TABLE 1

Typical DRTP Responsivity

Spectral Spectral
Wavelength Responsivity Wavelength Responsivity
(nm) (A) (nm) (am)
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


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 nn,
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 Q,,,(4) in electrons/photon (21, 22]

as follows:
x
RO) = QexeQ) 793975 TM

where the wavelength \ is in nm units. The external quantum efficiency
is related to the internal quantum efficiency via the reflection loss,
xQ),

35


Fig. 8. Typical (except DR-14, see text) detector uniformity map
at 1064 nm.

31


1.0

0.8

Quantum Efficiency
o
o

0.4

0.2
+0.05

% Difference
o

-0.05

Fig.

Internal

: 7
External
[~ 7
Z 7
~ 7
at 1 L 1 1 J
300 500 700 900 nm
t+ + +z f

11.

Calculated quantum efficiency of DR-14 (upper curve) and

comparison to measured values (lower curve).

40


M3lA YV3Y

Pps0D J8MOd

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“peey AeJoWOTpeI Tezsueiq esuodser JojDaQeq “[ “3Tq

MaIA LNOYS

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indino ONS


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
/_/Single 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


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 communication. ~

27) M. Born az Wolf, "Principles of Optics" (Pergamon Press, NY,
1964), S-G ed., p. 61.

28) J. ¢ . Zalewski, "The Quantum Yield of Silicon in the

appl. Phys. Lett. 35, 503 (1979).

29) F. J. Wilkinson, A. J. D Farmer, and J. Geist "The Near Ultraviolet
Quantum Yield of Silicon", J. Appl. Phys. 54, 1172 (1983).

48


UV and IR Response Transfer Intercomparison

NBS Detector Number Dr- Date:

Laboratory

Filter Number RT-

The 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(1) = Filtered Detector Output
Bare Detector Output

RESPONSE TRANSFER RATIO

WAVELENGTH RUN_1

a

RUN_2 RUN _3

320 nm

340

360

380

400

700

800

900

1000

1100

A6


Radiometer No. DR

ESTIMATED UNCERTAINTY IN ABSOLUTE RESPONSE MEASUREMENTS

Wavelength

250nm - 390nm + 6.0%

400nm - 490nm + 1.9%

500nm - 850nm + 0.8%

860nm - 960nm + 2.2%
101 4nm + 5.9%
1064. 2nm + 5.7%

Aperture Dimensions

Area: 0.5150 + 0.0005 om®

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

B4


References

1)

2)

3)

4)

5)

6)

7)

8)

9)

10)

11)

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).

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

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

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).

F, Grum and J. Cameron, "Detector Intercomparison Results"

Electro-Optical Systems Design, (Kiver Publications Inc., 222
W. Adams, Chicago, IL; November, 1974) p. 82.

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).

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.

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

Power Measurments Based on Photodetector Instrumentation",
Symp. 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.

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

Irradiance from Single Pulse Sources" NBS Tech. Note 935,
(1977).

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

Response Transfer and Intercomparison Package: The
Instrumentation", NBS Tech. Note 950, (1977).

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.

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


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 DRIP 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


Qexe A) = [1 - rQ)] Qat Q)- (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 SiO, [25] and Si [26]
in the Fresnel reflection equations [27] to calculate the thickness of

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

The calculated values of the SiO, 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 SiO, 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 SiO, 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.

36


TABLE OF CONTENTS

1) Introduction

1.1)
1.2)
1.3)
1.4)

Description of the calibration
Historical perspective
Overview of this document
Acknowledgements

2) Essential information for the user

2.1)
2.2)

2.3)
2.4)
2.5)
2.6)
2.7)

3) NBS cal

3.1)
3.2)
3.3)
3.4)
3.5)
3.6)

3.7)
3.8)

3.9)

Precautions
How to rent a detector response transfer
package (DRIP) .......... cee cece eee eee
Physical description of the DRTP ....
Other required instrumentation .
How to use the DRTP
Range of calibration ....
Description of the diagnostic intercomparison

ibration and characterization procedures
Overview of the calibration procedure ..
Safety considerations
Current amplifier calibration ..
Temperature controller adjustments ........
Linearity measurements .................6-5
Measurement of spatial and angular
characteristics
Aperture area measurements
Specular and diffuse reflectance
measurementS ........ 00. e cece cece eeeee
Absolute spectral response measurements

3.10) Interpolation of the external quantum

efficiency ........ 0... ccc e cece eee eee

3.11) Spectral comparator measurements

3.12) Estimation of the overall uncertainty ....

- quality control ....................

References 1... cece eee eee ene eeee
Appendix A: Intercomparison Format ........+eeee
Appendix B: Typical Calibration Report -+++++-+++

Verne

Al

BL


f

Fig. 6. Typical detector uniformity map at 407 nm.

29


TABLE 2

Summary 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

15


intensity light sources, such as lasers and are 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 SD1, through a 1
KW standard resistor. The anode side of the detector should be

connected to the negative side of the generator.

yaa


TRANSMISSION

NOMINAL

0.90

0.50

0.10
0.01
10-3
19-5

0.90

0.50

0.01
10-3

TUTTTTTTTT

2
=
1

Zaeaeee eet

10-5

0.90

0.50

0.10

0.01
10°3

TOT

1 1 1 L 1 1 L n 1 1 m n n 1

10-5

Fig.

300 400 500 600 700 800 900 1000
WAVELENGTH (NM)

2. Typical transmittances of the diagnostic filters.


-0.4

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


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.

P= , qd)

where P is optical power; R is 0.2137 A/W (a typical interpolated value
for the DRTP responsivity at 436 nm); G is 104 V/A (gain factor for the
DRIP 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


REPORT OF CALIBRATION Page 2

REFERENCES:

(1]

[2]

(3]

[4]

(5]

[6]

(7)

"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).

“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).

"Spectral Radiometry: A New Approach Based on Electro-
Optics, " J. Geist, M. A. Lind, A. R. Schaefer, and E.
F. Zalewski, NBS Technical Note 954 (1977).

"Silicon Photodiode Absolute Spectral Response Self-
Calibration", E. F. Zalewski and J. Geist, Applied
Optics 19, 1214 (1980).

"Spectral Response Self-Calibration and Interpolation
of Silicon Photodiodes", J. Geist, E. F. Zalewski, and
A. R. Schaefer, Applied Optics 19, 3795 (1980).

"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).

"Detector Spectral Response from 350 to 1200 nm Using a
Monochromator Based Spectral Comparator", A. Corrons
and E. F. Zalewski, NBS Technical Note 988 (1978).

NBS Test No.: 534/

Date:

B2


(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

4


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 nm.

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 1mm 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


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 +/- 0.04 KO;
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 KA.

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/cmTM?.

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

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

24


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°3 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


TABLE 1.

TABLE 2.
TABLE 3.

TABLE 4.

FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE
FIGURE

FIGURE
FIGURE

FIGURE

FIGURE

FIGURE

ON AUPE UNE

10.

11.

13.

LIST OF TABLES

Typical DRTP Responsivity
Summary of the DRTP Uncertainties.

Difference Between Measured and Interpolated
Responsivities .... 2... 2... 2 ee

Summary of the DRTP Uncertainty Estimates .....
LIST OF FIGURES

Detector Response Transfer Radiometer Head. .....
Typical Transmittances of the Diagnostic Filters
Circuit Diagram, Radiometer Head... ........
Non-linearity of DR-14 at 799m... ........
Typical Detector Uniformity Map at 254 nm.

Typical Detector Uniformity Map at 407 nm.

Typical Detector Uniformity Map at 633 nm.

Typical (Except DR-14, See Text) Detector Uniformity
Map at 1064 nm... .........00.

Spectral Absorptance of the DRTP Photodiodes

Change in Quantum Efficiency (Absorptance) Due to 2 nm
Change in the Thickness of the SiO, Layer .. .

Calculated Quantum Efficiency of DR-14 (Upper Curve)
and Comparison to Measured Values (Lower Curve). .

Calculated Quantum Efficiency of DR-15 (Upper Curve)
and Comparison to Measured Values (Lower Curve).

Spectral Comparator Measurements Versus DR-15 as the
Standard © 2... ee

vi

14
15

42
45

17
23
26
28
29
30

37

38

40

41

43


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


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.


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


NBS MEASUREMENT SERVICES:
THE NBS PHOTODETECTOR
SPECTRAL RESPONSE CALIBRATION
TRANSFER PROGRAM

Edward F. Zalewski

Center for Radiation Research
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


The absolute detector in these measurements was the ECPR or 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 40.1
568.2 -0.7 -0.4
632.8 -0.9 -0.7
799.3 -0.1 40.1

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


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.


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.


yonu WRS-448 APPENDIX B
(REV. 19-68)
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. DR-
has 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:

BI


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

26


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43


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;

Ai _ pAR
ap 7B + ap (3)

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


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

2


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 are 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


Visible Response Transfer Intercomparison

NBS Detector Number Dr- Date:
Laboratory
Filter Numbers £V-_ and SR-

The 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 interreflections) 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 RUN 2 RUN_3
450 nm
475
500
525

550

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?

AS


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 nm 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


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


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 104 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 R11 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.

22


Scattered Light Intercomparison
NBS Detector Numbs: oR- _ Date:

Laboratory __

The three absvioing 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 oucput 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 sv- _
250 sv- __
350 sv- __
450 RT- __
550 RT-
650 RT- __
750 _ ____ SR-
850 ___ SR-___ __
950 a SR- —
1050 SR- __
1150 SR-

A4


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


Quantum Efficiency

% Difference

1.0

0.8

0.6

0.4

0.2
+0.05

-0.05

Fig.

Internal

_] i ! _|
300 500 700 900 nm
7
+t ++ 4
x }
12. Calculated quantum efficiency of DR-15 (upper curve) and

comparison to measured values (lower curve).

41



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 quantum efficiency, e(\).

_ 1 - P) . __h
ce) = P+ GOB [1 = explant) - ar 6)

Here a is the wavelength dependent absorption coefficient of silicon.
From the literature values of the wavelength dependence of @ [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 DRTPs were at the extremes of the fitting
parameters, that is, their SiO, 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 DRIPs

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

39


Explicit mechanical and electronic descriptions of the DRTPs can be
found in NBS Tech Note 950 [9]. Some features of the instrument have
heen 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
em 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

9


Wavelength Accuracy Intercomparison

NBS Detector Number DR- Date:

Laboratory

Wavelength Calibration Filter Numbers WV- and WR-

The 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

T(1) = Filtered Detector Output
Bare Detector Output

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.

For WV, T = 0.200 at: WAVELENGTHS

RUN1 RUN _2 RUN 3
For WR, T = 0.200 at: WAVELENGTHS

RUN1 RUN2 RUN _3

AS


Radiometer No. DR

Spectral Spectral
Wavelength Responsivity Wavelength Responsivity

(am) (arWn1) == 60) 2 AWD)
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
340 0.099 730 0.541
350 0.100 THO 0.549
360 0.098 750 0.555
370 0.099 760 0.562
380 0.112 770 0.569
390 0.128 780 0.575
400 0.143 790 0.581
410 0.158 800 0.588
420 0.173 810 0.594
430 0.188 820 0.599
4uO 0.204 830 0.605
450 0.219 840 0.610
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
510 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 1014 0.57
610 0.431 1064.2 0.26
620 0.442
630 0.452

B3


and adjust the amplifier gain to 104, 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.

il


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 Publication (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 (6) 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 Chris E. Kuyatt
Acting Chief Director
Measurement Services Center for Radiation Research

aii


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


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


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 this 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. Government Printing Office, Washington, DC 20402-9325


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


ity map at 254 nm.

Fig. 5. Typical detector

28


Absorptance = 1- Reflectance

1.0

0.8

iad
ro)

S
b

0.2

! | L i ! ! !

300 500 700 900
Wavelength (nm)

Fig. 9. Spectral absorptance of the DRTP photodiodes.

37



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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 10* 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.


TABLE 4

Summary of the DRIP Uncertainty Estimates

Wavelength 250 to 400 to 500 to 860 to 1014 1064
(nm) 390 490 850 960

QED accuracy aoe 0.2% 0.2% Siete ciated ---
ECPR accuracy 0.5% ated otal 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 wre wee

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% 5.98 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


12)

14)

15)

16)

17)

18)

19)

20)

21)

22)

23)

24)

25)

W. R. Blevin and W. J. Brown, "Black Coatings for Absolute
Radiometers", Metrologia, 2, 139 (1966).

J. Geist ané W. R. Blevin, "Chopper-Stabilized Null Radiometer Based
on an Electrically Calibrated Pyroelectric Detector", Appl.
Opt. £532 (1973).

W. M. Doyle, 2. G. McIntosh, and J. Geist, "Implementation of a
System of Optical Calibration Based on Pyroelectric
Radiometry”, Opt. Engr. 15, 541 (1976).

A. R. Schaefer, E. F. Zalewski, M. A. Lind, and J. Geist,
"Linearity Studies on Silicon Photodiodes", Proc.
Flectro-Optics System Design Conf.(Industrial and Scientific
Conference Management, Chicago, 1977) p. 459.

A. B. Schaefer, E. F. Zalewski, and J. Geist, "Silicon Detector
Nonlinearity and Related Effects", Appl. Opt. 22, 1232 (1983).

J. B. Fowler, M. A. Linc, and E. F. Zalewski, "A Servo Controlled
Electro-Optic Modulator for cw Laser Power Stabilization and
Control", NBS Tech. Note 987, (1979).

These measurements were performed by Mr. Ralph Veale of the
Mechanical Processes Division of NBS.

E. F. Zalewski, J. Geist, and R. C. Willson "Cavity Radiometer
Reflectance", Proc. of the SPIE, 196, 152 (1979).

E. F. Zalewski and C. R. Duda, “Silicon Photodiode Device with 100%
External Quantum Effiency", Appl. Opt. 22, 2867 (1983).

E. F. Zalewski and J. Geist, "Silicon Photodiode Absolute Spectral
Response Self-Calibration", Appl. Opt. 19, 1214 (1980).

J. Geist, E. F. Zalewski, and A. R. Schaefer "Spectral Response
nd Interpolation of Silicon Photodiodes",

inc, ‘Silicon Photodiode Self-Calibration
ometry ia the Infrared", Proc. of the SPIE,

E. F. Zalewski and M.
as a Basis for Ra:
308, 2 (1981).

L. P. Boivin and F. 7. %e'
Radiometer Suitable fu:
554 (1986).

iy, “Electrically Calibrated Absolute
Measurement Automation", Appl. Opt. 25

I. H. Malitson, "Interspecimen Comparison of the Refractive Index of
Fused Silica", J. Opt. Soc. Am. 55, 1205 (1965).

47


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 Lemperature control at the front of the power supply box Lo 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