1 i 0' J S'*rrs Of @' 

e0 

REPORT OF TEST 

NIST Test # 39075s - Spectral Responsivity 

UNITED STATES DEPARTMENT OF COMMERCE 

National Institute of Standards and Technology 

Gaithersburg, Maryland 208990001 

for 

Acme Instruments Germanium Photodiode 

Model xx, SIN yyy 

Submitted by: 

Any Company 

Mr. Daniel Doe 

123 Calibration Court 

Measurement City, MD OOOOO-0000 

(See your Purchase Order No. XXXX-XX, dated January 1, 1993) 

1. Description of Test Material 

The test photodiode, Acme Instruments model xx, SIN yyy, consists of a thermoelectrically cooled 

germanium photodiode mounted in a cylindrical aluminum housin with the output signal available 

on a BNC connector. The active area of the photodiode is = x cm . 5 

The test photodiode was measured as supplied by Any Company. The test photodiode was also 

supplied with an Acme Instruments Thermoelectric Cooler Controller model zz, S/N zzz. 

2. Description of Test 

The test photodiode was compared to two cooled germanium photodiode working standards, 

Ge #x and Ge #x, using the NIST visible to near Infrared (VisINIR) monochromator-based 

comparator facility [ 11 from 700 nm to 1800 nm in 5 nm increments. The spectral comparisons 

between the test photodiode and working standard photodiodes were performed using a double 

monochromator and a quartz-halogen lamp as the tunable monochromatic source. 

The circular exit aperture of the VisINIR monochromator was imaged (= fl9) on the test 

photodiode, resulting in a beam diameter at the photosensitive area of 1.1 mm. The beam was 

centered on, and underfilled, the photosensitive area. The photocurrent was measured with the 

thermoelectric cooler controller set at - 30 "C. 

The wavelength scale of the monochromator was calibrated with several laser and emission lines 

and is accurate to k 0.1 nm over the entire spectral range. The bandpass of the monochromator 

was 4 nm. The short-circuit photocurrent from the test photodiode and each working standard 

photodiode was measured with a calibrated transimpedance amplifier. The test photodiode and 

each working standard photodiode were measured with zero bias voltage. Beam power 

fluctuations were monitored with a beamsplitter and cooled germanium photodiode. The absolute 

Laboratory Environment: 

Temperature: 23.x "C k 0.3 "C 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/3 Page I of 6 NbT 

A-15 
REPORT OF TEST 

NIST Test # 39075s - Spectral Responsivity 

Any Company 

Manufacturer: Acme Instruments 

Model#: xx 

Serial#: yyy 

spectral responsivity scale is based on a high accuracy cryogenic radiometer, with a relative 

expanded uncertainty (k = 2) to absolute (SI) units of 0.2 %. 

3. Results of Test 

The absolute spectral responsivity in amperes per watt of the test photodiode is presented as a 

function of wavelength in table 1 and is plotted in figure 1. The relative expanded uncertainty in 

the NIST absolute scale is described in Ref. [l]. The relative expanded uncertainty (k = 2) 

presented as a function of wavelength for this measurement is stated relative to absolute (SI) units 

and is listed in table 1 and plotted in figure 1. 

The reported relative expanded uncertainty does not include estimates for several components that 

are unknown for this test photodiode. The unknown uncertainty components are photodiode 

responsivity uniformity, polarization sensitivity, linearity, temperature coefficient, and long-term 

stability. These components could significantly add to the reported uncertainty. Errors larger than 

the stated uncertainties can occur if the irradiation geometry is significantly different from the test 

conditions described in section 2. 

Figure 1 

Absolute 

s 

.3 

> .i 

a 

3 

700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 

Wavelength [nm] 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/3 

A- 16 

YYY) 

Page 2 of 6 
REPORT OF TEST 

NIST Test # 39075s - Spectral Responslvity 

Any Company 

Manufacturer: Acme Instruments 

Model#: xx 

Serial #: yyy 

Table 1 

Absolute Spectral Responsivity of Acme Instruments Model xx (S/N yyy) 

Relative Relative 

Absolute Expanded Absolute Expanded 

Wavelength Responsivity Uncertainty Wavelength Responsivity Uncertainty 

(k = 2) [%I 

0.92 (k = 2) [%I [nml [MI 

1.8 900 2.758E- 1 

[nml [MI 700 8.55E-2 

705 

710 

715 

720 

725 

730 

735 

740 

745 

750 

755 

760 

765 

770 

775 

780 

785 

790 

795 

800 

805 

810 

815 

820 

825 

830 

835 

840 

845 

850 

855 

860 

865 

870 

875 

880 

885 

890 

895 

8.92E-2 

9.32E-2 

9.72E-2 

.O 1 E- 1 

.05E- 1 

.09E- 1 

.14E- I 

.18E- 1 

.22E- I 

.27E- I 

1.3 1E- 1 

I .36E- I 

1.40E- 1 

1.45E- I 

1.50E- I 

1.54E- 1 

.59E- 1 

.64E- 1 

.69E- I 

.73E- 1 

.78E- I 

.83E- 1 

.88E- I 

1.93E- I 

1.98E- 1 

2.03E- 1 

2.08E- I 

2.13E-I 

2.19E- I 

2.24E-1 

2.289E-1 

2.34 1E- I 

2.393E-1 

2.44E- 1 

2.50E- 1 

2.549E-1 

2.60E- 1 

2.65E-1 

2.71E-1 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/3 

1.8 

1.7 

1.6 

1.7 

1.6 

1.6 

I .5 

1.5 

1.5 

I .4 

1.3 

I .4 

1.4 

1.3 

I .2 

1.3 

1.3 

1.1 

1.1 

I .3 

I .2 

I .2 

1.1 

I .3 

1.2 

1.1 

1.2 

I .3 

1.3 

1.1 

0.98 

0.94 

0.94 

1 .o 

1 .O 

0.84 

1.1 

1 .O 

I .O 

905 

910 

915 

920 

925 

930 

935 

940 

945 

950 

955 

960 

965 

970 

975 

980 

985 

990 

995 

1 000 

1005 

1010 

1015 

1020 

1025 

1030 

1035 

1040 

1045 

1050 

1055 

1060 

1065 

1070 

I075 

1080 

1085 

I090 

1095 

A- 17 

2.8 1 OE- 1 

2.86 I E- I 

2.9 14E- 1 

2.968E- 1 

3.02E- 1 

3.08E- 1 

3.13E- 1 

3.18E-1 

3.24E-1 

3.29E- 1 

3.34E- 1 

3.40E- 1 

3.45E- 1 

3.50E- 1 

3.54E-1 

3.59E-1 

3.64E- I 

3.69E- 1 

3.75E- I 

3.81E-1 

3.86E-1 

3.92E-1 

3.97E- 1 

4.02E- 1 

4.07E-1 

4.12E-1 

4.16E- 1 

4.21E-I 

4.27E-1 

4.32E- 1 

4.38E-1 

4.43E-1 

4.49E- 1 

4.55E-1 

4.60E- 1 

4.66E- 1 

4.7 1 E- 1 

4.77E- 1 

4.82E-1 

0.68 

0.70 

0.62 

0.52 

2.0 

2. I 

2.0 

1.9 

1.9 

I .8 

I .7 

1.8 

1.7 

1.6 

1.6 

1.3 

.4 

.5 

.6 

.5 

.5 

.6 

.5 

1.5 

1.9 

I .7 

1.8 

I .7 

1.8 

1.7 

1.8 

I .7 

1.7 

1.7 

1.7 

1.7 

1.8 

1.8 

1.7 

Page 3 of 6 
REPORT OF TEST 

NIST Test # 39075s - Spectral Responsivity 

Any Company 

Manufacturer: Acme Instruments 

Model#: xx 

Serial #: yyy 

Table 1 (cont.) 

Absolute Spectral Responsivity of Acme Instruments Model xx (S/N yyy) 

Relative Relative 

Absolute Expanded Absolute Expanded 

Wavelength Responsivity Uncertainty Wavelength Responsivity Uncertainty 

1100 4.88E-1 1.6 I300 6.70E- 1 2.6 

[nml cm1 (k = 2) [%I [nml [WI (k = 2) [%I 

1105 

1110 

1115 

1120 

1125 

1130 

I135 

1140 

1145 

1150 

1155 

1160 

1165 

1170 

1175 

I180 

1185 

1190 

1195 

1200 

1205 

1210 

1215 

1220 

1225 

1230 

1235 

1240 

1245 

1250 

1255 

1260 

1265 

1270 

1275 

1280 

1285 

1290 

1295 

4.94E- 1 

5.00E- 1 

5.06E- 1 

5.12E-1 

5.18E-1 

5.23E- 1 

5.29E- 1 

5.34E-1 

5.39E-1 

5.44E- 1 

5.49E- I 

5.53E- I 

5.58E-1 

5.62E-1 

5.67E- 1 

5.71E-1 

5.75E- 1 

5.80E5.84E5.89E5.93E5.98E6.02E6.06E6.1OE-1 

6.15E- I 

6.19E-1 

6.23E-1 

6.28E- 1 

6.32E-1 

6.36E- 1 

6.40E- I 

6.43E- I 

6.47E- 1 

6.50E- 1 

6.54E- 1 

6.58E-1 

6.62E- 1 

6.66E-1 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/3 

1.8 

2.0 

1.8 

1.8 

2.0 

2. I 

1.8 

1.8 

2.3 

1.9 

I .8 

2.1 

2.1 

2.2 

2.0 

2. I 

2.4 

2.3 

2.0 

2.3 

2.2 

2.3 

2.8 

2.2 

2.5 

2.4 

2.3 

2.8 

2.8 

2.6 

2.7 

2.5 

1.9 

1.2 

1.2 

1.8 

I .8 

2.0 

2.4 

I305 

1310 

1315 

I320 

1325 

1330 

1335 

1340 

I345 

1350 

1355 

1360 

1365 

1370 

1375 

1380 

1385 

1390 

I395 

1400 

1405 

1410 

1415 

1420 

1425 

1430 

1435 

I440 

1445 

1450 

1455 

1460 

1465 

I470 

1475 

1480 

1485 

1490 

I495 

A-18 

6.75E- 1 

6.79E- I 

6.83E- I 

6.86E-1 

6.90E- 1 

6.94E- I 

6.97E- 1 

7.0 1 E- 1 

7.05E- I 

7.09E- 1 

7.12E-1 

7.16E-I 

7.20E- 1 

7.24E-1 

7.29E- 1 

7.33E- I 

7.38E-1 

7.43E- 1 

7.47E- 1 

7.52E-1 

7.56E- 1 

7.60E-1 

7.64E-1 

7.68E- 1 

7.72E- 1 

7.76E- 1 

7.8 I E- 1 

7.85E-1 

7.90E- 1 

7.94E-1 

8 .OOE- 1 

8.05E- 1 

8.1 1E-1 

8.17E-1 

8.23E- 1 

8.28E- 1 

8.32E- 1 

8.36E-1 

8.40E- 1 

2.5 

2.6 

2.6 

3.0 

2.6 

3.2 

3.0 

3.3 

3.6 

3.5 

3.5 

3.9 

3.8 

4.8 

5.0 

6.6 

5.5 

6.6 

5.3 

4.8 

5.0 

5.0 

4.4 

4.9 

4.6 

4.0 

4.4 

4.7 

5.4 

4.6 

4.9 

5.0 

4.5 

6.0 

5.2 

4.6 

4.6 

4.9 

5.2 

Page 4 of 6 
REPORT OF TEST 

NIST Test # 39075s - Spectral Responsivity 

Any Company 

Manufacturer: Acme Instruments 

Model#: xx 

Serial #: yyy 

Table 1 (cont.) 

Absolute Spectral Responsivity of Acme Instruments Model xx (S/N yyy) 

Relative Re1 ative 

Absolute Expanded Absolute Expanded 

Wavelength Responsivity Uncertainty Wavelength Responsivity Uncertainty 

[nml [ml (k = 2) [%] [nml [MI (k = 2) [%I 

1500 8.43E- 1 5.7 1650 3.23E- 1 5.0 

1505 

1510 

1515 

1520 

1525 

1530 

1535 

1540 

1545 

1550 

1555 

1560 

1565 

1570 

1575 

1580 

1585 

1590 

1595 

1600 

1605 

1610 

1615 

1620 

1625 

1630 

1635 

1640 

1645 

8.44E- 1 

8.42E- 1 

8.37E-1 

8.25E-1 

8.06E- 1 

7.80E- 1 

7.49E- 1 

7.13E- 1 

6.76E- 1 

6.39E- 1 

6.05E- 1 

5.74E- 1 

5.48E- 1 

5.26E- 1 

5.07E- 1 

4.90E- 1 

4.74E- 1 

4.59E-1 

4.45E- 1 

4.32E- 1 

4.20E-1 

4.08E- 1 

3.97E- 1 

3.87E-1 

3.76E- 1 

3.65E- 1 

3.54E- 1 

3.43E- I 

3.33E-1 

6.1 

5.5 

5.3 

6.0 

7.0 

5.0 

5.5 

5.1 

5.7 

4.9 

4.9 

5.3 

4.4 

4.4 

5.3 

4.3 

4.7 

4.6 

5.3 

4.3 

5.0 

4.8 

5.0 

4.7 

5.0 

5.1 

4.6 

5. I 

5.1 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/3 

A- 19 

1655 

1660 

1665 

1670 

1675 

1680 

I685 

1690 

1695 

1700 

1705 

1710 

1715 

1720 

1725 

1730 

1735 

1740 

1745 

1750 

1755 

1760 

1765 

1770 

1775 

1780 

1785 

1790 

1795 

1800 

3.13E-1 

3.03E- I 

2.93E- 1 

2.83E-1 

2.73E- 1 

2.62E-1 

2.52E- 1 

2.4 1 E- 1 

2.31E-1 

2.20E- 1 

2.09E- 1 

1.98E- 1 

1.87E- 1 

1.75E- 1 

1.64E- 1 

1.52E-1 

1.41 E- 1 

1.30E-1 

1.20E- 1 

l.lOE-1 

1 .OOE- 1 

9.14E-2 

8.3 1E-2 

7.55E-2 

6.86E-2 

6.23E-2 

5.66E-2 

5.14E-2 

4.65E-2 

4.2 1E-2 

5.4 

4.8 

4.5 

4.9 

5.0 

4.2 

5.2 

5.2 

4.4 

4.9 

5.0 

6.2 

5.2 

5.1 

5.1 

5.2 

5.1 

4.7 

4.9 

5.2 

4.3 

5.2 

6.0 

4.9 

5.8 

4.9 

4.5 

5.4 

5.8 

5.0 

Page 5 of 6 
REPORT OF TEST 

NIST Test # 39075s - Spectral Responsivity 

Any Company 

Manufacturer: Acme Instruments 

Model#: xx 

Serial#: yyy 

4. General Information 

The laboratory temperature is reported for information only. It is not intended that this data be 

used for corrections to the spectral responsivity data in this report. This report shall not be 

reproduced, except in full, without the written approval of NIST. 

9 

Prepared by: Reviewed by: 

Sally S. Bruce Thomas C. Larason 

Optical Technology Division Optical Technology Division 

Physics Laboratory Physics Laboratory 

(301) 975-2323 (301) 975-2334 

Approved by: 

Joseph L. Dehmer 

For the Director, 

National Institute of 

Standards and Technology 

(301) 975-2319 

Reference: 

[l] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services: 

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part I1 - 

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41 

(1998). 

Test Date: December 24, 1997 

NIST Test No.: 844/xxxxxx-97/3 

A- 20 

Page 6 of 6 
UNITED STATES DEPARTMENT 

National Institute of Standards 

Gaithersburg, Maryland 20899-0001 

3 

OF COMMERCE 

and Technology 

REPORT OF TEST 

NIST Test ## 3908 1 S - Responsivity Spatial Uniformity 

for 

Acme Instruments Silicon Photodiode 

Model aa, S/N bbb 

Submitted by: 

Dr. Matthew Metrologist 

Bigtime Government National Laboratory 

Group PPL2 (Mail Stop: F769) 

Secretcity, NM 87545-0001 

(See your Purchase Order No. YY-YYYY, dated April 1, 1997) 

1. Description of Test Material 

The test photodiode, Acme Instruments model aa, S/N bbb, consists of a silicon photodiode 

mounted in a cylindrical aluminum housing with the output signal available on a BNC connector. 

The active area of the photodiode is = 1 cm'. The test photodiode was measured as supplied by 

Bigtime Government National Laboratory. 

2. Description of Test 

The relative spatial uniformity of the responsivity across the test photodiode photosensitive area 

was measured using the NIST visible to near Infrared (Vis/NIR) monochromator-based 

comparator facility [I] from 400 nm to 700 nm in 100 nm increments. The relative uniformity 

was measured in 0.5 mm increments with a 1.1 mm diameter beam using a double monochromator 

and a quartz-halogen lamp as the tunable monochromatic source. The circular exit aperture of the 

Vis/NIR monochromator was imaged (= fl9) onto the photosensitive area resulting in a beam 

diameter of 1.1 mm. 

The wavelength scale of the monochromator was calibrated with several laser and emission lines 

and is accurate to f 0.1 nm over the entire spectral range. The bandpass of the monochromator 

was 4 nm. The short-circuit photocurrent from the test photodiode was measured with a calibrated 

transimpedance amplifier. The test photodiode was measured with zero bias voltage. Beam power 

fluctuations were monitored with a beamsplitter and silicon photodiode. 

3. Results of Test 

Figure la is a plot of the relative uniformity of the test photodiode photosensitive area, showing 

0.2 % contours at 400 nm of the deviations from the responsivity at the center of the photosensitive 

area. Figure lb is a 3-dimensional plot showing the responsivity relative to the center of the 

photosensitive area. 

Laboratory Environment: 

Temperature: 23.x "C f 0.3 "C 

Test Date: December 24, 1997 

NIST Test No.: 844fyyyyyy-97 

A-21 Page 1 of6 (VLl 
REPORT OF TEST 

NIST Test # 39081s - Responsivity Spatial Uniformity 

Bigtime Government National Laboratory 

Manufacturer: Acme Instruments 

Model#: aa 

Serial #: bbb 

Figures 2, 3, and 4 are similar plots of the relative uniformity at 500 nm, 600 nm, and 700 nm 

respectively. 

The relative expanded uncertainty (k = 2) for the responsivity values is 0.0024 %. This is the 

repeatability of the measured relative responsivity in the central portion of the active area during the 

measurement scan. The measurement repeatability uncertainty depends on the SNR of the detector 

and can vary spectrally. Note that the variation in responsivity over the measured area is much 

larger than this uncertainty value. The reported uncertainty is not an indication of the uniformity 

measurement reproducibility. The uncertainty analysis is described in Ref. [ 13. 

The intended primary use of the reported uniformity results is qualitative. That is, to indicate if any 

large discontinuities are present in the responsivity uniformity which can lead to larger than 

expected uncertainties in absolute responsivity measurements. Quantitative application of the 

reported uniformity results requires examination of the irradiance geometry and equipment 

involved. The generalized application of the uniformity measurement results is currently being 

studied. 

4. General Information 

The laboratory temperature is reported for information only. It is not intended that this data be 

used for corrections to the spectral responsivity data in this report. This report shall not be 

reproduced, except in full, without the written approval of NIST. 

Prepared by: 

Sally S. Bruce 

Optical Technology Division 

Physics Laboratory 

(301) 975-2323 

Reviewed by: 

Thomas C. Larason 

Optical Technology Division 

Physics Laboratory 

(301) 975-2334 

Approved by: 

Joseph L. Dehmer 

For the Director, 

National Institute of 

Standards and Technology 

(301) 975-2319 

Reference: 

[I] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services: 

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part I1 - 

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41 

(1998). 

Test Date: December 24, 1997 

NIST Test No.: 844/yyyyyy-97 

A- 22 

Page 2 of 6 
REPORT OF TEST 

NIST Test # 3908 1 S - Responsivity Spatial Uniformity 

Bigtime Government National Laboratory 

Manufacturer: Acme Instruments 

Model#: aa 

Serial #: bbb 

Figure la 

Responsivity Uniformity of Acme Instruments aa, S/N bbb 

0.2 % contours at 400 nm; 1.1 mm resolution; 0.5 "/Step 

-6 -4 -2 0 2 4 6 

mm 

Figure lb 

Surface Plot of Responsivity Relative to 

Center of Photosensitive Area for Acme Instruments aa, S/N bbb 

at 400 nm; 0.5 "/Step 

-6 

6 -6 

Test Date: December 24, 1997 

NIST Test No.: 844fyyyyyy-97 

1.010 

1.005 

1 .ooo 

0.995 

6 

Relative Responsivity 

Page 3 of 6 

A- 23 
REPORT OF TEST 

NIST Test # 3908 1s - Responsivity Spatial Uniformity 

Bigtime Government National Laboratory 

Figure 2a 

Responsivity Uniformity of Acme Instruments aa, S/N bbb 

0.2 % contours at 500 nm; 1.1 mm resolution; 0.5 "/Step 

Manufacturer: Acme Instruments 

Model#: aa 

Serial #: bbb 

-6 

-4 

-2 

0 

2 

4 

6 

-6 -4 -2 0 2 4 6 

mm 

Figure 2b 

Surface Plot of Responsivity Relative to 

Center of Photosensitive Area for Acme Instruments aa, S/N bbb 

at 500 nm; 0.5 "/Step 

Test Date: December 24, 1997 

NIST Test No.: 844/yyyyyy-97 A- 24 

1.010 

1.005 

1 .ooo 

0.995 

6 

6 -6 

Relative Responsivity 

Page 4 of 6 
REPORT OF TEST 

NIST Test # 3908 IS - Responsivity Spatial Uniformity 

Bigtime Government National Laboratory 

Manufacturer: Acme Instruments 

Model#: aa 

Serial #: bbb 

Figure 3a 

Responsivity Uniformity of Acme Instruments aa, S/N bbb 

0.2 % contours at 600 nm; 1.1 mm resolution; 0.5 "/Step 

-6 

-4 

-2 

0 

2 

4 

6 

-6 -4 -2 0 2 4 6 

mm 

Figure 3b 

Surface Plot of Responsivity Relative to 

Center of Photosensitive Area for Acme Instruments aa, S/N bbb 

at 600 nm; 0.5 "/Step 

Test Date: December 24, 1997 

NIST Test No.: 844/yyyyyy-97 

1.010 

1.005 

1 .ooo 

0.995 

6 

Relative Responsivity 

6 fi -6 

A- 25 

Page 5 of 6 
REPORT OF TEST 

NIST Test # 3908 1s - Responsivity Spatial Uniformity 

Bigtime Government National Laboratory 

Figure 4a 

Responsivity Uniformity of Acme Instruments aa, S/N bbb 

0.2 % contours at 700 nm; 1.1 mm resolution; 0.5 "/Step 

Manufacturer: Acme Instruments 

Model#: aa 

Serial #: bbb 

-6 

-4 

-2 

0 

2 

4 

6 

-6 -4 -2 0 2 4 6 

mm 

Figure 4b 

Surface Plot of Responsivity Relative to 

Center of Photosensitive Area for Acme Instruments aa, S/N bbb 

at 700 nm; 0.5 "/Step 

-6 

6 -6 

1.010 

1.005 

1 .ooo 

0.995 

6 

Relative Responsivity 

Test Date: December 24, 1997 

NIST Test No.: 844/yyyyyy-97 Page 6 of 6 

A- 26 :\U.S. GOVERNMENT PRINTING OFFICE:1998-433-235/93207 
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and Thermocouple Materials 

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