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NIST Special Publication 250-41 (2008)

Spectroradiometric Detector

Measurements:

Ultraviolet, Visible, and Near-Infrared

Detectors for Spectral Power

Thomas C. Larason

Jeanne M. Houston

I ^1 ^H^^ I National Institute of Standards and Technology • U.S. Department of Commerce

NIST Special Publication 250-41 (2008)

Spectroradiometric Detector

Measurements:

Ultraviolet, Visible, and Near-Infrared

Detectors for Spectral Power

Thomas C. Larason

Jeanne M. Houston

Optical Technology Division

Physics Laboratory

National Institute ofStandards and Technology

Gaithersburg MD 20899-8441

Supersedes NIST Special Publication 250-41 (1998)

Revised October 2008

U.S. Department of Commerce

Carlos M. Gutierrez, Secretaiy

National Institute of Standards and Technology

Patrick Gallagher, Depiit}- Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 250-41

Natl. Inst. Stand. Technol. Spec. Publ. 250-41, 119 pages (October 2008)

CODEN: NSPUE2

ii
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

"SP 250 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, SP 250-41 (2008), NIST Measurement Services: Spectroradiometric Detector

Measurements, is a revision of SP250-41 (1998). It covers the calibration and special test of

spectral radiant power responsivity for photodetectors from 200 nm to 1800 nm (Ser\'ice ID

numbers 39071S - 39081S in SP 250, 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.

Belinda L. Collins Katharine B. Gebbie

Director Director

Technology Services Physics Laboratory

iii
Abstract and Key Words

The National Institute of Standards and Technology supplies calibrated photodiode standards

and special tests of photodetectors for spectral radiant power responsivity for the wavelengths

from 200 nm to 1 800 nm. The scale of spectral radiant power responsivity is based solely on

detector measurements traceable to the Primary Optical Watt Radiometer (POWR), the reference

absolute cryogenic radiometer maintained by the National Institute of Standards and

Technology's Optical Technology Division. Solid-state transfer standard detectors are used to transfer the optical power unit, the optical watt, from this cryogenic radiometer to working

standard detectors used in monochromator-based facilities where routine measurements are performed. 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 the uncertainties of the reported values are also presented.

Key Words: calibration; detector; measurement; photodiode; photodetector; power responsivity;

quantum efficiency; radiometry; spectral; standard; uncertainty

IV
Table of Contents

1. INTRODUCTION 1

2. NIST SPECTRORADIOMETRIC DETECTOR MEASUREMENT SERVICE 4

2. 1 Description of Measurement Services 4

2.2 Measurement Limitations 7

2.3 How to Order Calibrations 8

2.4 Technical Contacts 9

3. MEASUREMENT THEORY 1

0

3 . 1 Measurement Equation 10

3.1.1 Approximations 13

3.2 Substitution Method 1

5

3.2. 1 General Substitution Method 15

3.2.2 Photodetector Substitution 16

3.2.3 Substitution Method with Monitor 1

7

3.2.4 Measurement Equation Applied to the SCFs 1

7

4. EQUIPMENT DESCRIPTION 20

4. 1 Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR SCF) 20

4. 1 . 1 Vis/NIR Source and Input Optics 21

4. 1 .2 Vis/NIR Monochromator 22

4. 1 .3 Vis/NIR Shutter and Output Optics 22

4. 1 .4 Vis/NIR Translation Stages - Detector Positioning 22

4. 1 .5 Vis/NIR Working Standards 22

4. 1 .6 Beam Splitter and Monitor Detector 23

4. 1 .7 Alignment Lasers 23

4.1.8 Enclosure 23

4.2 Ultraviolet Spectral Comparator (UV SCF) 23

4.2. 1 UV Source and Input Optics 24

4.2.2 UV Monochromator 25

4.2.3 UV Shutter and Output Optics 25

4.2.4 UV Translation Stages - Detector Positioning 25

4.2.5 UV Working Standards 25

4.2.6 Beam Splitter and Monitor Detector 26

4.2.7 Alignment Lasers 26

4.2.8 Enclosure 26

4.3 Electronics 26

4.3.1 Electronics - Signal Measurement 26

4.3.2 Electronics - Auxiliary Equipment 28

5. SPECTRAL POWER RESPONSIVITY SCALE REALIZATION 29

5.1 The Spectral Power Responsivity Scale Measurement Chain 29

5.2 Cryogenic Radiometry 30

5.3 Calibration of Transfer Standards with a Cryogenic Radiometer 32

5.4 Calibration of the Working Standards 35

V
5.4. 1 Calibration of the Visible Si Working Standards (Vis WS) 36

5.4.2 Calibration of the Ultraviolet Working Standards (UV WS) 37

5.4.3 Calibration of the Near-IR InGaAs Working Standards (IGA WS) 38

6. CALIBRATION PROCEDURES AND COMPUTER AUTOMATION 40

6.1 Spectral Radiant Power Responsivity 40

6. 1 . 1 Calibration Procedures 40

6. 1 .2 Quantum Efficiency 42

6.2 Spatial Uniformity 43

6.2. 1 Measurement Method and Calibration Procedure 44

6.2.2 Limitations 46

6.3 Computer Automation 47

6.3. 1 Computer Automated Equipment , 47

6.3.2 Computer Calibration Programs 49

7. UNCERTAINTY EVALUATION 51

7.1 Uncertainty Calculation from the Measurement Equation 51

7.2 Uncertainty Components in Test Detector Calibration 53

7.2. 1 Wavelength Uncertainty of the Monochromator 54

7.2.2 Effect of Bandpass 54

7.2.3 Ambient Temperature 56

7.2.4 Long-term Stability 56

7.2.5 Measurement Reproducibility 57

7.2.6 DVM Uncertainty 57

7.2.7 Spectral Stray-light 57

7.2.8 Other Components 58

7.3 Transfer from Cryogenic Radiometer to Working Standards 59

7.3.1 Visible Silicon Working Standards (Vis WS) Uncertainty 59

7.3.2 Near-Infrared InGaAs Working Standards (IGA WS) Uncertainty 61

7.3.3 Ultraviolet Silicon Working Standards (UV WS) Uncertainty 63

7.4 Transfer from Working Standards to Customer Detectors 65

7.4. 1 UV Silicon Customer Detector Example Uncertainty 67

7.4.2 Visible Silicon Customer Detector Example Uncertainty 68

7.4.3 Near-Infrared Customer Detector Example Uncertainties 70

7.4.4 Filtered Detector Example Uncertainty (Photometer) 76

7.5 Spatial Uniformity Measurement Uncertainty 78

8. QUALITY SYSTEM 79

8.1 Control Charts 79

8.2 Interlaboratory Comparisons 80

8.2.1 CCPR K2.a (NIR) 900 nm to 1650 nm 81

8.2.2 CCPR K2.b (Visible) 400 nm to 950 nm 81

8.2.3 CCPR K2.C (UV) 200 nm to 450 nm 82

8.3 Changes in the Spectral Power Responsivity Scale 82

8.3. 1 UV Scale (200 nm to 500 nm) 82

8.3.2 Visible Scale (350 nm to 1 100 nm) 83

8.3.3 NIR Scale (700 nm to 1800 nm) 84

vi
9. CHARACTERISTICS OF PHOTODIODES AVAILABLE FROM NIST 85

9.1 HamamatsuS1337-1010BQ 85

9.2 Hamamatsu S2281 87

9.3 OSI Optoelectronic UV-100 87

9.4 Detector Mounting Fixture Mechanical Drawings 89

10. FUTURE WORK 91

11. ACKNOWLEDGMENTS 92

12. REFERENCES 93

APPENDIX A: SAMPLE CALIBRATION REPORT A-1

APPENDIX B: BIBLIOGRAPHY B-1

vii
LIST OF FIGURES

Figure 2.1. The expanded uncertainties (k = 2) for spectral power responsivity measurements....

5

Figure 3.1. The geometry for detector spectral power responsivity measurements 11

Figure 3.2. Block diagram of photodetector substitution method 16

Figure 3.3. Block diagram of photodetector substitution method with monitor 17

Figure 4. 1 . Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR SCF) 20

Figure 4.2. The spectral power typically incident on the detectors 21

Figure 4.3. Ultraviolet Spectral Comparator Facility (UV SCF) 24

Figure 4.4. Electronic block diagram of the Vis/NIR SCF 27

Figure 4.5. NIST SCF precision transimpedance (I/V) amplifier circuit 27

Figure 5.1. Calibration chain for the spectral power responsivity measurements 29

Figure 5.2. The construction of the NIST Primary Optical Watt Radiometer (POWR) 31

Figure 5.3. The NIST L-1 ACR used in Spectral Irradiance and Radiance Responsivity

Calibrations using Uniform Sources (SIRCUS) 31

Figure 5.4. Comparison of POWR and the L-1 ACR measurements 32

Figure 5.5. Trap detector arrangement of photodiodes 33

Figure 5.6. Optical configuration for the SIRCUS facility 34

Figure 5.7. SIRCUS laser wavelengths used to calibrate the transfer standards 35

Figure 5.8. The calibration chain for the visible working standards (Vis WS) 36

Figure 5.9. The calibration chain for the ultraviolet working standards (UV WS) 37

Figure 5.10. The calibration chain for the InGaAs working standards (IGA WS) 38

Figure 5.11. The 1999 calibration chain for the InGaAs working standards (IGA WS) 39

Figure 6.1. Typical signals from Hamamatsu SI 337 and monitor photodiodes 41

Figure 6.2. Spectral responsivities of typical Si, InGaAs, and Ge photodiodes 42

Figure 6.3. Quantum efficiencies of typical Si, InGaAs, and Ge photodiodes 43

Figure 6.4. Spatial uniformities of typical Hamamatsu SI 337 and S2281 photodiodes 45

Figure 6.5. Spatial uniformities of typical OSI Optoelectronics UV-100 and EG&G Judson Ge

photodiodes 46

Figure 6.6. Vis/NIR SCF computer control block diagram 48

Figure 7. 1 . Error due to effect of bandpass for UV photodiode 55

Figure 7.2. Visible WS uncertainty 61

Figure 7.3. Near-Infrared IGA WS uncertainty 63

Figure 7.4. Ultraviolet WS uncertainty 64

Figure 7.5. Combined uncertainties for spectral power responsivity working standards 66

Figure 7.6. Example customer UV Si photodiode uncertainty..... 68

Figure 7.7. Example customer Si photodiode uncertainty 70

Figure 7.8. Example customer InGaAs photodiode uncertainty 73

Figure 7.9. Example customer TE cooled Ge photodiode uncertainty 75

viii
Figure 7.10. Example customer photometer uncertainty 77

Figure 8.1. Control chart example for a NIST Visible Working Standard (Vis WS) 80

Figure 8.2. NIST difference from KCRV in CCPR K2.b (Visible) Key Comparison 81

Figure 8.3. Change in the UV spectral power responsivity scale (UV WS) from 2001 to 2005

and expanded uncertainty (k=2) 82

Figure 8.4. Change in the visible spectral power responsivity scale (Vis WS) from 2005 to 2006

and expanded uncertainties (k = 2) 83

Figure 8.5. Change in the NIR spectral power responsivity scale (IGA WS) from 1999 to 2006

and expanded uncertainties (k=2) 84

Figure 8.6. Change in the NIR spectral power responsivity scale (Ge WS) from 1996 to 1999

and expanded uncertainties (k = 2) 84

Figure 9.1. Temperature coefficient of sihcon Hamamatsu SI 226 and SI 337 photodiodes 86

Figure 9.2. Linearity of Hamamatsu S1337-1010BQ at 633 nm 86

Figure 9.3. Linearity of UDT Sensors UVlOO at 442 nm 88

Figure 9.4. Responsivity dependence on bias voltage of UDT Sensors UVlOO at 442 rmi 88

Figure 9.5. Mechanical diagram of OSI Optoelectronics UV-lOO and Hamamatsu S2281

mounting fixture 89

Figure 9.6. Photograph of a sample NIST issued photodiode in mounting fixture, cap, and

wooden storage box 90

LIST OF TABLES

Table 2.1. NIST spectroradiometric detector measurement services 4

Table 2.2. NIST spectral power responsivity relative expanded (k = 2) uncertainties for typical

UV, visible, and NIR photodiodes 5

Table 7.1 Uncertainty components in photodetector calibration 53

Table 7.2. Visible working standard uncertainty (transfer from Vis Trap and Ge TS) 60

Table 7.3. Near-Infrared InGaAs working standard uncertainty (transfer from Vis WS, IGA TS,

and pyroelectric detector) 62

Table 7.4. UV working standard uncertainty (transfer from UV TS and Vis Trap) 64

Table 7.5 Relative combined standard uncertainties for NIST spectral power responsivity

working standards for the UV, visible, and NIR spectral regions 66

Table 7.6. Example uncertainty for customer OSI Optoelectronics UV-100 silicon photodiode 67

Table 7.7. Example uncertainty for customer Hamamatsu S1337-1010BQ or S2281 sihcon

photodiode 69

Table 7.8. Example uncertainty for customer InGaAs photodiode 72

Table 7.9. Example uncertainty for customer TE cooled Ge photodiode 74

Table 7.10. Example uncertainty for customer photometer 76

Table 7.11. Photodetector spatial uniformity measurement repeatability uncertainty 78

ix

1. Introduction

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

Measurement Service for spectral radiant power responsivity in the ultraviolet (UV), visible, and

near-infrared (NIR) spectral regions (200 nm to 1800 nm). This Measurement Service provides

calibrated silicon photodiodes (200 nm to llOOnm) and calibrated customer-supplied

photodetectors in the 200 nm to 1800 nm spectral region. This document supersedes NIST

Special Publication 250-41 (1998), "Detector Measurements: Part I - Ultraviolet Detectors and

Part II - Visible to Near-Infrared Detectors."

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

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

radiant power responsivity scale to the NIST reference absolute cryogenic radiometer and

detailed uncertainty estimates are discussed. Also presented are the spectral radiant power

responsivity measurement services provided by NIST and the quality system that complies with

ISO/IEC 17025 [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. NIST

is continually evaluating and improving the equipment, procedures, and services it offers. The

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

the spectral radiant power responsivity of photodiodes supplied by NIST to customers. The

procedures for characterizing customer supplied detectors are based on the procedures developed

for the NIST-fumished devices.

Note: This documentfollows the NISTpolicy of using the International System of

Units (SI). Only units of the SI and those units recognizedfor use with the SI are

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

SI values. The mechanical drawings in Sec. 9.4 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

spectral radiant power responsivity of photodetectors. The spectral radiant power responsivity is the ratio of the signal from the photodetector (amperes or volts) to the spectral radiant flux

(watts) incident on the photodetector. The spectral radiant power responsivity is often referred

to as spectral power responsivity or simply, spectral responsivity. Calibration of the spectral

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

Division and its predecessors for over 30 years.

Various techniques have been employed to determine photodetector 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 lasers [4. 5]

1
as the detector scale base. Relative uncertainties were reported on the order of 1 % to 3 %' over

the spectral range from 390 nm to 11 00 nm [6].

Note: This document conforms, to the ISO Guide to the Expression of

Uncertainty in Measurement [7], and follows the NIST policy [8-10] to use an expanded uncertainty with k=2 for the results of calibrations. Also, note that

uncertainty components are listed as standard uncertainties and expanded

uncertainty is used for the final results of calibrations.

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

[11, 12] and the subsequent introduction of 100 % quantum efficient (QE) photodiodes [13] 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, which is a light-trapping

device, constructed of three, windowless, UV inversion-layer 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 reverse-bias voltage

used. The spectral power responsivity was transferred to customers at this time by 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 ranged from 0.6 % to 4 %' over the spectral range from 250 nm to 1 100 nm [14].

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

Optronics QED-150^) was later used as the basis for the NIST spectral radiant power

responsivity scale [15]. Both second generation trap detectors are constructed with a different

type of silicon photodiode (Hamamatsu SI 337- 10 10). 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 [16]. Relative uncertainties were reported on the order of 0.2 % to 0.7%' over the spectral range from 250 nm to llOOnm (between 200 nm and 250 nm, the

relative uncertainty was reported as 3.5 %i)- The next advance came in the late 1980's when

cryogenic ESRs were reported with improved uncertainties over the 1 00 % QE detector-based

measurements [17-19]. The use of cryogenic ESRs in detector-based radiometry is discussed in

Refs. [20,21].

In the 1990's NIST improved and expanded the spectral responsivity measurements it provided

to its customers. The most significant change was to base the measurements on the NIST High

Accuracy Cryogenic Radiometer [22]. At the time, the HACR was the U.S. primary standard for

optical power. Today, a second generation reference cryogenic radiometer, the Primary Optical

'Expanded uncertainty with k = 2, converted from the originally stated uncertainty in "3 standard deviation

estimate" or "3a" used by NBS at the time.

^Certain 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.

^The QED 200 was originally manufactured by UDT Instruments; a part of United Detector Technology. UDT

Instruments was sold and became Graseby Optronics in Orlando, FL. UDT Instruments is now a part of Belfort Instruments with headquarters in San Diego, CA ( http:/7www.udlinstrumenis.com/).

2
Watt Radiometer (POWR), is the U.S. primary standard for optical power [23]. The spectral

responsivity scale [24] is transferred by various detectors, including second generation silicon

photodiode trap detectors, to working standards used with the NIST Visible to Near-Infrared [25]

and Ultraviolet Spectral Comparator Facilities (hereafter referred to as the Vis/NIR SCF and UV

SCF respectively in this document) where the Spectroradiometric Detector Calibration Service

measurements are performed. The next generation trap detectors, sometimes referred to as "tuimel-trap" detectors, [26] have external quantum efficiencies (EQE) (equal to Internal QE

times its reflection loss, defined to be 1 -reflectance) equal to 0.998 within 0.1 % between

500 nm and 900 nm [27].

3
2. NIST Spectroradiometric Detector Measurement Service

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

Technology Division. A complete listing of the calibration services offered by NIST can be

found in the NIST Calibration Services Users Guide NIST Special Publication 250 (NIST SP

250) on the NIST Calibration Services web page:

http://ts.nist.gov/calibrations .

The NIST Calibration Fee Schedule (SP 250 Appendix Fee Schedule) lists the current cost for all NIST calibration services. The Fee Schedule is typically updated in January and can be found on the above web page.

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; these services are for

unique customer-supplied test items.

The present spectral range for photodetector power responsivity measurements is from 200 nm to 1800 nm. Table 2.1 hsts the services offered along with typical measurement ranges and the

typical uncertainties of customer-supplied devices. All services listed are provided routinely.

The relative expanded uncertainties of the Spectroradiometric Detector Measurement Service are

listed in Table 2.2 and plotted in Fig. 2.1. See Sec. 7 for a detailed uncertainty explanation.

Table 2.1. NIST spectroradiometric detector measurement services

Service ID number

Item of test Range

Relative expanded

uncertainty (k = 2)

3907 IC Ultraviolet Silicon Photodiodes (OSI Optoelectronics UV-100) 200 nm to 500 nm 0.3 % to 4 %

39072C

Retest of Ultraviolet Silicon Photodiodes (UDT Sensors UVlOO

or OSI Optoelectronics UV-100)

200 nm to 500 nm 0.3 % to 4 %

39073C

Visible to Near-Infrared Silicon Photodiodes (Hamamatsu S228 1

)

350 nm to 1100 nm 0.2 % to 3 %

39074C

Retest of Visible to Near-Infrared Silicon Photodiodes (Hamamatsu S1337-1010BQ orS2281)

350nmto 1100 nm 0.2 % to 3 %

39075S Special Tests of Near-Infrared Photodiodes 700 nm to 1 800 nm 0.5 % to 4.6 %t

39077C UV to Near-Infrared Silicon Photodiodes (Hamamatsu S228I) 200 nm to 1100 nm 0.2 % to 4 %

39078C

Recalibration of UV to Near-Infrared Silicon Photodiodes (Hamamatsu S1337-1010BQ or S2281) 200 nm to llOOnm 0.2 % to 4 %

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

3908 IS Special Tests of Photodetector Responsivity Spatial Uniformity 200 nm to 1 800 nm 0.0024 % to 0.05 %^

'Depends on photodetector and signal level.

4
Table 2.2. NIST spectral power responsivity relative expanded ik = 2)

uncertainties for typical UV, visible, and NIR photodiodes.

Wavelength R-Clativc cxpa-iidcd unccrttiinly (A' — 2 1 r%i ^JX'"} rnml IJV '-^ V Vi'iihU- V 1 ri I U 1 ^ NIR

200 3 8 250 1.5 300 0.80 350 0 86 fl 77 400 0.74 0 38 450 0.36 0 24 500 0 96 0 77 550 0 20 600 0 20 650 0.20 700 0 20 U. JU 750 0 20 800 0 20 U.Ho 850 0 20 U.JO 900 0.20 0 62 950 0 20

1000 1.2 0 66

1050 2.0

1 100 3.0 0 62

1 150 1200 0 69 1250 \J .Jo 1300 ft ^9 1350 0 48

1400 0.48 1450 0.44 1500 0.42 1550 0.42 1600 0.5 1650 0.8 1700 1.9 1750 3.2 1800 4.6

UV SCF and Vis/NIR SCF Relative Expanded Uncertainties (k = 2)

5

0

• UV

= Visible A NIR

n

i. &

ii

&

(a) UV Relative Expanded Uncertainty (A = 2) J

(b) Vis Relative Expanded Unce """^^ / —

/

rtainty (A - 2) ^ Relative Expanded Un _ J:

&

"

it A

/

certaint> (A = 2) ^

200 400 600 800 1000 1200 1400 1600 1800 Wavelength |nm|

Figure 2.1. The relative expanded uncertainties (k = 2) for NIST spectral power

responsivity measurements in the (a) UV, (b) visible, and (c) NIR. Three

different detector types are used as working standards for the UV, visible, and

NIR regions.

5
Descriptions of each Service ID number are provided below.

3907 IC - UV Silicon Photodiodes

NIST will supply customers with an OSI Optoelectronics'* model UV-100 windowed silicon

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

includes the measured spectral power responsivity [A/W]^ from 200 nm to 500 nm in 5 nm steps.

The 1 cm^ photosensitive area of the photodiodes is underfilled for the measurements with a beam of diameter -1.5 mm. The spectral power responsivity is measured at radiant power levels

of less than 20 |liW. The relative expanded uncertainty ranges from 0.3 % to 4 %, depending on

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

measured at 350 nm.

39072C - Recalibration of UV Silicon Photodiodes

Recalibration of UV silicon photodiodes previously supplied by NIST (under 3907 IC) is performed by measuring spectral responsivity from 200 nm to 500 nm.

39073C - Visible to Near-Infrared Silicon Photodiodes

NIST will supply customers with a Hamamatsu model S2281 windowed sihcon photodiode

characterized in the visible to near-IR spectral region. The spectral power responsivity of the

photodiode is measured from 350 nm to 1 100 nm in 5 nm steps. The 1 cm"" photosensitive area of the photodiodes is underfilled for the measurements with a beam of diameter ~1 mm. The

spectral responsivity is measured at radiant power levels of less than 1 |liW. The relative

expanded uncertainty ranges from 0.2 % to 3 %, depending on the wavelength. The spatial

uniformity of responsivity over the photosensitive area is also measured at 500 nm.

39074C - Recalibrafion of Visible to Near-Infrared Silicon Photodiodes

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

39073C) is performed by measuring spectral power responsivity from 350 nm to 1 100 nm.

39075S - Special Tests of Near-Infrared Photodiodes

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

spectral power responsivity from 700 nm to 1800 nm. A beam of diameter ~1 mm is centered on

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

power levels of less than 1 |iW. The relative expanded uncertainty ranges from 0.5 % to 4.5 %

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

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

submitting a formal request.

39077C - UV to Near-Infrared Silicon Photodiodes

NIST will supply customers with a Hamamatsu model S2281 windowed silicon photodiode

characterized in the visible to near-IR spectral region. The spectral power responsivity of the

photodiode is measured from 200 nm to 1 100 nm in 5 nm steps. The 1 cm' photosensitive area

OSI Optoelectronics in Hawthorne, CA. acquired UDT Sensors, Inc. ( http://w'W"W'.udt.coni/).

^ 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
of the photodiodes is underfilled for the measurements with a beam of diameter ~1 mm. The

spectral power responsivity is measured at radiant power levels of less than 1 |iW. The relative

expanded uncertainty ranges from 0.2 % to 4 %, depending on the wavelength. The spatial

uniformity of responsivity over the photosensitive area is also measured at 500 nm.

39078C - Recalibration of UV to Near-Infrared Silicon Photodiodes

Recalibration of UV to near-infrared silicon photodiodes previously supplied by NIST (under

39077C) are performed by measuring spectral power responsivity from 200 nm to 1 100 nm

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 jiW. 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 Sec. 2.4 before

submitting a formal request.

3908 IS - 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 \iW, and a beam of diameter mm in the 400 nm to 1800 nm spectral region at power levels

less than 1 luW. 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 Sec. 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 in the NIST

calibration services. 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 lO^V/A to lO^V/A is typically required; thus the photodiode dynamic impedance (shunt

resistance) must be greater than 10 kQ. (See Sec. 6.1.1 for more detail.)

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.

The photodetector signal (output current) connection must be clearly identified and preferably

provided by a 50 ohm coaxial Bayonet Neill Concelman cormector (BNC) connector. The

radiometer or photometer must have an analog output voltage. Instruments without an analog

output, having only digital display (or analog meter) and/or only digital interface, will not be

accepted.

7
How to Order Calibrations

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

Details of Service ID numbers 39075S, 39080S, and 3908 IS should be discussed with one of

the technical contacts hsted in Sec. 2.4 prior to submitting a formal request.

2) Purchase orders should be sent to:

3) Include the following information on your purchase order:

A) Service ID number(s)

B) Manufacturer, model, and serial number of the test item(s)

C) User's name (technical contact), phone number, and email address

D) Name and mailing address of the person to receive the calibration report(s)

E) Billing address

F) Shipping address

G) Instructions for return shipment (the fee quoted does not include shipping costs)

[Note: If nothing is stated, NIST will return by common carrier, collect, and uninsured.]

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

customers are responsible for all shipping costs.

5) NIST policy requires that non-US customers prepay for calibration services. Please

contact Calibration Services to arrange for prepayment.

6) Please see the NIST Calibration Services web page (http ://ts. n ist. gov/calibrations) for a complete list of NIST calibration policies.

7) All test items should be shipped to the following address:

Jeanne Houston or Thomas Larason

NIST

Building 221 / Room B208

100 Bureau Drive, Stop 8441

Gaithersburg, MD USA 20899-8441

Calibration Services

National Institute of Standards and Technology

100 Bureau Drive, Stop 2300

Gaithersburg, MD USA 20899-2300

Phone number:

FAX number:

E-mail:

^ Web page:

(301)975-2092

(301) 869-3548

calibrations@nist.gov

http://ts.nist.gov/'calibrations

8
2.4 Technical Contacts

For technical information or questions contact:

Jeanne Houston (301) 975-2327 email: ieanne.houston(a-;nist.^ov

Thomas Larason (301) 975-2334 email: thomas.lara$on((6:nist.gov or fax (301) 869-5700

Technical information can also be found on the following web page:

http ://phvsics.nist.gov/photodiode

9
3. Measurement Theory

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

for the services outlined in this pubhcation. The measurement equation is presented and the

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

the uncertainties in Sec. 7. The calibration method, detector substitution, is described first in

general and then in detail.

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 associated assumptions and approximations. The measurement

equation is also fundamental to the analysis of the uncertainty of the measurement. In essence,

the detector comparator instrument is similar to a spectroradiometer but compares the response

of detectors rather than the irradiance of 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 [28] and is similar to the development of Eq. (7.18) in Ref [29] for spectral

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

for spectral responsivity is:

V(A,AX,A,)= llE,(x,y,A„A)-s{x,y,A)-6A-dA, (3.1)

AAA

where V{A,AA,Af^) is the output signal (typically in volts or amperes); E^{x,y,AQ,A) is the

spectral irradiance function in X of the comparator system at detector position x,y for a wavelength setting of Ao; s{x,y,A) is the spectral radiant power responsivity of the detector; A is the area of the radiant power beam at the detector (See Fig. 3.1.); and is the wavelength

interval for which the value of Ex is not zero (i.e., the full-width bandpass). This equation is equivalent to Eq. (7.1) [29], with some minor changes in the notation, where A and AA are left symbolically in V to indicate that V depends on how these elements are chosen.

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

s{x,y,A) = s{A) . (3.2)

Thus, the responsivity s is no longer dependent on position (x,y) and can be removed from the area integral:

V(A,AA,A,)= js{A)- jE,{x,y,A„A)-dA-dA. (3.3)

AA A

10
The area integral of irradiance gives the spectral radiant power function of the comparator

system at the detector:

^D,x(^'^)= lE,{x,y,AoaydA = T(Ay0,(A„A), (3.4)

A

where 0^(1^, A) is the output power from the monochromator and t{A) 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{AA,A^)= ^s{A)-T(A)-0^(A,,A)-dA. (3.5)

Equation (3.5) is the power (or flux) equivalent to Eq. (7.1b) [29]. Introducing the slit-scattering

function z(Aq -A) to the measurement equation allows the spectral radiant power function of the

monochromator 0^(Aq,A) to be written as the product of two functions (with appropriate

normalization):

0,(A„A)^z(\-Ay0,,(A), (3.6)

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

wavelength setting of the monochromator and the wavelength of the flux (power) and the

"spectral flux" factor 0^^ is dependent only on the wavelength of the flux. The factor <Z>,-, is

the spectral radiant flux at Xq, 0^{Aq), and is equivalent to the responsivity factor / introduced in

11
Eq. (7.12) [29]. Both the sHt-scattering function z{Aq-A) and the factor can be determined

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

(power) of the monochromator. The output signal Vcan now be expressed as:

F(AA,AJ= j^(A)T(/L)-z(^-/l)-0f,(/L)-dA. (3.7)

Equation (3.7) is the power equivalent to Eq. (7.13) [29] and is in a form that can be used for

finding the spectral power responsivity of a detector. It is a measurement equation for a

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

product s T-0^y^ is approximately constant (or linear) over the spectral range for which

z(/iQ -X) is significant (i.e., within AA) and z is approximately symmetrical with respect to Xq,

Eq. (3.7) can be written:

v{^x,x,)^s{x,)T[x,)0,^M (3.8)

A/i

Equation (3.8) is the equivalent flux version of Eq. (7.18) [29]. Implicit in this derivation is the

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

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

z{A,q -X) need only be measured once. Optical aberrations, scattering, and diffraction are also

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

bandpass, AA, or Xq of the monochromator is varied over a beam of fixed wavelength X.

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 Ai which is scattered into the

exit beam path and which contributes to the measured signal. Adding a stray-light term,

F,,(Ai,/Lo),to Eq. (3.8) gives:

v{^x,x,)=s(x,)T{x,)0,^M \z{x,-X)dx + vS^x,x,), (3.9)

where V^,(^X,X^)= ^s{X)-z{X^- X)-T(X)-0,y^(X)-dX . (3.10)

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 Vs\ term in Eq. (3.9) is typically small for radiometric measurement systems

(especially m a system using a double monochromator) and is normally ignored in the "routine"

measurement equation but is included in the uncertainty estimate calculations in Sec. 7.1.2.

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

factor 0^^ to give an expression for the power leaving the monochromator 0[

12
A/i

(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:

V(AA,A,) = sM-tM-0'M. (3.12)

Before extending the generalized measurement equation, Eq. (3.12), to the experimental

protocols used in the comparator facilities, it is important to reiterate the simplifying

assumptions that were made in its development.

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

[28] 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 will be polarized and the effect of

polarization on the responsivity of the detectors needs to be evaluated. However, polarization is not a problem when 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 radiadon 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^ photodiodes

NIST provides meet these requirements.

Humidity can affect the power reaching the detectors by changing the transmittance of the

system, but this does not affect the measurements since the optical path length is constant during

the measurement. Thus, if the humidity change is small over the time of the measurement, any

change in the power reaching the detectors is canceled out of the measurement. Humidity can

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

the absorption of water by the SiOi surface passivation layer changes the photodiode reflectivity,

which changes the detector responsivity [30]. Typically, for a windowed detector in the

laboratory, an effect due to water absorption (onto the detector or window) is not obser\'ed.

The temperature variation of the laboratory is small (typically < 0.5 °C) over the measurement

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

region. The temperature dependence can be applied as an additional uncertainty term when it is of concern, as near the band-edge of a semiconductor.

13
Diffraction effects can be estimated from the relationship [31, 32]:

d'=2A4--, (3.13)

d

where 6^ (in radian) is the diffraction angle (for the first Airy disk), X 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 /I ~ 2 |Lim andd- 1 mm which gives:

=2.44 x-i^ = 4.88x10 ' «5mrad. (3.14)

1 mm

The Vis/NIR SCF has a focal ratio (/^number or //#) of ~ 9 giving a beam angle of =^ 110 mrad

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

J~ 1.5 mm, and //#= 5) the diffraction effects are even smaller due to the shorter wavelengths

of the Hght. 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 [33] and are ignored in

the analysis of these comparator systems.

The responsivity, s, is assumed uniform throughout the area A of the incident beam on the

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

optically via imaging optics) so that s is uniform over A. This limits the amount of flux (power)

that can be delivered to the detector but this does not hinder the responsivity measurements for

typical photodiodes. Deviations from this approximation are treated as uncertainty terms.

Certain assumptions were made about the comparator system itself, [29] primarily the

monochromator. Two assumptions are that the dispersion remains the same for all wavelengths

and that the slit-scattering function z{\ - A) does not change with the wavelength setting, Xq, of

the monochromator. These approximations are not included in the uncertainty analysis because

their combined effect was determined to be negligible. This allows z(/Iq -A) to be measured

only once for an instrument and used over the entire spectral range. Another assumption is that

optical aberrations, scattering, and diffraction are the same whether the wavelength /I of a monochromatic source is varied over the monochromator bandpass AA or whether the

monochromator wavelength setting ao is varied over a monochromatic source of wavelength X. Also z{Aq-A) is assumed approximately symmetrical with respect to Aq; this assumption has

been experimentally verified for the two monochromators described in the spectral comparator

facilities.

The product s-r- 0^^^ is assumed to be linear over the monochromator bandpass, Al. In practice,

this is effected by reducing the slit widths until 5 -0^^^ is linear over AA. Reducing the bandpass

also reduces the amount of optical power (flux) that can be delivered to the detector, but this is not a limitation for typical measurements. Most measurements are made using a broadband

source to provide a wide range of measurement wavelengths. However, the assumption that

s 0f y is linear over AA may not be valid with some arc and discharge sources (primarily used in

14
the UV) that can have strong spectral variations in output intensity. Typically, the variations are small and are treated as uncertainty terms.

Stray-light is light from outside the monochromator bandpass Ai that is scattered into the beam.

The amount of stray-hght depends greatly on the design of the monochromator. The amount of

stray-light in the transmitted flux is typically negligible for double monochromator systems

(systems with two dispersing elements), but not for many single monochromator systems. The

two monochromators in the comparator facilities have double dispersion elements, prism-grating

or double-grating systems. Because the stray-light in the comparator systems is small in magnitude, it will not be included as a correction term in the measurement equation, but will be

evaluated and treated as a term in the uncertainty budget described in Sec. 7.

Spatially-scattered light is light that is scattered outside of the beam area, A. The amount of

spatially-scattered light depends on the quality and condition of the optics in the beam path.

Errors caused by spatially-scattered light cancels if the test and standard detectors have the same

size or are much larger than the beam area, A. It can be significant when the test detector area is small and approaches the diameter of the beam. For this reason, detectors with diameters of less

than 3 mm are not accepted for calibration. This error is evaluated and corrected if the detector

diameter is less than 5 mm. The uncertainty of this correction is included in the uncertainty

budget in the UV and Vis/NIR SCF measurements.

3.2 Substitution Method

The substitution method transfers the responsivity (output divided by input) of a standard

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

discussed in general and applied specifically to photodetectors. In this section, a measurement

equation will be developed for the substitution method. The general assumptions made in its development and their consequences are then discussed. Finally, the measurement equation as applied to the NIST Spectral Comparator Facilities is developed.

3.2.1 General Substitution Method

The output (signal) Y from a detector (or system) with a linear response can be written in

general:

where 7x is the signal (e.g., in ampere) from the detector, .^x is the responsivity of the detector

(e.g., in AAV), and Xy, is the input to the detector (e.g., in W). Similarly, the signal from a standard detector with a known responsivity 5s is given by:

(3.15)

(3.16)

Dividing Eq. (3.15) by Eq. (3.16) we have:

(3.17)

15
If the input X to each system is assumed to be constant (this is the basis of the substitution

method), solving for the detector responsivities Sx gives:

(3.18)

Thus, using the substitution method, the standard detector responsivity 5s is scaled by the ratio of

the outputs. In essence, the standard detector quantifies the input power X [W] 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. A block diagram of the photodetector substitution method is shown in Figure 3.2. The

radiant power (flux) <t> is the output power from the source that enters the monochromator. The

spectral radiant power is the output power from the monochromator and r is the transmittance

of any optics (and the atmosphere) between the monochromator and the detector. The spectral

radiant power received by the detectors is (P^-^^ =t-0^.

Source

Monochromator

(?i selection)

Optics Test

Standard

Figure 3.2. Block diagram of photodetector substitution method.

Using Eq. (3.18) assumes that the source is stable over the comparison time,

true for incandescent sources (i.e., quartz-halogen (QTH) lamps), used

comparator, but less true for arc sources, used in the UV comparator.

This is generally

in the VIS/NIR

Substitution can be done in two ways. One way is to substitute the test detector and standard

detector at each wavelength and another is to repeat the wavelength scan for each detector. The

former method has an advantage that the results are not affected by the drift of the source between wavelength scan, but the measurement is slower. The latter method is subject to possible drift of the source during the wavelength scan, but the measurement is faster. The latter method is employed in the NIST SCF facilities, with the addition of a monitor detector as described in the next section. The stability of the source can be the limiting factor in the signal

noise of the detectors.

16
3.2.3 Substitution Method with Monitor

Measurement errors due to source power fluctuations are minimized by introducing a beamsplitter and monitor detector into the measurement setup. Figure 3.3 illustrates the

photodetector substitution method with monitor. Here 0^ x - ^bs ' ^ ' ^) is the power received by

the detectors and 0^^ ^^ = Pbs " ^ " ^x the power received by the monitor. The transmittance and

reflectance of the beam splitter are ibs and pbs, respectively. The monitor detector and

beamsplitter are assumed to be stable (constant) over the time of the measurement. Thus the

monitor signal can be used to normalize the signals from the test and standard detectors. That is, the signal ratio of the test detector to monitor or standard detector to monitor will be constant

regardless of the source fluctuations and the signal levels. By simultaneously measuring both

the monitor detector signal and test detector or standard detector signal, source fluctuations on a time scale greater than the signal sampling rate are effectively eliminated.

Source

0

Monochromator

(k selection)

01

Optics

r0K

Beam

Splitter

0u).=phs-r0\

Test

Monitor Standard

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

3.2.4 Measurement Equation Applied to the SCFs

The actual measurement equation for the SCFs is now developed. Dropping the X notation and

changing the sub- and superscripts for clarity from Eq. (3.12), it can easily be shown that the

total signal, Ft,x, from a detector x and an amplifier is:

V,.-s^-G^-T^-0^+V^^, (3.19)

where 5x is the detector spectral power responsivity in A/W, Gx is an explicit gain term for a transimpedance amplifier in V/A, Tx is the transmittance of any optics (and the atmosphere)

between the monochromator and the detector, 0^ is the output power from the monochromator in W, and Fd,x is the dark signal (sometimes called the "output offset voltage") in V, i.e., the signal

measured when no flux is incident on the detector. In practice Fd,x is found by measuring the

signal from the detector with the shutter closed at the exit slit of the monochromator. For the

UV and Vis/NIR SCFs there is negligible difference between the background signal (with the source blocked before the monochromator and shutter open) and the dark/offset signal (with the

shutter closed). The net signal (or just signal) is the signal due to the radiant power recei\ ed

by the detector:

V.-V^.-V,,=s^-G^-T^-0^. (3.20)

17
This is the "routine" measurement equation for the substitution method depicted in Fig. 3.2. The

measurement equation for the "substitution method with monitor" (Fig. 3.3) is written by

expHcitly including the transmittance ibs and reflectance /?bs of the beam splitter assembly (which

also contains the reflectance of the monitor turning mirror). Implicit in this equation is the

assumption that the path length from the beam splitter to the monitor is the same as the beam

splitter to the working standard or test detector. Both comparator systems record simultaneous

measurements taken with both the detector and monitor. The signals from the test detector x and

monitor detector mx are:

^=^.x-^x=^x-Gx-^bsx-^x-^x, (3.21)

and = K,^^ - „^ = s^^ • p^sx ' ^x " ^x • (3-22)

The ratio of these two signals is: R^=^= ^x-^x-^^x-r.-^.

^ (3.23)

^mx "^mx -^^mx " Asx * ^x ' ^x

where t^^^^ • ' ^x the fraction of power (flux) received by the test detector, and yc^sx " ^x ' ^x

the fraction of power (flux) received by the monitor.

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

V <i -G -T T -0

r Jl _ _ "^s '^s ^bss (3 24")

X rr ^ JC^ ^

' ms '^ms ms /^bss s

Dividing Eq. (3.23) by Eq. (3.24) (taking the ratio of the ratios) gives:

^x _ '^x-<^x-^bsx-^x-^x -^ms-^^ms-Ass-^s-^s

^mx -^^mx • Asx -^x ' ^x h ^, ' ^bss

'

(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 transmittance and

reflectance, monitor detector responsivity, and amplifier gain are stable (constant) over the

comparison time, that is, r,,, ^r,^^, Asx = Pbss ^ = ^ms and G^, =G„^, these terms cancel,

leaving:

K^s_^:G^

(3.26)

Solving Eq. (3.26) for the test detector spectral responsivity 5x and substituting the signal

measurements into the ratios we have:

18
Vmx _ Gs (3.27)

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

(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 for spectral

responsivity, 5x, (in AAV) becomes:

where Cbw is a correction term due to the bandpass of the monochromator and is referred to as the bandpass error. Like the stray-light term, the bandpass error is small and can be ignored in

the "routine" measurement equation. The bandpass error is described in Sec. 7.2.2 as an

uncertainty term.

bw ' (3.28)

19
4. Equipment Description

In this section the components and associated electronics of the Visible to Near-Infrared Spectral

Comparator Facility (Vis/NIR SCF) and Ultraviolet Spectral Comparator Facility (UV SCF) are described.

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

The Vis/NIR SCF is a monochromator-based system that typically measures the spectral power

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

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

700 nm to 1800 nm using indium gallium arsenide (InGaAs) photodiodes as working standards.

The spatial uniformity of a detector's power responsivity can also be measured.

The Vis/NIR SCF uses 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 1 00

W quartz-halogen lamp is used as the source in the Vis/NIR SCF. A shutter is located just after

the monochromator exit slit. A monitor detector located after the monochromator measures

source fluctuations. The detectors and the exit optics are enclosed in a light tight box. A

diagram of the Vis/NIR SCF is shown in Fig. 4. 1 . Each of the primary components is now

described in greater detail.

Light Tight Enclosure

Beam

Splitter

Monitor

jcSx. Detector

013 C ^ ~ J?

Prism - Grating

Monochromator

Shutter

Sources

Figure 4.1.

SCF).

Visible to Near-Infrared Spectral Comparator Facility (Vis/NIR

20
4.1.1 Vis/NIR Source and Input Optics

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

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

integrating sphere with a spectral-line discharge lamp input is used as a source for cahbrating the

wavelength scale of the monochromator. A third source position is available for investigational

use. A helium-neon (HeNe) alignment laser can be used as a source in the Vis/NIR comparator

as well. The spectral output power (at the detector) of the 100 W quartz-halogen lamp with the

Vis/NIR SCF monochromator is shown in Fig. 4.2. 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.

Safety considerations

Quartz-halogen lamps, lasers, and arc sources are potential eye hazards. The HeNe lasers used

for alignment are Laser Safety Class II. Care is taken to never look directly into any of the sources or the laser beam. Shields mounted on the sides of the optical table inhibit direct visual

contact with the sources used with the comparator. Protective eyewear is worn when working in

proximity to these sources.

10

5 0.1

01o

0.

1385 nm

(a) Vis/NIR SCF (100 W Quartz-Halogen Lamp)

0.01 I I I' I I I I I

I I I I I I I 1 I—I I—I—I—I—I—I—I—I—I—

L

200 400 600 800 1000 1200 1400 1600 1800

Wavelength (nm)

Figure 4.2. The spectral power typically incident on the detectors in the (a)

Vis/NIR SCF using the 100 W quartz-halogen lamp and (b) UV SCF using the

argon mini-arc as a source.

21
4.1.2 Vis/NIR Monochromator

The visible comparator uses a modified Applied Physics Corporation Cary-14 prism-grating

monochromator. The Cary-14 employs a 30° fused silica prism in series with a 600 lines per

millimeter echelette grating. The monochromator' s spectral range is 186nm to 2.65 \im, the

spectral range used in the Vis/NIR SCF is 350 nm to 1800 nm. In the typical measurement

configuration, the monochromator slits are set to 1 . 1 mm, giving an instrument FWHM bandpass

of 4 nm. A circular 1 . 1 mm diameter aperture located just after the monochromator exit slit determines the beam size. The exit beam is //9. The monochromator has a stray-light rejection

of 10"^. Greater than 99 % of the beam power lies within an area of 1 .5 mm diameter around the

optical axis.

4.1.3 Vis/NIR Shutter and Output Optics

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

are used to optically fold the system and are not shown in Fig. 4. 1 . Mirrors do not produce

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 [34].

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 |im and an accuracy of 0.2 )nm per 100 mm. The

vertical stage travel range is 50 mm with a manufacturer specified resolution of 0. 1 jim and an

accuracy of 0.25 |Lim per 25 mm.

Each detector can be manually translated along the Vis/NIR SCF optical axis for focusing. A

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

optical axis for alignment.

4.1.5 Vis/NIR Working Standards

The visible working standards (Vis WS) are four Hamamatsu model SI 337-101 OBQ sihcon

photodiodes with ftised quartz windows. This is a p-n photodiode with a 1 cm" active area. The

Hamamatsu SI 337 diode is a popular diode for radiometric standards, and the linearity,

uniformity, and stability of the Hamamatsu SI 337 diode has been well documented [35, 36].

The Vis WS are used for measurements over the spectral range from 350 nm to 1 100 nm.

For near-infrared measurements (700 nm to 1 800 nm), four GPD Optoelectronics Corporation

(formerly Germanium Power Devices) model GAP5000 InGaAs photodiodes mounted in

temperature-controlled housings are used as working standards (IGA WS). These photodiodes

have a 5 mm diameter active area.

22
Typically, two working standards are used for each spectral comparison measurement. The

second working standard can be used as a "check" standard to verify that the measurement

process is operating normally, or is "in control."

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 Sec. 3.2.3. The beam splitter is a 50.8 mm x 50.8 mm flat fused quartz plate. A

35 mm x 46 mm flat oval mirror is used to optically fold the beam reflected off the beam splitter to a convenient physical location for the monitor detector. Hamamatsu S1337-1010BQ silicon

and EG&G Judson J16TE2-8A6-R05M-SC germanium photodiodes are used as monitor

detectors in the visible and NIR spectral regions, respectively. Ideally, the monitor detector has

the same spectral responsivity as the working standard used for the measurement. But for the

NIR region a small stray-light uncertainty component is included because of the IGA WS and Ge

monitor.

4.1.7 Alignment Lasers

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

positions of the monochromator entrance and exit slits). 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

Fig. 4.1.

4.1.8 Enclosure

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

This is essential because the routine measurements are all dc (optically unmodulated). The

enclosure is essentially a box that sits on the optical table and covers the exit portion of the

monochromator, shutter and output optics, and translation stages. The test detectors, working

standards, monitor detector, amplifiers, and other associated electronics are also housed inside

the enclosure. Doors allow easy access to the equipment and detectors.

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

exhaust fan is used to keep the temperature and humidity inside of the enclosure as close to that

in the laboratory as possible. The temperature and humidity are monitored inside the enclosure

and in the laboratory.

4.2 Ultraviolet Spectral Comparator (UV SCF)

The UV SCF is a monochromator-based system that measures the uniformity and spectral power

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 Fig. 4.3.

23
Light Tight Enclosure

UV WS Detector

Carousel

Beam Splitter

Monitor

Detector

Order

Sorting

Fnterji

tJ

Shutter

QJ U

> Q

Double Grating

Monochromator

Sources

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; 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. Motorized

translation stages position the test detector in the horizontal and vertical planes while the

working standards are positioned manually.

4.2.1 UV Source and Input Optics

The UV SCF 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 [37]. The argon mini-arc is an intense, uniform UV

source, with argon gas flowing through the arc structure at ~ 35 kPa. The arc can be operated

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

normal facility 5.6 °C chilled water. The water runs through plastic, non-conducting tubing. An

integrating sphere with a spectral-line discharge lamp input is used as a source for calibrating the

wavelength scale of the monochromator. A third source position is available for investigational

use. A helium-neon (HeNe) alignment laser can be used as a source in the UV comparator as well. The spectral radiant power at the detector on the UV SCF with the argon mini-arc source

is also shown in Fig. 4.2.

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.

24
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. The HeNe alignment lasers are Laser

Safety Class II. Shields mounted on the sides 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 electrodes 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. Non-conducting plumbing is always used.

4.2.2 UV Monochromator

The UV comparator uses a Spex 1680, 1/4 m double grating monochromator with 2840 lines per

millimeter 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 /15. Greater than 99 % of the beam power lies within an oval area of

diameters 2.0 mm and 2.5 mm around the optical axis.

4.2.3 UV Shutter and Output Optics

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

One 15.24 cm diameter spherical mirror and one 7.62 cm diameter flat mirror images the exit slit 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 ^m and an accuracy of 0.25 \im per

25 mm. The test detector can be manually translated along the optical axis for focusing.

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

Each detector can be manually 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.

4.2.5 UV Working Standards

The ultraviolet working standards (UV WS) are two UDT Sensors, Inc.*^ model UVlOO silicon

photodiodes with quartz windows and 1 cm/ circular active areas. The UVlOO is an inversion

6 UDT Sensors is now OSI Optoelectronics, 12525 Chadron Ave., Hawthorne, CA 90250, USA.

25
layer diode with enhanced resistance to UV radiation damage. Typically two working standards

are used for each spectral comparison measurement. The second working standard can be used

as a "check" standard to verify that the measurement process is operating normally, or is "in

control."

4.2.6 Beam Splitter and Monitor Detector

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

4.2.7 Alignment Lasers

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

comparator. The second laser is not shown in Fig. 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.)

26
Test Reports Computer

Instrument Control and Data Stroage Computer

Data Analysis

Computer

TE

Temperature

Controller

DVM with

Multiplexer

InputDVM

DVM

Other

Instruments

I t

i .

! 1

Monitor TE

Temperature

Controller

:><j-:-l

Monitor

Amplifier

Amplifiers

-•^ii

Temperature

Monitoring

Input

InGaAs

Wurkintf Standard

InC/aAs Working Standard

Silicon Working Standard "^^^ Silicon Working Standard

Test Detector

Test Detector

Silicon Monitor

Germanium Monitor

Figure 4.4. Electronic block diagram of the Vis/NIR SCF.

The amplifiers are identical in design and operate with variable gains ranging from 1 0"* V/A to lO^V/A. References [3, 38] 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.

\k

Input High I

Input Low <

100 pf

10 kQ 90 kQ 900 kn 9 MQ

}tCIZMZI>1-|

Output

Output Common

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

27
The SCFs are typically operated as described above (DC mode), but with the addition of a chopper in the optical beam, AC measurements are possible for special tests. AC measurements

are not discussed in this publication; see Ref. [39] for a detailed discussion of AC measurements.

4.3.2 Electronics - Auxiliary Equipment

Temperature-controlled photodiodes can be used which require thermoelectric (TE) temperature

controllers shown in Fig. 4.4. The following electronics not previously mentioned are also used

routinely: a 4-channel TE temperature controller for the InGaAs working standards; a standalone TE temperature controller for the germanium monitor; a digital I/O control unit for the

optical shutter; and a laboratory environmental monitoring system. The environmental

monitoring system measures the temperature and relative humidity in the laboratory and in each

comparator enclosure.

28
5. Spectral Power Responsivity Scale Realization

This section describes the determination of the NIST spectral power responsivity scale and its traceability to a primary standard. The propagation of the scale, the operation of a cryogenic

radiometer, and the calibration of the transfer standards are briefly reviewed. Then the

propagation the scale from the transfer standards to the working standards is described. Scale

realizations are scheduled every 12 to 18 months.

5.1 The Spectral Power Responsivity Scale Measurement Chain

The spectral radiant power responsivity measurement chain is shown in Fig. 5.1. The top of the

chain is the NIST-designed reference cryogenic radiometer called the Primary Optical Watt

Radiometer (POWR), which serves as the base of optical power unit. The second cryogenic

radiometer, the L-1 Absolute Cryogenic Radiometer (ACR) from L-1 Standards and Technology,

Inc. was verified for its low uncertainty by comparison with POWR. The L-1 ACR serves as the

secondary standard to calibrate transfer standard detectors at the Spectral Irradiance and

Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility [27] in the

200 nm to 1 800 nm region. The transfer detector is used to calibrate working standard detectors

at the Spectral Comparator Facilities (Vis/NIR SCF and UVSCF), and then customers' detectors

are routinely calibrated against the working standard detectors at the SCFs.

Primary Standard

(POWR)

1 SIRCUS

Secondary

(L-1 .

1 Standard

\CR)

SIRCUS

Transfer

Dete

Standard

jctor

1 SCF

Working Standard

Detector

I

SCF

Customer (Test)

Detector

Figure 5.1. Calibration chain for the spectral power responsivity measurements.

29
The POWR and L-1 ACR are described in Sec. 5.2. Transfer standards (TS) are detectors

designed to transfer the spectral radiant power responsivity scale from the cryogenic radiometer

to working standard (WS) detectors at the SCF. Transfer standards must measure the same laser sources as the primary standard and also operate at much lower power levels in the lampmonochromator-based SCF. Also, windowed photodiodes used as working standards at the SCF

tend to have interference problems with the coherent laser emission used with cryogenic

radiometers. Transfer standards are required to be stable only during the transfer measurements.

Working standards, on the other hand, must maintain their responsivity scale for an extended

period of time (between recalibrations) since they are used to calibrate customers' detectors

throughout the year. The uncertainties in the scale realization and dissemination from the

primary standard cryogenic radiometer to the working standards used in the SCF are described in

Sec. Error! Reference source not found..

5.2 Cryogenic Radiometry

Cryogenic radiometers provide an absolute basis for optical power (flux) measurements at the

lowest possible uncertainties. They are used as primary standards for optical power at many

other national laboratories as well [35, 40-45]. A cryogenic radiometer is an electrical

substitution radiometer (ESR) that operates by comparing the temperature rise induced by optical

power absorbed in a black receiving 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 electrical power, watt, via voltage and resistance standards maintained by

NIST^. There are several advantages to 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 ~ 10^, 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 nonequivalence of optical and

electrical heating resulting from heat dissipated in the wires. Consequently, most electrical

substitution radiometers, including NIST-maintained ESRs, operate at cryogenic temperatures.

The Optical Technology Division within NIST presently has two cryogenic radiometers that

provide the basis for the spectral radiant power responsivity scale: the NIST-designed POWR

(Figure 5.2), and the L-1 ACR [23] (Figure 5.3). The POWR is the primary U.S. national

standard for the unit of optical power. It has the capability to optimize its configuration for

measurements in different spectral regions and for different input laser power levels. For

optimized noise performance, it can operate at temperatures as low as 1.7 K for extended

periods. The L-1 ACR is also an absolute radiometer, but one whose operation is optimized for

the |LiW to mW power levels in the UV to NIR spectral region. In comparison with POWR, the

L-1 ACR is compact and mobile, which makes it a convenient instrument to use for scale

transfers. The relative combined standard uncertainty of the NIST cryogenic radiometer

measurements range from 0.01 % to 0.02 % in the visible region of the spectrum [27]. The

largest components of the uncertainty are those due to the systematic correction for the Brewster

angle window transmittance and the nonequivalence between electrical and optical heating. An

Electrical standards are maintained in the Electronics and Electrical Engineering Laboratory at NIST.

30
intercomparison between POWR and the L-1 ACR showed that these two standards agreed to within 0.02 %, which is within their combined uncertainties as shown in Figure 5.4 [23].

Figure 5.2. The construction of the NIST Primary Optical Watt Radiometer

(POWR).

Figure 5.3. The NIST L-1 ACR used in Spectral Irradiance and Radiance

Responsivity Calibrations using Uniform Sources (SIRCUS).

31
4

<>

O 488 nm

514 nm

O 633 nm

L-1 POWR

Figure 5.4. Comparison of POWR and the L-1 ACR measurements showing the

agreement between the ACRs. The error bars are the measurement uncertainty.

5.3 Calibration of Transfer Standards with a Cryogenic Radiometer

The cryogenic radiometers described above use lasers as their source and a variety of transfer

detectors to disseminate the spectral power responsivity scale. Historically, the scale was

realized using the High Accuracy Cryogenic Radiometer (HACR) [25] at nine discrete laser lines

in the visible wavelength range. A physical model was developed to interpolate the responsivity

of silicon trap detectors over the spectral range from 405 nm to 920 nm [24]. Outside of this

spectral range, the detector responsivity scale was based on pyroelectric detector with a

spectrally flat responsivity [21]. While the pyroelectric detector had a spectrally flat responsivity, its absolute responsivity value was low. While it could extend the scale, its noise

performance dramatically increased the uncertainties in the UV and the NIR spectral regions

because of the low flux available on the comparator facilities (see the 1998 version of this

document [46] for more information). The UV responsivity scale uncertainty was improved by

calibrating the UV WS at the NIST Synchrotron Ultraviolet Radiation Facility (SURF III) with an ACR-monochromator system [47, 48]. The NIR responsivity scale uncertainty was reduced

by more than a factor of two by also calibrating the pyroelectric detector with the SURF ACRmonochromator system [49].

In order to reduce the overall uncertainties in the scale, especially in the UV and NIR regions,

the HACR was replaced by POWR, the newly designed cryogenic radiometer described in

Sec. 5.2, and a new measurement protocol was developed with the L-1 ACR, the secondary

cryogenic radiometer. The cryogenic radiometers were installed in a new laser facility called the

Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS)

[27]. The SIRCUS facility has tunable laser sources that cover the range from about 200 nm to over 5 [im. Installing the cryogenic radiometers in the tunable laser facility enables the spectral

radiant power responsivity of the transfer detectors to be measured against the primary and

secondary standard cryogenic radiometers at numerous wavelengths spanning the range from the

UV to the NIR, instead of a few discrete laser lines in the visible. With access to these laser

systems, the physical model is no longer used to derive the responsivity scale. Instead, a curve

fit is used to interpolate between the data points.

p

c

-1

-2 h

-3

tr

<> o32
A practical means of disseminating the optical power scale to customers is to first transfer the

scale from a cryogenic radiometer in SIRCUS to working standard detectors used in the UV and

Vis/NIR SCFs. This transfer of scale requires the use of a detector that can be calibrated at the

lowest possible uncertainty by the cryogenic radiometer and yet operate at the flux levels

available with the lamp-monochromator sources to calibrate the working standards of the UV

and VIS/NIR SCF. The transfer standards are a set of three types of photodiodes that, in combination, covers the wavelength range from 200 nm to 1 800 nm.

In the wavelength range from 350 nm to 950 nm, two silicon photodiode trap detectors are used.

The trap detectors have excellent stability, spatial responsivity uniformity, linearity, low noise

performance [17, 24, 26], and no interference problem with laser emission. Figure 5.5 shows the

arrangement of the trap detectors using six windowless silicon photodiodes as the transfer

standard detectors. Such a trap configuration makes its responsivity independent from surface

reflectance, thus providing excellent spatial uniformity and long-term stability.

Figure 5.5. Trap detector arrangement of photodiodes maximizes the collection

of the optical beam by reducing reflection losses [26].

A UV trap detector has been built at NIST, but its FOV is smaller than the f5 beam from the UV

SCF monochromator system. The NIR photodiodes presently available are smaller in size than

those available for the visible and UV spectral regions. These NIR photodiodes in a trap

detector arrangement would have a FOV smaller than the fl9 Vis/NIR SCF monochromator

system beam.

The first step in the spectral responsivity scale determination is to transfer the specfral power

responsivity scale from the cryogenic radiometer to the transfer standards. Figure 5.6 shows a diagram of the setup used to transfer the scale from the cryogenic radiometer to the transfer

detectors using the substitution method. For the lowest possible measurement uncertainty, the

cryogenic radiometer requires a monochromatic, collimated, intensity stabilized, laser beam. In

this optical system, the combination of a laser intensity stabilizer that alters the beam

transmission through a liquid crystal device and a remote beam monitor that is located

33
downstream on the beam path controls the laser beam intensity to better than 0.01 % over the

duration of the measurement. The laser beam is then sent through the combination of a pinhole

to create an Airy diffraction pattern, a variable aperture to reduce the scatter, and two spherical

mirrors to collimate the beam. This creates a low scatter, collimated laser beam that can be

completely collected by both the transfer standard and cryogenic radiometer. In the direct

substitution method, both detectors are located in the same focal plane and translated into the

laser beam path. First the cryogenic radiometer which measures the laser beam's optical power,

then the transfer standard that also makes the same measurement.

Monitor Detector For Stabilizer

Laser

Intensity

Stabilizer

DUT 1 DUT 2 DUT 3

^ J Polarizer

eamsplitter

Spherical Mirrors and

Pinhole for Spatial

Filtering and CoUimation

Liquid Crystal for Controlling Laser Beam Intensity

Tunable Optical Parametric Oscillators: Frequency Doubled: 520 nm to 660 nm

Fundamental: 1040 nm to 1660 nm

<7

Tunable Ti:Sapphire Lasers: Fundamental: 680 nm to 1040 rm

Doubled: 350 nm to 500 nm

Tripled: 240 nmto 330 nm

Quadrupled: 210 nm to 240 nm

Figure 5.6. Optical configuration for the calibration of the transfer standards

with the cryogenic radiometer at the SIRCUS facility.

34
Two silicon trap detectors (Vis Trap), two germanium photodiode transfer standards (Ge TS)

that were temperature-controlled at -30 °C, two indium gallium arsenide photodiode transfer

standards (IGA TS), temperature-controlled to 25 °, several UV transfer standards, and UV trap

detectors have been measured by the cryogenic radiometer. The UV detectors were calibration

from 210 nm to 400 nm. The Vis Trap detectors were calibrated from 325 nm to 950 nm. The

Ge TS were calibrated from 850 nm to 1650 nm and the IGA TS were caHbrated from 700 nm to

1 800 nm. The spectral power responsivity scale realization is based on laser measurements on SIRCUS at the wavelengths shown in Figure 5.7. Because the SIRCUS calibration did not set the laser wavelengths at exact 5 nm intervals, the spectral responsivity data was interpolated

using a modified cubic-spline fit to the 5 nm intervals used in the SCF measurements.

Laser Wavelengths Measured at SIRCUS to Calibrate the Transfer Standards

0.0 I—I—I—I—i

'—I—I—I—I—'

'—^—'—I—'—I' ' ' I' ' I I I I I I > I 2su 200 400 600 800 1000 1200 1400 1600 1800

Wavelength [nm]

Figure 5.7. SIRCUS laser wavelengths used to calibrate the transfer standards. The

data points indicate the wavelengths used for calibrating the (a) UV TS, (b) Vis Trap,

(c) Ge TS, and (d) IGA TS.

5.4 Calibration of the Working Standards

Working standards are those used to calibrate customer's detectors routinely at the SCFs. Three

sets of working standards are used; one set each for the UV, Visible, and NIR regions. These

detectors are measured against the appropriate transfer standards at the SCF using the

measurement setup and procedure described in Sec. 6.1 or on another NIST facility, such as SIRCUS. A computer program is used to determine the center of the active area for each

detector. The measurements are then taken at this position. The working and transfer standards

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 and

transfer standards is typically set to 1 0^ V/A.

The spectral responsivity is found using eq. (3.27) where 55=^,' the transfer standard

responsivity determined from the cryogenic radiometer measurements. The spectral responsivity

of each working standard is the weighted mean of the measurements against two transfer

standards. Where the weighting constants are the inverse of a combination of the within TS and

35
between TS components of variance [50]. That is, the weighting includes both the statistical

uncertainty of the comparison with a TS but also the variation between each TS to account for

such things as drift during the measurement or (long-term) drift between the TS. The optical

power used for these measurements is on the order of 1 p.W or less as shown in Figure 4.2.

5.4.1 Calibration of the Visible Si Working Standards (Vis WS)

The spectral power responsivities of the four visible working standards (Vis WS) were measured

in the Vis/NIR SCF using the Vis Traps and the Ge TS. Sihcon trap detectors transfer the

calibration from 350 nm to 950 nm while temperature-controlled Ge TS transfer the scale to the

Vis WS from 955 nm to 1100 nm. Both Vis Traps and Ge TS were measured at SIRCUS in

varying step sizes against the L-1 ACR to determine the external quantum efficiencies. The

external quantum efficiency data was interpolated to 5 nm intervals and used to calculate the

spectral power responsivity scale for the transfer to the Vis/NIR SCF. The calibration chain for

the Vis WS is shown in Figure 5.8. Basing the calibration of the Vis WS on a combination of

transfer standards provides the lowest possible uncertainties over the entire wavelength range.

The uncertainties for the Vis WS are described in Sec. 7.3.1.

Cryogenic Radiometer

Silicon Trap Detector Ge Transfer Standard

350nmto950nm 955 nm to 1 100 nm

Silicon Visible WS

350nmto llOOnm

Figure 5.8. The calibration chain for the visible working standards (Vis WS).

The responsivity of Vis WS was determined by averaging 3 independent scans against each Vis

Trap detector and Ge TS. That is, the WS was removed from the SCF and then realigned

between each scan. The resulting data from the two sets of transfer standards were combined to create a single scale from 350 nm to 1 100 nm for the Vis WS.

36
5.4.2 Calibration of the Ultraviolet Working Standards (UV WS)

Two UV working standards (UV WS) were calibrated from 200 nm to 400 nm by a series of

measurements at the UV SCF, Vis/NIR SCF, SIRCUS, and SURF with various Si photodiode

UV transfer standards and trap detectors. The UV WS were calibrated with a Vis Trap at the

Vis/NIR SCF from 405 nm to 500 nm. The combination of the UV transfer standards and the

Vis Trap provides the lowest uncertainties over the entire UV WS calibration range. The

calibration chain for the UV working standards is shown in Figure 5.9. The uncertainties on the

UV working standards are described in Sec. 7.3.3.

Cryogenic Radiometer

UV Transfer Standards Silicon Trap Detector

200 nm to 400 nm 405 nm to 500 nm

Silicon UV WS

200 nm to 500 nm

Figure 5.9. The calibration chain for the ultraviolet working standards (UV WS).

The responsivity of the UV WS was determined by an average of 3 independent scans against

each UV TS and Vis Trap. As with the Vis WS, each UV WS was removed from the SCF and

realigned between each scan. The resulting data from the transfer standards were combined to create a single scale from 200 nm to 500 nm for the UV WS.

37
5.4.3 Calibration of the Near-IR InGaAs Working Standards (IGA WS)

The IGA working standards (IGA WS) responsivities are determined on the Vis/NIR SCF using

the Vis WS over the spectral region from 700 nm to 950 nm and the IGA TS from 955 nm to 1650 nm. The IGA TS were measured at the SIRCUS facility against a cryogenic radiometer in

varying step sizes from 700 nm to 1800 nm to determine the external quantum efficiencies. The

external quantum efficiency data was interpolated to 5 nm intervals and used to calculate the

spectral power responsivity scale for the transfer to the Vis/NIR SCF. The IGA TS were not

calibrated at enough wavelengths in the spectral region greater than 1650 nm to adequately

interpolate the responsivity due to the steep drop in the IGA responsivity in this spectral region.

The previous calibration of the IGA WS was used between 1655 nm and 1800 nm (as described

below). This combination of the Vis WS and the InGaAs TS provided the lowest uncertainties over the entire IGA working standards responsivity range. Figure 5.10 shows the NIR

measurement chain and the measurement uncertainties are described in Sec. 7.3.2.

Cryogenic Radiometer

Silicon Trap Detector

350 nm to 950 nm

t

InGaAs Transfer Standard

955 nm to 1650 nm

Sihcon Visible WS

700 nm to 950 nm

Pyroelectric (1999)

1655 nm to 1800 nm

InGaAs WS

700 nm to 1800 nm

Figure 5.10. The calibration chain for the InGaAs working standards (IGA WS).

The scale transfer of the IGA WS was determined by averaging 3 independent scans against the

Vis WS and the IGA TS. Each IGA WS was removed from the SCF and realigned between each

scan. The resulting data from the two sets of standards were combined to create a single scale

from 700 nm to 1 650 nm for the IGA WS

Until 1999, a slightly different cahbration method was used. The scale in the 1650 nm to 1800 nm spectral region is still based on this method at the time of this publication. The IGA

WS responsivities were determined at the Vis/NIR SCF using a Vis Trap over the spectral region

38
from 700 nm to 920 nm and a pyroelectric detector from 925 nm to 1800 nm. This calibration

occurred before the POWR cryogenic radiometer was operational. The previous primary

cryogenic radiometer, HACR, was used to calibrate the Vis Trap. The relative responsivity of

the pyroelectric detector was measured from 700 nm to ISOOnm with the SURF ACRmonochromator system [49]. The relative responsivity of the IGA WS was measured with the

pyroelectric detector from 700 nm to ISOOnm on the Vis/NIR SCF. The IGA WS relative

responsivity was scaled to the calibrated power responsivity from 700 nm to 920 nm. This

combination gave the lowest transfer uncertainty. Figure 5.1 1 shows the 1999 NIR measurement

chain.

Cryogenic Radiometer

Silicon Trap Detector

700 nm to 920 nm

3

SURF ASR

Pyroelectric TS

925 nm to 1800 nm

InGaAs WS

700nmto 1800 nm

Figure 5.11.

WS).

The 1999 calibration chain for the InGaAs working standards (IGA

The scale transfer of the IGA WS was determined by averaging 3 independent scans against each

Vis Trap and the pyroelectric detector. Each IGA WS was removed from the SCF and realigned

between each scan. The resulting data from the two sets of transfer standards were combined to create a single scale from 700 nm to 1 800 nm for the IGA WS.

39
6. Calibration Procedures and Computer Automation

6.1 Spectral Radiant Power Responsivity

This section describes the spectral radiant power responsivity measurement procedures for both

the UV SCF and Vis/NIR SCF. These correspond to Service ID numbers 3907 IC through

39078C. The primary operational difference between the two SCFs is that the UV SCF can accommodate one test detector per responsivity measurement run while the Vis/NIR SCF can measure two test detectors. The responsivity spatial uniformity for new photodiodes, which is included in 3907 IC, 39073C, and 39077C, is also measured as described in Sec. 6.2.

6.1.1 Calibration Procedures

New photodiodes are visually inspected for defects and placed in the specially designed

mounting fixtures described in Sec. 9.4. Each photodiode has a serial number engraved on the

mount. The photodiode dynamic impedance (shunt resistance) is measured with an HP 4 145

A

I-V plotter to confirm the manufacturer's specification. [Note: The shunt resistance of the

photodiode needs to be about 1 000 times greater than the input impedance of the transimpedance

amplifier for it not to deviate from linear operation [26, 39]. For the transimpedance amplifier

circuit typically used with the SCFs, the open-loop gain of the op-amp is 10 and an amplifier

gain (determined by the feedback resistor) of 10 is used, thus giving an input impedance of

Gamp/Goi = 1 Q. Thus the photodiode shunt resistance must be greater than 10 kQ for a nonlinearity in the current-to-voltage conversion of less than 0.01 %.] If needed, the diode window

is cleaned^ with lens tissue and spectral-grade acetone before any optical measurements are made. Methanol can be used immediately after acetone when cleaning optics to get rid of any

acetone residue or a mixture of methanol/acetone is sometimes used [51]. Ethanol has also been

used by others to clean photodiodes and optical windows [30].

The spectral responsivity is determined by substitution with two working standards using the

"substitution method with monitor" described in Sec. 3.2.3. 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. A computer program automatically centers the active area of each

detector to 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. 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 jiW. 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

Cleaning a photodiode can change its responsivity.

40
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 in a beam with small fluctuations in power level. 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.

300 400 500 600 700 800 900 1000 1100 Wavelength [nm]

Figure 6.1. Typical signals from Hamamatsu S1337 and monitor photodiodes.

In Figure 6.1 all curves are the means of 10 samples. The relative standard deviations of the

SI 337 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 SI 337 and monitor signal standard deviation curves are nearly indistinguishable because the source noise rather than detector noise dominates the measurement.

The laboratory environment (temperature and humidity) is monitored and recorded at the start of

each scan. This data is not used to correct the measurement results. The temperature of

detectors that have temperature sensors built into their housings can also be recorded. The

average temperature during the measurements is 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 [50] of all the scans with both working standard detectors. Examples of typical

photodiode spectral responsivities are shown in Figure 6.2.

41
T—I—I—

I—I—I—1—

I—I—I—I—

I—I—I—I—

I—I—I—I—I—I—I—I—I—I—I—I

\—I—I—

r

200 400 600 800 1000 1200 1400 1600 1800 Wavelength [nm]

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

6.1.2 Quantum Efficiency

The quantum efficiency is the photon-to-electron conversion efficiency of a photoelectric

detector. The quantum efficiency of a detector is often required for particular applications.

There is a simple calculation to convert spectral responsivity [AAV] to external quantum

efficiency. The external quantum efficiency, rjext, is given by [13, 24]:

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

velocity of light, 0 is the input radiant flux (power), n is the index of refraction of air, e is the

elementary electronic charge, and X is the spectral wavelength (in air). Substituting for the

constants h, c, n, and e gives:

77extW = 1239.48x^1^ = 1239.48x^ , (6-2)

where s{X) = i{X)l0{X) is the spectral responsivity [AAV], and for convenience, X is in nm.

Examples of typical photodiode EQEs are shown in Figure 6.3.

42
6.2 Spatial Uniformity

This section describes the procedures used in NIST Service ID numbers 3907 IC, 39073C,

39077C, and 3908 IS 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 as described in sections 4.1 and 4.2. Also, only one

detector can be measured at a time in the UV SCF while all four detector positions can be used

in the Vis/NIR SCF for spatial uniformity measurements since no working standard detector is needed.

The spatial uniformity is measured for all photodiodes issued from NIST, at 350 nm for the OSI

Optoelectronics UV-100 photodiodes (3907 IC), at 500 nm for the Hamamatsu S2281

photodiodes (for 39073C and 39077C). UV-100 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 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

(llOOnm). Nonuniformity is due to inhomogeneity in the photodiode material - t>^pically

inhomogeneity in surface recombination centers at shorter wavelengths [52, 53] and bulk

recombination centers at longer wavelengths [54]. The nonuniformity near the bandgap has also

been related to the nonuniformity of the bonding material's reflectance [55]. Because the

43
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.

Since 1993, the uniformity of the S1337-1010BQ and the S2281 diodes are also measured at 1000 nm [56]. 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 and S2281 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 in Sec. 6.1.1. The typical measurement consists of setting

the monochromator to the desired wavelength and scanning a 12 mm x 12 mm area in 0.5 mm

steps, for a 1 cm" test detector. 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 10^ V/A. Figure 4.2 shows the optical

power for these measurements is typically less than 1 |liW. 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 comer moving to the lower right comer 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 and humidity) is monitored and recorded at the

beginning and end of each scan. This data is not used to correct the measurement results. The

temperature detectors that have temperature sensors built into their housings can also be

recorded. The average temperature during the measurements is reported.

The test to monitor detector ratios are normalized to the mean of the center ratios. The reported

spatial uniformity figure is constmcted from these normalized ratios. Figure 6.4 shows the

spatial uniformities of the central portion of typical Hamamatsu S 1337-1 01 OBQ and S2281

photodiodes at 500 nm and 1000 nm. Similar spatial uniformity scans are shown in Figure 6.5a

for UV-100 at 350 nm and Figure 6.5b, c, and d for Judson EG&G thermoelectrically cooled Ge

photodiodes at 1000 nm, 1500 rmi, and 1600 nm, respectively.

44
X [mm]

(a) Hamamatsu SI 337 at 500 nm.

X [mm]

(b) Hamamatsu SI 337 at 1000 nm.

X [mm]

(c) Hamamatsu S2281 at 500 nm.

X [mm]

(d) Hamamatsu S2281 at 1000 nm.

Figure 6.4. Spatial uniformities of typical Hamamatsu SI 337 and S2281

photodiodes. The responsivities are normalized to the center values.

45
X [mm]

(a) OSI Optoelectronics UV-100 at 350 nm.

X [mm]

(b) EG&G Judson TE Ge at 1000 nm.

^- L"^J X [mm]

(c) EG&G Judson TE Ge at 1500 nm. (d) EG&G Judson TE Ge at 1600

Figure 6.5. Spatial uniformities of typical OSI Optoelectronics UV-100 and EG&G Judson Ge photodiodes. The responsivities are normalized to the center

values.

6.2.2 Limitations

The size of the active area to be measured has to be significantly larger than the beam size to avoid 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.

46
6.3 Computer Automation

This section describes the computer automation of the UV and Vis/NIR comparator facihties.

The automated equipment and computer software used for typical measurements are briefly

described. The operation of the spectral comparator facilities would not be practical without a high degree of computer automation.

6.3.1 Computer Automated Equipment

A block diagram of the computer controlled equipment for the Vis/NIR SCF is shown in Figure

6.6. 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)

cormects this computer with other computers which are used for data analysis, writing test reports, and calibration related file backups; allowing the SCF control computer to be dedicated

to taking measurements. Daily backups of the data files are made to an external hard drive

connected to the SCF control 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 with an absolute encoder

mounted on the wavelength drive shaft.

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 temperature

and humidity. Some test (customer) detectors have temperature monitoring circuitry that

produces a voltage signal or resistance proportional to their temperature. These signals can also

be multiplexed via computer control to the DVM. The WS temperature controllers are also

computer controlled via the IEEE 488 bus.

The UV SCF equipment and computer control is very similar to the Vis/NIR. The only

differences are in the monochromator wavelength control and the translation of the detectors into

the optical beam. The UV monochromator wavelength drive did not require any modification to be computer controlled and as mentioned, the UV SCF uses a rotary stage to move the detectors

and only one detector has automated horizontal and vertical (x,y) translation stages.

47
i

Test Reports

Computer

Instrument Control

and Data Strqage

Computer CQ

00

oo

I W

Data Analysis

and File Backup

Computer

TE

Temperature

Controller

InGaAs

Working Standards

DVM

T

Amplifiers

j Multiplexer

DVM

Working Standard

& Test Detectors

Temperature

Monitoring

Inputs

<1

Monitor Silicon Monitor

Monitor TE Amplifier

Temperature

Controller

Germanium Monitor

Environment

Monitor

X-Y i —I Translation

^Stages I Wavelength!

Drive i

j Source

'

l Selection

Drive

I Laboratory | Temperature

j & Humidity j

\ Sensor I

Enclosure | Temperature J Lsensor.

Digital I/O

Shutter

Controller

Shutter

Wavelength

Display

Wavelength I Encoder j Figure 6.6. Vis/NIR SCF computer control block diagram.

48
6.3.2 Computer Calibration Programs

SCF Setup Program

This program performs three functions: initiahze the SCF instruments, ahgn the test and working

standard (WS) detectors, and verify the signal levels from the detectors before starting

measurements. The operator uses this program interactively, via the front panel controls and a joystick to control the translation stages, to align new detectors in the SCF. This program

updates a file that contains the detector names and translation stage (jc,^) positions used by other

computer programs.

Spectral Power 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, 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. 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 display with the current scan number, the

detector name and position for the current measurement, filename, and the file pathname. 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, records are printed of the operator inputs and graphs of the detector to monitor

ratios and the percent standard deviation of the ratios.

Spectral Power Responsivitv Calculation Program

This program calculates the spectral power responsivity for the test detector. The results are stored on the hard disk and the responsivity and relative measurement uncertainty data are printed as graphs, along with the detector filenames, and control settings. The inputs to the

program are the test and working standard detector "raw" ratio data filenames, working standard

reference (calibration) data filename, and the selection of the transimpedance amplifier used with

the test detector.

The gain range and detector position in the SCF are extracted fi*om 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, or a factor of 1.0 is used if no amplifier gain correcfion is supplied,. Next, the program reads and

formats the ratio data files and the working standard reference data file. The program determines

49
the responsivity for a test detector by using Eq. (3.27) for each spectral scan combination of test detector and working standard. Then, if more than one spectral scan was taken, a weighted mean

is calculated. Where the weighting constants are the inverse of a combination of the within TS

and between TS components of variance. The uncertainty for the weighted average is also

calculated as the inverse of the square root of the sum of the weights [50].

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.

The program moves the detector to the first x and v 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.

50
7. Uncertainty Evaluation

The uncertainty evaluation for the detector spectral power responsivity measurements is explained in this section. First the uncertainty is evaluated for the measurement equation

developed in Sec. 3. In addition to the uncertainty terms that come directly from the

measurement equation there are 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 Refs. [7, 8, 57]. A general discussion of the sources of

error in radiometry can be found in Refs. [58, 59]. A detailed explanation of the evaluation and

expression of measurement uncertainty in radiometry is given in Refs. [60, 61].

Note: The uncertainty evaluation described in this section was prepared before the

pubhcation of Ref. [61] and thus differs somewhat from the analysis described

there. In the future, the uncertainty evaluation will follow Ref. [61].

7.1 Uncertainty Calculation from the Measurement Equation

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

quantities x, given by,

y = f(x^,x,,...,Xj^). (7.1)

If the input quantities are not correlated, the combined standard uncertainty udy) is given by the

law of propagation of uncertainty as the following sum,

,=1 ^dx.

(7.2)

where the partial derivatives df/dxi are the sensitivity coefficients and w(x,) are the standard

uncertainties of each input jc,. 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 iic(y)/y is more

convenient, and is given by

c,relW =

.{y)_

y

(7.3)

where {\lyy{df/dxi) is the relative sensitivity coefficient. The expanded uncertainty U is obtained

by multiplying udy) by a coverage factor k.

51
(7.4)

where k is chosen on the basis of the level of confidence desired. Replacing udy) with the

relative combined standard uncertainty Mc,rei(y) gives the relative expanded uncertainty Urd=U/y.

The coverage factor k = 2 is used in this document following the NIST policy [9] so that the

interval defined by f/has a level of confidence of approximately 95 %.

The uncertainty uo(sx) in the test detector spectral responsivity, calculated from the simplified

measurement equafion (Eq. (3.27)), is considered. The calculation is made by using the

propagation of standard uncertainty relationship in Eq. (7.2). The measurement equation for

spectral responsivity, Sy^(X), is:

The responsivity uncertainty of the test detector at a single wavelength uq{sx) is:

(7.5)

ds

dR

A2 +

+

J

+

+

ds

ds.^-u{sj

V

(7.6)

The relative uncertainty iiois^)/ s^ is:

\2

V -^s y or (7.7)

«0,rel (^x ) = ^JKi(K )+ W;i(^s )+ "relte )+ «rel (<^x )+ «relk)

where u(Rx) is the standard deviation of the mean of i ratios of the test detector and the monitor,

u{Rs) is the standard deviation of the mean of / ratios of the standard detector and the monitor,

u(Gs) is the standard detector amplifier gain uncertainty, w(Gx) is the test detector amplifier gain

uncertainty, u(ss) is the standard detector uncertainty, and i is the number of ratio measurements

taken. The quantities u(Rx) and iiiRs) are measurement noise.

The standard detector uncertainty, u(ss), is determined previously by the absolute cryogenic

radiometer transfer measurements. The amplifier gains, Gs and Gx, are calibrated using a precision current source and DVM. The amplifier gain uncertainties, u{Gs) and w(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 for

52
a measurement, then a weighted mean, is calculated. Where the weighting constants are the

inverse of a combination of the within scan and between scan components of variance. The

uncertainty for the weighted average is also calculated as the inverse of the square root of the

sum of the weights [50]. The uncertainty for the weighted mean is an indication of the

measurement repeatability. Repeated scans are used when the test detector exhibits short-term

drift.

Note that Eq. (3.27) is simplified with assumptions that several measurement conditions are

ideal. In real world detector measurements, additional uncertainty components from the

characteristics of the detector being measured and the measurement facility (discussed in the

next section) should be considered. Extending Eq. (7.7) and assuming that these additional

uncertainty contributions, wi,rei(^x), H2je\(sx), U3,Td{sx),-.- are not correlated, the relative combined

standard uncertainty, Wc,rei('?x), of the test detector spectral responsivity is given by:

7.2 Uncertainty Components in Test Detector Calibration

The uncertainty components not discussed in the previous section include 1) the wavelength

scale uncertainty in the monochromator, 2) effect of the monochromator bandpass, 3) ambient

temperature effects on the detectors, 4) long-term stability of working standards, 5) measurement

reproducibility, 6) uncertainty in the DVMs' calibration, 7) spectral stray-light and 8) other

factors not considered significant. All the uncertainty components, including those discussed in

Sec. 7.1 based on the measurement equation, are listed in Table 7.1.

Table 7.1 Uncertainty components in photodetector calibration

Symbol Type Component of uncertainty

"rel(^s) B Uncertainty of the working standard detector

Wrel(^x) A Noise in the test-to-monitor ratio A Noise in the standard-to-monitor ratio

Wrel(<Jx) B Amplifier gain for test detector

Wrel((js) B Amplifier gain for standard detector

Wl,rel(^x) B Wavelength scale uncertainty of monochromator

"2,rel(-Sx) B Effect of bandpass of monochromator

"3,rel('S'x) B Ambient temperature of detectors

"4,rel('S'x) B Long-term stabihty of the working standards A Measurement reproducibihty

W6,rel(>^x) B Calibration of DVMs used to measure Rx and Rj

W7,rel(5x) B Spectral stray-light effect A Other components (spatial nonunifonnit}' of responsivity, etc.)

The first five components were discussed in Sec. 7.1. Each of the components uijei{sx) to us,

Te[{sx) is discussed below and is evaluated in sections 7.3 and 7.4.

53
7.2.1 Wavelength Uncertainty of the Monochromator

The wavelength scale of both SCF monochromators is calibrated annually by using several laser

and spectral-line discharge lamp lines, and a correction curve is fitted. There are residual errors

(typically less than 0.1 nm) after the wavelength scale is corrected. Random shifts in the

wavelength scale due to the scanning mechanism and drift of the scale over time are negligible

compared to the residual scale errors.

The uncertainty in the measured spectral responsivity due to an uncertainty component is called an uncertainty contribution. The uncertainty contribution due to wavelength uncertainty is proportional to the slope of the ratio of the test to monitor and standard to monitor signal ratios,

drx/di, where r(/l) = R^{?i)/Rs(^) and drJdA is the sensitivity coefficient. The relative uncertainty

contribution u\je\(Sx) is calculated as the product of the sensitivity coefficient and the standard

uncertainty u{X) of the wavelength,

. ^..,(^U)) = ^^^^H^--^-W, (7.9)

where r^(M) is the signal ratio at the ith wavelength point, r^(li+\) is at the next measurement

point, separated by scanning interval Ai.

The wavelength errors tend to cancel out if the test detector and the standard detector are the

same type, having nearly identical relative spectral responsivity curves (Rx{^) ~ Rs(^))- Thus,

this uncertainty is normally insignificant for typical photodiode measurements, except in

wavelength regions where responsivity curve sharply rises or drops near the bandgap. This error can also be significant for filtered detectors, radiometers, and photometers, which tend to have

steep slopes in the responsivity curve and/or sharp cut-on and cut-off regions.

Also, it should be noted that wavelength errors also occur if the bandpass of the monochromator

is asymmetric and if the wavelength calibration is done using the peak of the bandpass. The

SCFs do not have this error, because the monochromators' bandpasses have been verified as symmetric and the centroid wavelength is used for wavelength calibration.

7.2.2 Effect of Bandpass

The bandpass of the monochromator can cause errors when the spectral responsivity curve is not

linear within the full-bandwidth of the bandpass, and can cause significant errors where spectral

responsivity curves change rapidly, e.g., near rising or falling parts of the curve, or at the peak or

valley of the curve. The measured spectral responsivity is a convolution of the true spectral

responsivity and the normalized bandpass function. At each wavelength point, the measured

responsivity ^'measC/l-o) is calculated as a product of the true spectral responsivity s{2.) and the

normalized bandpass function b{Xo- A), thus,

s^jA,)= \s{A)-b(A,-AydA with lb(A) = \. (7.10)

To estimate this error without knowing the true responsivity s(y{.), the measured responsivity data

is used as 5(/l) for an approximation. The spectral responsivity data is first interpolated to a 1 nm

54
interval, and the b(X) function (normally a triangular function) is also calculated at I nm intervals

and normalized so that the sum of all b(?i) values is equal to 1, then a sum-product calculation is performed as given in Eq. (7.10). The error due to the bandpass can be evaluated by looking at the difference between 5meas(/l-) and s()l).

•B.rel ^meas(^)-^(^)

(7.11)

In substitution measurements, this error also tends to cancel out if the test detector and standard

detector have similar spectral responsivity curves. In this case, the relative error for the test detector eB,reitest(>^) and for the working standard detector £B,rei,std(/l) can be calculated, and the

substitution error eB,sub(>^) in the measured test detector is given by:

3,sub('^)- ^B,rel,test ('^) ^B.rel.std i'^ )

• (7.12)

An example of a calculation of Eq. (7.12) for a UV photodiode for a triangular 4 nm bandpass

function is shown in Figure 7.1.

0.30

0.25

I

0.20

5 0.15 -

i 0.10

0.05

0.00

(a) UV Photodiode Spectral Responsivit> |AAV) «— Scale \

(b) Bandpass Effect Error

Scale

0.16

0.14

0.12 .|S

0.10 %

ou

0.08 i

0.06 I

p

-- 0.04 I

55 0.02

4^ 0.00 200 250 300 350 Wavelength |nni]

400 450 500

Figure 7.1. Error due to effect of bandpass for UV photodiode. The figure

shows the (a) UV photodiode spectral responsivity [A/W] and (b) the bandpass

effect error standard uncertainty contribution [%].

If a correction is not made for the bandpass error, the estimated error is converted to a standard

uncertainty using a rectangular distribution, thus.

"2,re]fe)=%sub(^)/V3 (7.13)

55
7.2.3 Ambient Temperature

The responsivity temperature coefficient is typically small for most detectors, but can be

significant depending on the type of detector and wavelength region, especially for wavelengths

near the bandgap of a semiconductor photodiode. The ambient temperature inside the SCF

enclosures is recorded and the average value during a calibration is reported in the NIST

calibradon report. The temperature variation in the enclosures is small during the measurements

(normally less than 0.5 °C). The temperature sensors have a small uncertainty, thus, altogether,

± 0.5 °C combined standard uncertainty is typically reported for the ambient temperature. Since

it is not possible to know in advance what the ambient temperature will be when the

customer uses a detector, a temperature coefficient uncertainty component is not included

in the customer detector uncertainty estimates.

The effect of ambient temperature changes for the UV WS and monitor is negligible in the

200 nm to 500 nm spectral region where calibrations are performed. The IGA WS and Ge

monitor are thermoelectrically temperature controlled, and thus their temperature does not

change. Thus, the temperature effects are negligible for the UV and IGA WS and are not

listed in the uncertainty budgets.

For the Vis WS and monitor detector the ambient temperature effect is negligible for most of the

spectral range. In the NIR spectral region (> 960 nm for the Hamamatsu SI 337 and S2281) the

temperature effect can be a significant uncertainty component. For the Vis WS, the change in

temperature is estimated to be a maximum of 1 °C from the time the Vis WS was calibrated to the time (possibly months later) when a customer detector is calibrated.

7.2.4 Long-term Stability

The working standards are calibrated on a 12 to 18 month schedule, and the long-term stability

of their responsivities between recalibrations is critical for reliable calibration results. The

working standard long-term stability is evaluated by the change in spectral responsivity between

each calibration. Stability data needs to be accumulated over several calibrations to obtain an

accurate estimate of the stability.

For the Vis WS and monitor detector the temperature effect is less than the long-term uncertainty

for most of the spectral range. In the NIR spectral region (> 960 nm for the Hamamatsu SI 337

and S2281) the temperature effect may be larger than the stated long-term uncertainty.

However, mounting the silicon working standards in recently acquired temperature-controlled

fixtures is expected to reduce the temperature effect significantly, and should reduce the

uncertainty component for long-term stability in the NIR spectral region.

The long-term changes in the UV WS and the temperature controlled IGA WS are not affected

significantly by temperature variations. The long-term uncertainty component is not

included in the customer detector uncertainty estimates. The customer should evaluate and

include the long-term uncertainty of their detector in their measurement uncertainty analysis.

56
7.2.5 Measurement Reproducibility

There is also a statistical uncertainty term (Type A) reflecting the reproducibility of the

measurements (e.g., setting up the measurement again the next day). The uncertainty due to reproducibility includes day-to-day temperature and alignment variations.

New detectors types (models) that have never been measured before by NIST are calibrated three

times with the detectors being re-mounted and aligned for each measurement. This produces

three independent measurements and the reproducibility component is the relative standard

deviation of the three scans.

7.2.6 DVM Uncertainty

The values and Rs are the ratios of signals which were taken simultaneously by two DVMs.

One DVM is used for both test detector and standard detector and the second DVM is used for

the monitor, thus systematic errors normally cancel out unless the test detector and standard

detector use different DVM ranges for the same wavelengths (i.e., very different responsivities).

Similarly, if the dark signal (shutter closed) and total signal (shutter open) use the same DVM

range then the systematic errors cancel when the (net) signal is calculated.

The stability of the two DVMs over the measurement time (typically 1 h) is of concern. The

24 h "Percent of Reading" manufacturer specification is used in lieu of a stability specification to calculate the uncertainty. The DVM uncertainty is typically found to be negligible (<0.01 %)

compared to other components, and is not listed in the subsequent uncertainty budget.

7.2.7 Spectral Stray-light

In principle, stray-light in a double monochromator as used in the SCFs is normally negligible.

To verify this, the stray-light error was estimated by using a simple model of a slit-scattering

function of the double monochromator:

Jl for(A,-AA/2)<A<{A,+AA/2)

z{A,-A) = ^

. (7.14)

[10 otherwise

The AX is the in-band (bandpass) region of the monochromator and is set to 4 nm for the SCFs.

The value of 1 0'^is based on measurements taken with a portable tunable laser system where the

laser was tuned over the in-band region at several wavelength settings of the monochromators.

The measurements for the Vis/NIR SCF monochromator were very similar to measurements

taken on an identical model monochromator [62]. These values are typical for a double

monochromator. From Eq. (3.7), the in-band signal Fib(Ao) of the detector at wavelength aq is given by:

VM- j^.(/l)-^,,e,(/^)T(i)-3(Ao-/l)-d/L, (7.15)

Ai

i where cPx.,rei(>^) is the relative spectral radiant flux entering the entrance slit of the

monochromator, and t{X) is the spectral transmittance of the monochromator system. For

simplicity, t{X) is assumed to be constant.

57
Similarly, from Eq. (3.10), the stray-light signal Ksl(^) of the detector is given by:

(7.16)

The spectral integration is done over the entire region where the spectral responsivity is not zero

(to the extent the data is available). The relative error due to stray-light is then estimated by:

This error also tends to be cancelled out if the test detector and standard detector have similar

responsivity curves. The value from Eq. (7.17) gives the worst case. This error was calculated

for each type of working standard detector used with the SCFs and the spectrum of the quartzhalogen lamp or the argon mini-arc. The error due to stray-light was found to be negligible in all cases at any wavelength. Because the errors were found to be negligible, this component is not listed in the subsequent uncertainty budgets.

7.2.8 Other Components

Several other factors that could contribute uncertainty components were mentioned in Sec. 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 [63-72]; 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. As mentioned earlier, diffraction and coherence effects are neghgible for these comparator systems.

The responsivity spatial uniformity is typically measured but not explicitly used in the

uncertainty analysis. Since the same beam size and geometry is used for both the calibration of

the WS detectors and the customer's detectors and the automatic alignment is very reproducible,

the spatial non-uniformity effects are negligible. No estimate for the uncertainty component is given for different beam sizes or geometries. These could significantly add to the estimated

uncertainty given in this publication.

Similarly, the monochromator beam profile and shape have been measured, but are not explicitly

used in the uncertainty analysis, because they have negligible effect on the measurement.

Spatially (or geometrically) scattered light is light scattered out of the nominal monochromator

beam and is thought to be primarily due to imperfections in the optics. The spatially scattered

light cancels when the test and standard detectors are the same size or 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 spatially scattered light is normally neglected. But,

SL (7.17)

IB

58
occasionally, test detectors having a different size of the sensitive area are measured, in which

case, the error due to the scattered light is considered. As mentioned in Sec. 3.2.4, the

background signal voltage from the detector amplifier has been measured (beam blocked and

shutter open) and found to be negligibly different than the signal with the shutter closed.

The reflected beam from the test detector(s) and working standard detectors travels "backwards"

along the optical axis and is assumed to be scattered inside the monochromator. Any of the

reflected beam that returns to the detector is considered 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 other 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 [3, 36, 38, 54, 73-79]. The

detector linearity has been measured for the Hamamatsu SI 337-1 OlOBQ and the OSI

Optoelectronics UV-100 sihcon photodiodes and is discussed in Sec. 9. At the power levels

used for routine measurements, nonhnearity 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.

Chopped measurements are not part of the typical calibration and are not considered here.

Information on uncertainties with chopped radiation measurements can be found in Ref [39].

7.3 Transfer from Cryogenic Radiometer 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; and abbreviated numerical tables are provided. The amplifier calibration uncertainties are all identical because identical amplifiers

are used and they are calibrated using the same equipment and procedure. Wavelength

calibration, bandpass effects, ambient temperature effects, long-term ambient stability,

reproducibility, DVM uncertainty, stray-light uncertainty components were calculated as described in Sec. 7.2 using typical data. All uncertainty components were considered

independent and combined by RSS to get the relative combined standard uncertainty shown.

7.3.1 Visible Silicon Working Standards (Vis WS) Uncertainty

Table 7.2 lists the uncertainty components for the two transfer calibrations used for the Vis WS.

The Vis Trap and Ge TS uncertainties are from the L-1 ACR transfer [23]. The bandpass effect

is negligible (<0.01 %). The amphfier uncertainty components (0.04 %) for the WS and TS are combined by RSS for a value of 0.06 % in the table. As mentioned in Sec. 7.2.3, the change in

ambient temperature is estimated to be a maximum of 1 °C. The long-term stability is estimated

by using the typical data for a Vis WS over a one year period. All the components are shown at 5 nm intervals in Figure 7.2a. The components were combined by RSS to get the relative

combined standard uncertainty listed in Table 7.2 and shown at 5 nm intervals in Figure 7.2b.

59
Table 7.2. Visible working standard uncertainty (transfer from Vis Trap and Ge TS)

Uncertainty conlribution from each component of uncertainty [%]

Wavelength

Transfer Std

calibration

Vis WS-tomonitor

ratio noise

Trati 1. 1 ullol^^l ^fpr Std-to- monitor

ratio noise

Vis WS &

Transfer Std

amplifier

calibration

Monochromator

wavelength

Vis WS

temperature

coefficient

Vis WS

long-term

stability

R pnrr\riiipihility and

other components

Relative combined

standard

uncertainty

[nm] Type B Type A Type A Type B TypeB Type B Type B Type A |%]

350 0.05 0.03 0.04 0.06 0.04 0.02 0.23 0.17 0.30 375 0.05 0.01 0.01 0.06 0.04 0.01 0.19 0.10 0.23 400 0.05 0.00 0.01 0.06 0.03 0.01 0.14 0.05 0.17 425 0.05 0.00 0.01 0.06 0.02 0.01 0.10 0.03 0.13 450 0.05 0.00 0.00 0.06 0.01 0.01 0.07 0.01 0.11 475 0.05 0.00 0.00 0.06 0.01 0.00 0.05 0.01 0.09 500 0.05 0.00 0.00 0.06 0.00 0.00 0.04 0.01 0.09 525 0.05 0.00 0.00 0.06 0.00 0.00 0.03 0.01 0.08 550 0.05 0.00 0.00 0.06 0.00 0.00 0.02 0.01 0.08 575 0.05 0.00 0.00 0.06 0.00 0.00 0.02 0.01 0.08 600 0.05 0.00 0.00 0.06 0.00 0.00 0.02 0.01 0.08 625 0.05 0.00 0.00 0.06 0.00 0.00 0.02 0.01 0.08 650 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 675 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 700 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 725 0.05 0.00 0.00 0.06 0.00 0.00 0.00 0.01 0.08 750 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 775 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 800 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 825 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 850 0.05 0.00 0.00 0.06 0.00 0.00 0.01 0.01 0.08 875 0.05 0.00 0.00 0.06 0.00 0.00 0.02 0.01 0.08 900 0.05 0.00 0.00 0.06 0.00 0.00 0.02 0.01 0.08 925 0.05 0.00 0.00 0.06 0.00 0.01 0.02 0.01 0.08 950 0.05 0.00 0.00 0.06 0.00 0.02 0.02 0.01 0.08 975 0.10 0.00 0.00 0.06 0.04 0.07 0.40 0.04 0.43 1000 0.10 0.00 0.00 0.06 0.07 0.19 0.52 0.09 0.57 1025 0.10 0.00 0.00 0.06 0.13 0.42 0.50 0.21 0.71 1050 0.10 0.00 0.00 0.06 0.21 0.76 0.29 0.37 0.93 1075 0.10 0.00 0.00 0.06 0.20 1.0 0.10 0.49 1.1 1100 0.10 0.00 0.00 0.06 0.22 1.2 0.02 0.58 1.3

60
Vis WS (Haraaraatsu S1337-1010BQ) Uncertainty Components

Vis WS (Hamamatsu S1337-1010BQ) Relative Combined Uncertainty

3.0 -|

2.5 -

— 2.0 -

1.5 -

0.0 -I , , ; , 1 1 : , ^ 1 , , , ,

350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100

Wavelength [nm]

Figure 7.2. Visible WS uncertainty: (a) Vis WS uncertainty components at 5 nm

intervals, (b) Vis WS relative combined standard uncertainty. The discontinuity

in the curve at 950 nm is the result of calibrations with different standards.

7.3.2 Near-Infrared InGaAs Working Standards (IGA WS) Uncertainty

Table 7.3 hsts the pov^er responsivity transfer measurement uncertainty components using the

Vis WS and IGA TS. The amplifier uncertainty components (0.04 %) for the WS and TS are combined by RSS for a value of 0.06% in the table from 700 nm to 1650 nm for the

measurements with the Vis WS and IGA TS. The amplifier uncertainty component is not

included for the relative measurements with the pyroelectric detector in 1999 from 1650 nm to 1800 nm. The long-term stability is the typical data for the IGA WS over one year. The

measurement uncertainty components are shown in 5 nm intervals in Figure 7.3a. The relative

combined standard uncertainty was determined by the RSS of all the components and is listed in

Table 7.3. Figure 7.3b shows the relative combined standard uncertainty at 5 nm intervals.

61
Table 7.3. Near-Infrared InGaAs working standard uncertainty (transfer from Vis WS,

IGA TS, and pyroelectric detector)

Uncertainty contribution from each component of uncertainty [%]

Wavelength

1 ranster Stcl calibration

IGA WS-tomonitor

ratio noise

Transfer

Std-tolllUllllUl ratio noise

IGA WS &

Transfer Std

aiiipiiiiCi calibration

Monochrowavelength error

IGA WS

long-term

stability

Reproducibility and

other components

Relative combined

standard

uncertainty

[run] Type B Type A Type A TypeB Type B Type B Type B Type A [%]

700 0.08 0.00 0.00 0.06 0.04 0.01 0.20 0.02 0.23 725 0.08 0.00 0.00 0.06 0.04 0.01 0.19 0.02 0.21 750 0.08 0.00 0.00 0.06 0.05 0.01 0.19 0.02 0.19 775 0.08 0.00 0.00 0.06 0.05 0.01 0.18 0.02 0.18 800 0.08 0.00 0.00 0.06 0.05 0.01 0.18 0.02 0.17 825 0.08 0.00 0.00 0.06 0.06 0.01 0.17 0.02 0.17 850 0.08 0.00 0.00 0.06 0.10 0.02 0.17 0.02 0.18 875 0.08 0.00 0.00 0.06 0.08 0.02 0.17 0.02 0.18 900 0.08 0.00 0.00 0.06 0.08 0.02 0.16 0.02 0.19 925 0.08 0.00 0.00 0.06 0.16 0.02 0.16 0.02 0.23 950 0.08 0.00 0.00 0.06 0.07 0.02 0.15 0.02 0.17 975 0.10 0.00 0.01 0.06 0.01 0.02 0.15 0.04 0.16 1000 0.10 0.00 0.00 0.06 0.01 0.02 0.15 0.03 0.15 1025 0.10 0.00 0.00 0.06 0.01 0.03 0.14 0.03 0.14 1050 0.10 0.00 0.00 0.06 0.01 0.03 0.14 0.03 0.14 1075 0.10 0.00 0.00 0.06 0.01 0.03 0.13 0.03 0.14

1 100 0.10 0.00 0.00 0.06 0.01 0.03 0.13 0.03 0.14

1 125 0.10 0.00 0.00 0.06 0.01 0.03 0.13 0.03 0.14

1 150 0.10 0.00 0.00 0.06 0.01 0.03 0.13 0.03 0.14

1 175 0.10 0.00 0.00 0.06 0.01 0.03 0.12 0.03 0.14 1200 0.10 0.00 0.00 0.06 0.01 0.03 0.12 0.02 0.15 1225 0.10 0.00 0.00 0.06 0.00 0.03 0.12 0.02 0.15 1250 0.10 0.00 0.00 0.06 0.00 0.03 0.11 0.02 0.15 1275 0.10 0.00 0.00 0.06 0.00 0.03 0.11 0.02 0.15 1300 0.10 0.00 0.00 0.06 0.00 0.03 0.12 0.02 0.15 1325 0.10 0.01 0.00 0.06 0.00 0.03 0.13 0.02 0.14 1350 0.10 0.00 0.00 0.06 0.00 0.03 0.12 0.03 0.14 1375 0.10 0.01 0.01 0.06 0.00 0.03 0.11 0.03 0.15 1400 0.10 0.01 0.01 0.06 0.00 0.02 0.10 0.03 0.15 1425 0.10 0.00 0.00 0.06 0.00 0.02 0.08 0.03 0.14 1450 0.10 0.01 0.01 0.06 0.00 0.02 0.07 0.03 0.14 1475 0.10 0.01 0.01 0.06 0.00 0.02 0.07 0.03 0.14 1500 0.10 0.00 0.01 0.06 0.01 0.02 0.07 0.03 0.14 1525 0.10 0.01 0.00 0.06 0.01 0.01 0.08 0.03 0.14 1550 0.10 0.01 0.01 0.06 0.01 0.01 0.08 0.03 0.15 1575 0.10 0.01 0.01 0.06 0.01 0.01 0.08 0.04 0.16 1600 0.10 0.01 0.01 0.06 0.01 0.02 0.08 0.05 0.18 1625 0.10 0.01 0.01 0.06 0.01 0.02 0.08 0.06 0.21 1650 0.10 0.02 0.01 0.06 0.01 0.02 0.08 0,12 0.27 1675 0.15 0.05 0.21 0.22 0.02 0.08 0.49 0.66 1700 0.15 0.09 0.23 0.31 0.02 0.08 0.57 0.80 1725 0.15 0.16 0.25 0.36 0.02 0.08 0.74 1.0 1750 0.15 0.22 0.27 0.39 0.02 0.08 1.1 13

1775 0.15 0.28 0.29 0.41 0.02 0.08 1.4 1.7 1800 0.15 0.35 0.31 0.43 0.02 0.08 1.7 2.0

62
IGA WS (GPD GAP5000) Uncertainty Components

0.0 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Wavelength [nm]

• Transfer Std calibration

" IGA WS-to-moniior ratio noise

A Transfer Std-to-monilor ratio noise

y IGA WS & Transfer Sid amplifier calibration M onochromaior wavelength

Bandpass error

-- IGA WS long-term stability

" Reproducibility + other components

(a)

IGA WS (GPD GAP5000) Relative Expanded Uncertainty (A = 2)

4.5 -.

4.0 •

3.5 - . . ^ 3.0 - I 2.5 -

1800

Wavelength (ninj (b)

Figure 7.3. Near-Infrared IGA WS uncertainty: (a) IGA WS uncertainty

components at 5 nm intervals, (b) IGA WS relative combined standard

uncertainty.

7.3.3 Ultraviolet Silicon Working Standards (UV WS) Uncertainty

The power responsivity transfer measurement uncertainty components for the UV WS which are traceable to the L-1 ACR are hsted in Table 7.4. The amplifier uncertainty components (0.04 %)

for the WS and TS are combined by RSS for a value of 0.06 % in the table. The long-term

stability is the typical data for a UV WS over a one year period and is a significant component in

the uncertainty. As with the Vis WS and IGA WS, the measurement uncertainty components are shown in 5 nm intervals in Figure 7.4a. The relative combined standard uncertainty listed in

Table 7.4 was determined by the RSS of each component. Figure 7.4b shows the relative

combined standard uncertainty at 5 nm intervals.

63
Table 7.4. UV working standard uncertainty (transfer from UV TS and Vis Trap)

Uncertainty contribution from each component of uncertainty [%] UV-100 WS

Wavelength

Transfer Std

calibration

UV-100

WS-tomonitor

ratio noise

Std-to- monitor

ratio noise

Std

amplifier

calibration

Monochromator

wavelength

Bandpass

error

UVlOO WS

long-term

stability

Ivcpi uuuci u

ility and

other components

ixclctll vt; combined

standard

uncertainty [nm] TypeB Type A Type A Type B Type B Type B TypeB Type A [%l 200 1.3 0.67 0.66 0.06 0.00 0.14 0.70 0.27 1.7 220 0.05 0.06 0.06 0.06 0.10 0.11 0.62 0.27 0.70 240 0.05 0.01 0.02 0.06 0.08 0.08 0.53 0.33 0.64 260 0.05 0.01 0.01 0.06 0.04 0.06 0.42 0.29 0.52 280 0.05 0.02 0.01 0.06 0.02 0.06 0.2! 0.20 0.31 300 0.05 0.01 0.01 0.06 0.03 0.04 0.13 0.20 0.26 320 0.05 0.01 0.01 0.06 0.04 0.01 0.21 0.11 0.26 340 0.05 0.01 0.01 0.06 0.05 0.02 0.26 0.12 030

360 0.05 0.01 0.01 0.06 0.06 0.03 0.27 0.15 0J3

380 0.05 0.01 0.01 0.06 0.06 0.02 0.25 0.25 0J7

400 0.05 0.01 0.01 0.06 0.04 0.01 0.21 0.19 0.30 420 0.05 0.00 0.01 0.06 0.03 0.00 0.15 0.05 0.18 440 0.05 0.01 0.00 0.06 0.03 0.00 0.08 0.03 0.12 460 0.05 0.01 0.01 0.06 0.03 0.00 0.05 0.02 0.10 480 0.05 0.01 0.00 0.06 0.02 0.00 0.06 0.02 0.10 500 0.05 0.01 0.01 0.06 0,02 0.00 0.06 0.01 0.10

UV WS (UDT UVlOO) Uncertainty Components

* Transfer Std calibration

« UV-100 WS-to-monitor ratio noise

* Transfer Std -to -monitor ratio Doise

X UV-100 WS& Transfer Std amplifier calibration

^ M ODOchromator wavelength

Bandpass error

^ UV-100 WS long-term stability

Reproducibility + other components

(a)

UV Si WS (UDT UVlOO) Relative Combined Uncertainty

350 Wavelength inm] (b)

Figure 7.4. Ultraviolet WS uncertainty: (a) UV WS uncertainty components at 5 nm intervals, (b) UV WS relative combined standard uncertainty.

64
7.4 Transfer from Working Standards to Customer Detectors

This section details the uncertainty components when transferring the spectral power

responsivity scale from the working standards to 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 customer's 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 customer's detector is used; NIST policy [9, 57] is to not include estimates

of the uncertainties introduced by transporting the detector or its use by the customer. These

additional uncertainties include long-term stability, differences in stray-light, beam geometry,

and laboratory environmental conditions.

The stray-light is 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 a germanium photodiode calibrated with the IGA WS, stray-light and bandpass are almost

completely negligible except near the ends of the responsivity spectral range where their

responsivities greatly differ.

The uncertainties were calculated at 5 nm intervals as shown in the figures. Abbreviated

numerical uncertainty tables are provided. The amplifier calibration uncertainties are all identical because the amplifiers are identical units and are calibrated using the same equipment

and procedure. Wavelength calibration, bandpass effects, ambient temperature effects,

long-term ambient stability, reproducibility, DVM uncertainty, stray-light uncertainty

components were calculated as described in Sec. 7.2 using typical data. All uncertainty

components were considered independent and were combined by RSS to get the relative

combined standard uncertainty shown.

The relative combined standard uncertainty as described in the previous section for the three

types of SCF WS is listed in Table 7.5 at 50 nm intervals and shown in Figure 7.5 at 5 nm

intervals. NIST policy is to report to customers the uncertainty using an expansion factor of

k^2. The example expanded uncertainties are listed in Table 2.2 at 50 nm intervals and shown

at 5 nm intervals in Figure 2.1. The uncertainty components are described and shown for

example customer detectors in the following sections.

65
Table 7.5 Relative combined standard uncertainties for NIST spectral power

responsivity working standards for the UV, visible, and NIR spectral regions.

Wavelength Relative combined standard uncertainty [%]

[nm] UV (UV WS) Visible (Vis WS) NIR (IGA WS)

200 1.7 250 0.62 300 0.26 350 0.32 0.30 400 0.30 0.17 450 0.10 0.11 500 0.10 0.09 550 0.08 600 0.08 650 0.08 700 0.08 0.23 750 0.08 0.19 800 0.08 0.17 850 0.08 0.18 900 0.08 0 19 950 0.08 0 17 1000 0.57 0 15 1050 0.93 0. 14 1100 1.3 0.14 1150 0.14 1200 0.15 1250 0 15 1300 0.15 1350 0.14 1400 0.15 1450 0.14 1500 0.14 1550 0.15 1600 0.18 1650 0.27 1700 0.80 1750 1.3 1800 2.0

2.5

2.0

1.5 -

1.0 -

0.5

UV SCF and Vis/NIR SCF WS Relative Combined Uncertainties

(a) UV WS Relative Combined Uncertainty

(b) Vis WS Relative Combined Uncertainty

• UV WS

o Vis WS

A IGA WS

0.0 4- 200 400 600 800 1000 Wavelength |nm|

1200 1400 1600 1800

Figure 7.5. Relative combined standard uncertainties for NIST spectral power

responsivity working standards (a) UV WS, (b) Vis WS, and (c) IGA WS.

66
7.4.1 UV Silicon Customer Detector Example Uncertainty

Table 7.6 lists the example uncertainty components for the calibration of a customer's UV

silicon photodiode using the UV WS. The UV WS calibration uncertainty is the relative

combined standard uncertainty from Table 7.4. The amplifier uncertainty components (0.04 %)

for the customer and WS are combined by RSS for a value of 0.06 % in the table. Uncertainty

components for the temperature coefficient and long-term stability are not included in the

uncertainty for transfer calibrations to customer photodiodes. The example relative combined

standard uncertainty was determined by the RSS of the components. The measurement

uncertainty components are shown in 5 nm intervals in Figure 7.6a. Table 7.6 lists the relative

expanded standard uncertainty {k = 2) and Figure 7.6b shows the relative expanded standard

uncertainty {k = 2) at 5 nm intervals.

Table 7.6. Example uncertainty for customer OSI Optoelectronics UV-100 silicon

photodiode

Uncertainty contribution from each component of uncertainty [%] Customer & Relative Relative

Wavelength

UV WS

calibration

Customerto-monitor

ratio noise

UV WS-tomonitor

ratio noise

UV WS

amplifier

calibration

Monochromator

wavelength

Bandpass

error

Reproducibility + other componentscombined

standard

uncertainty

expanded

uncertainty

(A = 2) [nm] Type B Type A Type A TypeB Type B Type B Type A [%| |%I

200 1,7 0.49 0.42 0.06 0.04 0.14 0.52 1.9 3.8 220 0.70 0.16 0.05 0.06 0.03 0.11 0.46 0.86 1.7 240 0.64 0.05 0.08 0.06 0.01 0.08 0.40 0.77 1.5 260 0.52 0.05 0.08 0.06 0.02 0.06 0.34 0.64 1.3 280 0.31 0.02 0.06 0.06 0.02 0.06 0.30 0.45 0.90 300 0.26 0.08 0.01 0.06 0.01 0.04 0.29 0.40 0.80 320 0.26 0.01 0.02 0.06 0.00 0.01 0.29 0.39 0.78 340 0.30 0.08 0.03 0.06 0.01 0.02 0.28 0.42 0.84 360 0.33 0.05 0.01 0.06 0.01 0.03 0.27 0.43 0.86 380 0.37 0.03 0.09 0.06 0.01 0.02 0.25 0.46 0.92 400 0.30 0.03 0.01 0.06 0.01 0.01 0.21 0J7 0.74 420 0.18 0.05 0.04 0.06 0.01 0.00 0.18 0.27 0.54 440 0.12 0.02 0.03 0.06 0.00 0.00 0.14 0.20 0.40 460 0.10 0.02 0.03 0.06 0.00 0.00 0.11 0.16 0.32 480 0.10 0.01 0.01 0.06 0.00 0.00 0.08 0.14 0.28

500 0.10 0.02 0.00 0.06 0.00 0.00 0.05 0.13 0.26

67
Example Customer UV Si Photodiode (OSI UV-100) Uncertainty Components

_ 3.0 -

(a)

Example Customer UV Si Photodiode (OSI UV-100) Relative Expanded Uncertainty {k = 2)

200 250 300 350 400 450 500 Wavelength [nm]

(b)

Figure 7.6. Example customer UV Si photodiode uncertainty: (a) Example

uncertainty components at 5 nm intervals for transfer to a customer's OSI

Optoelectronics UV-100 silicon photodiode, (b) Example relative expanded

uncertainty (k = 2).

7.4.2 Visible Silicon Customer Detector Example Uncertainty

Table 7.7 lists the uncertainty components for the calibration of a customer's Hamamatsu S1337-

lOlOBQ or S2281 visible silicon photodiode using the Vis WS. The Vis WS calibration

uncertainty is the relative combined standard uncertainty from Table 7.1. The amplifier

uncertainty components (0.04 %) for the customer and WS are combined by RSS for a value of

0.06 % in the table. The bandpass uncertainty component is negligible, but included for

comparison to the UV and NIR customer photodiode uncertainty tables. Uncertainty

components for the temperature coefficient and long-term stability are not included in the

uncertainty for transfer calibrations to customer photodiodes. The example relative combined

68
standard uncertainty was determined by the RSS of the components and is listed in Table 7.7. Figure 7.7a shows the measurement uncertainty components in 5 nm intervals. Figure 7.7b

shows the relative expanded standard uncertainty {k = 2) at 5 nm intervals.

Table 7.7. Example uncertainty for customer Hamamatsu S 1337-1 01 OBQ or S2281

silicon photodiode

Uncertainty contribution from each component of uncertainty [%]

Wavelength

V IS W o

calibration

Customerto-monitor

ratio noise

Vis WS-tomonitor

ratio noise

Customer &

Vis WS

amplifier

calibration

Monochromator

wavelength

Bandpass

error

Reproducibility ^ other components

Rtlalivu combined

standard

unccrtaintv

Relative expanded

uncertainty

(A = 2) [nm] TypeB Type A Type A Type B Type B Type B Type A l%l i%|

350 0.30 0.03 0.04 0.06 0.00 0.02 0.17 0J6 0.72 375 0.23 0.01 0.01 0.06 0.00 0.01 0.10 0.26 0.52 400 0.17 0.00 0.01 0.06 0.00 0.01 0.05 0.19 0J8

425 0.13 0.00 0.00 0.06 0.00 0.00 0.03 0.15 OJO

450 0.11 0.00 0.00 0.06 0.00 0.00 0,01 0.12 0.24 475 0.09 0.00 0.00 0.06 0.00 0.00 0.01 0.11 0.22 500 0.09 0.00 0.00 0.06 0.00 0,00 0.01 0.11 0.22 525 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20 550 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

575 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

600 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

625 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

650 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

675 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20 700 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

725 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20 750 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

775 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

800 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

825 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

850 0.08 0.00 0.00 0,06 0.00 0.00 0.01 0.10 0.20

875 0.08 0.00 0.00 0.06 0.00 0.00 0,01 0.10 0.20

900 0.08 0.00 0.00 0.06 0.00 0.00 0,01 0.10 0.20

925 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

950 0.08 0.00 0.00 0.06 0.00 0.00 0.01 0.10 0.20

975 0.43 0.00 0.00 0.06 0.00 0.01 0.04 0.44 0.88

1000 0.57 0.00 0.00 0.06 0.01 0.01 0.09 0.58 1.2

1025 0.71 0.00 0.00 0.06 0.02 0.02 0.21 0.74 1.5

1050 0.93 0.00 0.00 0.06 0.02 0.02 0.37 1.0 2.0

1075 1.1 0.00 0.00 0.06 0.01 0.02 0,49 IJ 2.6

1100 1.3 0.00 0.00 0.06 0.00 0.03 0.58 1.5 3.0

69
Example Customer Visible Si Photodiode (Hamamatsu S2281/S1337-1010BQ) Uncertainty Components

1.6

1.4

I

1.0-1

1 0 8 % 0.6 -

i 0.4

OS

0.2 A

0.0 350 400 450 500 550 600 650 700 750 800 850

Wavelength |nm]

900 950 1000 1050 1100

Vis WS calibration

w Customer-to-monitor ratio noise

A Vis WS-to-monitor ratio noise

X Customer & Vis WS amplifier calibration

Monochromator wavelength

+ Bandpass error

Reproducibility and other components

(a)

Example Customer Visible Si Photodiode (Hamamatsu S2281/S1337-1010BQ)

Relative Expanded Uncertainty {k = 2)

700 750

Wavelength [nm]

Figure 7.7. Example customer Hamamatsu 1337-1 01 OBQ or S2281 Si

photodiode uncertainty: (a) Example uncertainty components at 5 nm intervals,

(b) Example relative expanded uncertainty {k = 2). The discontinuity in the curve

at 950 nm is the result of Vis WS calibration with different standards.

(b)

7.4.3 Near-Infrared Customer Detector Example Uncertainties

The calibration uncertainties for example customer indium gallium arsenide (InGaAs) and

germanium (Ge) photodiodes are presented in Table 7.8 and Table 7.9 respectively. Several of

the data columns are detector-dependent, thus these tables are presented as examples of typical

uncertainties for these types of photodiodes. The reported uncertainty does not include several

components - such as responsivity uniformity, polarization sensitivity, linearity, temperature

coefficient, and long-term stability - that are unknown for customer's photodetectors and

radiometers in general.

7.4.3.1 InGaAs Detector Example Uncertainty

Table 7.8 lists the uncertainty components for the calibration of a customer's InGaAs photodiode

using the IGA WS. The IGA WS calibration uncertainty is the relative combined standard

uncertainty from Table 7.3. The amplifier uncertainty components (0.04 %) for the customer

70
and WS are combined by RSS for a value of 0.06 % in the table. The bandpass uncertainty

component is negligible, but included for comparison to the other customer uncertainty tables.

Uncertainty components for the temperature coefficient and long-term stability are not included

in the uncertainty for transfer calibrations to customer photodiodes. The example relative

combined standard uncertainty was determined by the RSS of the components and is listed in Table 7.8. The measurement uncertainty components are shown in 5 nm intervals in Figure 7.8a.

Figure 7.8b shows the relative expanded uncertainty {k = 2) at 5 nm intervals.

The example relative expanded uncertainty {k = 2) presented in Table 7.8 is for a GPD

Optoelectronics GPD5000 InGaAs photodiode. Several of the uncertainty components are detector-dependent in Table 7.8, thus this table is presented as an example representing typical

uncertainties for this type of photodiode. The uncertainty is analyzed for each type of

customer-supplied InGaAs photodiode.

71
Table 7.8. Example uncertainty for customer InGaAs photodiode

Uncertainty contribution from each component of uncertainty [%]

Relative Customer- IGA WS-to- IGA WS Monochro- Reproducib- combined expanded IGA WS to-monitor monitor amplifier mator Bandpass ility + other standard uncertainty

Wavelength calibration ratio noise ratio noise calibration wavelength error components uncertainty (A = 2) [nm] Type B Type A Type A TypeB TypeB TypeB Type A |%] [%]

700 0.23 0.02 0.02 0.06 0.01 0.00 0,08 0.25 0.50 725 0.21 0.02 0.02 0.06 0.01 0.01 0.10 0.24 0.48 750 0.19 0.02 0.02 0.06 0.01 0.01 0.12 0.24 0.48 775 0.18 0.02 0.03 0.06 0.01 0.01 0.14 0.24 0.48 800 0.17 0.02 0.03 0.06 0.00 0.01 0.16 0.24 0.48 825 0.17 0.02 0.03 0.06 0.00 0.01 0.17 0.25 0.50 850 0.18 0.02 0.03 0.06 0.01 0.01 0.19 0.28 0.56 875 0.18 0.02 0.02 0.06 0.01 0.01 0.22 0.29 0.58 900 0.19 0.01 0.02 0.06 0.01 0.01 0.24 0.31 0.62 925 0.23 0.01 0.01 0.06 0.01 0.01 0.26 0.35 0.70 950 0.17 0.01 0.01 0.06 0.01 0.01 0.28 0.33 0.66 975 0.16 0.00 0.01 0.06 0.01 0.01 0.29 0.34 0.68 1000 0.15 0.00 0.00 0.06 0.01 0.01 0.29 0.33 0.66 1025 0.14 0.00 0.00 0.06 0.01 0.01 0.28 0.32 0.64 1050 0.14 0.00 0.00 0.06 0.01 0.01 0.28 0.32 0.64 1075 0.14 0.00 0.00 0.06 0.01 0.01 0.27 031 0.62 1100 0.14 0.00 0.00 0.06 0.01 0.01 0.27 0.31 0.62 1125 0.14 0.00 0.00 0.06 0.01 0.01 0.27 0.31 0.62 1150 0.14 0.00 0.00 0.06 0.01 0.01 0.28 0.32 0.6 1175 0.14 0.00 0.00 0.06 0.01 0.01 0.28 0J2 0.6 1200 0.15 0.00 0.00 0.06 0.00 0.01 0.27 0.31 0.6 1225 0.15 0.00 0.00 0.06 0.00 0.01 0.26 0.31 0.6 1250 0.15 0.00 0.00 0.06 0.00 0.01 0.24 0.29 0.6 1275 0.15 0.00 0.00 0.06 0.00 0.01 0.22 0.27 0.5 1300 0.15 0.00 0.00 0.06 0.00 0.01 0.20 0.26 0.52 1325 0.14 0.00 0.00 0.06 0.00 0.01 0.19 0.24 0.48 1350 0.14 0.00 0.00 0.06 0.00 0.01 0.18 0.24 0.48 1375 0.15 0.00 0.01 0.06 0.00 0.01 0.18 0.24 0.48 1400 0.15 0.00 0.01 0.06 0.00 0.02 0.18 0.24 0.48 1425 0.14 0.00 0.00 0.06 0.00 0.01 0.17 0.23 0.5 1450 0.14 0.00 0.00 0.06 0.00 0.01 0.16 0.22 0.44 1475 0.14 0.00 0.00 0.06 0.00 0,01 0.15 0.22 0.44 1500 0.14 0.01 0.00 0.06 0.00 0.01 0.15 0.21 0.42 1525 0.14 0.01 0.01 0.06 0.00 0.01 0.14 0.21 0.42 1550 0.15 0.01 0.01 0.06 0.00 0.02 0.13 0.21 0.42 1575 0.16 0.01 0.01 0.06 0.00 0.02 0.13 0.22 0.44 1600 0.18 0.01 0.01 0.06 0.00 0.03 0.16 0.25 0.5 1625 0.21 0.01 0.01 0.06 0.00 0.05 0.22 0.31 0.6 1650 0.27 0.02 0.01 0.06 0.02 0.08 0.30 0.42 0.8 1675 0.66 0.02 0.02 0.06 0.08 0.10 0.40 0.78 1.6 1700 0.80 0.03 0.02 0.06 0.04 0.11 0,52 1.0 1.9 1725 1.0 0.04 0.04 0.06 0.04 0.10 0.64 1.2 2.4 1750 1.3 0.06 0.05 0.06 0.04 0.09 0.76 1.6 3.2 1775 1.7 0.08 0.07 0.06 0.01 0.07 0.88 1.9 3.8 1800 2.0 0.11 0.08 0.06 0.01 0.05 1.0 2.3 4.6

72
Example Customer InGaAs Photodiode Uncertainty Components

0.0 700 800 1000 1100 1200 1300 1400 Wavelength |nml

1500 1600 1700 1800

IGA WS calibraiion

» Cusiomer-lo-monilor ratio noise

i, IGA WS-to-monitor ratio noLse

X Customer & IGA WS amplifier calibration

- Monochromalor wavelenglb

Bandpass error

Reproducibility + other components

(a)

Example Customer InGaAs Photodiode Relative Expanded Uncertainty (A: = 2)

1200 1300 Wavelength |nm|

Figure 7.8. Example customer InGaAs photodiode uncertainty: (a) Example

uncertainty components at 5 nm intervals, (b) Example relative expanded

uncertainty {k = 2).

(b)

7.4.3.2 Ge Detector Example Uncertainty

Table 7.9 lists the uncertainty components for the calibration of a customer's thermoelectrically

cooled Ge photodiode using the IGA WS. The IGA WS uncertainty is the relative combined

standard uncertainty from Table 7.3. The amplifier uncertainty components (0.04 %) for the

customer and WS are combined by RSS for a value of 0.06 % in the table. The bandpass

uncertainty component is negligible, but included for comparison to the other customer

uncertainty tables. Uncertainty components for the temperature coefficient and long-term

stability are not included in the uncertainty for transfer calibrations to customer photodiodes.

The example relative combined standard uncertainty was determined by the RSS of the

components and is listed in Table 7.9. Figure 7.9a shows the measurement uncertainty

components at 5 nm intervals. Figure 7.9b shows the relative expanded uncertainty {k = 2) at 5 nm intervals.

The example relative expanded uncertainty {k = 2) presented in Table 7.9 is for a Judson

EG&G J16TE2-8A6-R05M-SC Ge photodiode. As with the InGaAs transfer uncertaint>' in Table 7.8 several of the uncertainty components are detector-dependent, thus this table is

73
presented as an example representing typical uncertainties for this type of photodiode. The

uncertainty is analyzed for each type of customer-supplied photodiode.

Table 7.9. Example uncertainty for customer TE cooled Ge photodiode

Uncertainty contribution from each component of uncertainty [%] Customer & Relative Relative Customer- IGA WS-to- IGA WS Monochro- Reproducib- combined expanded IGA WS to-monitor monitor amplifier mator Bandpass ility + other standard uncertaintv Wavelength calibration ratio noise ratio noise calibration wavelength error components uncertainty (k = 2)

'

[nm] TypeB Type A Type A Type B TypeB Type B Type A 1%] [%1

700 0.23 0.02 0.01 0.06 0.04 0.01 0.11 0.26 0.52 725 0.21 0.02 0.01 0.06 0.02 0.01 0.10 0.24 0.48 750 0.19 0.02 0.02 0.06 0.01 0.01 0.09 0.22 0.44 775 0.18 0.02 0.02 0.06 0.01 0.01 0.08 0.21 0.42 800 0.17 0.02 0.02 0.06 0.01 0.01 0.08 0.20 0.40 825 0.17 0.02 0.02 0.06 0.03 0.01 0.07 0.20 0.40 850 0.18 0.02 0.02 0.06 0.05 0.01 0.06 0.21 0.42 875 0.18 0.02 0.02 0.06 0.05 0.01 0.06 0.21 0.42 900 0.19 0.01 0.01 0.06 0.07 0.01 0.05 0.22 0.44 925 0.23 0.01 0.01 0.06 0.08 0.01 0.05 0.26 0.52 950 0.17 0.01 0.01 0.06 0.05 0.01 0.05 0.19 0.38 975 0.16 0.00 0.00 0.06 0.02 0.01 0.05 0.18 0.36 1000 0.15 0.00 0.00 0.06 0.01 0.01 0.06 0.17 0.34 1025 0.14 0.00 0.00 0.06 0.01 0.01 0.06 0.17 0.34 1050 0.14 0.00 0.00 0.06 0.01 0.01 0.07 0.17 0J4

1075 0.14 0.00 0.00 0.06 0.02 0.01 0.07 0.17 034

1100 0.14 0.00 0.00 0.06 0.01 0.01 0.07 0.17 0.34 1125 0.14 0.00 0.00 0.06 0.01 0.01 0.07 0.17 034

1150 0.14 0.00 0.00 0.06 0.00 0.01 0.06 0.17 0.34 1175 0.14 0.00 0.00 0.06 0.00 0.01 0.06 0.17 0.34 1200 0.15 0.00 0.00 0.06 0.01 0.01 0.05 0.17 0.34 1225 0.15 0.00 0.00 0.06 0.01 0.01 0.05 0.17 03

1250 0.15 0.00 0.00 0.06 0.01 0.01 0.04 0.17 0.34 1275 0.15 0.00 0.00 0.06 0.00 0.01 0.04 0.16 0.32 1300 0.15 0.00 0.00 0.06 0.00 0.01 0.04 0.16 032

1325 0.14 0.00 0.00 0.06 0.00 0.01 0.03 0.16 032

1350 0.14 0.00 0.00 0.06 0.01 0.01 0.03 0.16 032

1375 0.15 0.00 0.00 0.06 0.01 0.02 0.03 0.16 032

1400 0.15 0.00 0.00 0.06 0.01 0.02 0.03 0.16 032

1425 0.14 0.00 0.00 0.06 0.01 0.02 0.03 0.16 032

1450 0.14 0.00 0.00 0.06 0.01 0.02 0.03 0.16 032

1475 0.14 0.00 0.00 0.06 0.01 0.02 0.03 0.16 032

1500 0.14 0.00 0.00 0.06 0.02 0.02 0.04 0.16 032

1525 0.14 0.00 0.00 0.06 0.06 0.02 0.05 0.17 0.34 1550 0.15 0.01 0.01 0.06 0.08 0.02 0.05 0.19 038

1575 0.16 0.01 0.01 0.06 0.07 0.02 0.06 0.20 0.40 1600 0.18 0.01 0.01 0.06 0.06 0.03 0.08 0.21 0.42 1625 0.21 0.01 0.01 0.06 0.06 0.03 0.09 0.24 0.5 1650 0.27 0.01 0.01 0.06 0.09 0.03 0.11 0.32 0.6 1675 0.66 0.02 0.02 0.06 0.19 0.03 0.14 0.70 1.4 1700 0.80 0.02 0.02 0.06 0.24 0.03 0.17 0.86 1.7 1725 1.0 0.03 0.04 0.06 0.25 0.03 0.19 1.1 2.2 1750 1.3 0.05 0.06 0.06 0.19 0.03 0.22 1.4 2.8 1775 1.7 0.07 0.08 0.06 0.14 0.03 0.25 1.7 3.4 1800 2.0 0.10 0.10 OOf) 0.16 0.03 0.28 2.0 4.0

74
Example Customer TEC Ge Photodiode Uncertainty Components

700 800 900 IJdII HIKI 1411(1

Wavelength Inm)

151)1) 1600 1700 1800

* IGA WS calibration

« Customcr-to-raonitor ratio noise ^ IGA WS-to-monitor ratio noise X Customer & IGA WS amplifier calibration

'// Monochromator wavelength

"- Bandpass error

O Reproducibility + other componails

Example Customer TEC Ge Photodiode Relative Expanded Uncertainty (Ar = 2)

O

C

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

Wavelength jntn]

Figure 7.9. Example customer TE cooled Ge photodiode uncertainty: (a)

Example uncertainty components at 5 nm intervals, (b) Example relative

expanded uncertainty {k = 2).

75
7.4.4 Filtered Detector Example Uncertainty (Photometer)

Special tests of filtered detectors (such as photometers) can be made under service ID number

39080S. It should be noted that the relative uncertainty due to the reproducibility will increase

due to the decrease in signal in the "wings" of the response. As mentioned in Sec. 7.2.1, when

the slope of the responsivity curve is steep, the uncertainty due to wavelength calibration

increases. Also the uncertainty due to stray-light and bandwidth can increase by orders of

magnitude relative to the uncertainties within the bandpass of the filtered detector.

Table 7.10 lists the uncertainty components for the calibration of a customer's photometer using

the Vis WS. The Vis WS calibration uncertainty is the relative combined standard uncertainty

from Table 7.2. The amplifier uncertainty components (0.04 %) for the customer and WS are combined by RSS for a value of 0.06 % in the table. The bandpass uncertainty component is not

negligible for a detector with sharp changes in responsivity like a photometer. Also, the DVM

and stray-light uncertainty components are significant for this measurement and are combined by

RSS with the reproducibility. Uncertainty components for the temperature coefficient and

long-term stability are not included in the uncertainty for transfer calibrations to customer

detectors. The example relative expanded uncertainty (k = 2) was determined by the RSS of the

components and is listed in Table 7.10. The measurement uncertainty components are shown in

5 nm intervals in Figure 7.10a. Figure 7.10b shows the relative expanded uncertainty (k = 2) at 5 nm intervals.

Because several of the uncertainty components are photometer dependent, Table 7.10 is presented as an example illustrating typical uncertainties for a customer's photometer. The

uncertainty is analyzed for each type of customer-supplied photometer.

Table 7.10. Example uncertainty for customer photometer

Uncertainty contribution from each component of uncertainty [%]

Reproducib- Customer & ility, DVM, Relative Relative Customer- Vis WS-to- Vis WS Monochro- stray-light, combined expanded

Vis WS to-monitor monitor amplifier mator Bandpass and other standard uncertainty

Wavelength calibration ratio noise ratio noise calibration wavelength error components uncertainty (k = 2) [nm] Type B Type A Type A Type B Type B TypeB Type A |%] [%l

400 0.18 0.26 0.01 0.06 1.6 0.31 4.3 4.6 9.2 425 0.14 0.02 0.01 0.06 0.39 0.17 0.68 0.82 1.6 450 0.11 0.01 0.00 0.06 0.45 0.15 0.14 0.51 1.0 475 0.10 0.00 0.00 0.06 0.48 0.13 0.06 0.51 1.0 500 0.09 0.00 0.00 0.06 0.40 0.09 0.09 0.44 0.88 525 0.09 0.00 0.00 0.06 0.18 0.07 0.07 0.23 0.46 550 0.08 0.00 0.00 0.06 0.03 0.04 0.06 0.13 0.26 575 0.08 0.00 0.00 0.06 0.11 0.02 0.06 0.16 0.32 600 0.08 0.00 0.00 0.06 0.22 0.02 0.08 0.26 0.52 625 0.08 0.00 0.00 0.06 0.35 0.06 0.13 0.39 0.78 650 0.08 0.00 0.00 0.06 0.48 0.15 0.21 0.55 1.1 675 0.08 0.00 0.00 0.06 0.60 0.30 0.35 0.76 1.5 700 0.08 0.02 0.00 0.06 0.70 0.46 0.60 1.0 2.0 725 0.08 0.21 0.00 0.06 0.74 0.51 4.5 4.6 9.2

76
Example Customer Photometer Uncertainty Components

0.1

0.01

0.001

0.000] 400 425 450 475 500 525 550 575 600 625 650 675 700 725 Wavelength |nni|

• Vis WS calibration

« Customcr-lo-monilor ralio noise

i Vis WS-to-monilor ralio noise

s! Customer & Vis WS amplifier calibration

X M onochromator wavelength

'-• Bandpass error

* Reproducibility & DVM & stray-light & other components

Example Customer Photometer Relative Expanded Uncertainty (A: = 2)

450 475 525 550 575

Wavelength |niii|

625 650 725

Figure 7.10. Example customer photometer uncertainty: (a) Example uncertainty

components at 5 nm intervals, (b) Example relative expanded uncertainty (k = 2).

77
7.5 Spatial Uniformity Measurement Uncertainty

The uncertainty components for the responsivity spatial uniformity measurement of a typical Si

photodiode are listed in Table 7.11. Measurement repeatability, i.e., short-term drift, is the

relative standard deviation of three measurements in the center of the active area of the detector

during the spatial scanning measurements. The measurement noise is the average standard

deviation of the mean of the three measurements. The relative combined standard uncertainty is the RSS of the measurement repeatability, 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 measurement noise varies spectrally, primarily due to the change in

monochromator flux magnitude with wavelength. Note that the variation in responsivity over

the measured area is typically 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 [56] 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. The reported

uniformity indicates if any large discontinuities are present in the responsivity uniformity that can lead to larger than expected uncertainties in responsivity measurements. Quantitative

application of the reported uniformity results requires examination of the irradiance geometry

and equipment involved.

Table 7.11. Photodetector spatial uniformity measurement repeatability uncertainty

Uncertainty contribution from each component of

uncertainty [%]

Relative combined

Measurement standard Wavelength Repeatability noise DVM uncertainty [nm] TypeB Type A Type A [%]

500 0.0020 0.0012 0.0007 0.0025

1000 0.0056 0.0033 0.0007 0.0065

78
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 the NiST Quality

System for Measurement Services [8, 80] based on the ISO/IEC 17025:2005 [1, 81]. Although

quality procedures have always been 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.

In 1993 the Optical Technology Division began developing a calibration quality system based on ANSI/NCSL Z540-1-1994 (the predecessor of ISO/IEC 17025) [82-84]. The outcome was to unify all the calibration services offered by the Division with standard formats and similar

procedures. Balancing fiinctionality and bureaucracy was a concern from the start. Efforts were

directed toward developing a usefiil 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 [85]. For

calibrations, the working standards are randomly chosen each week allowing 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 Fig. 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 standard deviation of the responsivity reproducibility 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.

I 79
1

I I I I

I

1 I 1 I

I

I I I I I

I I I I z ~ UCL

m

- Center

; Line

• _I

^ LCL

I I

I I I I I I I I I 1 I I I I I I I I I :-1992 19-Apr-1993 14-Sep-1995 6-Aug-1996 3-Oct-1996

Time [days]

Figure 8.1. Control chart example for a NIST Visible Working Standard

(Vis WS).

8.2 Interlaboratory Comparisons

NIST has historically been involved with many interlaboratory comparisons of spectral power

responsivity. In 1993 NIST participated in an international intercomparison carried out by the

Bureau International des Poids et Measures (BIPM) over the 250 nm to 1000 nm wavelength

region [36, 86]. There have also several bilateral intercomparisons, such as a NIR bilateral

intercomparison between NRC (Canada) and NIST in 2001 [87].

The International Committee of Weights and Measures (Comite' International des Poids et Mesures, CIPM), under the authority given to it in the Metre Convention, crafted a Mutual

Recognition Arrangement (MRA). In 1999, the CIPM MRA was signed by the directors of the

national metrology institutes (NMIs) of thirty-eight Member States of the Metre Convention

(including NIST, the U.S. NMI) and representatives of two international organizations. Sixty- seven other institutes have now signed the MRA^. The MRA objectives are to establish the

degree of equivalence of national measurements standards maintained by NMIs; to provide for

the mutual recognition of calibration and measurement certificates issued by NMIs; and to thereby provide governments and other parties with a secure technical foundation for wider

agreements related to international trade, commerce and regulatory affairs.

Under the MRA, the metrological equivalence of national measurement standards are to be

determined by a set of key comparisons chosen and organized by the Consultative Committees

of the CIPM working closely with Regional Metrology Organizations (RMOs).

At the annual meeting in 1997 of the Consultative Committee for Photometry and Radiometry

(CCPR), several Key Comparisons were identified in the field of optical radiation metrology, one of which, K2, was spectral (power) responsivity. The Key Comparison of Spectral

Responsivity was divided into three separate comparisons covering different spectral regions:

0.31310

P

0.31305 so

g 0.31300

0.31295

0.31290

0.31285

0.31280

o

n.

_l I I U

21 -Sep- 1992 1-De(

More information about the MRA can be found at the BIPM website http://vvwvv.bipm.org/en/cipm-Tnra.^

80
K2.a (900 nm to 1600 nm), K2.b (300 nm to 1000 nm), and K2.c (200 nm to 400 nm). 1 hcse

intercomparisons and NIST's participation are described in the following sections.

8.2.1 CCPR K2.a (NIR) 900 nm to 1650 nm

The Key Comparison K2.a which covered the spectral range from 900 nm to 1600nm was

piloted by NIST. InGaAs photodiodes were used as the intercomparison artifacts. All of the

measurements are complete, but the final results have not been published [88] at the time of this

document's publication.

8.2.2 CCPR K2.b (Visible) 400 nm to 950 nm

The Key Comparison CCPR-K2.b (300 nm to 1000 nm) was piloted by the BIPM. A total of 17

NMIs participated in this comparison. Four silicon detectors (single element and trap type) were measured by each participant and by the pilot laboratory (BIPM). Figure 8.2 summarizes the

results.

The following graph is given to provide more information on the transfer detectors (they are not

graphs of equivalence): they show the relative differences from the KCRV (base line) when

either traps or single diodes are used as transfer detectors. The uncertainty bars (coverage factor

k^l) correspond to NIST's uncertainty combined with the uncertainty of the transfer. They do

not include the uncertainty of the reference.

o

£

Is•a

250 300 350 400 450 500 550 600 650 700

Wavefength / nm

750 800 850 900 950 1000 1050

NIST

Figure 8.2. NIST difference from KCRV in CCPR K2.b (Visible) Key

Comparison (NIST measured in Dec. 2000.).

81
8.2.3 CCPR K2.C (UV) 200 nm to 450 nm

The third spectral responsivity Key Comparison (K2.c) started in December 2003 and covers the

spectral region of 200 nm to 400 nm. This comparison is still in progress and no results are

available at the time of the publication [88] of this document.

8.3 Changes in the Spectral Power Responsivity Scale

8.3.1 UV Scale (200 nm to 500 nm)

The most recent realization of NIST's spectral radiant power responsivity scale in the 200 nm to 500 nm spectral region (UV SCF) was 2005. The changes in the UV WS assigned values from

the previous realization in 2001 are shown in Figure 8.3. The difference between the two

realizations includes changes in both the realized scale and in the responsivities of the UV WS.

Some of the changes seen are larger than the 2005 expanded uncertainty {k = 2). Because of

these changes, new UV photodiodes were investigated to replace the current UV WS. The

photodiodes chosen as new UV WS are the International Radiation Devices (IRD) model UVG100 photodiodes^^. These diodes have a 10 mm x 10 mm active area. The characterization and

calibration of a group of four UVG-100 diodes is continuing at this time. It is expected that they

will replace the current UV WS in 2008.

UV WS Change from 2001 Scale.

200 250 300 350 400 450 500

Wavelength |nm]

—^UVWS Change 2001 to 2005 ^. UV WS Exp Uncert 2005 (+) UV WS Exp Uncert 2005 (-)

Figure 8.3. Change in the UV spectral power responsivity scale (UV WS) from

2001 to 2005 and expanded uncertainty {k = 2).

International Radiation Detectors (IRD), Torrance, CA (htip://www.ird-inc.com/l.

82
8.3.2 Visible Scale (350 nm to 1100 nm)

The current responsivity scale from 350 nm to 1 100 nm for spectral radiant power at the NIST

Vis/NIR SCF was realized in 2006. The changes in the Vis WS assigned values from the

previous reahzation in 2005 are shown in Figure 8.4. The uncertainties were significantly

reduced beyond 950 nm because of the Ge TS calibrated by SIRCUS. The uncertainty still increases beyond 950 nm mainly due the temperature coefficient of the photodiodes. (See the

Vis WS uncertainty Sec. 7.2.1.) New temperature controlled mounts were acquired to keep the

Vis WS at a constant temperature which will reduce this uncertainty. The uncertainty beyond

950 nm will sdll be larger than in the visible region due to the increasing spadal non-uniformity

of the diodes as the wavelength approaches the Si bandgap.

700 Wavelength (nm]

900 1000 1100

-Vis WS Change WSExp Uncert 2005 (+) _s>-- WSExp Uncert 2005 {-) -^-WSExp Uncert 2006 (+) WSExp Uncerl 2006 (-)

Figure 8.4. Change in the visible spectral power responsivity scale (Vis WS)

from 2005 to 2006 and expanded uncertainties {k = 2).

83
8.3.3 NIR Scale (700 nm to 1800 nm)

The NIR spectral radiant power responsivity scale (IGA WS: 700 nm to 1800 nm) at the Vis/NIR

SCF was last realized in 2006, except in the 1650 nm to 1800 nm region where scale is still based on the 1999 realization [49]. The previous NIR WS were a group of four Ge WS

described in the 1998 edition of this document [46]. The Ge WS and IGA WS were calibrated

concurrently in 1999 as described in Sec. 5.4.3. The change of the assigned values of the

IGA WS from the previous realization in 1999 (700 nm to 1650 nm) is shown in Figure 8.5. The

change of the assigned values to the Ge WS from the realization in 1 996 to 1 999 is shown in

Figure 8.6.

.2.0 -L. -— —

700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

Wavelength [nm| —»— IGA WS Change IGA WS Exp Uncert 1999 (+) IGA WS Exp Uncert 1999 (-) --^!>IGA WS Exp Uncert 2006 (+) -*-IGA WS Exp Uncert 2006 (-)

Figure 8.5. Change in the NIR spectral power responsivity scale (IGA WS) from

1999 to 2006 and expanded uncertainties {k = 2).

700 800 900 1000 1100 1200 1300 MOO 1500 1600 1700 1800 Wavelength |nin| —^GeWS Change 1996 to 1999 —s—WSExp Uncertainly 1996 (+) —»— WS Exp Uncertainty 1996 (-) -*~WSExp Uncertainty 1999 (+) ~-*~WS Exp Uncertainty 1999 (-)

Figure 8.6. Change in the NIR spectral power responsivity scale (Ge WS) from

1996 to 1999 and expanded uncertainties {k = 2). The current IGA WS were

calibrated concurrently with the Ge WS in 1999.

84
9. Characteristics of Photodiodes Available from NIST

This section describes the characteristics of the siHcon photodiodes provided by NIST under

Service ID numbers 3907 IC, 39073C, and 39077C. The physical and electrical characteristics

of the photodiodes are discussed and the results of linearity measurements are given.

9.1 Hamamatsu S1337-1010BQ

Hamamatsu SI 337-10 lOBQ silicon photodiodes have been supplied by NIST as spectral radiant

power responsivity standards. NIST currently supplies a similar diode, the Hamamatsu S2281

for this purpose under NIST Service ID numbers 39073C and 39077C. The major differences

between the SI 337-1 OlOBQ and the S2281 photodiodes are how they are packaged and the

shape of the active area. The S1337-1010BQ is packaged in a square ceramic package with a square active area. The S2281 is packaged in a round package with a round active area. As

mentioned previously, the Hamamatsu SI 337 series diode is a popular diode for radiometric

standards and it has been extensively characterized [35, 36, 89, 90]. Hamamatsu describes" 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 < 190nm to IlOOnm with a peak at 960 nm. Changes in spectral responsivity have been noted with the Hamamatsu SI 337 and S2281 models

when exposed to light < 250 nm [64, 91, 92]. The S1337-1010BQ also has a high shunt

resistance (dynamic impedance), with a typical value of 200 MQ and a minimum value of

50 MQ".

The typical measured spectral responsivity and quantum efficiency are shown in Figure 6.2 and

Figure 6.3, respectively. The typical spatial uniformities measured at 500 nm and 1000 nm are shown in Figure 6.4a and b, respectively. Because the responsivity of a photodiode can be

affected by temperature, the temperature coefficient of several SI 337 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 SI 337 series photodiode. A second common

silicon photodiode series, the SI 226, 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 level

from 0.5 mW/cm'' to 6.6 mW/cm''. 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 [79]. The linearity is dependent on

the irradiation geometry and will differ for spot sizes significantly smaller than the aperture size.

These values are from the manufacturer's catalog, Photodiodes, Cat. No. KPD 0001 E05. Aug. 1996 T. Hamamatsu Photonics K. K., Sohd State Division, 1 126-1, Ichino-cho, Hamamatsu City, 435-91, Japan

85
0.014

0.012

0.010 PAS

SAT

0.006 -

0.004 t0.002

-0.002

I I I I

I

I I I I II I I I

I I I I II

I I I I

I

I I I I

I I I I I

S1337 Si (25 °C)

Si;126 Si (25 ° C) \ 88

M 2 £

S" -s-»-®-iB-»-«w-a e-s

I I I I I I I I I I 1I I I I I 1I I I I I I I I I I I I I I I I I I I I 300 400 500 600 700 800 900 1000 1100 Wavelength [nm]

Figure 9.1. Temperature coefficient of silicon Hamamatsu S1226 and S1337

photodiodes.

'tos

oav>

>—

1.005

1.000

0.995

0.990

0.985 -

0.980 L—i—i

-1—I—I—I—I—I—I—I—1—I—I—I—I—I—I—I—r—I—I—I—I—

r

0

I _i I I I I I I I r I I I I I I L.

1 2 3 4 5

Irradiance [mW/cm^]

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

86
9.2 Hamamatsu S2281

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

responsivity (NIST Service ID numbers 39073C and 39077C). The characteristics are

essentially identical to the S1337-1010BQ. Since the spectral responsivity and quantum

efficiency of the S2281 is almost indistinguishable from the SI 337, they are not shown in Figure

6.2 and Figure 6.3. The significant differences between the two diode models are that the S2281

has a 1 cm"" 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 mounting since no electrical wiring is required.

9.3 OSI Optoelectronic UV-100

UDT Sensors UVlOO silicon photodiodes have been supplied by NIST as spectral radiant power

responsivity standards. UDT Sensors was acquired by OSI Optoelectronics in 1990 and stopped

using the name "UDT Sensors, Inc." around 2004. The OSI Optoelectronics UV-100 silicon

photodiodes are currently provided as spectral responsivity standards in the UV (NIST Service

ID number 3907 IC). OSI Optoelectronic literature describes the UV-100 as having an

inversion layer structure, excellent UV responsivity, and high shunt resistance. The UV-100 has

a quartz window, a 1 cm^ circular active area, and is housed in a metal case with a BNC

connector similar to the Hamamatsu S2281. The spectral response range is from < 200 nm to

1 100 nm with a peak around 760 nm. The typical shunt resistance value is 10 MQ'-.

The linearity of the UV-100, shown in Fig. 9.3 at 442 nm, spans irradiance levels from

0.1 mW/cm" to 1.1 mW/cm" 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 [79].

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 dark current and 1//noise introduced which will limit the minimum usable

signal.

These values were taken from the OSI Optoelectronics 2007 Optoelectronic Components Catalog

(htlp://vvww.udL.com/OSI-Optoelectronics-Cata]o!£-2007.pdf), OSI Optoelectronics. 12525 Chadron Ave.. Hawthorne, CA 90250, USA.

87
S

o

a.

<u>

9J

1.1

1.0

0.9

0.8

0.7

0.6

1- 1 1

- ® » —

-

1 . 1

• » No Bias Volta

1 1 1

®

ge e

1 —

•

1 1 1

1

1 1 1 — -

-

1 • VBias

-

•

1 1 1

1 1 1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Irradiance [mW/cm ]

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

1.2

1.0

0.8

'>So^ 0.6 >

0.4

0.2

-*--»-*«»«4

• V : «

^ IS ^ i — --

• ImW/cm^ : >^

: ^ : — 1 :

0.0 0.5 1.0 1.5 2.0 2.5

Bias Voltage [V]

3.0 3.5 4.0

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

88
9.4 Detector Mounting 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 mounting fixtures for the Hamamatsu S2281 and the OSI Optoelectronics UV-100

silicon photodiodes are designed for convenient handhng 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 mounts. Most of the fixtures also have a threaded hole (1/4-20: a common

English thread size and 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 during storage and shipment.

The OSI Optoelectronics UV-100 and Hamamatsu S2281 photodiodes are housed in metal cases with BNC outputs. This simplifies the mounting fixture and reduces the construction time since no electrical wiring is required. The mounting fixture mechanical diagram for the UV-100 and

S2281 diodes is shown in Figure 9.5.

Material: Aluminum

Finish: Black Anodized

0.125

1_

1" 0.625"

0.200'^

4 X 2-56 tapped

blind hole

4-40 tapped

through

0.25'

0.375"

1" 2"

0.25'

Figure 9.5. Mechanical diagram of OSI Optoelectronics UV-100 and

Hamamatsu S2281 mounting fixture.

The detectors NIST issues to customers are supplied in a wooden box with a form-fitting foam

insert to hold the photodiode fixture. An anodized aluminum cap is pro\ ided to protect the

detector when not in use. Figure 9.6 is a photograph of the NIST issued photodiode, fixture, cap,

and wooden storage box.

89
Figure 9.6. Photograph of a sample NIST issued photodiode in mounting fixture,

cap, and wooden storage box.

90
10. Future Work

This section will discuss the future modifications and improvements to the UV and Vis/NIR

SCFs. 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.

In the near future, single element nitrided Si detectors will be used as working standards in the

UV. Nitrided sihcon photodiodes have been reported with higher responsivities, better spatial

uniformity, and a lower sensitivity to UV-induced changes than other Si photodiodes [89-91, 93,

94]. To reduce the uncertainty component due to laboratory temperature fluctuations,

temperature-controlled mounts will be used with the Vis WS. New windowless InGaAs

photodiodes will be used as transfer standards from the SIRCUS facility to the SCF. These

photodiodes are mounted in temperature-controlled housings.

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 new prism pre-disperser for the Vis/NIR SCF

monochromator made from Supracil 300 will improve the throughput out to 3 ^m.

Improvements are being studied in the SCF irradiance measurement method [95] that can reduce

the amount of time needed for the calibrations. New methods for irradiance calibration are also

being explored [96]. Continued improvements in the calibration procedures will be made by

applying the Once-is-Enough model [97] throughout the calibration service.

The 'Once-is-Enough' model is really just applying good process control techniques to our

calibration services. For example, a transfer artifact is typically thought of as being well

characterized, that is, its performance is understood, including its stability. If the same facility

and method is used, there is no need to remeasure the artifact' s stability every time it is returned

to NIST for calibration. For certain artifacts with limited lifetimes, such as lamps, it is detrimental to take measurements that are in essence redundant.

Also the measurement system must also be 'under control', meaning that it too is well

characterized and its performance is understood. This means that not only is the system

routinely calibrated, but its performance is monitored, for instance, with check standards. If

control parameters exceed set limits, calibrations are stopped until the system is brought back

under control.

91
11. Acknowledgments

The development, operation, and evolution of this measurement service have involved several

NIST personnel. The major contributors over the past decades have been: Sally Bruce, Chris

Cromer, Bob Saunders; Ed Zalewski.

The authors would like to thank Tom Gentile, Jonathan Hardis, Chris Cromer, Chris Classon,

George Eppeldauer, Keith Lykke, and Steve Brown for their work with the cryogenic

radiometers, silicon trap detectors, and other transfer standard detectors. Additional thanks go to Randy Canfield, Rob Vest, and Ping-Shine Shaw for their work with SURF III and help with UV

detector studies and measurements; and to Joel Fowler for providing the transimpedance

amplifiers.

Thanks are also given to the staffs of the NIST Measurement Services Division and National

Voluntary Laboratory Accreditation Program (NVLAP) of Technology Services and the

Statistical Engineering Division of the Information Technology Laboratory for their help with

maintaining and improving the measurement service.

92
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lEC, Geneva, Switzerland (2005).

F. Hengstberger, Determination of the Spectral Responsivity of Optical Radiation

Detectors, Bureau central de la CIE, Paris, France (1984).

W. Budde, Optical Radiation Measurements, Vol. 4, Physical Detectors of Optical

Radiation, Academic Press, New York, NY (1983).

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, Applied Optics 12 (1 1), 2773-1776(1973).

J. Geist, M. A. Lind, A. R. Schaefer, and E. F. Zalewski, Spectral Radiometry: A New

Approach Based on Electro-Optics, NBS Technical Note 954 (1977).

Preprint of sequel to NBS Technical Note 950 describing the radiometric characteristics

of the DRTIP radiometers. Unpublished work. National Bureau of Standards (1980?).

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E. F. Zalewski and J. Geist, Silicon photodiode absolute spectral response self- calibration, Applied Optics 19 (8), 1214-1216 (1980).

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E. F. Zalewski and C. R. Duda, Silicon photodiode device with 100 % external quantum

efficiency, Apphed Optics 22 (18), 2867-2873 (1983).

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

NBS Special Publication 250-17 (1988).

J. M. Houston and E. F. Zalewski, Photodetector spectral response based on 100%

quantum efficient detectors, in Optical Radiation Measurements II. J. M. Palmer, ed., Int Soc for Optical Engineering, Bellingham, WA, USA, SPIE 1 109 (1989) 268-277.

93
[16] C. L. Cromer, A new spectral response calibration method using a silicon photodiode trap

detector, Presented at the 1991 Measurement Science Conference, Anaheim, CA (1991).

[17] N. P. Fox, Trap Detectors and Their Properties, Metrologia 28 (3), 197-202 (1991).

[18] J. Geist, Current status of and future directions in silicon photodiode self-calibration, in

Publ by Int Soc for Optical Engineering, Bellingham, WA, USA, 1 109 (1989) 246-256.

[19] E. F. Zalewski and C. C. Hoyt, Comparison between Cryogenic Radiometry and the

Predicted Quantum Efficiency of Pn Silicon Photodiode Light Traps, Metrologia 28 (3),

203-206(1991).

[20] N. P. Fox, Radiometry with cryogenic radiometers and semiconductor photodiodes,

Metrologia 32 (6), 535-543 (1996).

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

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99

Appendix A: Sample Calibration Report

UlUrrEO STATES DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

for

namamatsu StJiconPhotodiode Model S2281, S/N UMU

Issued to:

Big-time Laborator)' Atm.: Dr. Aiyssa Scientist

1 00 Laboratory Road

Caisbratiori Citv, CA xxxxx-xxxx

(See your Purchase Order No. ##-###. dated August 10. 2007)

1 . Description of CalibratioB Material

llie test photodiode, serial number H###, is a If amamatsu model S22S I silicon photodiode in a 2 in diatueier anodized alummuni mousl. The active area of the photodiode is '

- 1 cm'.

2. Description of CallbratioM

The spectral radiant power responsivity of the test photodiode was deienirined horn .'^50 nni to

1 100 mil in 5 nm increments by comparisons to silicon photodiode working standards, H630 and

H633. using the monochrotuator-based NISI Visible to Near-hifrared Spectral Comparator

Facility (V'is/IMIR SCF) |1J, The spectra! comparisons between the test photodiode and working

standard photodtodes were perTormed lining a double monoehroniator iUuniinated by a qnortz- halogen lamp as the tunable monockromatic soitrce. The circular e.Kit aperture of Uie Vis-'NIR SCF moBOehromator was imaged { on the test phot(.>diode resulting in a beam diameter of

1 .! mm at the diode. The beam was centered on, and underfilled, tlie photosensitive area.

The waveietifth scale of the monochromator was cahbr-ated with se\'ei-ai laser and emission lines to within an imcettainty of a- 0.1. nm t>ver the entire spectral range. The bandpass {TWfiM) of

the monoehrtimator was 4 nm. The short-circuit photocurrent from the lest phottrdiode and each

workitia standard photodiode was tneasured with a calibrated tnnisimpedance ainplilier. Beam

power tluctuations were monitored whh a beam.sphtter and silicon photodiode. The si)ectral radiant power responsivity scale is based on the NIST reference absolute cryogenic radiometer,

'fhe laboratory temperature during this calibration was 23.8 *C .-i: 0.5 'C.

Calibration Date: September 25, 2007

NIST Test No.: S44 -',.;^,;»?-07

A-1
REPORT OF ( AIJBRATION

NISI' Test 4 39073C - Spectral RespDn*.i\-ity

Big-time Laboratory

Manufacturer: Ilamamatsw

Model #: S2281

Serial #; HMM

Tlif spatial uniformity of the responsixity across the test photodiode phtnosensitivc area was

measured at 500 iniii using the described comparaKir liicility. The unilbrnmy was. measured in 0.5 mm increments using a l.l mm diatneter beam.

3, Re.<iults of C'aHbration

The spectra] radiaat power resp<msivit"y of the lest photodiode is listed iii .'\.-W at each uavekiisrh in Tabk^ i and is plotted in Fig. 1. The uncertainty in the NIST spectral radiatit: power responsivtty scale is described in Rel'. 1. llie relative expanded unceilainty {/; 2) for

this measurement is hsted at each wav elength m Table f and plotted in Fig 1. The uncertainty

budget of this spectral power re.sponsi\ ity calibration is summarized in Table 2.

Figure 2a is a plot of the uniformity of the test photodiode at 500 nm showing 0,2 % contours of

the tiex iations from the responsivity al the pln»todiode center, f igure 2b is a .^-dimensional plot

showing the respoiisivity relatiie to the center of the photodiode.

0 6

OA

0 2

0 1

0.0

'"i"'r"»"'r"')' T '

' ' 'j

Povvei-Rcsponsivin- W] ^^^^^

- »

.

r %

\ Relative Expanded ; k

\ \ Uncertainty ["J^-] ; g

:

' * I

^'

*. s

f- V

»

J

! 1 ) < t 1 > 1 1 > ) 1 t < I 1 1 ! 1 1 ! I 1 t i 1 ! t t ( 1 1 1 i"

2.0

1.5

1,0

0:5

0.0 300 406 500 <500 70ft 800 900 1000 1100

Wavck'tigth [nm\ Figure 1- Spectral power responsiv-jty of Ilamamatsu sihcon photodiode model 82281,

Calitetion Date: Sej^tember 25, 2007

NIST Test. N'o.; 844/^f#####-07 Page 2 of 8

A-2
REPORT OF C AIJBRATiON

NIS I Test # 39073C - Spectral Respoiisivity

JBig-Ume Laboratory

Manufacturer: Hamaniatsu

Model ^: S2281

Serial lititit;

I aWe I Spectral power l esponsivify of MamaniatKu silicon piiotodiode moilel S2281. S/N H###

Spectral ReliUhe Spectral Rehtlive

Wavelengih

power expanded power expanded

respoiisi^itv uiiccrlainty Wavelength rcsponsivity uncertainly

InaiJ iA/WJ

' (A--2)[%J [nmj IA/WJ (k " 2) [%}

35*0 355

... ^--^j^^- 0.70 P" """550 'y_2g76

' 0722 0.1556 0.66 555 0.2905 0.22 360 0.1533 0.62 560 0.2934 0.22 365 0.151! 0.60 565 0.2962 0.22 370 0.1526 0.58 570 0.299

1

0.22 375 0.1572 0.58 575 0.3020 0.22 380 0.1636 0.58 580 0.3049 0.22 385 0,1705 0.52 585 0.3077 0.20 390 0.1769 0.48 590 0.3106 0.20 395 0,1824 0.44 595 0,3134 0.20 400 0.1870 0.42 600 0,3162 0.20 405 0.1908 0.40 605 0,3191 0.20 410 0.1953 0.36 610 0,3219 0.20 415 0.1996 0.34 615 0..3247 0.20 420 0.2037 0.34 620 0,3275 0.20 425 0.2076 0.32 625 0.3303 0.20 430 0.2114 0,30 630 0.3332 0.20 435 0.215! 0.30 635 0.3360 0.20 440 0.2187 0.2$ 640 0.3387 0.20 445 0.2222 Q2H 645 0..3416 0.20 450 0.2257 0.26 650 0.3443 0.20 455 0,229,1. 0.26 655 03471 0,20

460 0.2325 0.24 660 0.3499 0.20 465 0.2358 0.24 665 0,3527 0.20 470 0.2390 0.24 670 0..3555 0.20 475 0.2422 0.24 675 0.3582 0.20 480 0.2454 0.24 680 O..36I0 0.20 485 0.2485 0.22 685 0.3638 0.20 490 0.2517 0.22 690 0..3666 0.20 495 0.2548 0.22 695 0..3693 0.20 500 0.2578 0.22 700 0.3721 0.20 505 0,2609 0.22 705 0.3748 0.20 510 0.2639 0.22 710 0.3776 0.20 515 0.2669 0.22 715 0 ^804 0.20 520 0.2699 0.22 720 0.3831 0.20 525 0 2*'29 0.22 725 0.3859 0.20 530 0^2758 0.22 730 0.3S86 0.20 535 0.27S8 0

^'^ 735 0.3914 0.20 540 0.2817 0.22 740 0.3941 0.20 545 0,284? 0.22 745 0,3968 0.20

Calibration Date: September 25, 2007

NISI Test No.: 844/###^##-07 Page 3 of S

A-3
REPORT OF C"ALIBRATION Maiiuiljccurer: Hamamami

NIS r l est # 39073C - Spectral Responsivity Model #: S228}

Big-tjme Laboratory Serial. #r H###

Tsible 1 (coHt.) Spectral pom'i- responsivity of IIt<inamiitsu silicon pliotodiodc model S2281, S/N H###

Spectral Relative power t'xj>anded

Wavelength rcsponsi\iiy uiucitamsy

"~750" (0996

'"^ 0.20 755 0.402.^ 0.20 760 0.4050 0.20 765 0.4078 0.20 770 0.4! 05 0.20 775 0.413.3 0,20

780 0.4160 0.20 785 0.4187 0-20 790 0.4215 0.20 795 0.4242 0.20 800 0.4269 0.20 805 0.4297 0.20 810 0.4.^24 (,>.20 815 0.4351 0.20 820 0.43 7S 0,20

825 0,4406 0.20 S30 0.4433 0.20 835 0.4460 0.20 840 0.4487 0.20 845 0.4515 0.20 850 0.4542 0,20

S55 0.4569 0.20 860 0.4597 0.20 865 0.4623 0.20 870 0.4650 0.20 $75 0.467S 0.20 880 0.4704 0.20 885 0.4732 0,20

890 0.4759 0,20

895 0.4786 0.20 900 0.4814 0.20 905 0.4841 0.20 910 0.4868 0.20 915 0.4895 0.20 920 0,4923 0.20

Spectral Relative power expanded

Wavelength respoiisivilv unecilatnlv

Inra] ... . 925 0.4951 0.20 930 0,4969 0,20

935 0.4993 0.20 940 0.5020 0.20 945 0,5040 0.20 950 0,5057 0.20 955 0.508 0.66 960 0.509 0.70 965 0.510 0,76

970 0.510 0,82

,

975 0.510 0,86

980 0.509 0.92 985 0.507 0.98 990 0.503 1,0 995 0.499 1,1 1000 0,493 1,2 1005 0.485 L2

1010 0,475 1.3 1015 0464 14

1 020 0,450 1.4 1025 0,435 1,5 10.30 0.417 1.6 1035 0.396 1.7 1040 0,374 1.8 1045 0.350 1.9

1 050 0.325 2.0 1055 0.299 2.2 1060 0.271 '> 1065 0.245

2

'5

1 070 0.226 2,4 1075 0.207 2,6

1 080 0.190 2.6 1085 0.173 2,5

1 090 0,157 2,8

1 09."^ 0.142 2 8

! 100 0.128 VO

Calibration Dale: September 25, 2007

NIST Test No.: 844/#####s-07 Page 4 of 8

A-4
REPORT OF C-AIJBRATION Manufacturer: Hamainatsu

NIST Test tt 39073C - Spectral Responsivify M.nk) S228 1. Biy-time Laburatory Scn.il - Htiftfi

y |nim

1.010 005

1.000

0 090

X [nun]

Relative Rcspoash'iiy

Figure 2b. Siirfitce pkiE ul'dse i^'sponsiviiy relative to the center of phoiodiode for Haniamaisu

siUcoti phi>tod)t>de model S2281, S/1Si H### at 500 nni; 0.5 mni/Siep.

Califoration Date: September 25. 2007

NIST Test No..; 844/##f???#f?-07 Pace 5 of 8

A-5
REPORT OF CALIBRATION Mainilacutrer: Hamamats.ii HIST Test # 39073C • Spectral Responsmty Model #: S228

1

Big-time Laboratoty Serial

Tabic 2 Spectral pawei- fespoiislvity ijiiffrtaiiity butlget summary for Hamautatsu Mioclel S2281, S/N H###

iH^iii c.i.:!i >:<>

Vi'ave&isstf)

V» WS

csiifcjiBisjti

fiistdsicr. Vtt. \VS..!0.

BIOS';! Ulf & Vii WS

*!;(i:n

i>!!!!y isuii OtliCl ttm latuty

Rtlatlvv

aacfi-iaiiuv

B Type A Type A Type 8 Jypi it Type B Tyjx; A t%| 350 0.30 0.03 0.0,1 O.OfS 0,01 0,02 0.17 o.i.s: 0,70

s75 0 25 0 01 001 0.06 0,09 0,01 0.10 0,2'> 0.58

401) O.SS 0 l» 0,01 0.06 <!.06 0,01 0,05 0,21 0,42 42? 0. 1-4 0.00 0,00 0-OiS 0,04 0,00 0.03 O.IS 0.J2 450 0,11 0,00 0.00 0.0<' 0.03 0,00 O.OS 0.13 0.26

4''5 0.10 O.Mi 0,00 0,06 0.03 O,0<.* O.Oi 0,J2 0,24 300 0.09 O.tK! 0,00 0.06 0,03 0,0i5 0.01 0.1

1

0.22 525 0 09 0.00 o.oo 0.06 0.02 0.00 O.OJ O.ti 0.22 550 O.OS O.M 0,00 0.06 o,o: 0,00 0,01 o,u 0.22 575 0.08 0.00 0.00 0,06 0.02 0,00 0 0! o.u 0.22 600 &M 0,00 0.00 0.06 0.02 0,00 0.01 O.M) (t.20 0.08 o.m 0.00 0.06 0,03 0.(K! O.OI 0.10 0.20 «30 O.OS (ym 0.00 O.OS 0.02 0,00 0.0J o.to 0.20

&7'i O.OS 0iX> i)M) 0.06 0.02 0.00 O.OI O.iO 0.20

7t)t) O.OS 0.(tti 0.00 0.06 0,03 o,w 0,01 O.JO 0.20 735 om ot» 0.00 0.06 O.Oi 0,00 0,01 0.10 0.20 750 OOSs 0.(K> 0,00 0.06 0.01 0.00 0.01 0.10 0.20 775 0.0« 0.(H> 0,00 0.06 0,01 (i.m 0,<ii! 0.10 0,20 mi oos o.oo 0 00 0.06 0.01 o,«x> 001 O.lfl 0.20 B25 0,0!* O.iXi 0,00 0,06 0.«£ 0,00 0.01 O.IO 0.20

*i50 o.os 0{»i! 0.00 0,06 0,01 i'iM O.OI O.iO 0.20

87 s om ooo 0.00 0,06 0,01 ooo 001 0.10 0.20 Q.m 0.(.H> 0.00 0.06 0,01 0,0i,> 0.01 0.1» 0.20 n5 o.os 0.tX» 0.00 0 06 0,01 0.01 0.10 0.20

o.«w 0 (K! O.Ofi 0.06 O.tXi 0.00 0,01 0.10 0.20 975 0,,1? OMi 0.00 0,06 0.01 o.ei OiM 0,43 OM

0.57 0 00 0.00 0,06 0,05 0.01 &M 0.5S 1.2 1025 0 70 0(>0 0.0(,i o.oe 0.11 0.02 0.2.1 0.75

1050 0,9 i OM 0,00 0.06 O.IS 0,02 0.37 1.0 2.0 1075 1.1 Om 0 1)0 0 06 0.!7 0.02 1.3 2.6

i im 1.3 ami 0 0(i 0,0<- 0 i'5 0.0.^ 0 5S 1.5 i.o

CaJibration Date; Seplt;)«ber 25. 2007

NIST T«*{ No.: i:44'«-'-«--<«^-07 Paae6of8

A-6
REPORT OF C AI JBRATION

NIS I Test P 39073C - Spectral Responsivity

Big-timc Laboratory

4. General Intbrmatioii

Manufacturer: Hamamatsu

Model #: S228I

Scnal fi: imn

TUe uncertainty of the specira! radiant pow er resptuisivity w ill be hiraer than reported in Sec. 3 if the inadiation geometry is sigHificantly diflercnt from the test conditious described in Sec. 2. The uncertainty will also increase if the innbient temperature ib s-ignificantly diflereni from

reported in section 2 (The temperature coefficient of this photodiode is typically less than

0.01 % / in the 380 nm to 900 nm region and increases outside this region.). Such uncertainty

components should be e\-ahiated by the customer. Ihe spectral responsl^'lty of photodiodes is subject to drift over time. l*eriodic calibration is recommended.

Tlie appetidix pnn ides v>perating instructions for the test photodiode.

This report sliall not be reprcxiuced, except ui full, without the w ritten approval of NIST.

Prepared by: RcNiewed by:

Jeanne M. Houston

Optical Technology Division Physics Laboratory'

(301) 975-2327

Thomas C. Larason

Opiical Technology Division Physics Laboratory

(301)975-2334

Approved by:

Yoshihiro Ohno

For the Director. National Institute of Standards and Feehiiolagy

(301)975-2321

References:

|lj T, C, Larason and j. M. llouslon, Spectroradiometric detector measurements: part 1- ultraviolef detectors and part il-vjsjble to near-infrared detectors, Natl, Inst. Stand.

Technol,,. Spec, Publ, 250-41 2007 revision (2(X)7).

Caltbratioit Date: September 25. 2007

NIST Test No.; S44^##^###-07 Page 7 of 8

A-7
REPORT OF C ALIBRATION

NIS r Test f? 39073C - Spectral Responsivity

Big-time Laboraton'

Mamifacmrer: Hams»matsu

M.odeT#: S2281

Serial #: H##f?

AppewtHx: <>PERATt>SG INSTRl C TIONS FOR NIST {>HO I OIMOOE

Tiie NIST characteiizec! phoU>diode coa.sis.ts of a silicon phoiodiode *A ith a quanz u iiidou and a BNC connector.

A. Hie phoiodiode should be rigidly mounied on a dxiaJ-axis tik mount such that the photodiode can be tiUcd about twu orthogonal axei>. The photcKiiode *.hotiId be adjusted

to be perpendicular to the incident radiatioii.

B. The incident beam ol" radiation should be smaller than the active area, and shoiik! be

centered ja the photodiode acii\-e area.

C. lire phoiodiode should be comiected with a BNC cable to an electrometer grade amplifier

(iran.snnpedance amplifier) which nieasnrcs the cunens from ihc photodiode.

D. The diode window can be cleaned with lens tissue and spectral grade solvent.

Calibration Date: SejUcn-iher 25, 2007

NIST Test No.; 844 -r ^; -.^-07 Page 8 ol' 8

A-8
Appendix B: Bibliography

Optics

E. Hecht, Optics, 2nd edition, Addison-Wesley Publishing Company, 1 987.

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. Bom 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 ofModern 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 I'Eclairage, Paris, 1984). Available for sale on-line through the

International Commission on Illumination (CIE) website, http:/Avww.cie.co.at/cie/.

E. L. Dereniak and D. G. Crowe, Optical Radiation Detectors, Wiley, New York, NY, 1984.

E. L. Dereniak and G. D. Boreman, Infrared Detectors and Systems, Wiley, New York, NY,

1996.

G. H. Rieke, Detection of Light: from the Ultraviolet to the Submillimeter, Cambridge

University Press, New York, NY, 1994.

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

NY, 1990.

B-1
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 ofElectronics, 2nd edition, Cambridge University Press, New

York, NY, 1989.

S. M. Sze, Physics ofSemiconductor Devices, 2nd edition, Wiley, New York, NY, 1981.

S. M. Sze, Semiconductor Devices, Physics and Technology, Wiley, New York, NY, 1985.

Radiometry

Optical Radiometry, A. C. Parr, R. U. Datla, and J. L. Gardner, editors, Elsevier Academic Press,

New York, NY, 2005.

Reliable Spectroradiometry, H. J. Kostkowski, Spectroradiometry Consulting, La Plata, MD

(1997).

The Self-Study Manual on Optical Radiation Measurements (NBS 910-1 through 910-8)

website, http:/7physics. nist.gov/Divisioiis/Div844/maniial/studvmanuai.htmJ (2007).

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 ofRadiation, 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.

B-2
New Developments and Applications in Optical Radiometry III, Metrologia 28 (3) 1991

.

New Developments and Applications in Optical Radiometry IV, Metrologia 30 (4) 1993.

New Developments and Applications in Optical Radiometry V, Metrologia 32 (6) 1996.

New Developments and Applications in Optical Radiometry VI, Metrologia 35 (4) 1998.

New Developments and Applications in Optical Radiometry VII, Metrologia 37 (5) 2000.

New Developments and Applications in Optical Radiometry VIII, Metrologia 40 (1) 2003.

New Developments and Applications in Optical Radiometry IX, Metrologia 43 (2) 2006.

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, Mill Valley, CA, 1982.

ANSI/NCSL Z540-2-1997, American National Standard for Expressing Uncertainty-U.S. Guide •

to the Expression of Uncertainty of Measurement. Available for sale on-line through the NCSL

International website, http :// www.ncsli . org/.

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.). Also

available on-line, hitp://phvsics.nist.gov/T)ocumenl/tnl297.pdf.

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.

Qualitv and Laboratory Accreditation

ANSI/ISO/IEC 17025:2005, General Requirements for the Competence of Testing and

Calibration Laboratories. Available for sale on-line through the NCSL International website,

http:/7www.ncsli.org/.

NIST Handbook 150, National Voluntary Laboratory Accreditation Program Procedures and

General Requirements, C. D. Faison, J. Horlick, W. R. Merkel, V. R. White, Editors, 2006.

Available on-line at http:/7ts.nist.gov/Standards/214.cfm.

B-3