NIST Measurement Services:

Spectroradiometric Detector Measurements:

Part I–Ultraviolet Detectors and

Part II–Visible to Near-Infrared Detectors

NIST

Special

Publication

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
UNITED STATES OF AMERICA

DEPARTMENT OF COMMERCE

NIST Special Publication 250-41

NIST 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 250-17

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. 250–41, 118 pages (Feb. 1998)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1998

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402–9325

ii
iii

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, SP250-41 (1998), 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 - 39081S 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) Katharine B. Gebbie

Director Director

Measurement Services Physics Laboratory
iv

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 μm 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
v

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

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 ...........................................................................................13

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

3.2.4 Measurement Equation Applied to the SCFs.................................................................14

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

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

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

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

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

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

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

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

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

4.1.8 Enclosure .....................................................................................................................19

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

4.2.1 UV Source ...................................................................................................................20

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

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

4.2.4 UV Translation Stages - Detector Positioning...............................................................21

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

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

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 (1800 nm)..........................................29
vi

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

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

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

5.5 UV Silicon Working Standards............................................................................................30

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 UV100 ..........................................................................................................68

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

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

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

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

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

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

APPENDIX - SAMPLE TEST REPORTS

39071S - UV Silicon Photodiode Spectral Responsivity Test Report Sample............................A-1

39073S - Visible to Near IR Silicon Photodiode Spectral Responsivity Test Report Sample .....A-8

39075S - Near IR Photodiode Spectral Responsivity Test Report Sample...............................A-15

39081S - 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 (I/V) 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 S1337 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 S1337 and S2281 photodiodes...............38

Figure 6.5. Spatial uniformities of typical UDT Sensors UV100 and EG&G Judson Ge

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

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

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

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

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

Figure 9.3. Linearity of UDT Sensors UV100 at 442 nm ..........................................................69

Figure 9.4. Responsivity dependence on bias voltage of UDT Sensors UV100 at 442 nm .........69

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

Figure 9.6. Mounting pieces for the Hamamatsu S1337-1010BQ photodiode and BNC connector

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

Figure 9.7. Exploded view diagram of Hamamatsu S1337-1010BQ fixture ...............................72

Figure 9.8. Mechanical diagram of UDT Sensors UV100 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 7.4. Transfer Uncertainty to Test (Customer) UV100 Silicon Photodiodes......................61

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

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
1

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 Z540-1-1994 [1].

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 NIST policy of using the International System of

Units (SI). 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, 3]. 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
2

lasers [4, 5] as the detector scale base. Relative uncertainties were reported on the order of 1.5 %

to 5 % (3 standard deviation estimate1) over the spectral range from 390 nm to 1100 nm [6].

Note: This document follows, to the extent possible, the ISO 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. [7] 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, 9] and the subsequent introduction of 100 % quantum efficient (QE) photodiodes [10] 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 [11], 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 [14].

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 % QE detector-based

measurements [15, 16, 17]. The current state of cryogenic ESRs and future direction of

detector-based radiometry are discussed in Refs. [18 and 19].

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

 

1The term “3 standard deviation estimate” or “3σ” in the context of this paper is the estimated standard deviation

multiplied by 3. Prior work at NIST was generally reported with “3σ” 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.
3

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

Vis/NIR 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 [23].

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 39071S

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

number Item of test Range Relative expanded

uncertainty (k = 2)

39071S Ultraviolet Silicon Photodiodes (UDT UV100) 200 nm to 500 nm 0.4 % to 13 %

39072S Retest of Ultraviolet Silicon Photodiodes (UDT UV100) 200 nm to 500 nm 0.4 % to 13 %

39073S Visible to Near-Infrared Silicon Photodiodes

(Hamamatsu S2281)

350 nm to 1100 nm

Can be extended to 200 nm

0.2 % to 4 %

0.2 % to 13 %

39074S Retest of Visible to Near-Infrared Silicon Photodiodes

(Hamamatsu S1337-1010BQ or S2281)

350 nm to 1100 nm

Can be extended to 200 nm

0.2 % to 4 %

0.2 % to 13 %

39075S Special Tests of Near-Infrared Photodiodes 700 nm to 1800 nm 0.5 % to 7 %

†

39080S Special Tests of Radiometric Detectors 200 nm to 1800 nm 0.2 % to 13 %

†

39081S Special Tests of Photodetector Responsivity Spatial Uniformity 200 nm to 1800 nm 0.0024 % to 0.05 %

†

†Depends on photodetector and signal level.
4

0

1

2

3

4

5

6

7

8

200 400 600 800 1000 1200 1400 1600 1800

UV Photodiodes

Visible to Near IR Photodiodes

Near IR Photodiodes

Relative Expanded

Uncertainty [%]

Wavelength [nm]

UV

Power < 20μW

Near IR

Power < 1 μW

Visible to Near IR

Power < 1μW

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

Figure 2.1. NIST UV, visible, and near-IR 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) [%]

Wavelength

[nm] UV100 (UV) S1337 (Visible) Ge (NIR) InGaAs (NIR)

200 13.02

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 1.32 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

1050 2.66 1.74 1.74

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
5

Descriptions of each Service ID number are provided below.

39071S - UV Silicon Photodiodes

NIST will supply customers with a UDT Sensors, Inc. model UV100 windowed silicon

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

includes the measured spectral responsivity [A·W-1]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

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

0.4 % 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 39071S) are

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 Hamamatsu

S1337-1010BQ) 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 cm

2

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

from 0.2 % to 4 %, 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.

39074S - Retest of Visible to Near-Infrared Silicon Photodiodes

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

39073S) 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.

39075S - Special Tests of Near-Infrared 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 μW. 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.

 

3 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.
6

39080S - 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

μW. 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.

39081S - 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 μW, and a beam of

diameter 1.1 mm in the 400 nm to 1800 nm spectral region at power levels less than 1 μW. 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 105 to 107 is typically required, thus the

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

Physical size and weight are limited by the translation stages used in the UV SCF and Vis/NIR

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 from 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.

2.3 How To Order Photodiodes Or Special Tests

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

Details of Service ID numbers 39075S, 39080S, and 39081S should be discussed with one of

the technical contacts listed in section 2.4 prior to submitting a formal request.
7

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/230/233/233.htm

3) The purchase order must include the following:

A) Bill to address.

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

C) Method of return shipment (the costs below do not include shipping costs).

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

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

E) Service ID number(s).

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://physics.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
8

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

V( A,Δl,l0 ) = El( x, y,l0,l)

A

∫

Δl

∫ ⋅SF ( x, y,l)⋅dA⋅dl [U], (3.1)

where V( A, Δl,l0 ) is the output signal (in U units, typically volts or amperes); El(x, y,l0,l) is

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

wavelength setting of l0; SF (x, y,l) 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 Δl is the wavelength interval

for which the value of El 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 Δl are left symbolically in V

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

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

SF (x, y,l) = S F (l) [U·W-1]. (3.2)

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

area integral

V( A,Δl,l0 ) = SF (l)⋅ E l( x, y,l0 ,l)

A

∫

Δl

∫ ⋅dA⋅dl [U]. (3.3)
9

Optical Axis

x

y xd

A

yd

F D,l( l0,l)= ∫El(x,y,l0,l)·dA [W]

Detector A

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

F D,l(l0 ,l) = El(x, y,l0 ,l)

A

∫ ⋅dA = t(l)⋅F l(l0,l) [W], (3.4)

where 

F l(l0,l) is the output flux from the monochromator and 

t (l) is the transmittance of the

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

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

can be written

V(Δl,l0 ) = SF (l)⋅t(l)⋅F l(l0 ,l)

Δl

∫ ⋅dl [U]. (3.5)

Equation (3.5) is the flux equivalent to eq (7.1b) [26]. Introducing the slit-scattering function

z(l0 − l) to the measurement equation allows the spectral radiant flux function of the

monochromator 

F l(l0,l) to be written as the product of two functions (with appropriate

normalization)

F l(l0,l) = z(l0 − l)⋅Ff ,l(l) [W], (3.6)

where the slit-scattering function z(l0 − l) 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 f ,l is dependent only on the wavelength of the flux. The factor F f ,l is the spectral

radiant flux at l0, 

F l(l0 ), and is equivalent to the responsivity factor r

f

 introduced in

eq (7.12) [26]. Both the slit-scattering function z(l0 − l) and the factor 

F f ,l can be determined

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

monochromator.

Thus the output signal V can now be written
10

V(Δl,l0 ) = SF (l)⋅t(l)⋅z(l0 − l) ⋅F f ,l(l)

Δl

∫ ⋅dl [U]. (3.7)

Equation (3.7) is the flux equivalent to eq (7.13) [26] 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 SF ⋅t ⋅F f ,l is approximately constant (or linear) over the spectral range for which

z(l0 − l) is significant (i.e., within Δl) and z is approximately symmetrical with respect to l0,

eq (3.7) can be written

V(Δl,l0 ) ≅SF (l0 )⋅t(l0 )⋅F f,l(l0 )⋅ z(l0 − l)

Δl

∫ ×dl [U]. (3.8)

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(l0 − l) does not change with the wavelength setting

l0 of the monochromator (that is, the dispersion is the same for all wavelengths) and thus

z(l0 − l) need only be measured once. Optical aberrations, scattering, and diffraction are also

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

bandpass, Δl, or l0 of the monochromator is varied over a beam of fixed wavelength l.

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 Δl which is scattered into Δl and

which contributes to the measured signal. Adding a stray light term, Vsl(Δl,l0 ), to eq (3.8) gives

V(Δl,l0 ) ≅SF (l0 )⋅t(l0 )⋅F f,l(l0 )⋅ z(l0 − l)

Δl

∫ ⋅dl+Vsl(Δl,l0 ) [U], (3.9)

where Vsl(Δl,l0 ) ≅ SF (l)⋅z (l0 − l)⋅t(l)⋅F f ,l(l)

l≠Δl

∫ ⋅dl [U]. (3.10)

The Vsl 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.

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

factor F f ,l to give an expression for the flux leaving the monochromator F ′ l,

F ′ l(l0 ) =F f,l(l0 )⋅ z(l0 − l)

Δl

∫ ⋅dl≅F l(l0,l) [W]. (3.11)

Using eq (3.11) 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
11

V(Δl,l0 ) ≅SF (l0 )⋅t(l0 )⋅F ′ l(l0 ) [U]. (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 function 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 (i.e., 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 cm

2

photodiodes NIST provides meet these requirements.

Humidity can affect the measurement by changing the transmittance t 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 SiO2 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.

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 estimated from [28]

q

d = 2.44⋅

l

d [rad], (3.13)

where q

d

 is the diffraction angle (for the first Airy disk), l 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 l≈2 μm and d ≈1 mm which gives
12

 

qVis/NIR

d = 2.44⋅ 2 μm

1 mm = 4.88×10 - 3 ≈5 mrad . (3.14)

The Vis/NIR SCF has a focal ratio (f-number or f /#) of ≈9 giving a beam angle of ≈110 mrad

which is over 20 times greater than the diffraction limit. For the UV SCF (l≈0.5 μm,

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 responsivity 

SF 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 

SF 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 [26] (primarily the monochromator). One

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

function z(l0 − l) does not change with the wavelength setting l0 of the monochromator. These

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

negligible. This allows z(l0 − l) 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 l of a monochromatic source is varied over the monochromator

bandpass Δl or whether the monochromator wavelength setting l0 is varied over a

monochromatic source of wavelength l. Also z(l0 − l) is assumed approximately symmetrical

with respect to l0 which is the case for the monochromators described in this publication.

The product SF ⋅t ⋅F f ,l is assumed to be constant (or linear) over Δl. In practice, this is affected

by reducing Δl so that SF ⋅F f ,l is constant (or linear) over Δl. 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.
13

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

Yx = Sx ⋅Xx [U], (3.15)

where Yx is the signal from the detector, Sx is the responsivity of the detector, and Xx is the input

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

Ys = Ss ⋅Xs [U]. (3.16)

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

Yx

Ys

= Sx ⋅Xx

Ss ⋅Xs

. (3.17)

If the input X to each system is assumed to be constant (this is the basis of the substitution

method), solving for the detector responsivity Sx gives

Sx = Yx

Ys

⋅Ss [U·input units-1]. (3.18)

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

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

X to the test detector.

3.2.2 Photodetector Substitution

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) F is the output flux from the source that enters

the monochromator. The spectral radiant flux F l is the output flux from the monochromator and

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

detector. The spectral radiant flux received by the detectors is F D ,l = t ⋅F l.
14

Standard

Source

Monochromator

(lselection) Test

F l F

Focusing

Optics F D,l=t · F l

Figure 3.2. Block diagram of photodetector substitution method.

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

for incandescent sources (i.e., 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 F D,l = tbs ⋅t ⋅F l is the flux received by the detectors and 

F M,l = rbs ⋅t ⋅F l is the

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

rbs, 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.

Monitor

Beam

Splitter

Monochromator

(λ selection) F F l

Focusing

Optics t · F l

F D,l=tbs · t · F l

F Μ ,l=rbs · t · F l

Test

Standard

Source

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

3.2.4 Measurement Equation Applied to the SCFs

The actual measurement equation is now developed. Dropping the l 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

Vx = Sx ⋅Gx ⋅tx ⋅F x + Vd,x [V], (3.19)

where Sx is the spectral responsivity in A·W-1, Gx is an explicit gain term for a transimpedance

amplifier in V·A-1, tx is the transmittance of any optics (and the atmosphere) between the

monochromator and the detector, F x is the output flux from the monochromator in W, and Vd,x 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
15

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

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

received by the detector

ΔVx =Vx − Vd,x = Sx ⋅Gx ⋅tx ⋅F x [V]. (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 rbs 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

ΔVx =Vx − Vd,x = Sx ⋅Gx ⋅t bsx ⋅tx ⋅F x [U], (3.21)

and ΔVmx =Vmx − Vd, mx = S mx ⋅G mx ⋅rbsx ⋅tx ⋅F x [U]. (3.22)

The ratio of these two signals is

Rx = ΔVx

ΔVmx

= Sx ⋅Gx ⋅t bsx ⋅tx ⋅F x

S mx ⋅Gmx ⋅rbsx ⋅tx ⋅F x

, (3.23)

where 

tbsx ⋅tx ⋅F x is the fraction of light received by the test detector, and 

rbsx ⋅tx ⋅F x 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,

Rs = ΔVs

ΔVms

= Ss ⋅Gs ⋅t bss ⋅ts ⋅F s

Sms ⋅Gms ⋅rbss ⋅ts ⋅F s

. (3.24)

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

Rx

Rs

= Sx ⋅Gx ⋅t bsx ⋅tx ⋅F x

S mx ⋅Gmx ⋅rbsx ⋅tx ⋅F x

⋅

S ms ⋅Gms ⋅rbss ⋅ts ⋅F s

Ss ⋅Gs ⋅t bss ⋅ts ⋅F s

. (3.25)

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

Rx

Rs

= Sx ⋅Gx

Ss ⋅Gs

. (3.26)

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

measurements into the ratios we have
16

Sx = Rx

Rs

⋅

Gs

Gx

⋅Ss =

Vx − Vd,x

Vmx − Vd, mx

Vs − Vd,s

Vms − Vd, ms

⋅

Gs

Gx

⋅Ss [A·W-1]. (3.27)

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

Sx = Yx

Ys

⋅Ss [U·input units-1]. (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

Rs

⋅

Gs

Gx

⋅Ss + Cbw [A·W-1], (3.28)

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.

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
17

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.

Sources Light Tight Enclosure

Monitor

Detector

Alignment Laser

Baffle

Wavelength

Drive

Shutter

Beam

Splitter

Scanning Detector Carriage

Prism - Grating

Monochromator

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.

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 II. Protective eyeware is worn when working in proximity to

these sources.
18

0.01

0.1

1

10

200 400 600 800 1000 1200 1400 1600 1800

Monochromator Output Flux (Power) [μW]

Wavelength (nm)

840 nm

600 nm 985 nm

1270 nm

1385 nm

420 nm

Argon Mini-Arc

100 W Quartz-Halogen Lamp

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 μm. 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 f /9. The

monochromator has a stray light rejection of 10-8. 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.

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

aperture of the monochromator with 1:1 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 [30].
19

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 μm and an accuracy of 0.2 μm per 100 mm. The vertical

stage travel range is 50 mm with a manufacturer specified resolution of 0.1 μm and an accuracy of

0.25 μm 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 S1337-1010BQ silicon photodiodes

with fused quartz windows. This is a p-n photodiode with a 1 cm

2

 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, 32]. The Vis

WS are normally used for 350 nm to 1100 nm measurements. For near-infrared measurements

(700 nm to 1800 nm), four EG&G Judson J16TE2-8A6-R05M-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.

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-R05M-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.

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.
20

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.

Detector

Carousel

Light Tight Enclosure

Monitor

Detector

Sources

Baffle

Alignment Laser

Scanning

Carriage

Wavelength

Drive

Shutter

Order

Sorting

Filter

Beam Splitter

Double Grating

Monochromator

UV WS

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
21

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 °F) chilled water. The water runs through plastic nonconducting

tubing. An FEL lamp, tungsten strip lamp, xenon arc lamp, 100W 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 II.

4.2.2 UV Monochromator

The UV comparator uses a Spex 1680, 1/4 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.

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 1:1 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 μm and an accuracy of 0.25 μm per

25 mm.
22

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 UV100 silicon photodiodes

with quartz windows and 1 cm

2

 circular active areas. The UV100 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 UV100 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 UV 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.

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.)
23

DVM

Other

Instruments

M

u

l

t

i

p

l

e

x

e

r

DVM with

Multiplexer

Input

DVM

TE

Temperature

Controller

Monitor

Amplifier

Silicon Monitor

Amplifiers

1

2

3

4

1

2

3

4

1

2

Test Detector

Test Detector

Temperature

Monitoring

Input

Germanium Monitor

Monitor TE

Temperature

Controller

Silicon

Working Standard

Silicon

Working Standard

Germanium

Working Standard

Germanium

Working Standard

Data Analysis

Computer

Test Reports

Computer

NIST Local Area Network

IEEE-488 Bus

Instrument Control

and Data Stroage

Computer

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

The amplifiers are identical in design and operate with gains of 104 V·A-1 to 109 V·A-1.

References [34 and 35] and several titles in the bibliography describe similar transimpedance

amplifiers and their operation. The schematic for the precision transimpedance (I/V) 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

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].
24

_

+

Out

_

+

Out

OPA

27AJ

OPA 128

Input High

Input Low

Output

Output

Common

10 kž

100 pf

1000 Mž

100 Mž

90 kž 900 kž 9 Mž

Figure 4.5. NIST SCF precision transimpedance (I/V) amplifier circuit.

Data Analysis

Computer

Test Reports

Computer

NIST Local Area Network

TE

Temperature

Controller

Monitor

Amplifier

Silicon Monitor

Amplifiers

1

2

3

4

1

2

3

4

Test Detector

Test Detector

Temperature

Monitoring

Inputs

Silicon

Working Standard

Silicon

Working Standard

Germanium Monitor

Monitor TE

Temperature

Controller

IEEE-488 Bus

Other

Instruments

Lock-In

Amplifer

DVM

Instrument Control

and Data Stroage

Computer

Germanium

Working Standard

Germanium

Working Standard

1

2

Multiplexer

Lock-In

Amplifer

DVM

Multiplexer

Chopper

Sync. Signal

Figure 4.6. Vis/NIR detector, amplifier, lock-in, DVM, and computer control block diagram.
25

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, 38]. 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 ≈107

,

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 % [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

27

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-of-squares

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 %.

1

2

(b) 3

Bottom

View

1

2

3

(a)

Front

View

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

Turning

Mirror

Laser Stabilizer Shutter

Monitor Detector

Wedged

Beamsplitter

Entrance Window

Electro-optic

Laser Stabilizer Aperture

Laser(s)

HeCd, Ar, Nd:YAG,

HeNe, Ti SAF

Trap

Detector

Substitution

High Accuracy

Cryogenic

Radiometer

0.975

0.980

0.985

0.990

0.995

1.000

400 500 600 700 800 900 1000

Wavelength [nm]

(a) Measured (circles) and modelled (solid line) external quantum efficiency

of a trap detector. Error bars are the uncertainties given in Ref. 21. (b) Measurement setup.

Figure 5.3. Scale transfer by substitution method with the HACR.
28

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.

HACR

via

Trap Detector

UV WS

200 nm to

500 nm

Pyroelectric

Detector

Pyroelectric

Detector

 Visible WS

405 nm to

920 nm

Ge WS

700 nm to

1800 nm

Visible WS

350 nm to

400 nm

Visible WS

920 nm to

1100 nm

Relative

Transfer

- or - - or -

Absolute

Transfer

Absolute

Transfer

Absolute

Transfer

Absolute

Transfer

Absolute

Transfer

Relative

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 f/9 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 106

.

The spectral responsivity is found using eq (3.27) where SS = ST, 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 μW.
29

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 [41].

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(l) is given as

 

a p(l) = 1− rp(l) [unitless], (5.1)

where rp(l) is the reflectance and the transmittance tp(l) is assumed to be equal to zero. The

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

Sp (l) = tw(l)⋅ap (l)⋅CFp = t w(l)⋅ 1− rp ( (l))⋅CFp [U·W-1], (5.2)
30

where tw(l) is the pyroelectric detector window transmittance and CFp 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 sp(l) is determined. Note that the lower case “s” is used to designate relative

responsivity [2].

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

Lambda-19 spectrophotometer [42]. The window transmittance tw(l) 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(l) of a test detector x can be

written

sx (l) = Rx (l)

Rp (l)

⋅sp (l) [arbitrary U·W-1], (5.3)

where sp(l) 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 (NIR) 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.

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
31

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.
32
33

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 39075S. 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 106

 is used, requiring that the

photodiode shunt resistance must be greater than 10 kΩ. 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 μW. 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.
34

10-3

10-2

10-1

100

101

300 400 500 600 700 800 900 1000 1100

Ratio of Signals, Signals [V], and

Relative Standard Deviations [%]

Wavelength [nm]

Ratio of S1337/Monitor

S1337 & 

Monitor 

Signal

Rel. Std. 

Devs. [%]

S1337 Signal [V] Monitor Signal [V]

Ratio Rel.

Std. Dev. [%]

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 S1337 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 the 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.
35

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 400 600 800 1000 1200 1400 1600 1800

Absolute Responsivity [A/W]

Wavelength [nm]

UDT Sensors

UV-100 Hamamatsu S1227

Hamamatsu S1337

S1337 Trap

EG&G Judson

TE Cooled Ge

InGaAs

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 efficiency.

The external quantum efficiency EQE is given by [10]

EQE(l) = I(l) ⋅h⋅c

F (l)⋅n⋅e⋅l , (6.1)

where I is the photocurrent (output current minus the dark output), h is Planck’s constant, c is the

velocity of light, F is the input radiant flux (power), n is the index of refraction of air, e is the

elementary electronic charge, and l is the spectral wavelength. Substituting for the constants h,

c, n, and e gives

EQE(l) = 1239 .85⋅ I(l)

F (l) ⋅l= 1239.85⋅

S (l)

l , (6.2)

where S(l) = I (l) F (l) is the spectral responsivity [A·W-1], and for convenience, l is in nm.

Examples of typical photodiode EQEs are shown in figure 6.3.
36

0

20

40

60

80

100

200 400 600 800 1000 1200 1400 1600 1800

Quantum Efficiency [%]

Wavelength [nm]

UDT Sensors

UV-100

Hamamatsu S1227

Hamamatsu S1337

S1337 Trap

EG&G Judson

TE Cooled Ge

InGaAs

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

39081S 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 UV100 photodiodes

(39071S). The spatial uniformity for the Hamamatsu S1337-1010BQ and, more recently, the

Hamamatsu S2281 photodiodes is measured at 500 nm (39073S). UV100 photodiodes are not

issued if the responsivity nonuniformity (i.e., slope) is greater than 1 % over the active area or

greater than 0.5 % 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 S1337-1010BQ and S2281 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 % 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

(1100 nm). Nonuniformity is due to inhomogeneity in the photodiode material - typically

inhomogeneity in surface recombination centers at shorter wavelengths [45, 46] and bulk

recombination centers at longer wavelengths [47]. The nonuniformity near the bandgap has also

been related to the (non)uniformity 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.
37

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 % 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 % is found within the center 90 % 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 cm

2

 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 106. Figure 4.2 shows the optical power

used for these measurements is typically less than 1 μW. 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 S1337-1010BQ and S2281 photodiodes

at 500 nm and 1000 nm. Similar spatial uniformity scans are shown in figure 6.5a for UV100 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.
38
39
40

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 IEEE-488 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.
41

Shutter

Controller

Shutter

Wavelength

Encoder

Wavelength

Display

Working Standard

& Test Detectors

TE

Temperature

Controller

Monitor

Amplifier

Silicon Monitor

Amplifiers

Temperature

Monitoring

Inputs

Germanium Monitor

Monitor TE

Temperature

Controller

Germanium

Working Standards

IEEE-488 Bus

Laboratory

Temperature

& Humidity

Sensor

Enclosure

Temperature

Sensor

Wavelength

Drive

Source

Selection

Drive

X-Y

Translation

Stages

DVM

Multiplexer

Enviromnent

Monitor

DVM

Servo

Motor

Controller

Digital I/O

Future Addition

Future Addition

Data Analysis

and File Backup

Computer

Test Reports

Computer

NIST Local Area Network

Instrument Control

and Data Stroage

Computer

Figure 6.6. Vis/NIR SCF computer control block diagram.
42

Shutter

Controller

Shutter

NIST Local Area Network

Working Standard

& Test Detectors

Amplifiers

Temperature

Monitoring

Inputs

DVM

Multiplexer

IEEE-488 Bus

UV

Monochromator

Digital I/O

Order Sorting

Filter Selector

Future Addition

Data Analysis

and File Backup

Computer

Test Reports

Computer

Instrument Control

and Data Stroage

Computer

Monitor

Amplifier

Silicon Monitor

DVM

Rotary

Stage

Source

Selection

Drive

X-Y

Translation

Stages

Servo

Motor

Controller

Laboratory

Temperature

& Humidity

Sensor

Enclosure

Temperature

Sensor

Enviromnent

Monitor

TE

Temperature

Controller

New TE Working

Standards

Future Addition

Figure 6.7. UV SCF computer control block diagram.
43

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.
44
45

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 [40] 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 (l) . 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.
46

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 51]. A detailed explanation of the evaluation and

expression of measurement uncertainty is given in Ref. [52].

7.1 Uncertainty Components

In general, a measurement result y can be expressed as a functional relationship f of N input

quantities 

xi given by,

y f N = (x , x ,..., x ). 1 2 (7.1)

The combined standard uncertainty uc ( y) is given by the law of propagation of uncertainty as the

following sum,
47

uc ( y) = ∂f

∂xi

⋅u (xi )  

 

  

  

2

i =1

N

∑ , (7.2)

where the partial derivatives 

∂f ∂xi are the sensitivity coefficients and u(x i ) are the standard

uncertainties of each input 

xi which are assumed to be uncorrelated. This method is also called

the root-sum-of-squares or “RSS” method.

When the functional relationship involves products and quotients, as the measurement equation

described in this document, the relative combined standard uncertainty uc ( y) y is more

convenient, and is given by

uc, r( y ) ≡ uc ( y)

y = 1

y

⋅

∂f

∂xi

⋅u (xi )  

 

  

 

 

2

i=1

N

∑ , (7.3)

where (1 y)⋅ ∂f ∂x ( i ) is the relative sensitivity coefficient. The expanded uncertainty U is

obtained by multiplying uc ( y) by a coverage factor k,

U = k ⋅u c ( y) , (7.4)

where k is chosen on the basis of the level of confidence desired. Replacing uc ( y) with the

relative combined standard uncertainty uc, r( y ) gives the relative expanded uncertainty Ur ≡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 uc, 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 u0(S ws) in the

working standard spectral responsivity is defined. The uncertainty u0(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

S ws = Rws

Rs

⋅

Gs

Gws

⋅S s [A·W-1]. (7.5)

The uncertainty u0(S ws) is

u0(S ws ) = ∂S ws

∂Rws

⋅u (Rws)  

 

  

  

2

+

∂S ws

∂Rs

⋅u (Rs)  

 

  

  

2

+

∂S ws

∂Gs

⋅u (Gs )  

 

  

  

2  

 

 

+

∂S ws

∂Gws

⋅u (Gws)  

 

  

  

2

+

∂S ws

∂S s

⋅u (Ss )  

 

  

  

2

 

 

 

1 2 [A·W-1]. (7.6)
48

The relative uncertainty u0(S ws ) S ws is

u0(S ws )

S ws

= u (R ws )

Rws

 

 

  

  

2

+

u (R s

 )

Rs

 

 

  

  

2

+

u (G s )

Gs

 

 

  

  

2

+

u (Gws)

Gws

 

 

  

  

2

+

u (S s

 )

S s

 

 

  

  

2

, (7.7)

where u (Rws) is the standard deviation of the mean of i ratios of the working standard and the

monitor, 

u (Rs ) is the standard deviation of the mean of i ratios of the calibration standard and the

monitor, u (Gs) is the calibration standard amplifier gain uncertainty, u (Gx ) is the working

standard amplifier gain uncertainty, u (Ss ) is the calibration standard uncertainty, and i is the

number of ratio measurements taken.

The calibration standard uncertainty, u (Ss ), is determined previously by the HACR transfer

measurements. The amplifier gains, Gs and Gx , are calibrated using a precision current source

and DVM. The amplifier gain uncertainties, u (Gs) and u (Gx ), 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, Rw ,

of the ratio R = Rws Rs is calculated [40] using the RSS of u (R ws ) and u (Rs ) from each scan as

weighting factors. An uncertainty term u (R w) is calculated for the weighted average and is an

indication of the measurement repeatability.

There is also a statistical uncertainty term u (R r) reflecting the reproducibility of the

measurements (e.g., setting up the measurement again the next day). The relative uncertainty

term, u (R r) Rr is combined by RSS with u (Rw ) Rw to get the relative measurement

uncertainty, u (R m ) Rm . 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 uc (S ws) 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.
49

The relative combined uncertainty uc (S ws ) S ws is

uc (Sws )

Sws

= u (Rm)

Rm

 

 

  

  

2

+

u (G s

 )

Gs

 

 

  

  

2

+

u (G ws )

Gws

 

 

  

  

2

+

u (S s )

Ss

 

 

  

  

2

+

u (S l)

S l

 

 

  

  

2  

 

 

+

u (V)

V

 

 

  

 

 

2

+

u (S lt )

Slt

 

 

  

  

2

+

u(g)

g

 

 

  

  

2

+

u (R sl )

Rsl

 

 

  

  

2

+

u (S bw )

Sbw

 

 

  

  

2

 

 

 

1 2 , (7.8)

where u (Sl ) Sl is the monochromator wavelength relative uncertainty, u (V ) V is the DVM

relative uncertainty, u (Slt ) S lt is the relative uncertainty due to the long-term stability of the

working standards, u(g) g is the scaling factor relative uncertainty for the relative responsivity

measurements, u (R sl) Rsl is the relative uncertainty due to stray light, and u (Sbw ) S bw 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 (l) introduces an

uncertainty proportional to the slope of the responsivity dSx/dl [53]

u (Sl ) = dS l

dl ⋅u(l) = ddl

Rws

Rs

⋅G⋅S s

 

  

 

  ⋅u (l) [U·W -1 ], where (7.9)

Sl = Sx = Rx

Rs

⋅G⋅S s [U·W -1 ], (7.10)

and G = Gs Gx is the ratio of the amplifier gains, which is a constant with respect to wavelength.

Carrying out the differentiation gives

u (Sl ) = S s

Rs

2 ⋅ Rs ⋅

dRx

dl − Rx ⋅

dRs

dl

 

 

  

  +

Rx

Rs

⋅

dS s

dl

 

  

 

  ⋅u (l) [U·W -1 ]. (7.11)
50

The wavelength uncertainty of the monochromator u (l) is the standard deviation of the residuals

to a fit of the wavelength calibration.

The relative uncertainty can be shown to be

u (Sl )

S l

= u (l)

G⋅S s

⋅

Rs

Rx

⋅

dS x

dl = u (l)

G ⋅ 1Rx

⋅

dRx

dl − 1

Rs

⋅

dRs

dl

+

1

S s

⋅

dS s

dl

 

  

 

  [unitless]. (7.12)

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 Rx = Rs and G = 1, eq (7.12) reduces to

u (Sl )

Sl

= u (l)

Ss

⋅

dS s

dl [unitless]. (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 Rws and Rs 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 ) S lt 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.
51

Scaling factor

The scaling factor g 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(g) g is the relative standard

deviation of the mean of the gi’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(l0) be the signal (in U units) from a

detector x when the monochromator is set to wavelength l0

Vx (l0 ) = Vx,bw (l0 ) + Vx,sl (l0 ) [U], (7.14)

where Vx,bw (l0 ) is the signal due to light within the full-width bandpass Δl and Vx,sl(l0) 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 Δl is

Vx,bw (l0 ) = Sx

Δl

∫ (l) ⋅F D,l(l,l0 ) ⋅dl [U]. (7.15)

Similarly, the signal due to stray light is

Vx,sl(l0) = S x

l≠ Δl

∫ (l)⋅F D,l(l,l0 )⋅dl [U], (7.16)
52

where the integral spans the detector responsivity spectral range but is not inclusive of the

bandpass Δl. The incident flux F D,l(l,l0 ) on the detector from the monochromator can be

expressed as a product with the slit-scattering function

F D,l(l,l0 ) = z(l0 − l) ⋅Ff ,l(l) [W], (7.17)

where z(l0 − l) is the same slit-scattering function introduced in eq (3.6), and Ff ,l(l) is the

spectral radiant flux on the detector at l0. (Note: the term Ff ,l(l), 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 l0 is thus

Vx (l0 ) = S x

Δl

∫ (l)⋅z(l0 − l)⋅F f ,l(l)⋅dl+ S x

l≠Δl

∫ (l) ⋅z (l0 − l) ⋅Ff ,l(l)⋅dl [U]. (7.18)

Similarly, the measurement equation for a working standard detector s is

Vs(l0) = S s

Δl

∫ (l)⋅z (l0 − l) ⋅F f ,l(l)⋅dl+ S s

(l)⋅z (l0 − l) ⋅F f ,l(l)⋅dl

l≠Δl

∫ [U]. (7.19)

For estimating the stray light contribution to the measured signal, the slit-scattering function is

assumed to have the computationally convenient form

z(l0 − l) = 1 for l0 − Δl≤l≤l0 + Δl

10− 7 otherwise

   

 . (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 Δl. The relative error due to

stray light when the monochromator is set to l0 is estimated as

δRsl(l0 )

Rsl (l0 ) ≈RT(l0 ) − Rbw (l0 )Rbw (l0 ) [unitless], (7.21)

where RT(l0 ) =

Vx(l,l0 )

Vs (l,l0)

 and Rsl (l0 ) = Rbw (l0 ) = Vx,bw (l,l0 )

Vs,bw (l,l0)

 . (7.22a,b)

The relative error was calculated over the measurement spectral range at 5 nm intervals of l0 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.
53

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

monochromator,

Sx = Rx

Rs

⋅Ss − b⋅b 2

6 ⋅

dS x

dl [U·W-1], (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 

2b = Δl= full-width bandpass ( 

b = Δl 2 = FWHM for triangular bandpass)

b = s

F D − l0 ⋅s

 [m-1], where s = dF D

dl [W·m-1], (7.24a,b)

and 

dF D 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 (18) in Ref. [53],

Sx = Rx

Rs

⋅ Ss +

b⋅b 2

6 ⋅

dS s

dl

 

  

 

  − b⋅b 2

6 ⋅

dS x

dl [U·W-1], (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

δSbw = Cbw = − b⋅b 2

6 ⋅

dSx

dl [U·W-1]. (7.26)

The relative error is

δS bw

S bw

= δS bw

Rx

Rs

⋅S s

 [unitless], (7.27)
54

and

δSbw

Sbw

= δSbw

Ss

⋅

Rs

Rx

= − b⋅b 2

6 ⋅Ss

⋅ Rs

Rx

⋅

 δSx

dl [unitless]. (7.28)

Similarly, for eq (7.25) the relative error is eq (7.27) where the bandwidth-effect uncertainty

component is

δSbw = Cbw = Rx

Rs

⋅

b⋅b 2

6 ⋅

dSs

dl − b⋅b 26 ⋅

dSx

dl [U·W-1], (7.29)

and

δSbw

Sbw

= δSbw

Ss

⋅

Rs

Rx

= b⋅b2

6 ⋅Ss

⋅ dSs

dl − RsRx

⋅

dSx

dl

 

  

 

  [unitless]. (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
55

uniformity directly affects these measurements (i.e., 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, 56]. The detector

linearity has been measured for the Hamamatsu S1337-1010BQ and the UDT Sensors UV100

silicon photodiodes and is discussed in section 9. 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
56

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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

UV WS

calibration

UV WS

amplifier

gain

Visible WS

amplifier

gain

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidth-e

ffect

Visible WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SUV)/SUV u(GUV)/GUV u(GVis)/GVis u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SVis)/SVis

Wavelength

[nm] Estimated value [%] Root-sumof-squares

350 0.87 0.56 0.83 0.04 0.04 0.04 0.000 -0.001 0.06 1.33

400 0.23 0.12 0.71 0.04 0.04 -0.03 0.000 0.001 0.03 0.76

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Trap

detector

calibration

Trap

detector

amplifier

gain

Visible WS

amplifier

gain

Wavelength

calibration

(± 0.1 nm) Stray light Bandwidth-e

ffect

Visible WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(ST)/ST u(GT)/GT u(GVis)/GVis u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SVis)/SVis

Wavelength

[nm] Estimated value [%] 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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Ge WS

calibration

Ge WS

amplifier

gain

Visible WS

amplifier

gain

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidth-e

ffect

Visible WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SGe)/SGe u(GGe)/GGe u(GVis)/GVis u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SVis)/SVis

Wavelength

[nm] Estimated value [%] Root-sumof-squares

950 0.90 0.02 0.90 0.04 0.04 -0.02 0.002 0.001 0.22 1.29

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 1.31

1100 0.40 0.02 0.78 0.04 0.04 0.18 -0.002 0.001 1.86 2.06
57

0.0

0.5

1.0

1.5

2.0

2.5

300 400 500 600 700 800 900 1000 1100

Relative combined

standard uncertainty [%]

Wavelength [nm]

Trap Detector

UV WS Ge WS

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.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Relative combined

standard uncertainty [%]

Wavelength [nm]

Trap Detector Pyroelectric Detector

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

standard uncertainty.
58

Table 7.2. Germanium Working Standard Uncertainty

Transfer from pyroelectric (relative) and scaling with trap (absolute)

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Trap

detector

calibration

Trap

detector

amplifier

gain

Ge WS

amplifier

gain

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidth-e

ffect

Ge WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(ST)/ST u(GT)/GT u(GGe)/GGe u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SGe)/SGe

Wavelength

[nm] Estimated value [%] Root-sumof-squares

700 0.05 0.03 0.05 0.04 0.04 -0.09 0.008 0.002 0.87 0.88

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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Pyroelectric

relative

calibration

Scaling

factor g

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidth-e

ffect

Ge WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(sp)/sp u(g)/g u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SGe)/SGe

Wavelength

[nm] Estimated value [%] Root-sumof-squares

950 0.68 0.02 0.52 0.14 -0.02 0.000 0.001 0.26 0.90

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 1.30

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 2.31

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.
59

0

1

2

3

4

5

6

7

200 250 300 350 400 450 500

Relative combined

standard uncertainty [%]

Wavelength [nm]

Trap Detector

Pyroelectric Detector

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

uncertainty.

Table 7.3. UV Working Standard Uncertainty

Transfer from Pyroelectric (relative) and Scaling with Visible WS (absolute)

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Pyroelectric

relative

calibration

Scaling

factor γ

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidth-e

ffect

UV WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(sp)/sp u(g)/g u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SUV)/SUV

Wavelength

[nm] Estimated value [%] Root-sumof-squares

200 5.87 0.38 0.52 0.18 -0.98 0.160 0.002 0.72 6.03

250 0.34 0.01 0.52 0.18 -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 0.77 1.02

350 0.44 0.01 0.52 0.18 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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Visible WS

calibration

Visible WS

amplifier

gain

UV WS

amplifier

gain

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidth-e

ffect

UV WS

long-term

stability

Relative

combined

standard

uncertainty

Type A B B B B B B B A [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SVis)/SVis u(GVis)/GVis u(GUV)/GUV u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw u(Slt)/Slt uc(SUV)/SUV

Wavelength

[nm] Estimated value [%] Root-sumof-squares

450 0.12 0.04 0.09 0.04 0.04 -0.05 0.0000 -0.002 0.00 0.17

500 0.11 0.03 0.08 0.04 0.04 -0.04 -0.0001 -0.001 -0.04 0.16
60

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 [7] 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.
61

0.0

1.0

2.0

3.0

4.0

200 400 600 800 1000 1200 1400 1600 1800

Relative combined

standard uncertainty [%]

Wavelength [nm]

UDT Sensors

UV-100

Hamamatsu S1337-1010BQ

& S2281

EG&G Judson

TE Cooled Ge

(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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

UV WS

calibration

UV WS

amplifier gain

Test detector

amplifier gain

Wavelength

calibration

(± 0.1 nm)

Stray light† Bandwidtheffect†

Relative

combined

standard

uncertainty

Type A B B B B B B B [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SUV)/SUV u(GUV)/GUV u(GTest)/GTest u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw uc(STest)/STest

Wavelength

[nm] Estimated value [%] Root-sumof-squares

200 2.41 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

500 0.05 0.01 0.16 0.04 0.04 -0.04 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 S2281 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.
62

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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Visible WS

calibration

Visible WS

amplifier gain

Test detector

amplifier gain

Wavelength

calibration

(± 0.1 nm)

Stray light† Bandwidtheffect†

Relative

combined

standard

uncertainty

Type A B B B B B B B [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SVis)/SVis u(GVis)/GVis u(GTest)/GTest u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw uc(STest)/STest

Wavelength

[nm] Estimated value [%] Root-sumof-squares

350 0.31 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 1.33

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-R05M-SC test photodiode. For different customer-supplied germanium

photodiodes the uncertainty is reanalyzed for that device.
63

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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Ge WS

calibration

Ge WS

amplifier gain

Test detector

amplifier gain

Wavelength

calibration

(± 0.1 nm)

Stray light† Bandwidtheffect†

Relative

combined

standard

uncertainty

Type A B B B B B B B [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SGe)/SGe u(GGe)/GGe u(GTest)/GTest u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw uc(STest)/STest

Wavelength

[nm] Estimated value [%] Root-sumof-squares

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

1000 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

1800 0.04 0.26 2.49 0.04 0.04 0.20 0.00 0.00 2.51

†

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.
64

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

Source of

uncertainty

Relative

measurement

uncertainty

DVM

uncertainty

Ge WS

calibration

Ge WS

amplifier gain

Test detector

amplifier gain

Wavelength

calibration

(± 0.1 nm)

Stray light Bandwidtheffect

Relative

combined

standard

uncertainty

Type A B B B B B B B [%]

Relative

uncertainty u(Rm)/Rm u(V)/V u(SGe)/SGe u(GGe)/GGe u(GTest)/GTest u(Sl)/Sl u(Rsl)/Rsl u(Sbw)/Sbw uc(STest)/STest

Wavelength

[nm] Estimated value [%] Root-sumof-squares

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 0.11 0.03 0.45 0.04 0.04 -0.11 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

1800 0.54 0.57 2.49 0.04 0.04 0.28 0.54 0.00 2.68

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
65

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 [49] for a Hamamatsu S1337-1010BQ 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

Source of

uncertainty

Relative

measurement

noise†

DVM

uncertainty

Relative

combined

standard

uncertainty

Type A B [%]

Relative

uncertainty u(R)/R u(V)/V uc(SUnif)/SUnif

Estimated value [%] Root-sum-ofsquares

0.0009 0.0007 0.0012

†

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 ANSI/NCSL

Z540-1-1994 [57, 58, 59]. 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 ANSI/NCSL Z540-1-1994 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 s tandard detectors are used for each test (customer) detector measurement. Thus, one
66

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 charts.

0.31280

0.31285

0.31290

0.31295

0.31300

0.31305

0.31310

21-Sep-1992 1-Dec-1992 19-Apr-1993 14-Sep-1995 6-Aug-1996 3-Oct-1996

Absolute Responsivity at 600 nm [A/W]

Time [days]

UCL

LCL

Center

Line

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) II [61, 62]. The SURF II 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 III [63] and a second absolute cryogenic

radiometer being installed at SURF III.

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 % at

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.
67

Currently there is a spectral responsivity comparison underway between NIST and Centro

Nacional De Metrología (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 39071S 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 S1337-1010BQ silicon photodiodes have been supplied by NIST as spectral (power)

responsivity standards. NIST currently supplies the Hamamatsu S2281 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 [31, 32].

Hamamatsu describes [65] the S1337-1010BQ 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 MΩ and a minimum value of 50 MΩ.

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 S1226, 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.
68

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

300 400 500 600 700 800 900 1000 1100

Responsivity

Temperature Coefficient [%/°C]

Wavelength [nm]

S1226 Si (25 °C)

lResp peak = 720 nm

S1337 Si (25 °C)

lResp peak = 960 nm

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

0.980

0.985

0.990

0.995

1.000

1.005

0 1 2 3 4 5 6 7

Irradiance [mW/cm2]

Relative Responsivity

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

9.2 Hamamatsu S2281

Hamamatsu S2281 silicon photodiodes are currently provided by NIST as standards of spectral

responsivity (NIST Service ID number 39073S). The characteristics are essentially identical to

the S1337-1010BQ. Since the S2281 is almost indistinguishable from the S1337, 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 cm

2

 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 UV100

UDT Sensors UV100 silicon photodiodes are provided as spectral responsivity standards in the

UV (NIST Service ID number 39071S). UDT Sensors literature [67] describes the UV100 as an

inverted channel diode with enhanced resistance to damage from UV radiation. The UV100 has a

quartz window, a 1 cm2

 circular active area, and is housed in a metal case with a BNC connector
69

similar to the Hamamatsu S2281. The spectral response range is from 200 nm to 1100 nm with a

peak around 760 nm. The typical shunt resistance value is 10 MΩ.

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 [66].

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.

0.6

0.7

0.8

0.9

1.0

1.1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Relative Responsivity

Irradiance [mW/cm2]

1 V Bias

No Bias Voltage

Figure 9.3. Linearity of UDT Sensors UV100 at 442 nm.

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Bias Voltage [V]

Relative Responsivity

1 mW/cm2

4 mW/cm2

Figure 9.4. Responsivity dependence on bias voltage of UDT Sensors UV100 at 442 nm.
70

9.4 Detector Apertures

The precision apertures provided with the photodiodes by NIST Service ID numbers 39071S 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

2

. 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 NIST policy of

using the International System of Units (SI). 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

UV100 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 S1337-1010BQ 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 UV100 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 UV100 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 struck

the countersunk area around the aperture. An improved design shown in figure 9.10 has been
71

used since 1993. Later productions of the aperture plate are engraved with the detector fixture

serial number facing the diode.

0.625"

4 x 2-56

tapped

blind hole

1" 0.75" 0.625" 0.875" 1" 2"

1.25"

0.375" 0.25"

0.125"

3 x 2-56 clearance

countersunk

0.25"

0.125"

0.125"

Material: Aluminum

Finish: Black Anodized

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

0.75"

0.25"

0.050"

0.125"

0.050"

dia.

0.025"

dia.

Material: Black Delrin®

0.625"

0.25"

120°

Drill and Tap for

supplied BNC connector

Material: Aluminum

Finish: Black Anodized

3 x 2-56 tapped through

Figure 9.6. Mounting pieces for the Hamamatsu S1337-1010BQ photodiode and

BNC connector.
72

Mount Body

Aperture Plate

BNC

Mounting

Ring

Sockets

Photodiode Delrin® Disk

Precision

Aperture

Figure 9.7. Exploded view diagram of Hamamatsu S1337-1010BQ fixture.

0.625" 1" 2"

0.375" 0.25"

0.25"

0.125"

Material: Aluminum

Finish: Black Anodized

0.200"

1"

4-40 tapped

through

4 x 2-56

tapped

blind hole

Figure 9.8. Mechanical diagram of UDT Sensors UV100 and Hamamatsu S2281 fixture.
73
74

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 III [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 [68], 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 (NEP) 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, 70] designed as a standard with the new IR SCF covering 2 μm to 20 μm

[71, 72]. Since the bolometer has a flat response, it can also, in principle, be used below 2 μm 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
75

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. References

[1] American National Standard for Calibration - Calibration Laboratories and Measuring and

Test Equipment - General Requirements, ANSI/NCSL Z540-1-1994. (Currently available

through the National Conference of Standards Laboratories, 1800 30th Street, Suite

305B, Boulder, CO 80301.)

[2] Determination of the Spectral Responsivity of Optical Radiation Detectors, Publ. 64

(Commission Internationale de L’Éclairage, 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.)

[3] W. Budde, Optical Radiation Measurements, Volume 4: Physical Detectors of Optical

Radiation, Academic Press, Inc., Orlando, FL (1983) pp. 38-59.

[4] 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 (1973).

[5] 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 (1977).

[6] Unpublished preprint of sequel to NBS Technical Note 950 describing the radiometric

characteristics of the DRTIP radiometers, p. 9.

[7] 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.).

[8] E. F. Zalewski and J. Geist, “Silicon Photodiode Absolute Spectral Response

Self-Calibration,” Appl. Opt. 19, 1214-1216 (1980).

[9] J. Geist, E. F. Zalewski, and A. R. Schaefer, “Spectral Response Self-Calibration And

Interpolation of Silicon Photodiodes,” Appl. Opt. 19, 3795-3799 (1980).

[10] E. F. Zalewski and C. R. Duda, “Silicon Photodiode Device with 100 % External

Quantum Efficiency,” Appl. Opt. 22, 2867-2873 (1983).

[11] The QED-200 is now manufactured by UDT Instruments (formerly Graseby Optronics),

Orlando, FL.
76

[12] E. F. Zalewski, The NBS Photodetector Spectral Response Calibration Transfer Program,

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-17, p. 45 (1988).

[13] J. M. Houston and E. F. Zalewski, “Photodetector Spectral Response Based on 100 %

Quantum Efficient Detectors,” Optical Radiation Measurements II, James M. Palmer,

Editor, Proc. SPIE 1109, pp. 268-277 (1989).

[14] 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. 31-Feb. 1, 1991 (unpublished).

[15] N. P. Fox, “Trap Detectors and Their Properties,” Metrologia 28, 197-202 (1991).

[16] J. Geist, “Current Status of, and Future Directions In, Silicon Photodiode

Self-Calibration,” Optical Radiation Measurements II, James M. Palmer, Editor, Proc.

SPIE 1109, 246-256 (1989).

[17] 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).

[18] N. P. Fox, “Radiometry with Cryogenic Radiometers and Semiconductor Photodiodes,”

Metrologia 32, 535-543 (1995/96).

[19] A. C. Parr, A National Measurement System for Radiometry, Photometry, and Pyrometry

Based Upon Absolute Detectors, Natl. Inst. Stand. Technol., Tech. Note 1421 (1996).

[20] 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).

[21] 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 (1996).

[22] 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).

[23] NIST Calibration Services Users Guide, Joe D. Simmons, Editor, Natl. Inst. Stand.

Technol., Spec. Publ. 250 (1991).

[24] NIST Calibration Services Users Guide Fee Schedule, Natl. Inst. Stand. Technol., Spec.

Publ. 250 Appendix (1997).
77

[25] 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

(1978).

[26] 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 910-4 (1979).

[27] R. Köhler, R. Goebel, R. Pello, and J. Bonhoure, “Effects of Humidity and Cleaning on

the Sensitivity of Si Photodiodes,” Metrologia 28, 211-215 (1991).

[28] A description of diffraction 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 (1993) or E. Hecht, Optics, 2nd edition, Addison-Wesley Publishing

Company, Reading, MA (1987).

[29] 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).

[30] J. L. Gardner, “Astigmatism Cancellation in Spectroradiometry,” Metrologia 28, 251-254

(1991).

[31] F. Lei and J. Fischer, “Characterization of Photodiodes in the UV and Visible Spectral

Region Based on Cryogenic Radiometry,” Metrologia 30, 297-303 (1993).

[32] R. Köhler, R. Goebel, and R. Pello, Report on the International Comparison of Spectral

Responsivity of Silicon Detectors, Rapport BIPM-94/9, document CCPR/94-2, dated July

27, 1994, Bureau International des Poids et Mesures, Pavillon de Breteuil, 93212 Sevres,

Cedex, France.

[33] 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).

[34] G. Eppeldauer and J. E. Hardis, “Fourteen-Decade Photocurrent Measurements with

Large-Area Silicon Photodiodes at Room Temperature,” Appl. Opt. 30, 3091-3099

(1991).

[35] W. Budde, Optical Radiation Measurements, Volume 4: Physical Detectors of Optical

Radiation, Academic Press, Inc., Orlando, FL (1983) pp. 265-268.
78

[36] G. Eppeldauer, “Electronic Characteristics of Ge and InGaAs Radiometers,” Infrared

Technology and Applications XXII, Björn F. Andresen and Marija Strojnik, Editors, Proc.

SPIE 3061, pp. 833-838 (1997).

[37] J. E. Martin, N. P. Fox, and P. J. Key, “A Cryogenic Radiometer for Absolute

Radiometric Measurements,” Metrologia 21, 147-155 (1985).

[38] 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.

[39] Electrical standards are maintained in the Electronics and Electrical Engineering

Laboratory at NIST.

[40] R. C. Paule and J. Mandel, “Consensus Values and Weighting Factors,” J. Res. Natl. Bur.

Stand. (U.S.) 87, 5 (1982).

[41] 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

(1984).

W. Budde, Optical Radiation Measurements, Volume 4: Physical Detectors of Optical

Radiation, Academic Press, Inc., Orlando, FL (1983).

[42] 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).

[43] 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. 250-6 (1987).

[44] See Ref. 27 and personal communication with one of the authors. Note: Cleaning a

photodiode can change its responsivity.

[45] 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).

[46] K. D. Stock, “Internal Quantum Efficiency of Ge Photodiodes,” Appl. Opt. 27, 12-14

(1988).
79

[47] A. R. Schaefer, E. F. Zalewski, and J. Geist, “Silicon Detector Nonlinearity and Related

Effects,” Appl. Opt. 22, 1232-1236 (1983).

[48] Private communication with Richard Austin of Gamma Scientific, San Diego, CA.

[49] 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).

[50] E. F. Zalewski, Radiometry and Photometry, Chapter 24 in Handbook of Optics, Vol. II,

2nd edition, M. Bass, Editor-in-Chief, McGraw-Hill, New York, NY (1995).

[51] C. L. Wyatt, Radiometric Calibration: Theory and Methods, Academic Press, San Diego,

CA (1978).

[52] American National Standard for Expressing Uncertainty - US Guide to the Expression of

Uncertainty in Measurement, ANSI/NCSL Z540-2-1997. (Currently available through the

National Conference of Standards Laboratories, 1800 30th Street, Suite 305B, Boulder,

CO 80301.)

[53] 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

(1978).

[54] R. D. Saunders and J. B. Shumaker, “Apparatus Function of a Prism-Grating Double

Monochromator,” Appl. Opt. 25, 3710-3714 (1986).

[55] N. M. Durant and N. P. Fox, “Evaluation of Solid-State Detectors for Ultraviolet

Radiometric Applications,” Metrologia 32, 505-508 (1995/96).

R. Goebel, R. Köhler, and R. Pello, “Some Effects of Low-Power Ultraviolet Radiation

on Silicon Photodiodes,” Metrologia 32, 515-518 (1995/96).

G. Eppeldauer, “Longterm Changes of Silicon Photodiodes and Their Use for Photometric

Standardization,” Appl. Opt. 29, 2289-2294 (1990).

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 (1989).

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).
80

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, 143-146

(1991).

J. L. Gardner and F. J. Wilkinson, “Response Time and Linearity of Inversion Layer

Silicon Photodiodes,” Appl. Opt. 24, 1531-1534 (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,

1555-1558 (1979).

[57] 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).

[58] S. S. Bruce and T. C. Larason, “Building a Quality System Based on ANSI/NCSL

Z540-1-1994 - An Effort by the Radiometric Physics Division at NIST,” Proc. NCSL

1995 Workshop and Symposium (National Conference of Standards Laboratories), Dallas,

TX July 16-20, 1995.

[59] S. S. Bruce and T. C. Larason, Developing quality system documentation based on

ANSI/NCSL Z540-1-1994 -- the optical technology division’s effort, Natl. Inst. Stand.

Technol., Internal Report 5866 (1996).

[60] 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 (1992).

[61] L. R. Canfield, “New Far UV Detector Calibration Facility at the National Bureau of

Standards,” Appl. Opt. 26, 3831-3837 (1987).

[62] L. R. Canfield and N. Swanson, Far Ultraviolet Detector Standards, Natl. Bur. Stand.

(U.S.), Spec. Publ. 250-2 (1987).

[63] NIST is currently upgrading SURF II, creating SURF III. Improvements will be made in

many areas, including beam current monitoring, magnetic structure uniformity, and

electron energy.

[64] R. Köhler, R. Goebel, and R. Pello, “Results of an International Comparison of Spectral

Responsivity of Silicon Photodetectors,” Metrologia 32, 463-468 (1995/96).
81

[65] 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, 435-91, Japan

[66] R. D. Saunders and J. B. Shumaker, “Automated Radiometric Linearity Tester,” Appl.

Opt. 23, 3504-3506 (1984).

[67] These values are from the manufacturer’s catalog (obtained Å 1993), Optoelectronic

Components Catalog, UDT Sensors, Inc., 12525 Chadron Ave., Hawthorne, CA, USA

90250

[68] 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 (1995).

[69] G. Eppeldauer, A. L. Migdall, and C. L. Cromer, “Characterization of a High Sensitivity

Composite Silicon Bolometer,” Metrologia 30, 317-320 (1993).

[70] 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.

[71] G. Eppeldauer, “Near Infrared Radiometer Standards,” Optical Radiation Measurements

III, James M. Palmer, Editor, Proc. SPIE 2815, pp. 42-54 (1996).

[72] A. L. Migdall and G. Eppeldauer, NIST Measurement Services: Spectroradiometric

Detector Measurements: Part III -- Infrared Detectors, Natl. Inst. Stand. Technol., Spec.

Publ. 250-42 (in preparation).
82

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 of Optics, 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 II, 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.
83

J. D. Vincent, Fundamentals of Infrared Detector Operation and Testing, Wiley, New York, NY,

1990.

T. M. Frederiksen, Intuitive Operational Amplifiers, McGraw-Hill, New York, NY, 1988.

J. G. Graeme, Photodiode Amplifiers: 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, Vol. 1: 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 III, Metrologia 28, (1991).

New Developments and Applications in Optical Radiometry IV, Metrologia 30, (1993).

New Developments and Applications in Optical Radiometry V, Metrologia 32, (1996).
84

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: The Study of Uncertainties in Physical

Measurements, University Science Books, Sausalito, CA, 1982.

ANSI/NCSL Z540-2-1997, 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 (1994 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 Z540-1-1994, 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 (1990). Currently available through the American National Standards Institute (ANSI),

11 West 42nd Street, 13th Floor, New York, NY 10036.
REPORT OF TEST

NIST Test # 39071S - Spectral Responsivity

Laboratory Environment:

Temperature: 23.x °C ± 0.8 °C

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-1 Page 1 of 7

for

UDT Sensors UV100 Silicon Photodiode, U1xxx

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, 1997)

1. Description of Test Material

The test photodiode, labeled U1xxx, is a UDT Sensors UV100 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

U5xx, using the NIST Ultraviolet (UV) monochromator-based comparator facility [1] 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 ± 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.
REPORT OF TEST Manufacturer: UDT Sensors

NIST Test # 39071S - Spectral Responsivity Model #: UV100

Any Company Serial #: U1xxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-2 Page 2 of 7

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. [1]. 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.

Figure 1

0.05

0.10

0.15

0.20

0.25

0.30

0

2

4

6

8

10

12

14

200 300 400 500

Absolute Responsivity [A/W]

Relative Expanded Uncertainty ( =2) [%] k

Wavelength [nm]

Absolute Spectral Responsivity of Silicon Photodiode U1xxx

Absolute Responsivity [A/W]

Relative Expanded

Uncertainty [%]
REPORT OF TEST Manufacturer: UDT Sensors

NIST Test # 39071S - Spectral Responsivity Model #: UV100

Any Company Serial #: U1xxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-3 Page 3 of 7

Table 1

Absolute Spectral Responsivity of Silicon Photodiode U1xxx

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

200 6.28E-2 13 350 1.24E-1 1.7

205 6.68E-2 8.4 355 1.22E-1 1.6

210 6.95E-2 5.1 360 1.20E-1 1.6

215 7.34E-2 4.1 365 1.18E-1 1.5

220 7.78E-2 2.8 370 1.21E-1 1.5

225 8.42E-2 1.5 375 1.27E-1 1.5

230 9.45E-2 1.7 380 1.35E-1 1.4

235 1.06E-1 1.5 385 1.43E-1 1.4

240 1.13E-1 1.5 390 1.50E-1 1.4

245 1.17E-1 1.4 395 1.57E-1 1.4

250 1.19E-1 1.4 400 1.63E-1 1.5

255 1.21E-1 1.3 405 1.689E-1 0.60

260 1.20E-1 1.3 410 1.750E-1 0.56

265 1.17E-1 1.4 415 1.816E-1 0.58

270 1.13E-1 1.5 420 1.882E-1 0.76

275 1.12E-1 1.7 425 1.935E-1 0.60

280 1.13E-1 2.0 430 1.993E-1 0.54

285 1.15E-1 2.0 435 2.049E-1 0.50

290 1.19E-1 1.9 440 2.100E-1 0.42

295 1.24E-1 2.0 445 2.160E-1 0.42

300 1.27E-1 2.1 450 2.220E-1 0.38

305 1.29E-1 1.8 455 2.281E-1 0.38

310 1.29E-1 1.8 460 2.341E-1 0.40

315 1.28E-1 1.8 465 2.401E-1 0.38

320 1.27E-1 1.8 470 2.462E-1 0.42

325 1.27E-1 1.9 475 2.523E-1 0.40

330 1.26E-1 1.7 480 2.583E-1 0.38

335 1.26E-1 1.6 485 2.643E-1 0.40

340 1.25E-1 1.6 490 2.704E-1 0.38

345 1.25E-1 1.7 495 2.764E-1 0.40

500 2.824E-1 0.38

Table 2

Aperture Dimensions

Area: 0.5xxx + 0. 0005 cm2

Distance from aperture plane

to photodiode surface: 4.8 + 0.4 mm
REPORT OF TEST Manufacturer: UDT Sensors

NIST Test # 39071S - Spectral Responsivity Model #: UV100

Any Company Serial #: U1xxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-4 Page 4 of 7

 Figure 2a

Responsivity Uniformity of Silicon Photodiode U1xxx

0.2 % contours at 350 nm; 1.5 mm resolution; 0.5 mm/Step

-6 -4 -2 0 2 4 6

-6

-4

-2

0

2

4

6

mm

mm

Figure 2b

Surface Plot of Responsivity Relative to

Center of Photodiode for Silicon Photodiode U1xxx

at 350 nm; 0.5 mm/Step

-6

-4

-2

0

2

4

6

mm

-6

-4

-2

0

2

4

6

mm

0.995

1.000

1.005

1.010

Relative Responsivity
REPORT OF TEST Manufacturer: UDT Sensors

NIST Test # 39071S - Spectral Responsivity Model #: UV100

Any Company Serial #: U1xxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-5 Page 5 of 7

4. General Information

The linearity of this type of photodiode is shown in figure 3 at 442 nm at 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. 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 UV100 at 442 nm

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Relative Responsivity

Irradiance [mW/cm2]

1 V Bias

No Bias Voltage

Figure 4

Responsivity dependence to bias voltage of UDT Sensors UV100 at 442 nm

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Bias Voltage [V]

Relative Responsivity

1 mW/cm2

4 mW/cm2
REPORT OF TEST Manufacturer: UDT Sensors

NIST Test # 39071S - Spectral Responsivity Model #: UV100

Any Company Serial #: U1xxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-6 Page 6 of 7

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:

[1] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services:

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part II -

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41

(1998).
REPORT OF TEST Manufacturer: UDT Sensors

NIST Test # 39071S - Spectral Responsivity Model #: UV100

Any Company Serial #: U1xxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/1 A-7 Page 7 of 7

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.

B. The incident beam of radiation should be smaller than the aperture, and should be centered

in the photodiode aperture.

C. The photodiode should be connected with a BNC cable to an electrometer grade amplifier

(transimpedance amplifier) which measures the current from the photodiode.

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.
REPORT OF TEST

NIST Test # 39073S - Spectral Responsivity

Laboratory Environment:

Temperature: 23.x °C ± 0.3 °C

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/2 A-8 Page 1 of 7

for

Hamamatsu S2281 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 ± 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.

REPORT OF TEST Manufacturer: Hamamatsu

NIST Test # 39073S - Spectral Responsivity Model #: S2281

Any Company Serial #: Cxxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/2 A-10 Page 3 of 7

Table 1

Absolute Spectral Responsivity of Silicon Photodiode Cxxx

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

350 1.50E-1 3.0 550 2.848E-1 0.20

355 1.49E-1 2.6 555 2.877E-1 0.20

360 1.47E-1 2.6 560 2.906E-1 0.20

365 1.45E-1 2.2 565 2.935E-1 0.20

370 1.47E-1 2.1 570 2.965E-1 0.20

375 1.51E-1 2.0 575 2.994E-1 0.20

380 1.58E-1 1.8 580 3.023E-1 0.20

385 1.65E-1 1.8 585 3.052E-1 0.20

390 1.71E-1 1.6 590 3.081E-1 0.20

395 1.77E-1 1.7 595 3.109E-1 0.20

400 1.82E-1 1.6 600 3.138E-1 0.20

405 1.860E-1 0.36 605 3.166E-1 0.20

410 1.909E-1 0.34 610 3.195E-1 0.20

415 1.959E-1 0.32 615 3.223E-1 0.20

420 2.008E-1 0.30 620 3.252E-1 0.20

425 2.041E-1 0.28 625 3.280E-1 0.20

430 2.079E-1 0.26 630 3.308E-1 0.20

435 2.113E-1 0.26 635 3.337E-1 0.20

440 2.141E-1 0.24 640 3.365E-1 0.20

445 2.178E-1 0.24 645 3.393E-1 0.20

450 2.214E-1 0.24 650 3.421E-1 0.20

455 2.250E-1 0.24 655 3.449E-1 0.20

460 2.284E-1 0.24 660 3.477E-1 0.20

465 2.319E-1 0.22 665 3.505E-1 0.20

470 2.352E-1 0.22 670 3.534E-1 0.20

475 2.385E-1 0.22 675 3.562E-1 0.20

480 2.418E-1 0.22 680 3.589E-1 0.20

485 2.450E-1 0.22 685 3.617E-1 0.20

490 2.482E-1 0.22 690 3.645E-1 0.20

495 2.513E-1 0.22 695 3.673E-1 0.20

500 2.545E-1 0.22 700 3.701E-1 0.20

505 2.576E-1 0.22 705 3.729E-1 0.20

510 2.607E-1 0.22 710 3.756E-1 0.20

515 2.638E-1 0.22 715 3.784E-1 0.20

520 2.668E-1 0.22 720 3.812E-1 0.20

525 2.699E-1 0.22 725 3.840E-1 0.20

530 2.729E-1 0.22 730 3.868E-1 0.20

535 2.759E-1 0.22 735 3.896E-1 0.22

540 2.788E-1 0.22 740 3.923E-1 0.22

545 2.818E-1 0.20 745 3.951E-1 0.20
REPORT OF TEST Manufacturer: Hamamatsu

NIST Test # 39073S - Spectral Responsivity Model #: S2281

Any Company Serial #: Cxxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/2 A-11 Page 4 of 7

Table 1 (cont.)

Absolute Spectral Responsivity of Silicon Photodiode Cxxx

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

750 3.978E-1 0.22 925 4.93E-1 2.8

755 4.007E-1 0.22 930 4.95E-1 2.8

760 4.034E-1 0.22 935 4.97E-1 2.8

765 4.061E-1 0.22 940 5.00E-1 2.5

770 4.088E-1 0.22 945 5.02E-1 2.4

775 4.116E-1 0.22 950 5.03E-1 2.6

780 4.144E-1 0.22 955 5.04E-1 2.4

785 4.171E-1 0.22 960 5.06E-1 2.3

790 4.199E-1 0.22 965 5.05E-1 2.2

795 4.227E-1 0.22 970 5.05E-1 2.1

800 4.254E-1 0.22 975 5.04E-1 1.9

805 4.282E-1 0.22 980 5.02E-1 1.6

810 4.309E-1 0.22 985 4.98E-1 1.6

815 4.336E-1 0.22 990 4.93E-1 1.7

820 4.364E-1 0.22 995 4.87E-1 1.9

825 4.391E-1 0.24 1000 4.79E-1 1.7

830 4.418E-1 0.22 1005 4.69E-1 1.8

835 4.446E-1 0.24 1010 4.57E-1 1.8

840 4.473E-1 0.24 1015 4.42E-1 1.8

845 4.502E-1 0.24 1020 4.26E-1 1.8

850 4.528E-1 0.22 1025 4.08E-1 2.2

855 4.556E-1 0.22 1030 3.87E-1 2.5

860 4.584E-1 0.22 1035 3.63E-1 2.1

865 4.610E-1 0.22 1040 3.38E-1 1.9

870 4.638E-1 0.22 1045 3.13E-1 2.6

875 4.665E-1 0.22 1050 2.86E-1 2.7

880 4.693E-1 0.24 1055 2.60E-1 2.5

885 4.720E-1 0.22 1060 2.33E-1 2.0

890 4.747E-1 0.22 1065 2.09E-1 2.4

895 4.774E-1 0.22 1070 1.91E-1 2.9

900 4.802E-1 0.22 1075 1.75E-1 3.7

905 4.833E-1 0.22 1080 1.60E-1 3.2

910 4.861E-1 0.24 1085 1.46E-1 3.2

915 4.876E-1 0.22 1090 1.33E-1 3.7

920 4.902E-1 0.22 1095 1.20E-1 4.4

1100 1.09E-1 4.2
REPORT OF TEST Manufacturer: Hamamatsu

NIST Test # 39073S - Spectral Responsivity Model #: S2281

Any Company Serial #: Cxxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/2 A-12 Page 5 of 7

Figure 2a

Responsivity Uniformity of Silicon Photodiode Cxxx

0.2 % contours at 500 nm; 1.1 mm resolution; 0.5 mm/Step

-6 -4 -2 0 2 4 6

-6

-4

-2

0

2

4

6

mm

mm

Figure 2b

Surface Plot of Responsivity Relative to

Center of Photodiode for Silicon Photodiode Cxxx

at 500 nm; 0.5 mm/Step

-6

-4

-2

0

2

4

6

mm

-6

-4

-2

0

2

4

6

mm

0.995

1.000

1.005

1.010

Relative Responsivity
REPORT OF TEST Manufacturer: Hamamatsu

NIST Test # 39073S - Spectral Responsivity Model #: S2281

Any Company Serial #: Cxxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/2 A-13 Page 6 of 7

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:

[1] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services:

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part II -

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41

(1998).
REPORT OF TEST Manufacturer: Hamamatsu

NIST Test # 39073S - Spectral Responsivity Model #: S2281

Any Company Serial #: Cxxx

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/2 A-14 Page 7 of 7

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.

B. The incident beam of radiation should be smaller than the aperture, and should be centered

in the photodiode aperture.

C. The photodiode should be connected with a BNC cable to an electrometer grade amplifier

(transimpedance amplifier) which measures the current from the photodiode.

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.
REPORT OF TEST

NIST Test # 39075S - Spectral Responsivity

Laboratory Environment:

Temperature: 23.x °C ± 0.3 °C

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/3 A-15 Page 1 of 6

for

Acme Instruments Germanium Photodiode

Model xx, S/N yyy

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, Acme Instruments model xx, S/N yyy, consists of a thermoelectrically

cooled germanium photodiode mounted in a cylindrical aluminum housing with the output signal

available on a BNC connector. The active area of the photodiode is ≈ x cm2

.

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 (Vis/NIR) monochromator-based

comparator facility [1] 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 Vis/NIR monochromator was imaged (≈f/9) 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 ± 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

REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39075S - Spectral Responsivity Model #: xx

Any Company Serial #: yyy

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/3 A-17 Page 3 of 6

Table 1

Absolute Spectral Responsivity of Acme Instruments Model xx (S/N yyy)

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

700 8.55E-2 1.8 900 2.758E-1 0.92

705 8.92E-2 1.8 905 2.810E-1 0.68

710 9.32E-2 1.7 910 2.861E-1 0.70

715 9.72E-2 1.6 915 2.914E-1 0.62

720 1.01E-1 1.7 920 2.968E-1 0.52

725 1.05E-1 1.6 925 3.02E-1 2.0

730 1.09E-1 1.6 930 3.08E-1 2.1

735 1.14E-1 1.5 935 3.13E-1 2.0

740 1.18E-1 1.5 940 3.18E-1 1.9

745 1.22E-1 1.5 945 3.24E-1 1.9

750 1.27E-1 1.4 950 3.29E-1 1.8

755 1.31E-1 1.3 955 3.34E-1 1.7

760 1.36E-1 1.4 960 3.40E-1 1.8

765 1.40E-1 1.4 965 3.45E-1 1.7

770 1.45E-1 1.3 970 3.50E-1 1.6

775 1.50E-1 1.2 975 3.54E-1 1.6

780 1.54E-1 1.3 980 3.59E-1 1.3

785 1.59E-1 1.3 985 3.64E-1 1.4

790 1.64E-1 1.1 990 3.69E-1 1.5

795 1.69E-1 1.1 995 3.75E-1 1.6

800 1.73E-1 1.3 1000 3.81E-1 1.5

805 1.78E-1 1.2 1005 3.86E-1 1.5

810 1.83E-1 1.2 1010 3.92E-1 1.6

815 1.88E-1 1.1 1015 3.97E-1 1.5

820 1.93E-1 1.3 1020 4.02E-1 1.5

825 1.98E-1 1.2 1025 4.07E-1 1.9

830 2.03E-1 1.1 1030 4.12E-1 1.7

835 2.08E-1 1.2 1035 4.16E-1 1.8

840 2.13E-1 1.3 1040 4.21E-1 1.7

845 2.19E-1 1.3 1045 4.27E-1 1.8

850 2.24E-1 1.1 1050 4.32E-1 1.7

855 2.289E-1 0.98 1055 4.38E-1 1.8

860 2.341E-1 0.94 1060 4.43E-1 1.7

865 2.393E-1 0.94 1065 4.49E-1 1.7

870 2.44E-1 1.0 1070 4.55E-1 1.7

875 2.50E-1 1.0 1075 4.60E-1 1.7

880 2.549E-1 0.84 1080 4.66E-1 1.7

885 2.60E-1 1.1 1085 4.71E-1 1.8

890 2.65E-1 1.0 1090 4.77E-1 1.8

895 2.71E-1 1.0 1095 4.82E-1 1.7
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39075S - Spectral Responsivity Model #: xx

Any Company Serial #: yyy

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/3 A-18 Page 4 of 6

Table 1 (cont.)

Absolute Spectral Responsivity of Acme Instruments Model xx (S/N yyy)

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

1100 4.88E-1 1.6 1300 6.70E-1 2.6

1105 4.94E-1 1.8 1305 6.75E-1 2.5

1110 5.00E-1 2.0 1310 6.79E-1 2.6

1115 5.06E-1 1.8 1315 6.83E-1 2.6

1120 5.12E-1 1.8 1320 6.86E-1 3.0

1125 5.18E-1 2.0 1325 6.90E-1 2.6

1130 5.23E-1 2.1 1330 6.94E-1 3.2

1135 5.29E-1 1.8 1335 6.97E-1 3.0

1140 5.34E-1 1.8 1340 7.01E-1 3.3

1145 5.39E-1 2.3 1345 7.05E-1 3.6

1150 5.44E-1 1.9 1350 7.09E-1 3.5

1155 5.49E-1 1.8 1355 7.12E-1 3.5

1160 5.53E-1 2.1 1360 7.16E-1 3.9

1165 5.58E-1 2.1 1365 7.20E-1 3.8

1170 5.62E-1 2.2 1370 7.24E-1 4.8

1175 5.67E-1 2.0 1375 7.29E-1 5.0

1180 5.71E-1 2.1 1380 7.33E-1 6.6

1185 5.75E-1 2.4 1385 7.38E-1 5.5

1190 5.80E-1 2.3 1390 7.43E-1 6.6

1195 5.84E-1 2.0 1395 7.47E-1 5.3

1200 5.89E-1 2.3 1400 7.52E-1 4.8

1205 5.93E-1 2.2 1405 7.56E-1 5.0

1210 5.98E-1 2.3 1410 7.60E-1 5.0

1215 6.02E-1 2.8 1415 7.64E-1 4.4

1220 6.06E-1 2.2 1420 7.68E-1 4.9

1225 6.10E-1 2.5 1425 7.72E-1 4.6

1230 6.15E-1 2.4 1430 7.76E-1 4.0

1235 6.19E-1 2.3 1435 7.81E-1 4.4

1240 6.23E-1 2.8 1440 7.85E-1 4.7

1245 6.28E-1 2.8 1445 7.90E-1 5.4

1250 6.32E-1 2.6 1450 7.94E-1 4.6

1255 6.36E-1 2.7 1455 8.00E-1 4.9

1260 6.40E-1 2.5 1460 8.05E-1 5.0

1265 6.43E-1 1.9 1465 8.11E-1 4.5

1270 6.47E-1 1.2 1470 8.17E-1 6.0

1275 6.50E-1 1.2 1475 8.23E-1 5.2

1280 6.54E-1 1.8 1480 8.28E-1 4.6

1285 6.58E-1 1.8 1485 8.32E-1 4.6

1290 6.62E-1 2.0 1490 8.36E-1 4.9

1295 6.66E-1 2.4 1495 8.40E-1 5.2
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39075S - Spectral Responsivity Model #: xx

Any Company Serial #: yyy

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/3 A-19 Page 5 of 6

Table 1 (cont.)

Absolute Spectral Responsivity of Acme Instruments Model xx (S/N yyy)

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

Wavelength

[nm]

Absolute

Responsivity

[A/W]

Relative

Expanded

Uncertainty

(k = 2) [%]

1500 8.43E-1 5.7 1650 3.23E-1 5.0

1505 8.44E-1 6.1 1655 3.13E-1 5.4

1510 8.42E-1 5.5 1660 3.03E-1 4.8

1515 8.37E-1 5.3 1665 2.93E-1 4.5

1520 8.25E-1 6.0 1670 2.83E-1 4.9

1525 8.06E-1 7.0 1675 2.73E-1 5.0

1530 7.80E-1 5.0 1680 2.62E-1 4.2

1535 7.49E-1 5.5 1685 2.52E-1 5.2

1540 7.13E-1 5.1 1690 2.41E-1 5.2

1545 6.76E-1 5.7 1695 2.31E-1 4.4

1550 6.39E-1 4.9 1700 2.20E-1 4.9

1555 6.05E-1 4.9 1705 2.09E-1 5.0

1560 5.74E-1 5.3 1710 1.98E-1 6.2

1565 5.48E-1 4.4 1715 1.87E-1 5.2

1570 5.26E-1 4.4 1720 1.75E-1 5.1

1575 5.07E-1 5.3 1725 1.64E-1 5.1

1580 4.90E-1 4.3 1730 1.52E-1 5.2

1585 4.74E-1 4.7 1735 1.41E-1 5.1

1590 4.59E-1 4.6 1740 1.30E-1 4.7

1595 4.45E-1 5.3 1745 1.20E-1 4.9

1600 4.32E-1 4.3 1750 1.10E-1 5.2

1605 4.20E-1 5.0 1755 1.00E-1 4.3

1610 4.08E-1 4.8 1760 9.14E-2 5.2

1615 3.97E-1 5.0 1765 8.31E-2 6.0

1620 3.87E-1 4.7 1770 7.55E-2 4.9

1625 3.76E-1 5.0 1775 6.86E-2 5.8

1630 3.65E-1 5.1 1780 6.23E-2 4.9

1635 3.54E-1 4.6 1785 5.66E-2 4.5

1640 3.43E-1 5.1 1790 5.14E-2 5.4

1645 3.33E-1 5.1 1795 4.65E-2 5.8

1800 4.21E-2 5.0
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39075S - Spectral Responsivity Model #: xx

Any Company Serial #: yyy

Test Date: December 24, 1997

NIST Test No.: 844/xxxxxx-97/3 A-20 Page 6 of 6

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: 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:

[1] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services:

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part II -

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41

(1998).
REPORT OF TEST

NIST Test # 39081S - Responsivity Spatial Uniformity

Laboratory Environment:

Temperature: 23.x °C ± 0.3 °C

Test Date: December 24, 1997

NIST Test No.: 844/yyyyyy-97 A-21 Page 1 of 6

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 cm2

. 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 [1] 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 (≈f/9) 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 ± 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 1a 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 1b is a 3-dimensional plot showing the responsivity relative to the

center of the photosensitive area.
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39081S - Responsivity Spatial Uniformity Model #: aa

Bigtime Government National Laboratory Serial #: bbb

Test Date: December 24, 1997

NIST Test No.: 844/yyyyyy-97 A-22 Page 2 of 6

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. [1].

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: 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:

[1] T. C. Larason, S. S. Bruce, and A. C. Parr, NIST Measurement Services:

Spectroradiometric Detector Measurements: Part I - Ultraviolet Detectors and Part II -

Visible to Near-Infrared Detectors, Natl. Inst. Stand. Technol., Spec. Publ. 250-41

(1998).
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39081S - Responsivity Spatial Uniformity Model #: aa

Bigtime Government National Laboratory Serial #: bbb

Test Date: December 24, 1997

NIST Test No.: 844/yyyyyy-97 A-23 Page 3 of 6

Figure 1a

Responsivity Uniformity of Acme Instruments aa, S/N bbb

0.2 % contours at 400 nm; 1.1 mm resolution; 0.5 mm/Step

-6 -4 -2 0 2 4 6

-6

-4

-2

0

2

4

6

mm

mm

Figure 1b

Surface Plot of Responsivity Relative to

Center of Photosensitive Area for Acme Instruments aa, S/N bbb

at 400 nm; 0.5 mm/Step

-6

-4

-2

0

2

4

6

mm

-6

-4

-2

0

2

4

6

mm

0.995

1.000

1.005

1.010

Relative Responsivity
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39081S - Responsivity Spatial Uniformity Model #: aa

Bigtime Government National Laboratory Serial #: bbb

Test Date: December 24, 1997

NIST Test No.: 844/yyyyyy-97 A-24 Page 4 of 6

Figure 2a

Responsivity Uniformity of Acme Instruments aa, S/N bbb

0.2 % contours at 500 nm; 1.1 mm resolution; 0.5 mm/Step

-6 -4 -2 0 2 4 6

-6

-4

-2

0

2

4

6

mm

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 mm/Step

-6

-4

-2

0

2

4

6

mm

-6

-4

-2

0

2

4

6

mm

0.995

1.000

1.005

1.010

Relative Responsivity
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39081S - Responsivity Spatial Uniformity Model #: aa

Bigtime Government National Laboratory Serial #: bbb

Test Date: December 24, 1997

NIST Test No.: 844/yyyyyy-97 A-25 Page 5 of 6

Figure 3a

Responsivity Uniformity of Acme Instruments aa, S/N bbb

0.2 % contours at 600 nm; 1.1 mm resolution; 0.5 mm/Step

-6 -4 -2 0 2 4 6

-6

-4

-2

0

2

4

6

mm

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 mm/Step

-6

-4

-2

0

2

4

6

mm

-6

-4

-2

0

2

4

6

mm

0.995

1.000

1.005

1.010

Relative Responsivity
REPORT OF TEST Manufacturer: Acme Instruments

NIST Test # 39081S - Responsivity Spatial Uniformity Model #: aa

Bigtime Government National Laboratory Serial #: bbb

Test Date: December 24, 1997

NIST Test No.: 844/yyyyyy-97 A-26 Page 6 of 6

Figure 4a

Responsivity Uniformity of Acme Instruments aa, S/N bbb

0.2 % contours at 700 nm; 1.1 mm resolution; 0.5 mm/Step

-6 -4 -2 0 2 4 6

-6

-4

-2

0

2

4

6

mm

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 mm/Step

-6

-4

-2

0

2

4

6

mm

-6

-4

-2

0

2

4

6

mm

0.995

1.000

1.005

1.010

Relative Responsivity