NBS Measurement Services:

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Research Information Center

| (National Bureau of Standards

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Spectral Response Calibration

Transfer Program

NBS

Special

Publication

250-17

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National

Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support Systemfor ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, conopiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radia- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Physics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • Spectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quantun radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities

Including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instrunentation for

radiation programs and facilities; provides national leadership

in the standardization of nuclear instrunentation; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

• Accelerator Research

• Linac Operations

• Electronic Instrumentation

• Mechanical Instrunentation

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic • data for applications of ionizing radiation; develops new methods of • chemical and physical dosimetry; conducts theoretical and experimental • research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and • measurement support services to the National Measurement Support • System for Ionizing Radiation; and develops and operates radiation • sources needed to provide primary radiation standards, fields, and • well-characterized beams of radiation for research on radiation • Interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES:

THE NBS PHOTODETECTOR

SPECTRAL RESPONSE CALIBRATION

TRANSFER PROGRAM

Edward F. Zaiewski

Center for Radiation Researcli

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1988
Library of Congress Catalog Card Number: 88-600509

National Bureau of Standards Special Publication 250-17

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-17, 65 pages (Mar. 1988)

CODEN: XNBSAV

Commercial products -materials and instruments -are identified in tills document for the sole pur- pose of adequately describing experimental or test procedures. In no event does such identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

For sale by the Superintendent of Documents, U.S. Govemment Printing Office, Washington, DC 20402-9325
PREFACE

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publicat ion (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (5) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document ( SP 250-17), NBS Measurement Services: The NBS

Photodetector Spectral Response Calibration Transfer Program, by E. F.

Zalewski, is the seventeenth to be published in this new series of

special publications. It describes the methods and procedures for

calibrating a silicon-photodiode-based radiometer and explains how to

apply the radiometer to the measurement of absolute spectral response

in the 250 to 1064 nm region of the spectrum (see test number 39060C in

the SP 250 Users Guide) . Inquiries concerning the technical content of

this document or the specifications for these services should be

directed to the author or one of the technical contacts cited in SP

250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

.

Joe D. Simmons

Acting Chief

Measurement Services

Chris E. Kuyatt

Director

Center for Radiation Research
ABSTRACT

A silicon-photodiode-based radiometer is supplied to

transfer the calibration of absolute spectral response in units of A/W

(Amperes/Watt) and A cm^/W in the 250 to 1064 nm region of the spectrum.

The radiometer is also characterized for linearity over the four-decade

range of the amplifier gain settings. Also included with the radiometer

are components and a procedure for measurement error diagnosis. The

methods for obtaining and using this calibrated radiometer are described, as well as the calibration and characterization procedures

used at NBS

.

KEY WORDS: calibration, linearity, photodetector , photoelectric effect,

quantum efficiency, radiometry, responsivity , silicon photodiode,

spectral response.

iv
TABLE OF CONTENTS

1) Introduction

1.1) Description of the calibration 1

1.2) Historical perspective 2

1.3) Overview of this document 4

1.4) Acknowledgements 5

2) Essential information for the user

2.1) Precautions 6

2.2) How to rent a detector response transfer

package (DRTP) 7 2.3) Physical description of the DRTP 7 2.4) Other required instrumentation 9 2.5) How to use the DRTP 10

2.6) Range of calibration 13

2.7) Description of the diagnostic intercomparison 15

3) NBS calibration and characterization procedures

3.1) Overview of the calibration procedure 19

3.2) Safety considerations 20

3.3) Current amplifier calibration 21

3.4) Temperature controller adjustments 22

3.5) Linearity measurements 24

3.6) Measurement of spatial and angular

characteristics 27

3.7) Aperture area measurements 32

3.8) Specular and diffuse reflectance

measurements 33

3.9) Absolute spectral response measurements 34

3.10) Interpolation of the external quantum

efficiency 35

3.11) Spectral comparator measurements

- quality control 42

3.12) Estimation of the overall uncertainty 44

References 46

Appendix A: Intercomparison Format Al

Appendix B: Typical Calibration Report Bl

V
LIST OF TABLES

TABLE 1. Typical DRTP Responsivity 14

TABLE 2. Summary of the DRTP Uncertainties 15

TABLE 3. Difference Between Measured and Interpolated

Responsivities 42

TABLE 4. Summary of the DRTP Uncertainty Estimates 45

LIST OF FIGURES

FIGURE 1. Detector Response Transfer Radiometer Head 8

FIGURE 2. Typical Transmittances of the Diagnostic Filters .... 17

FIGURE 3. Circuit Diagram, Radiometer Head 23

FIGURE 4. Non-linearity of DR-14 at 799 nm 26

FIGURE 5. Typical Detector Uniformity Map at 254 nm 28

FIGURE 6. Typical Detector Uniformity Map at 407 nm 29

FIGURE 7. Typical Detector Uniformity Map at 633 nm 30

FIGURE 8. Typical (Except DR-14, See Text) Detector Uniformity

Map at 1064 nm 31

FIGURE 9. Spectral Absorptance of the DRTP Photodiodes 37

FIGURE 10. Change in Quantum Efficiency (Absorptance) Due to 2 nm

Change in the Thickness of the Si02 Layer 38

FIGURE 11. Calculated Quantum Efficiency of DR-14 (Upper Curve)

and Comparison to Measured Values (Lower Curve) 40

FIGURE 12. Calculated Quantum Efficiency of DR-15 (Upper Curve)

and Comparison to Measured Values (Lower Curve) 41

FIGURE 13. Spectral Comparator Measurements Versus DR-15 as the

Standard 43
The NBS Photodetector Spectral Response

Calibration Transfer Program

1) Introduction

1.1) Description of the calibration

The NBS photodetector spectral response calibration transfer

program is a means of obtaining an absolute spectral response

calibration in the visible and near-visible spectral region. It is

listed as item number 39060C (formerly 7 . 5H) in NBS Special Publication

250 [1] and its Appendix.

The instruments used to transfer the calibration are rented from

NBS rather than purchased. During the rental period the user transfers

the responsivity calibration from the NBS instrument to his own set of

in-house secondary standard photodetectors . To do the spectral

comparison the user needs to have a tunable monochromatic source, such

as a monochromator or a set of narrow-band filters combined with a suitable light source. The range of radiant power levels at which the

spectral comparison can be performed is 10" to 10'^ W. The linearity of

the photodetectors and amplifiers is calibrated over this range.

The NBS photodetectors are calibrated in units of A/W

(Amperes/Watt) in 10 nm intervals from 250 to 960 nm. The photodetectors

are also calibrated at two additional spectral lines in the infrared:

1014 and 1064 nm. A calibrated 0.5 cm^ aperture is included to enable

the calibration to be transfered in units of A cm^/W. In the first case

1
the output beam from the spectral comparator is focussed into a spot,

i.e. underfills the active area of the photodetector . In the second case

the output beam overfills the detector aperture.

1.2) Historical perspective

In the early 1970s NBS ceased to offer detector spectral response

calibrations. Before that time only relative, not absolute,

photodetector spectral response calibrations were offered. That program

was halted for several reasons. The photodetectors then available were

judged to be unreliable as calibration transfer standards;

radiometric-quality silicon photodiode technology was just emerging. The

NBS photodetector comparison instrumentation had deficiencies requiring

major revisions. The number of calibration requests was very small.

And, finally, it was thought that the NBS-calibrated spectral irradiance

lamps could serve as the calibration base for detectors.

The photodetector calibration picture changed quickly because of the

rapid advances made in photodetector technology, particularly with

respect to silicon photodiodes. Some types of silicon photodetectors

were reported to have superior radiometric characteristics, such as linearity over a large dynamic range, spatial response uniformity [2], no light memory (hysteresis) effects [3], and long-term stability.

Furthermore, large measurement differences arising from using lamp

standards for photodetector calibrations were demonstrated by the newly

formed Council for Optical Radiation Measurements (CORM) . Under the

aegis of CORM, Franc Grum of the Eastman Kodak Co. organized an intercomparison of detector spectral response measurements [4] . The

differences among the participating laboratories were much larger than

expected. It was clear that suitable detector standards were needed by

the optical radiation community and that NBS had to take steps to remedy

the situation.

Because of the very encouraging reports on the characteristics of

silicon photodiodes and earlier work at NBS [3], these detectors
appeared to be the most suitable choice as calibration transfer

standards. However, their behavior in regards to all the critical

radiometric characteristics, especially long-term stability, were not

yet well understood. Furthermore, it was not clear from the CORM

intercomparison what portion of the problem was due to the lack of a standard detector, and what portion was due to inadequate calibration

transfer measurement techniques. Therefore, a pilot program was undertaken in which silicon photodiodes were used as the calibration

transfer standards and an experiment was designed to test the

state-of-the-art of detector calibration transfer measurements.

Concurrently, lasers and other electro-optic devices were being applied

to radiometry [5] and new absolute response measurement techniques [6]

were being developed at NBS

.

The pilot calibration program and measurement diagnosis experiment

showed that a number of laboratories could perform detector spectral

response transfer measurements with an uncertainty of +/- 2% or less

[7], and that silicon photodiodes could serve as reliable calibration

transfer standards. The results of other work at NBS [2,8], however,

indicated that the behavior of silicon photodiodes was not well enough

understood and that there might yet be some hidden problems. One of the

findings of the measurement diagnosis experiment [7] was that many

laboratories did not have the appropriate electronics to correctly

amplify the output of a silicon photodiode.

The successor to the pilot calibration program is now called the

NBS Detector Response Transfer and Intercomparison Program" . In this

program, calibrated silicon detectors are not issued as separate

entities. Rather, the DRTPs [9] are complete radiometers

* The acronym is DRTIP, but, the calibration transfer instruments are commonly referred to as the more pronounceable DRIPs. Since we are referring only to the calibration transfer and not the intercomparison

aspect of the program in this publication, the acronym we will use is

DRTP.
(temperature-controlled silicon photodiode , amplifier, and power

supplies) that are characterized for other critical parameters besides

the absolute spectral response. Because of the large expense of

development and characterization, the DRTPs are not sold but rented.

The rental period, one month scheduled in advance, is sufficient to

enable a laboratory to transfer the calibration to several of its

in-house standard detectors and to perform the diagnostic tests that are

the "Intercomparison" part of the program (see Appendix A)

.

It should be noted that, because of staff and funding limitations,

the NBS portion of the intercomparison has not been completed. Even

though the measurement diagnosis portion of the NBS detector calibration

program has not been implemented, the DRTPs have served over the years

to significantly improve the state-of-the-art in absolute spectral

response measurements of photodetectors

.

Recently, the addition of new staff members has facilitated the

development of the instrumentation that will enable NBS to complete its

part of the DRTIP intercomparison. It is hoped that this important part

of the NBS photodetector calibration program, the measurement diagnosis

service, will be reactivated shortly.

1.3) Overview of this document

Section 2 contains most of the information needed by the general

user of the NBS detector response transfer package (DRTP) . It includes

the physical description of the instrument as well as a description of

the range and accuracy of the calibration. Other pertinent information

in Section 2 includes how to obtain, use, and return a DRTP.

Readers interested in more details regarding the photodetector

calibration and characterization methods can find them in Section 3. It

includes discussions of the instrumentation and experimental procedures

used in characterizing the DRTPs, the absolute spectral response

calibration procedure, the interpolation procedure, verification
experiments, and the estimation of the uncertainties associated with

this calibration.

1.4) Acknowledgements

Many people have worked with the author on the development and

improvement of the NBS photodetector spectral response transfer program

over the years. Most of them are recognized in the list of references.

However, much of the work exists as unpublished data. The many

contributions to the NBS photodetector calibration program are hereby

gratefully acknowledged. Redesign and ruggedization of the electronic

circuitry: Joel B. Fowler. Radiometric characterization measurements:

A. Russell Schaefer and Warren K. Gladden. Spectral response

interpolation/extrapolation calculations: Warren K. Gladden and Jon

Geist. Interpolation/extrapolation verification measurements: Warren K.

Gladden and Robert L. Booker. Spectral comparator instrument

development and quality control measurements: Robert D. Saunders and

Jeanne B. Houston.

5
2) Essential information for the user

2.1) Precautions

Before beginning a description of the instrument, several

precautions must be noted regarding the care and handling of the

detector, aperture, and filters. The user is cautioned to observe the

following instructions:

DO NOT TOUCH the surface of the detector, the filters or the inner

rim of the aperture.

DO NOT EXPOSE these components to a dusty environment or contaminating vapors.

DO NOT OPEN THE DRTP EXCEPT IN A NO SMOKING AREA

DO NOT ATTEMPT TO CLEAN these surfaces except by a stream of dry,

oil-free nitrogen or other inert gas (compressed freon is generally NOT

oil-free)

.

KEEP THE LENS CAP ON the detector whenever it is not in use. Also

keep the filters wrapped in lint- free lens tissue and in their

containers when they are not in use.

DO NOT OPEN THE RADIOMETER HEAD. The aperture holder may be removed

if an active detector area of greater than 8 mm diameter is required.

Please exercise care when doing so.

In order to insure the most accurate transfer measurements, NO

ELECTRONIC ADJUSTMENTS OR ALTERATIONS TO THE INSTRUMENT SHOULD BE MADE IN

THE FIELD BY THE USER.

6

i
2.2) How to rent a detector response transfer package (DRTP)

Current price and ordering information for the DRTP is published in

the Appendix to NBS Special Publication 250 [1]. The Appendix to SP 250

is updated semi-annually. To obtain a current copy contact the NBS

Office of Physical Measurement Services by phone: (301) 975-2007; or by

mail: Room B-362 Bldg. 221; National Bureau of Standards; Gaithersburg

,

MD 20899.

Presently the Radiometric Physics Division staff member to contact

regarding a DRTP rental is Ms. Jeanne Houston: phone number, (301) 975-

2327; mail address, Room A-222 Bldg. 221. Six months is a typical lead

time

.

The instrument is shipped by second-day-delivery air freight from NBS

and it should be returned to NBS by air freight. The special shipping

crate should be saved and used to return the instrument. We have found

that, besides saving time, the instruments arrive in better condition

when shipped by an air freight service.

2.3) Physical description of the DRTP

The detector response transfer package consists of the radiometer,

three broad-band absorbing glass filters, and two narrow-band

interference filters. The radiometer consists of a cylindrical head

6.35 cm in diameter by 18 cm long, connected by a 1.5 m umbilical cord

to a power supply (15 cm x 33 cm x 46 cm). All the sensitive

electronics are contained in the radiometer head (see Figure 1) . These

are a temperature-stabilized silicon photodiode, current-to-voltage

amplifier, gain control, and output buffer amplifier. The gain settings

vary in decade steps from lO'' to 10^ V/A and are adjustable by means of

a rotary switch at the rear of the radiometer head. The output voltage

connection is a female BNC connector also located at the rear of the

radiometer head.

7

Explicit mechanical and electronic descriptions of the DRTPs can be

found in NBS Tech Note 950 [9]. Some features of the instrument have

been changed, however, to ruggedize the control box for shipping. The

digital panel meter, overload indicator, and variable- time-constant

circuits described in reference [9] have been eliminated. The control

box now contains simply the power supplies for the radiometer head. As

noted above, the amplifier gain adjustment and the amplifier output are now at the rear of the radiometer head. Also at the rear of the

radiometer head is a miniature gold-plated connector for a thermistor

readout that is used only for calibrations at NBS. The radiometer head

can not be disconnected from the control box. Finally, because the

plastic lens cap was suspected to be a possible source of contamination

it has been replaced by an aluminum cap which is held in place by a single #2-56 screw.

The instrument operates on 110 to 130 VAC at 50 to 60 Hz.

2.4) Other required instrumentation

A calibrated voltmeter and a male BNC connector cable will be

needed to read the amplifier output at the rear of the radiometer head.

A stable mounting assembly will be required to hold the radiometer

head. This mounting should have adjustable pitch and yaw motions. A 6.4 cm inside diameter clamping ring with a 1.27 cm diameter protruding rod

is included in the DRTP package to facilitate mounting the radiometer.

The user is expected to have a detector response comparator

instrument to transfer the DRTP calibration to a set of in-house

detector standards. Descriptions of this type of instrumentation along

with complete discussions of possible sources of error are given in CIE

Publication 64 [ 10]

.

NBS Technical Note 988 [11] is a study of the characteristics of a particular spectral comparator instrument based on a pair of grating
monochromators . This NBS Tech Note is similar in some respects to CIE

Publication 64 [10] in that it deals with the pitfalls in designing and

working with a detector spectral comparator. Also included in reference

[11] is some early work on the use of a spectral irradiance standard

lamp as a standard for detector calibrations via a black coated

thermopile. It demonstrates the problems encountered in assuming the

spectral non-selectivity of black thermal detectors, and may be a useful

reference for those applications where a black thermal detector is used

to extend the spectral range of the DRTP calibration. Other useful

references for extending the spectral range of the DRTPs are [12, 13 and

14] where gold-black coatings and electrically calibrated pyroelectric

radiometers are discussed.

2.5) How to use the DRTP

This calibration transfer standard has been made as rugged as possible to minimize physical damage to the instrument. However, as with any calibrated instrument, special care should be taken to protect

the instrument from physical abuse. In particular the protective cap

that covers the detector should always be replaced when the detector is

not in use. In order to insure the highest accuracy, it is essential

that the detector surface be kept free of contamination and that the

aperture not sustain any physical damage.

The actual operation of the device has been kept as simple as possible to avoid operator error. Switch the power (110-130 V, 50-60 Hz)

and the temperature control at the front of the power supply box to the

ON positions. Allow 30 minutes for the detector temperature to

stabilize to its operating temperature of 30° C. Over the course of the

calibration transfer measurements, it is preferable to leave the

instrument on continuously to insure maximum temperature stability. The

ambient temperature must be below 30° C for proper operation.

Set the spectral comparator source to the wavelength of maximum

radiant power output. Remove the protective cap from the detector head

10
and adjust the amplifier gain to 10^ , the least sensitive setting.

Initial measurements should be made using this range and then changing

to more sensitive ranges if necessary. If the output voltage reads 12

V, the amplifier in the detector package is saturated and the incoming

beam of radiation must be attenuated, typically with neutral density

filters. For optimum linearity at any gain setting, the amplifier

output should be 10 V or less.

The linearity of the detector response is a function of the radiant

power density [15, 16]. That is, a high power density focused spot may

cause the detector to be non- linear even though the photocurrent is

below the maximum allowable for the amplifier. To avoid saturation, the

radiant power density on the detector should not exceed 25 mW/cm^

.

The detector should be mounted so that the incident radiation is

normal to the surface and underfills the active area. Because the

detector uniformity degrades near the edges of the active area [2], the

radiation should be restricted to the central 8 mm diameter area within

the confines of the precision aperture. However, for images larger than

8 mm but not larger than the active area of the detector (11 mm

diameter) the aperture can be removed -- exercising care as noted above.

The accuracy of the calibration transfer will of course be degraded.

In order to use the calibration values as supplied in units of A/W,

the radiation should always underfill the aperture (or the active area)

of the photodiode. If the radiation field is larger than this, the

precision aperture should be used. The calibration will then be in units

of amps per irradiance (A cm^/W) and the measured area of the aperture as supplied in the calibration report should be used to determine these

calibration values.

In the irradiance measurement mode, the non-uniformities of the source irradiance and detector responsivity could lead to large,

position dependent errors [11]. As noted above, the active area of the

detector is not the correct limiting area.

11
Careful attention must be given to the geometry of the incoming

radiation and the placement of lenses and filters. Since the detector

has an optical quality surface, the user must be mindful of possible

stray radiation due to inter-reflections between the detector and lenses

or filters.

As a simple example of the use of the DRTP calibration, the

following is a typical calculation for an optical power measurement at a mercury emission line.

where P is optical power; R is 0.2137 A/W (a typical interpolated value

for the DRTP responsivity at 436 nm) ; G is 10^ V/A (gain factor for the

DRTP amplifier); and X is 3.756 V (DRTP output). Therefore, P equals

1.758 mW.

By placing another detector in this beam and reading its output,

say 3.626 x 10'^ A, its responsivity at 436 nm is determined to be

0.2063 A/W.

If the radiation had overfilled the aperture of the DRTP, then the

measurement is one of irradiance responsivity. If the aperture area is

0.5113 cm^ , then the irradiance measured by the DRTP is 3.438 mW/cm^

.

The irradiance responsivity of the transfer detector is 0.1055 A cm^/W.

Because the detectors are linear, they can be used to verify the

linearity of other detectors. In addition, they can be used to

characterize the irradiance versus distance behavior of light sources such as spectral irradiance or luminous intensity standards. That is,

the DRTP can be used to check the inverse square "law" behavior of lamp

standards [8], thereby extending the dynamic range of a lamp

calibration.

12

i
2.6) Range of calibration

The silicon photodiode is calibrated for spectral response (A/W) at

10 nm intervals from 250 to 960 nm and at two discrete wavelengths in

the infrared: 1014 and 1064.2 nm. The responsivity calibration of a typical photodiode is listed in Table 1 . The typical uncertainties in

the calibration relative to SI (absolute units) are listed in Table 2.

The uncertainties summarized in Table 2 are the upper bound of the total

error. They are more completely described in Section 3.12.

13
TABLE 1

Typical DRTP Responsivity

Spectral Spectral

Wavelength Responsivity Wavelength Respons ivity

(run) (A/W) (run) (A/W)

250 0.115 620 0.4457

260 0.109 630 0.4562

270 0.097 640 0.4660

280 0.091 650 0.4756

290 0.093 660 0.4850

300 0.098 670 0.4940

310 0.100 680 0.5024

320 0. 100 690 0.5113

330 0. 102 700 0. 5196

340 0. 101 710 0. 5275

350 0.098 720 0.5352

360 0.095 730 0. 5429

370 0. 102 740 0. 5503

380 0.117 750 0. 5572

390 0.132 760 0.5641

400 0. 148 770 0.5708

410 0. 162 780 0.5773

420 0.177 790 0.5838

430 0.192 800 0.5903

440 0.207 810 0.597

450 0.222 820 0.602

460 0.237 830 0.608

470 0.251 840 0.614

480 0.266 850 0.620

490 0.281 860 0. 625

500 0.2940 870 0.630

510 0.3077 880 0.636

520 0. 3223 890 0.641

530 0.3364 900 0.646

540 0.3504 910 0.651

550 0. 3632 920 0.656

560 0. 3762 930 0.659

570 0.3886 940 0.661

580 0.4011 950 0.661

590 0.4126 960 0.656

600 0.4246 1014 0. 577

610 0.4353 1064.2 0.277

14
TABLE 2

Sununary of the DRTP Uncertainties

Wavelength Uncertainty

250 to 390mm +/- 6..0%

400 to 490mm +/- 1

,

,9%

500 to 850mm +/- 0,.8%

860 to 960mm +/- 2..2%

1014mm +/- 5..9%

1064.2mm +/- 5..7%

A complete description of the calibration and interpolation method

along with a discussion of the various sources of uncertainty are presented in Section 3.

A typical calibration report is presented in Appendix B. The most

recent calibration results of each DRTP are included with the instrument

as it is sent to the user. After checking the stability of the absolute

response of the DRTP upon return to NBS , a report is issued of the

actual calibration of the DRTP at the time that it was in the hands of

the user.

2.7) Description of the diagnostic intercomparison

An accurate absolute spectral response calibration of a detector

from NBS will enable other laboratories to determine the absolute

spectral response of many other different detectors and radiometers

.

However, the other key component in the quality of the measurement

chain, besides the accuracy of the NBS calibration and the various

pertinent characteristics of the detector and the electronics, is the

accuracy of the user's transfer of this calibration. The detector

response transfer package contains several components selected to
diagnose and intercompare each laboratory's response transfer

capabilities. The diagnostic tests have been designed to evaluate the

performance of monochromator based detector comparators, since these are by far the most common type.

Two of the most common sources of error in detector response

transfer measurements are inaccuracy in the monochromator wavelength

readings and out-of-band (stray) radiation transmitted by the

monochromator. To test for the accuracy of the wavelength setting, two

mounted interference filters are supplied with each instrument. One

filter has a transmission peak at approximately 410 nm (serial numbers

begin with WV) and the other at approximately 940 nm (WR serial

numbers) . Each laboratory is requested to use its monochromator and

source to measure the wavelength at the 20% transmission points of each

filter when mounted on the DRTP radiometer head.

The other principal source of error, stray radiation, is of two

types: radiation from all other wavelengths scattered into the

monochromator ' s transmission band and the second order diffraction

present in grating instruments. For instruments employing an incandescent source, the first type of stray radiation is usually a big

problem in the blue and uv spectral regions whereas the second type is

most apparent in the red and ir . Instruments employing arc sources with

intense uv lines are a special case since the second order scatter will occur in the visible. To test for stray radiation two absorbing glass

cut-off filters are supplied. The filter to check for stray radiation

at wavelengths shorter than 400 nm (SV serial number) is a 1 mm thick

Schott VG-5 glass. For stray radiation at wavelengths longer than 800 nm the filter (SR serial number) is a 1 mm thick Schott BG-18 glass.

The nominal transmittances of these filters are displayed in Fig. 2.

Each laboratory is requested to measure the output of its monochromator

at specified uv and ir wavelengths using the NBS detector both with and

without the cut-off filters.

16
17
A good indicator of each laboratory's response transfer capability

is how well it can transfer the absolute spectral response from the

calibrated silicon detector to another detector having a very different

spectral sensitivity. The third absorbing glass filter in the detector

response transfer package (RT serial number) is a 2 mm thick filter of

Schott UG-1 glass. This is a visible absorbing filter that has

transmission peaks in both the uv and ir. (See Fig. 2.) Each

laboratory is requested to measure the spectral response of a "test"

detector (the NBS detector with the RT filter in place) against that of

the DRTP detector calibration as supplied by NBS. When the SR and SV

filters are used in tandem with the silicon detector the response is

limited to only visible wavelengths. The laboratories are also

requested to measure the response of this combination at specified

wavelengths

.

From the data on the above two test response functions as supplied

by the various laboratories, it is possible to compare interlaboratory

spectral response capabilities in three distinct spectral regions: near

uv, visible and near ir.

The intercomparison format along with the instructions are presented in Appendix A.

18
3) NBS calibration and characterization procedures

3.1) Overview of the calibration procedure

This Section covers the procedures followed at NBS in the

characterization and calibration of the DRTPs . The procedures described

here are not to be undertaken on the DRTP by the user. In order to

insure the most accurate transfer measurements, no electronic adjustments

or alterations to the instrument should be made in the field by the

user

.

Before the DRTPs were radiometrically characterized, the electronic

circuits were adjusted and the temperature control set as described

below. These adjustments do not have to be repeated.

The key radiometric characteristics of linearity, uniformity, and

aperture area were then measured. These were followed by the principal

calibration of reflectance and absolute spectral response.

The experimentally determined values of the spectral response and

reflectance were used to calculate the internal quantum efficiency at

each measurement wavelength. Using theoretical models of the internal

quantum efficiency of silicon and the reflectance of a thin film

dielectric on a conductor, the internal quantum efficiency and

reflectance were interpolated over the spectral range of the

calibration: 250 to 960 nm in 10 nm intervals. These two spectral

functions were combined to yield the absolute spectral response as presented in each DRTP calibration report.

The stability of the calibration of the DRTPs is checked each time

they are returned to NBS by intercomparing their spectral responsivities

by means of a conventional monochromator spectral comparator. Changes in

the spectral response of each DRTP relative to the group are monitored.

19
The radiometric characterizations were all performed using the

laser-based facility described in NBS Tech Note 954 [5]. The heart of

the facility is the amplitude-stabilized cw laser. This is a nearly

ideal source of monochromatic radiation due to its high degree of

collimation and high power. The associated optics are simpler and the

dynamic range of radiant power available for most experiments is orders

of magnitude greater than from conventional monochromatic sources. For

many experiments , the concentrated radiant power in the laser beam is

too great and must be attenuated. Neutral density filters can be used if

the attendant scattered radiation is acceptable. A preferred technique

is to use a beam expanding telescope and spatial filter to provide a more uniform laser beam and reduce its radiant power density.

In some experiments it is important that there be very low scatter

around the collimated beam. One successful method of forming a well

collimated and clean beam of radiation is to defocus the microscope

objective in the beam expanding telescope so that it slightly overfills

the spatial filter pinhole. This produces a circular diffraction

pattern. The central spot of the diffraction pattern can be selected by

intersecting the first dark ring in the pattern with an iris diaphragm.

In this way diffraction from the iris diaphragm and other subsequent

apertures is avoided. An aperture might be needed close to the

experimental area to remove the small amount of scattered radiation from

the telescope or other optical components.

It should be noted that for optimum performance of the laser power

stabilizer system, the beamsplitter and monitor detector should be

positioned close to the experimental area. It may be necessary to

position the final aperture after the beamsplitter in order to reduce

the scatter from it.

3.2) Safety considerations

The main hazards that occur in the calibration and characterization

of the DRTPs arise from the electrical instrumentation and from the high

20
intensity light sources, such as lasers and arc lamps. In addition, high

pressure arc lamps can explode and may produce hazardous levels of ozone during operation.

Electrical connections on all equipment are well insulated and

these instruments are connected with approved grounded power cords. High

voltage sources are well shielded.

Laser and ultraviolet (from the arc lamp) radiation hazard signs

are posted appropriately. In addition the lasers and arc lamps are shielded to reduce the level of exposure as a protection for the

personnel working in the area. Protective goggles should be worn by the

working personnel and additional sets are readily available for

visitors

.

Adequate shielding is especially important when working with high

pressure arc lamps because of the danger of explosion. Such precautions

are especially important when the lamps have to be handled during

replacement

.

3.3) Current amplifier calibration

Access to the detector head electronics is achieved in the

following manner. Remove the aperture plate by removing the three

countersunk Allen screws. Next remove the three outer countersunk Allen

screws in the detector plate. Replace the lens cap to protect the

detector. Loosen the Allen set screw on the side near the back (cable)

end of the detector head and slide the outer shell back over the cables.

With the silicon detector covered to insure zero light input,

attach the positive side of a stable current generator capable of a 1 mA

to 100 nA output to the cathode of the silicon detector SDl , through a 1 KW standard resistor. The anode side of the detector should be

connected to the negative side of the generator.

21
A high accuracy DVM is then used to monitor the current output of

the generator by measuring the voltage drop across the standard

resistor. Locate R2 and R3 using the diagram in Fig. 3. With the current

generator set for zero current output, adjust R2 for a minimum voltage

at the output of Ul . Next adjust R3 for a minimum output of U3

.

Adjust the current generator to 1.0000 mA with the gain switch set

to the 10^ V/A position. Adjust R14 to read 10.000 V at the output of

U3 (the detector head BNC connector). Next adjust the current to 1.0000

mA and set the gain switch to the 10^ V/A position. The output of U3

should again read 10.000 V. Readjust R14 slightly if necessary. Try

the other gain settings to insure that the amplifier is scaling

properly. Using an iterative procedure it is possible to bring the

output of U3 to the correct value within 0.01% for all gain settings.

The linearity of the amplifiers was measured by checking the

amplifier calibration over an output range of 0.1 to 13 V. In the range

from 0.1 to 10 V, the amplifiers were linear to within +/- 0.01%. For an output above 10 V, the amplifier begins to saturate showing a one percent effect at about 12 V. The linearity between ranges is +/-

0.01%. The uncertainty in the amplifier gain, i. e. the absolute

accuracy of the current to voltage conversion, is +/- 0.03%. The

uncertainty in the amplifier gain does not contribute to the uncertainty

of the absolute response calibrations. Rather, it is a contribution to

the linearity uncertainty that is included in the detector linearity

measurement

.

3.4) Temperature controller adjustments

The temperature is set by adjusting Rll in the detector head (Fig.

3). In order to insure the best possible transfer measurements, all

characterizations of the detectors have been performed with the

temperature of the substrate set at approximately 30° C and controlled to

better than +/" 0.1°C.
O Q

UJ CO 2

UJ 3

^ Q 5 Q ^^'^

O (J

<J ^ UJ - oa _| d a UJ

CO LU 00

55

o

O Z

< 1

I s

- I

_JCO<u

Q

TO

UJQ u

UJ a.

_j 2

iri i

41J1 CON DA Q

GB 2 CH ON

Q. Z tj O Ul > It) z (J

St

I- o <jO <i |2< <

0. 2 O <

Q

>

rr- «

UJ U

Q O

U

< SQ

UJ < o o

9

X

o o ^

(- ^

00 . II

O en

=^5 ii

0)

£

O

•Hu

4J

•1-1 Ou

bO

•i-l fa

23
A thermistor probe (Fenwal GB41J1) has been installed in each DRTP

to enable the temperature set point to be read easily. This thermistor

is connected to the minature gold-plated connector at the rear of the

radiometer head. At the temperature set point the resistance of the

thermistor in each of the six DRTPs was as follows:

DR-10, 8.08 +/- 0-02 KQ; DR-11, 6.64 +/" 0-0^ KQ;

DR-12, 6.77 +/- 0.02 KQ; DR-13, 6.63 +/" 0.01 KQ;

DR-14, 6.89 +/- 0.05 KQ; DR-15, 5.43 +/" 0.02 KQ.

The uncertainty in the absolute response calibration due to

temperature fluctuations (+/- 0.1° C) is only significant in the ir

spectral region. From the temperature coefficient of the responsivity at

1064 nm the upper bound on the systematic error due to temperature

fluctuations is estimated to be +/- 0.2%.

3.5) Linearity measurements

The linearity of the DRTP photodiodes was checked at several

wavelengths using amplitude stabilized [17] laser lines as the

monochromatic source. The wavelengths used were: 406, 530, 647 and 799

nm. Significant departures from linear responsivity (greater than 0.1%

per decade) were observed only for DR-14 and only at 799 nm. This

nonlinearity was manifest as an increase in responsivity at high radiant

power input, the phenomenon known as super-responsivity [15, 16].

Besides super-responsivity, the other non-linearity effect is

saturation of the photodiode at high radiant power densities. Saturation

at the 0.1% level occured in all the DRTPs (except DR- 14) at radiant

power densities above 25 mw/cm^ . The radiant power density in the case

of DR-14 should not exceed 5 mW/cm~^.

Two linearity measurement methods were used: the AC-DC method [15,

16] and a beamsplitter reflectance measurement versus power. In the

24
AC-DC method the laser output is split into two beams. One beam is

chopped (AC) and then superimposed with the steady (DC) beam on the

detector. The AC output of the detector is measured with a lock- in

amplifier. The radiant power in the DC beam is varied. If the detector

is non-linear the responsivity , R, will be a function of the input

power

.

i = R(P) P. (2)

where i and p are the output current and input power respectively. The

AC measurement is analogous to the first derivative of Eq. 2;

fi = R.^ (3) Ap Ap

If the AC output of the detector remains constant as the power in the DC

beam changes, the derivative of i with respect to p must be a constant.

The right-hand side of Eq. 3 is therefore also a constant function of

radiant power, i. e. the detector must be linear.

The advantage of the AC-DC method is that it is simple to

implement. The disadvantage is that if the detector is non- linear then

there is no clear-cut way to obtain a correction factor for the

non-linearity

.

The second method, the beamsplitter reflectance measurement versus

power, was used to compare linear and non- linear detectors to determine

a correction for the latter. The measurements for DR-14 at 799 nm are shown in Fig. 4. This shows a super-responsivity non-linearity of about

0.5% per decade at about 1 mW, the highest input power where the DRTP is

calibrated. The non-linearity decreases rapidly at lower power levels.

The super-responsivity non-linearity effect is greater at longer

wavelengths. From these and other measurements made with a conventional

spectral comparator, the non-linearity corrections to the absolute

response calibration values were obtained for DR-14. The corrections to

25
T

1.010

1.008

E

c

o> 1.006

o>

(0

? 1.004

w

c

o

Q.

0)

a 1.002

o

o

N

(0

E 1.0

0.998

0.996

20

L_

200

Power (/>tw)

Fig. 4. Non-linearity of DR-14 at 799 nm.

26
be applied when operating at any power Level less than 0.1 mW: - 0.5% at

800 nm; - 1% at 900 nm; and - 2% at 950 nin.

At the long wavelength end of the calibration range (700 nm and up)

all the other DRTPs were found to be reasonably linear over the entire

range of the amplifier gain settings; i. e. no extreme

super-responsivity effect. The non-linearity uncertainty (upper bound)

that applies when using any of the DRTPs at short wavelengths and at

radiant power levels less than 0.1 mW, is +/- 0.1% per decade. At

wavelengths longer than 700 nm the linearity uncertainty (upper bound)

for all the DRTPs, including DR-14 with the correction as stated above,

increases to +/- 0.3% for the first decade, from 1 to 0.1 mW, and for

radiant power levels below 0.1 mW it reduces to 0.1% per decade.

3.6) Measurement of spatial and angular characteristics

Spatial uniformity was measured by focussing a 1 mm square spot on

the detector and then recording its output while moving the detector in

a 1 mm step, two dimensional scan. Translation of the detector, rather

than scanning the beam by tilting mirrors, was preferred because of the

possible fluctuations introduced by the variable reflectance of a rotated mirror. A square 1 mm x 1 mm spot was obtained by expanding the

laser beam onto a 1 mm square aperture. The image of this aperture was then focussed on the detector. The detector output was AC amplified.

The detectors all showed similar spatial uniformity of response. As an example, the uniformity maps for one of the DRTPs at four wavelengths

(257.3, 406.7, 632.8 and 1064.0 nm) are shown in Figs. 5, 6, 7 and 8. In

the blue and uv spectral regions, the maximum variations observed ranged

from 2% across the detector to as much as 6%. In the mid-visible to red

spectral region the variation in response ranged from 0.5 to 2%. The

uniformity does not vary monotonically across the surface but exhibits

peaks and valleys.

27
Fig. 5. Typical detector uniformity map at 254 nm.

28
29
Fig. 7. Typical detector uniformity map at 633 nm.

30
Fig. 8. Typical (except DR-14, see text) detector uniformity map

at 1064 nm.

31
At 1064 nm the character of the uniformity variations is markedly

different. Typically, the maximum variations of response at 1064 nm

ranged from 5 to 8%, except for DR-14 which varied by 39%. See reference

[16] for an explanation of the large ir non-uniformity in some silicon

photodiodes

.

The uncertainty due to the non-uniformity of response depends on

the size of the spot of radiation used in measuring the DRTP and also

the wavelength of the radiation. In the responsivity calibration, the

spot size was 4 +/- 1 mm in diameter. The upper bound of the systematic

error associated with the non-uniformity of the photodiode was estimated

to be one-half of the maximum observed non-uniformty

.

Angular responsivity was measured by irradiating a small region

(approximately 3 mm diameter spot) in the center of the detector and

recording the output while rotating the detector about an axis in the

plane of its surface. Rotation of +/- 5 degrees from normal incidence

produced negligible, less than 0.1%, change in the detector output.

3.7) Aperture area measurements

The area of the apertures was measured [18] by a microprobe

analysis of the roundness of the opening (Tallyround method) . The

average diameter was then used to calculate the area. From the

variations in the diameters, the upper bound on the systematic error in

the area of the apertures is estimated to be +/- 0.02%. The measured

value for the area of each DRTP aperture is included in the calibration

report

,

The distance to which the plane of the aperture is recessed from

the front of the holder was also measured and is included in each

calibration report. The exact position of the plane of the aperture is,

of course, important in transferring detector response in the irradiance

mode (A cm^/W)

.

32
3.8) Specular and diffuse reflectance measurements

Specular reflectance (within a 1° cone around a 5° angle of

reflection) was measured with an amplitude stabilized HeNe laser at

632.8 run and a HeCd laser at 441.6 and 325.0 nm. These measurements were performed with a separate silicon detector as the sensor to receive the

reflected radiation. The sensor detector was then moved to intercept and

measure the incident beam. Care was taken to insure that the incident

and reflected spots were approximately the same size and in the same position on the sensor detector to preclude any non-uniformity effects.

The sensor detector's linearity was verified over the dynamic range of

this measurement. The measurements were performed in the AC mode (with a lock- in amplifier).

Diffuse reflectance (specular excluded) was measured with a 50 mm

diameter silicon detector that had a 9 mm diameter hole in the center

[19]. This detector was checked for linearity by comparing it to

detector DR-10 and for uniformity by the method described in Section

3.6. With the laser radiation passing through the hole and reflected

back out of the hole from one of the DRTP detectors, the diffusely

reflected radiation could be detected. By increasing the distance

between the two detectors, the collection angle was reduced so as to

measure the radiation that had escaped through the center hole when the

detectors were at the closer position. The diffuse reflectance was measured from within 1° of specular (at nearly normal incidence) to 68°

.

It was found that the diffuse reflectance was approximately

proportional to the specular reflectance. The estimate used was that the

diffuse component was 0.009 times the value of the specular component of

reflection. The upper bound on the systematic error in the absolute

response calibration due to this approximation of the diffuse reflection

component is negligible.

33
3.9) Absolute spectral response measurements

The absolute spectral response of the DRTPs was measured by

comparison to an absolute detector [20, 21, 22] at 325.0, 441.6, 632.8

and 951 nm. The first two lines are from a HeCd laser and the third from

a HeNe laser. The fourth line is a xenon emission line obtained from a high pressure arc lamp and interference filter. The laser lines were amplitude stabilized [17] with the experimental set-up as described in

NBS Tech Note 954 [5]. The filtered image of the high pressure arc lamp was focussed through a beamsplitter simultaneously onto a monitor

detector and the detectors being calibrated. The active areas of the

detectors were underfilled; the image size ranged from 3 to 5 mm

diameter

.

In addition to the calibrations at the above mentioned wavelengths,

the DRTPs were measured at 1014 and 1064 nm [23]. The first wavelength

is that of a mercury emission line and the second is that of a NdYAG

laser. The absolute basis for these measurements was the Electrically

Calibrated Pyroelectric Radiometer (ECPR) [13, 14]. These data were not

used in the interpolation process (see below), but are reported as measured.

Since these are radiant power calibrations, care was taken to

insure that all the radiation was collected and that the image spot was not surrounded by appreciable scattered radiation. In the case of the

laser-based measurements, this was checked by measuring the radiant

power along the beam with an apertured DRTP. Radiant power variations

were much less than 0.1%; therefore, the error from this effect was negligible. In the case of the arc source, the image quality was visually inspected (in the green spectral region) by passing the image

through a hole in a white card and estimating the amount of scatter

around the beam. By comparison with the visual appearance of the laser

based measurements, the scatter observed from the arc source was estimated to be no more than 0.1%.

34
The absolute detectors upon which the response measurements are based are of the 100% quantum efficient type [20] . These are standards

based on the silicon photodiode self -calibration or predictable quantum

efficiency technique [21, 22]. The upper bound on the systematic error

in the absolute response of these devices is +/- 0.2% with respect to SI

base units [20, 24] in the 360 to 700 nm wavelength range.

Direct comparison to a 100% quantum efficient device was done at

only 441.6 and 632.8 nm. At the uv and ir lines the comparison detector

was an ECPR that previously had been calibrated with the 100% quantum

efficient device [23]. The upper bound on the systematic error of the

ECPR calibration, i.e. the maximum range of the variation of the

correction factor is +/- 0.5%.

The worst case estimate of the standard deviation of the mean (one

sigma value) at each wavelength was: 325.0 nm, 0.22% (5); at 441.6 nm,

0.07% (4); at 632.8 nm, 0.10% (5); and at 951 nm, 0.58% (5). At 1014 and

1064 nm, the estimated standard deviations of the mean were somewhat

higher: 0.99% (8) at 1010 nm and 0.88% (8) at 1064 nm. The number of

measurements in each set is given in parentheses.

3.10) Interpolation of the external quantum efficiency

The absolute spectral response R(A) in A/W is directly proportional

to the external quantum efficiency Qext(^) electrons/photon [21, 22] as follows:

R(A) = Qe,t(A) 1239.5 ^""^

where the wavelength A is in nm units. The external quantum efficiency

is related to the internal quantum efficiency via the reflection loss,

r(A),

35
[1 - r(A)] Qi,t(^) (5)

The spectral variation of reflectance was calculated as follows.

The value of the reflectance as measured at 441.6 nm was used with the

literature values of the indices of refraction of Si02 [25] and Si [26]

in the Fresnel reflection equations [27] to calculate the thickness of

the oxide layer. The Si02 thicknesses obtained ranged from 115 to 122 nm

.

The calculated values of the Si02 thickness were used in the

Fresnel equations to calculate the spectral variation of the reflectance

at 10 nm intervals over the range from 250 to 960 nm. The reflectance

curves for 115 and 122 nm thickness of SiOg on Si are shown in Fig. 9.

The effect on absorptance (one minus the reflectance) due to a 2 nm

error in a 120 nm thick oxide is shown in Fig. 10. It can be seen in

Fig. 10 that the fitting errors in the absorptance are expected to be

greatest in the blue and uv.

Using the values of the oxide layer thicknesses obtained from the

441.6 nm data, the reflectances at 325.0 and 632.8 nm were calculated to

check the goodness of the fit. Since the Si02 layer acts as an antireflection coating in the red-visible region, the agreement was quite good at 632.8 nm. The largest difference in terms of the effect on absolute response in this region was 0.5%. However, the opposite is true

in the blue-visible and ultraviolet. The reflection is near a maximum so that an error in the estimation of the oxide thickness has a more pronounced effect. At 325.0 nm the largest difference in the reflectance

fitting was 3.5% in terms of the absolute response.

From the measurements of the absolute spectral response and the

calculated values of the spectral reflectance, the internal quantum

efficiency of each DRTP was obtained at 325.0, 441.6, 632.8, and 951 nm.
1.0

0.8

0)

o

c

COo

i> 0.6

0)O

COQ.

o

CO .Q

<

0.4

0.2

115 nm //

122 nm

1 1

300 500 700

Wavelength (nm)

900

Fig. 9. Spectral absorptance of the DRTP photodiodes

37
38
The experimental values of the internal quantum efficiency were used to

determine the P, T and L parameters in the theoretical model [28] of the

internal quantiom efficiency, e(A).

Here a is the wavelength dependent absorption coefficient of silicon.

From the literature values of the wavelength dependence of a [26] , the

spectral dependence of e was calculated at 10 nm intervals from 250 to

960 nm.

The quantum yield of silicon (production of electron-hole pairs per

photon absorbed) is unity in the 360 to 960 nm region; however, it

becomes greater than unity below 360 nm. In the ultraviolet region of

the spectrum from 250 to 360 nm, the values of the quantum yield of

silicon as measured by Wilkinson, Farmer, and Geist [29] were used to

calculate the internal quantum efficiency. The upper bound on the

systematic error contribution to the uncertainty of the absolute

response is +/- 3% at 250 nm and decreasing to effectively zero at 360

nm.

From the spectral variation of the reflectance and internal quantum

efficiency, the external quantum efficiency was calculated at 10 nm

intervals from 250 to 960 nm. These values were then converted to the

conventional units of absolute spectral response, A/W. The resulting

quantum efficiency curves are shown for DR-14 in Fig. 11 and for DR-15

in Fig. 12. These two DRTFs were at the extremes of the fitting

parameters, that is, their Si02 thicknesses and P, T, and L values

differed by the greatest amounts.

The accuracy of the interpolation (and extrapolation) was checked

experimentally by measuring the absolute spectral response of the DRTPs

at several laser lines and atomic (mercury and xenon) emission lines.

39

41
The absolute detector in these measurements was the ECPR cr the 100%

quantum efficient detector depending on the wavelength (see Section

3.9). The results at several laser lines including those used in the

interpolation are shown for DR-14 and DR-15 in Table 3.

TABLE 3

Difference between measured and interpolated responsivities

Wavelength DR-14 DR-15

325.0 nm -0.8% -0.4%

406.7 +0.9 +0.3

441.6 +0.2 +0.1

568.2 -0.7 -0.4

632.8 -0.9 -0.7

799.3 -0.1 +0.1

The results in Table 3 as well as the results of the measurements

at the atomic emission lines are shown in the bottom portion of Figs. 11

and 12. The error bars on the data points represent the three sigma

precision of the DRTP-absolute comparison measurements.

3.11) Spectral comparator measurements - quality control

In addition to the absolute detector based measurements discussed

at the end of the last Section, the responsivities of all the DRTPs were intercompared by means of a conventional (monochromator-based) spectral

comparator. See reference [10] for a description of this type of

instrument. The results of those measurements are shown in Fig. 13.

Here DR-15 was chosen as the "standard" detector and the

responsivity of the other DRTPs calculated with respect to DR-15. The

42

calculated DRTP responsivity was then compared to the responsivity as determined by the interpolation procedure described in the previous

Section. The percent differences between the comparator measurements

based on DR-15 and the actual calibration of each DRTP are plotted in

Fig. 13.

3.12) Estimation of the overall uncertainty

The verification of the estimation of the uncertainties of the DRTP

calibrations is contained in Table 3 and Figs. 11, 12 and 13. The major

components of the uncertainty at short wavelengths are the quantum yield

of silicon (peak at 250 nm) and the reflectance fitting (peaks around

300 and 450 nm) . In the visible region of the spectrum all the

contributions to the uncertainty go to a minimum. At long wavelengths

the uncertainty is dominated by the imprecision of the response

comparison and the non-uniformity effects. A summary of the uncertainty

estimates for the DRTPs is presented in Table 4.

44
TABLE 4

Summary of the DRTP Uncertainty Estimates

Wavelength 250 to 400 to 500 to 860 to 1014 1064

(nm) 390 490 850 960

QED accuracy --- 0.2% 0.2%

ECPR accuracy 0.5% --- --- 0.5% 0.5% 0.5%

Response

comparison

precision* 0.7 0.2 0.3 1.7 3.0 2.7

Silicon

quantum

yield 3.0 0.0 0.0 0.0

Reflectance

fitting 4.0 1.5 0.5 0.7

Quantum

efficiency

fitting 1.0 0.5 0.2 0.5

Non-uniformity 3.0 1.0 0.5 1.0 5.0 5.0

{DR-14) {20 20}

Temperature

fluctuations 0.0 0.0 0.0 0.0 0.2 0.2

Linearity,

0 . 1 mW and

below, % per {250 to 700 nm, 0.1; 700 to 1064 nm, 0.3:

decade except DR-14, see Section 3.5}

TOTAL

(quadrature

addition) 6.0% 1.9% 0.8% 2.2% 5.9% 5.7%

{DR-14} {20% 20%}

* This is a three sigma estimate of the precision (three times the

standard deviation of the mean) . All the other uncertainty estimates

are upper bounds of a systematic error and are considered to have the

nature of a three sigma confidence limit.

45
References

1) G. A. Uriano, E. L. Garner, R. K. Kirby, and W. P. Reed eds

.

"NBS Calibration Services User's Guide 1986-88", Natl. Bur.

Stand. (U. S.) Spec. Publ. 250 (1986).

2) M. A. Lind and E. F. Zalewski "Silicon Photodetector Instabilities

in the UV" Appl. Opt. 15, 1377 (1976).

3) K. Mohan, A. R. Schaefer, and E. F. Zalewski, "Stability and

Temperature Characteristics of Some Silicon and Selenium

Photodetectors" NBS Tech. Note 594-5, (1973).

4) F. Grujn and J. Cameron, "Detector Intercomparison Results"

Electro-Optical Systems Design, (Kiver Publications Inc., 222

W. Adams, Chicago, IL; November, 1974) p. 82.

5) J. Geist, M. A. Lind, A. R. Schaefer, and E. F. Zalewski,

"SPECTRAL RADIOMETRY: A New Approach Based on Electro

Optics" NBS Tech. Note 954, (1977).

6) M. A. Lind, "Measurement of the Absolute Spectral Response of

Detectors", Proc . Electro-Optics System Design Conf.

(Industrial and Scientific Conference Management, Chicago,

1976) p. 55.

7) E. F. Zalewski and M. A. Lind ," Improving the Accuracy of Radiant

Power Measurments Based on Photodetector Instrumentation"

,

Syrap. on Bio. Effects and Meas . of Light Sources, D. G.

Hazzard, Ed. HEW Publication (FDA) 77-8002 (U. S. Government

Printing Office, Washington, DC, 1977) p. 117.

8) A. R. Schaefer and E. F. Zalewski, "Measurement of the Energy

Irradiance from Single Pulse Sources" NBS Tech. Note 935,

(1977).

9) M. A. Lind, E. F. Zalewski, and J. B. Fowler, "The NBS Detector

Response Transfer and Intercomparison Package: The

Instrumentation", NBS Tech. Note 950, (1977).

10) CIE Publication No. 64 (1984) "Determination of the Spectral

Responsivity of Optical Radiation Detectors". To obtain a copy

of this publication contact Mr. Robert A. McCully, North

American Philips Lighting Corp., Philips Square, CN6800,

Somerset, NJ 08873-6800; phone (201) 563-3471.

11) A. Corrons and E. F. Zalewski, "Detector Spectral Response from 350

to 1200 nm Using a Monochromator Based Spectral Comparator",

NBS Tech. Note 988, (1978).

46
12) W. R. Blevin and W. J. Brown, "Black Coatings for Absolute

Radiometers", Metrologia, 2, 139 (1966).

15) J. Geist anc V. R, Blevin, "Chopper-Stabilized Null Radiometer Based

on an Electrically Calibrated Pyroelectric Detector", Appl.

Opt. 22, im (1973).

14) W. K. Doyle, 2. C. Mcintosh, and J. Geist, "Implementation of a SysLem of Optical Calibration Based on Pyroelectric

Radiometry", Opt. Engr . 15, 541 (1976).

15) A. R. Schaefer, E. F. Zalewski, M. A. Lind, and J. Geist,

''Linearity Studies on Silicon Photodiodes" , Proc

.

Riectro-Optics System Design Conf .( Industrial and Scientific

Conference Management, Chicago, 1977) p. 459.

16) A. F.. Schaefer, E. F. Zalewski, and J. Geist, "Silicon Detector

Ncniinearity and Related Effects", Appl. Opt. 22, 1232 (1983).

17) J. B. Fowler, M. A. Line, and E. F. Zalewski, "A Servo Controlled

Electro-Optic Modulator for cw Laser Power Stabilization and

Control", NBS Tech. Note 987, (1979).

18) Tnese measurements were performed by Mr. Ralph Veale of the

Mechanical Processes Division of NBS.

19) E. F. Zalewski, J. Geist, and R. C. Willson "Cavity Radiometer

Reflectance", Proc. of the SPIE, 196, 152 (1979).

20) E. F. Zalewski and C. R. Duda, "Silicon Photodiode Device with 100%

External Quantum Effiency" , Appl. Opt. 22, 2867 (1983).

21) E. F. Zalewski and J. Geist, "Silicon Photodiode Absolute Spectral

Response Self -Calibration" , Appl. Opt. 19, 1214 (1980).

22) J. Geist, E. F. Zalewski, and A. R. Schaefer "Spectral Response

Self-Calibration and Interpolation of Silicon Photodiodes",

Appl. Opt. 19, 3735 (1980).

23) E. F. Zalewski and M. '.^Jif inc , 'Silicon Photodiode Self-Calibration

as a Basis for F<.ad:- omatry in the Infrared", Proc. of the SPIE,

308 . 2 <1981).

24) L. P. Boivin and F. T. :^cNe?.ly, "Electrically Calibrated Absolute

Radiometer Suitable fur Measurement Automation", Appl. Opt. 2_5 554 (1986)

.

25) I. H. Malitson, "Interspecimen Comparison of the Refractive Index of

Fused Silica", J. Opt. Soc. Am. 55, 1205 (1965).

47
26) H. R. Philipp, "Influence of Oxide Layers on the Determination of

the Optical Properties of Silicon", J. Appl. Phys . 43, 2835

(1972), and personal conununication.

27) M. Born and F V/olf, "Principles of Optics" (Pergamon Press, NY,

1964), 3 :o ed. , p. 61.

28) J. Seise c-nd \i. F. Zalewski, "The Quantum Yield of Silicon in the

>/isiDlfc--, Appl. Phys. Lett. 35, 503 (1979).

29) F. J. Wilkinson, A. J. D Farmer, and J. Geist "The Near Ultraviolet

Qu.ant'.irn Yield of Silicon", J. Appl. Phys. 54, 1172 (1983).

48
Appendix A

Intercomparison Format

On the following pages are the instructions for the intercomparison

phase of the detector response transfer program. This intercomparison

consists of three parts: a wavelength accuracy measurement, a scattered

light measurement, and spectral response transfer measurements in the

ultraviolet, visible and infrared spectral regions.

The detailed information requested on the first page will be kept

confidential and will be used only by NBS staff members to elucidate

possible systematic errors in the measurements. The data requested on

the subsequent pages will likewise be kept confidential. When the

results of these intercomparisons are prepared for publication the

laboratories will not be identified except by code numbers.

Al
Description of Instrumentation

NBS Detector Number DR- Date:

Laboratory

Person(s) to contact regarding measurements:

Phone Number: ( )

Address

:

Monochromator

:

/_/Grating /_/Prism /_/Interference Filters /_/Other, identify

/_/S ingle Monochromator /_/Double Monochromator

If Double, /_/Additive or /_/Subtractive Dispersion?

Bandpass (full width at half -maximum) . Is the bandpass the same throughout the spectrum? If not please identify the wavelength or clarify below.

Wavelength calibration (which lines)

Size and Shape of Output Beam on Detector Surface

Clarification of above information or other pertinent instrument

characteristics

Light Source:

Description of light source (i.e. incandescent strip lamp, mercury arc,

etc . )

Was an image of the light source focussed or diffused on the entrance

slit of the monochromator?

Did you use a monitor detector to adjust the intercomparison data for

possible light source fluctuations?

A2
Wavelength Accuracy Intercomparison

NBS Detector Number DR- Date

:

Laboratory

Wavelength Calibration Filter Numbers WV- and WRThe nominal peak transmission wavelength is 410 nm for the WV filter

and 940 nm for the WR filter. You are asked to identify the wavelengths

of the 20% transmission points on both of these filters. That is,

measure the detector output both with and without the filter at the

smallest wavelength intervals you can obtain. Compute the following

ratio

then report the two wavelengths (for each filter) at which T = 0.200.

Please repeat this procedure at three different times (different days if

possible) and preferably after some change in the instrument such as switching gratings.

T(l) = Filtered Detector Output

Bare Detector Output

For WV, T = 0.200 at: WAVELENGTHS

RUNl RUN 2 RUN 3

For WR, T = 0.200 at: WAVELENGTHS

RUNl RUN2 RUN 3

A3
Scattered Light Intercomparison

NBS Detector Nuinbi.: ^R- Date:

Laboratory

The three ab5:oiDivig glass filters have been selected to enable a test of out of -band radiation in three separate regions: the uv from 200

to 350 nm (SV filter), the visible from 450 to 650 nm (RT filter), and

the ir from 750 to 1150 nm (SR filter) . You are asked to first measure

the ovicput of your moncchromator in each of these regions using the bare

detector. Then repeat the measurements with the appropriate filter in

place and the gain set at 10"^ A/V or as high as possible.

Bare Detector Filter Filtered Detector

Wavelength Output Gain Number Output Gain

200 nm SV250 SV350 ' SV450 RT550 RT650 RT750 SR850 SR950 SR1050 SR1150 SRA4
Visible Response Transfer Intercompar ison

NBS Detector Number Dr- Date:

Laboratory

Filter Numbers SV- and SRThe SV and SR filters when used together on the detector result in a response function limited to the visible spectral region. The order of

stacking the filters may be significant (because of interref lections ) so put the SV filter in place first, then the SR filter over that. You are asked to measure the absolute spectral response of the filtered detector,

basing the measurements on the values of response as supplied for the

bare detector. Please repeat these measurements at three different times

(different days if possible) in order that an estimate of your precision

can be made

.

ABSOLUTE RESPONSE (A/W)

WAVELENGTH RUN 1 RUN 2 RUN 3

450 nm

475

500

525

550

575

600

625

^

650

675

Has a correction been made for the effect of the monochromator bandpass?

Was the aperture in place between the detector and the filters? filters?

A5
UV and IR Response Transfer Intercomparison

NBS Detector Number Dr- Date:

Laboratory

Filter Number RTThe RT filter when placed on the detector results in a two part

response function: one section in the uv and one in the ir. You are asked

to measure at three different times the detector output both with and

without the filter in place. Then report the response transfer ratio:

T(l) = Filtered Detector Output

Bare Detector Output

RESPONSE TRANSFER RATIO

WAVELENGTH RUN 1 RUN 2 RUN 3

320 nm

340

360

380

400

700

800

900

1000

1100

A6
FORM NBt-MS

(RCV. IJ-M) APPENDIX B

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Gaithersburg, MD 20899

REPORT OF CALIBRATION

of

Photodetector Absolute Spectral Response

Requested by:

Company Name

Company Address

(See your purchase order number ###### dated: .)

The NBS detector response transfer radiometer [ ] No. DRhas been supplied to the above-mentioned laboratory for the

purpose of transferring the NBS scale of detector spectral

response. The responsivity in amps/watt of this radiometer is

presented in the accompanying table as a function of wavelength.

A second table lists the estimated uncertainty relative to

absolute (SI) units as a function of wavelength. Also included

is the measured area of the aperture supplied with the radiometer

to enable measurements of radiant power density (irradiance)

.

The absolute spectral response of the NBS detector response

transfer radiometer is based on electrical power versus radiant

power substitution measurements [2,3] and on the silicon

photodiode self-calibration method [4,5]. Amplitude stabilized

cw lasers [6] and a HgXe arc lamp with narrow-band filters were

used as the monochromatic sources. The incident radiation was

normal to the detector surface and underfilled the active area.

Each time the radiometer is returned to NBS, a measurement of the

absolute spectral responsivity is performed at a few key

wavelengths to verify the stability of the calibration. For

maximum accuracy in the detector response transfer measurements,

detector uniformity must be considered. It is recommended that

the incident radiation be restricted to the central 8 mm diameter

area of the detector. Further recommendations for performing

detector response transfer measurements can be obtained in

reference [ 7 ]

.

Prepared by: Approved by:

Jeanne M. Houston Donald A. McSparron

Radiometric Physics Division Radiometric Physics Division

Center for Radiation Research Center for Radiation Research

NBS Test No.

:

Date:

Bl
REPORT OF CALIBRATION Page 2

REFERENCES

:

[1] "The NBS Detector Response Transfer and Intercomparison

Package: The Instrumentation," M. A. Lind, E. F.

Zalewski and J. B. Fowler, NBS Technical Note 950

(1977).

[2] "Fundamental Principles of Absolute Radiometry and the

Philosophy of This NBS Program (1968 to 1971)," J.

Geist, NBS Technical Note 594-1 (1972) ; also

"Comparison of the Laser Power and Total Irradiance

Scales Maintained by the National Bureau of Standards,"

J. Geist, L. B. Schmidt and W. E. Case, Applied Optics

12, 2773 (1973).

[3] "Spectral Radiometry: A New Approach Based on ElectroOptics, " J. Geist, M. A. Lind, A. R. Schaefer, and E.

F. Zalewski, NBS Technical Note 954 (1977)

.

[4] "Silicon Photodiode Absolute Spectral Response SelfCalibration", E. F. Zalewski and J. Geist, Applied

Optics 19, 1214 (1980).

[5] "Spectral Response Self-Calibration and Interpolation

of Silicon Photodiodes" , J. Geist, E. F. Zalewski, and

A. R. Schaefer, Applied Optics 19, 3795 (1980).

[6] "A Servo Controlled Electro-Optic Modulator for cw

Laser Power Stabilization and Control," J. B. Fowler,

M. A. Lind and E. F. Zalewski, NBS Technical Note 987

(1979) .

. [7] "Detector Spectral Response from 3 50 to 12 00 nm Using a

Monochromator Based Spectral Comparator", A. Corrons

and E. F. Zalewski, NBS Technical Note 988 (1978)

.

NBS Test No. : 534/

Date:

B2
Radiometer No. DR

Spectral Spectral

Wavelength Responsi vi ty Wavelength Responsi vity

(nm) (A-W-1

)

(nm

)

( A-W"'" )

250 0. 105 640 0.462

260 0.104 650 0. 471

270 0.095 660 0. 481

280 0.091 670 0.490

290 0.093 680 0.500

300 0.097 690 0.508

310 0. 100 700 0.517

320 0.099 710 0.526

330 0. 101 720 0.534

3>^0 0.099 730 0.541

350 0. 100 740 0.549

360 0.098 750 0.555

370 0.099 760 0.562

380 0.1 12 770 0.569

390 0. 128 780 0.575

ilOO 0.1 43 790 0.581

il10 0. 158 800 0.588

i420 0.173 810 0.594

430 0. 188 820 0.599

440 0.204 830 0.605

450 0.219 840 0.61 0

460 0.234 850 0.616

470 0.248 860 0.621

480 0.263 870 0.626

490 0.277 880 0.631

500 0.292 890 0.636

51 0 0. 306 900 0.640

520 0.320 910 0.646

530 0.334 920 0.650

540 0.347 930 0.653

550 0.361 940 0.654

560 0.373 950 0.654

570 0.384 960 0.649

580 0.397

590 0.409

600 0.422 101 4 0.57

610 0.431 1064.2 0.26

620 0.442

630 0.452

B3
Radiometer No. DR

ESTIMATED UNCERTAINTY IN ABSOLUTE RFSPONSE MEASUREMENTS

Wavel ength

250nm - 390nm +_6.0%

ilOOnm - 490nm ^ .9%

500nm - 850nm ^0.8%

860nm - 960nm 2.2%

1 01 nnm +^5.9%

1064. ?nm + 5.7%

Aperture Dimensions

Area: 0.5150 +_ 0.0005 cm^

Distance from aperture plane to front of holder: 4.85 + 0.05 mm

B4

i
FORM NBS-1 14A (REV. 11 84)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See in struct/on sj

1. PUBLICATION OR

REPORT NO. NBS/SP-250/17

2. Performing Organ. Report No. 3. Publication Date

March 1988

4. TITLE AND SUBTITLE

NBS Measurement Services:

The NBS Photodetector Spectral Response Calibration Transfer Program

5. AUTHOR(S)

Edward F. Zalewskl

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

SAME AS ITEM //6 ABOVE.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 88-600509

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnifi cant information. If document includes a significant

bibliography or literature survey, mention it here)

A silicon-photodiode-based radiometer is supplied to

transfer the calibration of absolute spectral response in units of A/W

(Amperes/Watt) and A cm^/W in the 250 to 1064 nm region of the spectrum.

The radiometer is also characterized for linearity over the four-decade

range of the amplifier gain settings. Also included with the radiometer

are components and a procedure for measurement error diagnosis. The

methods for obtaining and using this calibrated radiometer are described, as well as the calibration and characterization procedures

used at NBS.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semi colon s) calibration, linearity, photodetector, photoelectric effect, quantum

efficiency, radiometry, responsivity , silicon photodiode , spectral

response

.

13. AVAILABILITY

[]J Unl Imlted Q For Official Distribution. Do Not Release to NTIS

reorder From Superintendent of Documents, U.S. Government Printing Office, Washington, DC

20402.

Order From National Technical Information Service (NTIS), Springfield, VA 22161

14. NO. OF

PRINTED PAGES

65

15. Price

USCOAAM-DC 85-6006

I
i
THE SP 250 SERIES ON NBS MEASUREMENT SERVICES*

SP 250-1 Spectral Radiance Calibrations 003-003-02792 $3.50

SP 250-2 Far Ultraviolet Detector Standards 003-003-02810-0 $4.25

SP 250-3 Radiometric Standards in the Vacuum

Ultraviolet 003-O03-02806-1 $6.50

SP 250-4 Fricke Dosimetry in High-Energy Electron Beams

003-003-02816-9 $2.75

SP 250-5 Alpha-Partide Calibrations SN003-003-0283-1 $2.00

SP 250-6 Regular Spectral Transmtttance 003-003-02805-3 $3.25

SP 250-7 Radiance Temperature Calibrations 003-003-02827-4 $2.25

SP 250-8 Spectral Reflectance 003-003-02812-6 $7.00

SP 250-9 Calibration of Beta-Partide-Emttting

Ophthalmic Applicators

003-003-02817-7 $2.00

SP 250-10 Radioactivity Calibrations with the "4-ir" Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities

003-003-02824-0 $2.25

SP 250-1 1 Dosimetry for High-Dose Applications

SP 250-12 Neutron Personnel Dosimetry

003-003-02811-8 $2.50

SP 250-13 Activation Foil irradiation with Californium Rssion Sources

SP 250-14 Activation Foil irradiation by Reactor Cavity

Fission Sources

SP 250-15 Photometric Calibrations SN003-003-02835-5 $4.25

SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuring instmments

SP 250-17 The NBS Photodetector Spectral Response

Calibration Transfer Program

SP 250-18 Neutron Source Strength Calibrations

SP 250-19 Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

SP 250-20 Spectral InBdiance Calibrations 003-003-02829-1 $5.50

SP 250-21 Calibration of Beta-Partide

Radiation Instrumentation

SP 250-22 Platinum Resistance Thermometer

Calibrations 003-003-02831-2 $17.00

SP 250-23 Liquid-in-Glass Thermometer

Calibration Sen/ice

SP 250-24 Standard Cell Calibrations 003-003-02825-8 $2.75

SP 250-25 Calibration Service for Inductive Voltage Dividers

SP 250-26 Phase Angle Calibrations

SP 250-27 AC-DC Difference Calibrations

SP 250-28 Solid-state DC Voltage Standard Calibrations SN003-003-02842-8 $2.00

SP 250-29 Traceable Frequency Calibrations SN003-003-02844-4 $2.25

SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation

SN003-003-02845-2 $2.75

SP 250-31 Mass Calibrations at the National Bureau

of Standards

* Those entries containing a stock numt)er (003-003 ) and price can be purchased from the Superintendent of Documents, U.S. Govemment Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and Mastercharge. For more Information, or to place an order, call (202) 783-3238. Be sure to use the stock numtjer in all orders. Titles without stock numbers are in preparation.
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