NBS TECHNICAL NO

418

Radiometric Methods for Measuring

Laser Output

y**<«* °F c,

A' o

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
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UNITED STATES DEPARTMENT OF COMMERCE

Alexander B. Trowbridge, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 418

ISSUED NOVEMBER 1967

Radiometric Methods for Measuring

Laser Output

D. A. McSparron, C. A. Douglas,

and H. L. Badger

Metrology Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

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CONTENTS

PAGE

1. Introduction 1

2. Experimental Setup 2

3. Calibration 2

3.

1

General Considerations 2

3.

2

Phototube Calibration 4

4. Laser Measurements with the Phototubes 6

5. Thermopile Calibration 8

6. Comparison with Calorimeter 9

7. Discussion of Possible Sources of Error 9

8. Conclusions 11

9. References 12

iii
LIST OF ILLUSTRATIONS

FIGURE PAGE

1. Experimental Setup for Measuring Laser Energy 3

2. Experimental Setup for Calibrating Receivers 3

3. Photoelectric Receiver 5

4. Block Circuit Diagram of the Phototube Measuring Circuit 5

5. Block Circuit Diagram of the Thermopile Measuring Circuit 10

6. Experimental Setup for Measuring Laser Energy with a Calorimeter 10

iv
RADIOMETRIC METHODS FOR MEASURING LASER OUTPUT

D. A. McSparron, C. A. Douglas, and H. L. Badger

The output of pulsed lasers may be measured with reasonable

accuracy by modifications of established radiometric methods. The

receiver, thermopile or phototube, is used ballistically. It is

calibrated by an incandescent-lamp standard of spectral irradiance.

The laser beam is attenuated to make its irradiation on the receiver

comparable to that from the calibrating source. Attenuation is

accomplished by intercepting the entire laser beam on a diffusely

reflecting surface. The attenuation factor is determined from direct

measurement of the reflectance of this diffusing surface and the geometric constants of the system. The accuracy of the measurement of

laser output is dependent upon the accuracy of calibration of the

standard of spectral irradiance and the accuracy with which the

spectral sensitivity of the receiver, the spectral transmittance of

the band-pass filters, and the reflectance of the diffuser can be

determined. Measurements made with different photoelectric receivers

agree to about 1.5% using a 1/4-joule pulsed ruby laser as a source. Total uncertainty is estimated to be about 57 . Measurements made

using these radiometric methods have been compared with calorimetric

measurements and a discrepancy of 97o was observed.

Key Words: Laser, radiometry, laser energy measurement, radiometric calibration, photoelectric photometry, optical attenuator.

1. Introduction

The rapid development of laser technology has left workers in the field without recognized methods of measuring the parameters of interest. The situation is particularly bad

in the measurement of the energy emitted from pulsed lasers. Discrepancies of 25% or more

have been common. Two widely differing approaches to the problem have been popular, namely,

radiometric and calorimetric. A common calorimetric approach entails the absorption of the

entire laser beam in a liquid. The temperature rise of the liquid is then a measure of the

total energy contained in the pulse. Calorimetric measurements are difficult to perform

because of the low signal levels obtained from low energy pulsed lasers, usually a few

microvolts, and because of the long recycling times, thirty minutes or more. In addition,

no information can be obtained from calorimetric measurements on the peak power levels of

the pulse. The work described in this paper employs a radiometric approach. In addition

to the advantages of relatively high signal levels and fast recycling times, a radiometric

method can provide information about the peak power levels in the pulse through the display

of the receiver output on an oscilloscope. Although the work to date has involved only a relatively low powered, approximately 1/4-joule, conventional ruby laser operated at 694.3

nm, the measurement techniques and procedures should be readily applicable to a wide range

of energies and wavelengths.

A pulsed laser beam differs from conventional sources in that it is highly monochromatic, extremely intense, of short time duration, and in some cases polarized. These properties require modification of conventional radiometric measurement techniques if existing

standards are to be used for the calibration. Such receivers as vacuum phototubes, photomultipliers and thermopiles can tolerate maximum incident power levels only of the order of

milliwatts/cm for conventional tubes or at most watts/cm^ for present biplanar photodiodes.

Beyond these levels physical damage occurs. Since instantaneous power levels in laser

beams run to megawatts/cm^ and higher, some attenuation device is necessary. An ideal
attenuator should be simple, rugged, and readily standardized through existing techniques.

Attenuation factors that range from 10"^ to 10"8 are necessary. A diffusing surface of

high reflectance meets these requirements.

2. Experimental Setup

The experimental setup used is shown in Figure 1. Energy from the laser is incident

normally on a pressed block of USP grade magnesium carbonate. Receivers are placed at a distance d from the diffuser, in tracks that form a 20° angle with the incident beam. The

attenuation factor, &, is given by:

ga cos 20°

rr d2 (i)

where a is the area of the receiver, |3 is the 0° :20° (incidence angle: reflectance angle)

reflectance factor 1/ of the MgCC>3 and d is the distance from the MgC03 to the receiver.

MgCC>3 was chosen for the diffusely reflecting surface and has proved satisfactory for the

wavelength and energy used as will be discussed later. £! An angle of 20° was chosen to

be sufficiently far from the normal that the effects of specular components of the reflected beam are negligible and yet to be sufficiently small so that cos 8 is not a rapidly

varying function. The p of this equation was measured at 20° on a goniophotometer with

substantially monochromatic light of the same wavelength as the ruby laser used in this

work. Thus, no assumptions about the diffuse reflection properties of MgCOo have been

made. For typical values, a one-centimeter-radius receiver and a receiving distance of

100 cm, a is approximately 10" . Increased attenuation may be obtained by increasing d or, if necessary, by introducing a second diffuser.

Two types of receivers have been used in this work. The first is a type PJ14B vacuum phototube. These phototubes have very low dark currents and an S-7 response which

is relatively flat at the ruby wavelength. This tube is no longer being manufactured. It was originally developed for use in the Hardy (General Electric) recording spectrophotometer.

The second type of receiver, so far used for preliminary investigations only, is a Moll

large-surface thermopile. It has 80 const ant an -manganin junctions placed in a circular

array one centimeter in radius with a platinum black surface.

3. Calibration

3. 1 General Considerations

Typically in radiometry employing a photoelectric or thermoelectric receiver, the

primary problem is not the measurement of the electrical output of the receiver to the

desired degree of accuracy and precision. Instead the primary problem is the calibration

of the receiver in terms of electrical output per unit of incident energy (or power). The

problem is particularly acute when the sensitivity of the receiver is not constant but

varies with the wavelength of the incident energy as does the sensitivity of a phototube,

and a calibrating source having a spectral energy distribution identical with that of the source to be measured is not available. The calibration procedure is as follows.

— Reflectance factor is the ratio of the reflected flux to that reflected by a perfect

diffuser identically irradiated.

2/ — Although MgCOo was the chosen material other diffusers of high reflectance such as MgO

and BaSOA should also prove satisfactory. These materials would be more stable chemically when irradiated by high-energy laser pulses. MgC0~ was chosen mainly because of

the ready availability of pressed blocks with large surface areas.
RECEIVER No.l

LASER

RECEIVER No.2

Figure 1. Experimental Setup for Measuring Laser Energy

ENTRANCE APERTURE

RECEIVER

\

BAFFLESDIFFUSING

GLASS SHUTTER FILTER

LAMP

G\

\ /

Figure 2. Experimental Setup for Calibrating Receivers
The receivers were calibrated in a separate experiment. The calibration setup is

shown in Figure 2. A lamp calibrated for spectral irradiance is used as an energy source. Energy from this lamp is passed through a filter, then a shutter and finally on to the

receiver. The filter is introduced in order to spectrally limit the output of the lamp.

A special two-part shutter was used. The first part consists of a 60-r/min. synchronous

motor which drives a sector disk through a multiple-speed reduction gear-head. The disk is

thus rotated at various known slow speeds to provide series of light pulses of accurately

known time durations. The second part is a simple open-close shutter that allows selection of a single pulse from the series of pulses provided by the sector disk. The combination thus simulates the pulsed characteristic of the laser beam except that the pulse

lengths are longer.

Care was taken to fully and uniformly irradiate the sensitive surface of the receiver

because receivers might exhibit large sensitivity variations across the receiving surface.

[1,2]* The requirement for uniform irradiance was met by using source-to-receiver distances which were large in comparison to the size of the source and the receiver aperture.

In addition, since the available energy and phototube sensitivity allowed it, a diffusely

transmitting glass was incorporated as part of each phototube receiver as shown in Figure

3. The baffling shown in Figures 1 and 2 is required to eliminate the effects of stray

light and interref lections. The laser head emits stray light in two ways, namely flashtube

light escaping through the vent holes and light emitted through the rod which has not

undergone laser action. In contrast to the laser beam proper, the flux incident on the

MgCOo diffuser from these sources of stray light obeys the inverse-square-law and hence, can be reduced to any degree desired by the use of large laser-head-to-diffuser distances.

In this work a distance of five meters was used.

3.2 Phototube Calibration

Figure 4 is a block circuit diagram of the measuring circuit used with the phototubes.

The photocurrent charges a capacitor, C. An electrometer-amplifier is used to determine

the voltage to which the capacitor is charged and the result is displayed on a chart

recorder.

3/ At a wavelength \, a steady-burning lamp with a spectral radiant intensity, I , — and

at a distance, d, from the receiver aperture, will produce a spectral irradiance, a , on the receiver aperture where E, «= I, /d8

. If an interference filter of spectral transmittance, t(\), is placed between the lamp and the phototube and if the phototube has a relative spectral response, R(X), the phototube output will be proportional to the triple

product: E t(\)R(\). In the calibration setup (Figure 2) the shutter allows irradiation

of the phototube receiving surface for a measured time, T. Hence, the voltage to which

the capacitor is charged, V, is given by:

V

p -Kp

T

aTf E^ T (\)R(\)d\ (2a)

where a is the area of the receiver aperture, K^ is the receiver calibration factor,

and \ x and \g are the lower and upper limits or the pass band of the filter. Thus:

Numbers in [] refer to similarly numbered references at the end of the paper.

1/ In this paper quantities which are restricted to a narrow wavelength band are indicated by adding the word spectral. The corresponding symbols are indicated by a subscript, e.g., E , for a spectral concentration or a A. in parenthesis, e.g., t(\),

for a function of wavelength. This usage is in accord with the recommendations of

ISO, IEC, CIE, and SUN Commission of IUPAP and in forthcoming American Standard YlO.6.
PHOTOTUBE

Figure 3. Photoelectric Receiver

ELECTROMETER

AMPLIFIER

DIFFUSING

GLASS

ENTRANCE

APERTURE

o

o

—

o

o-o

^-RECORC

T

o-o

1

r\ r\c l U.Ub /*t

CAPACITOR

Figure 4. Block Circuit Diagram of the Phototube Measuring Circuit

5
*P1

aTf^"

3 Ex

T(\)R(\)d\

(2b)

This factor is a function of the transmittance of the diffusing glass, the size of the

capacitor, and the absolute sensitivity of the phototube at the wavelength at which R(\)

is unity.

In the laser-measuring setup (Figure 1), the voltage, V , developed on the capacitor

as the result of a laser shot will be:

V

L sKPT aR

(694.3) Q

where a is the attenuation factor given by Equation 1, and R/fiq / ^ is the relative

spectral response of the phototube at 694.3 ran. Hence, the laser energy is:

(3)

St* R

(694.3)

(4)

4. Laser Measurements with the Phototubes

In order to establish the validity of the phototube calibrations, a complete

factorial calibration experiment was performed. Systematically, all combinations of

three phototubes, two interference filters, two exposure times, two lamps, and two

receiver housings with diffusers were used to determine six calibration factors, one each for each phototube -housing combination. The results are shown in Table I.

Table I: Phototube Calibrations

Phototube

Receiver

Hous ing Average K^,

(Volts/Joule) b

S Sing

D. le of a Measurement

(%)

S.D. of

Avg. c

(%)

2134 B 9,528 2.44 0.85

A 6,982 0.91 .32

2136 B 8,049 2.41 .85

A 5,893 2.59 .92

2138 B 12,493 2.11 .75

A 8,892 1.47 .53

Average 1.99 0.70

Based upon the relative response of the phototube at 694.3 nm (R, n . -)

Volts per joule incident on the receiver aperture.

Based on 8 independent determinations of IC, T

.

6

1.00
Each calibration factor, KpT

, shown in Table I is an average of eight different

combinations of lamp, filter, and exposure time. The standard deviations quoted are indicative of the precision with which the various measured parameters, Vp, R(\)> T (X) , and E^

can be determined. Analysis of the data shows no systematic variation with filter,

exposure time, or receiver housing. A shift of 1.3% was traceable to the standard lamp used.

However, the direction and magnitude of this shift are such as to be explained by the fact

that one of the lamps has functioned as a working standard while the other has been held as

a reference standard.

In order to check the consistency of the calibration factors thus determined, an additional experiment was performed. Laser energy was measured by using the arrangement

shown in Figure 1. Each phototube -housing combination was used in each of the 2 positions

shown to measure the energy of three laser shots. Since only two phototube-housing combinations could be used at one time, an experimental design involving a total of 36 laser shots

was employed. This design allowed separation of the effects due to varying laser energy

from those due to calibration uncertainties. The data from these two experiments were analyzed statistically. The results for the various factors involved in the calibration

experiment were as stated above. Two additional effects were found. Measurements made

with one of the phototubes, Number 2136, appear to differ from the other two by about 1%. The exact cause has not been traced as yet. The second effect was a k% shift depending

only on whether the receiver was located at position 1 or position 2 of Figure 1. This

effect was traced to interref lected energy reaching the receiver at one position and not

the other, emphasizing again the need for adequate shielding of the equipment. Table II shows the experimental design and the results of the energy measurements. (All results

obtained by measurements at receiver position 1 have been corrected by subtracting k% from

the measured value.)

Table II: Laser Energy Measurements

Phototube

(Position 1) ReceiverHousing

Laser

Output

Joules a

Phototube

(Position 2)

Re

He ceiver

using

Laser

Output

Joules 3

Difference

(Pos. 1 - Pos.

(%)

2)

2134 A .2245 2138 B .2190 2.3

2136 A .2157 2138 B .2189 -1.4

2136 A .2199 2134 B .2264 -2.7

2138 A .2249 2134 B .2279 -1.3

2138 A .2322 2136 B .2300 0.9

2134 A .2339 2136 B .2346 -0.4

2136 B .2328 2134 A .2353 -1.7

2138 B .2327 2134 A .2381 -2.1

2138 B .2316 2136 A .2204 4.9

2134 B .2305 2136 A .2157 6.8

2134 B .2227 2138 A .2312 -3.5

2136 B .2269 2138 A .2312 -1.8

The energies reported are in each case the average of three separate firings of the

laser.

The results of this statistical analysis show that the physical parameters, exposure

time, lamp energy, relative spectral response of the receivers, and particularly notable,

interference filter transmit tances, have been evaluated and combined in a mutually con- sistent manner. The experimental parameters, such as receiver housing and receiver position,

are also under adequate control. Hence, a set of mutually consistent measurements has
been achieved under widely varied conditions.

Several experiments were run to check on possible sources of systematic error. An

investigation was made to determine if the phototubes were saturating under the high

power density produced by the laser. The usual aperture was replaced with a larger

aperture (area ratio 11.6) and the supply voltage across the phototube was varied. A

second phototube was used to monitor the laser output. Table III indicates that phototube

saturation was not biasing the experiments.

Table III: Phototube Saturation Experiment

Supply Test Phototube Monitor Ratio

Voltage Output Output (Test/Monitor)

(Volts) (Volts) (Volts)

520 2.06 0.189 10.90

270 2.08 .193 10.78

180 2.02 .188 10.74

100 1.81 .177 10.22

100

b 1.82 .188 9.68

The normal supply voltage on the phototube was 220 volts.

This measurement was made with the test phototube 12 cm closer to the

MgCOQ block than the previous ones. The inverse-square law has been

applied to put this measurement on the same basis as the others.

In order to check for possible polarization effects, a linear polarizer was placed in

front of one of the two receivers. No change in the measured values was observed as the

plane of polarization was rotated.

Several experiments were run to check on the stability and suitability of MgCOo for the

diffusing surface. The reflectance factor, (3 of Equation 1, was remeasured one year after

the original measurement. The measurements agreed to within 1/2%, the precision of the

gonioref lectometer. A converging lens was introduced into the laser beam in order to focus

the beam on the MgC03 block. Although the energy density was concentrated by a factor of

10 , no effects were observed either in the measured energy or in the physical condition

of the block. Finally the usual measuring angle of 20° was varied and the phototube

responses noted. Although the angle was varied from 15° to 45° no anamalous effects were

observed.

5. Thermopile Calibration

A block circuit diagram of the thermopile measuring circuit is shown in Figure 5. The

circuit is one that allows for calibration against an external emf and thus obviates the

necessity of relying on amplifier or recorder stability. The output of the thermopile is stated in terms of the microvolts developed at the amplifier input terminals.

For a thermopile, R(\) in Equation 2b is 1.00 and hence no significant error is introduced by the relative spectral sensitivity parameter. However, in contrast with the PJ14B

phototubes whose spectral sensitivity is negligible beyond one micron, thermopiles respond

to energy far into the infrared. Since the spectral irradiance values, E , of the

standard lamps become uncertain beyond 2.5 microns, it is necessary to introduce a filter

to absorb long-wavelength energy. A "heat-absorbing" glass with a transmittance of 1% or less for wavelengths beyond 1. 1 microns was used. However, even when the filter is used,

16% of the transmitted energy from the standard lamps is contained in the region
beyond 1.1 microns.

With the modification that R(\) = 1.00 the calibration procedure described for the

phototubes is applicable. When this was tried experimentally, difficulties were encountered. The lamp energies available during calibration were such that exposure times in

the range 1/2 to 3 seconds were required to keep the thermopile response far enough above

the noise level for accurate measurements. Experiments showed that the thermopile had a 1/e time constant of about 3 seconds. The amplifier-recorder system also had a response

time on the order of seconds. The effects due to the thermopile and those due to the

measuring system proved inseparable and new equipment having a faster response time is

being obtained. It is expected that when this equipment is put in operation thermopile

measurements of the laser energy will be possible.

Although accurate calibration of a thermopile is uncertain at this time, the short

time duration of the laser pulse allows self-consistent measurements. This experiment

was performed by placing a phototube at one of the receiver positions shown in Figure 1 and

the thermopile at the other one. The ratio of their readings should be constant. Experimentally the standard deviation of a single measurement was 2.8%. A similar experiment performed with two phototubes gave a standard deviation of a single measurement of

0. 77%. This internal consistency of thermopile energy measurements indicates that future

work with the thermopile will be profitable.

6. Comparison with Calorimeter

A check of the accuracy of the calibrations was made by making a limited comparison

of results with a calorimeter built at the NBS Boulder Laboratory. [3] Since the calorimeter accepts the entire laser beam, an auxiliary monitoring system is required. Figure 6 shows the arrangement employed. The monitor was first calibrated relative to the phototubes by removing the calorimeter and allowing the laser beam to impinge on the MgCC>3

block as in Figure 1. The calorimeter was then put in place and a transfer comparison

obtained. In placing the calorimeter, care was taken not to align it with its entrance

window perpendicular to the incoming laser beam. The calorimeter was canted with respect

to the laser beam enough to insure that the energy reflected from the entrance windows did not proceed back along the beam track to the laser head. A simple calculation (assuming

16% reflectance from the calorimeter windows, 907o transmittance and 8% surface reflectance

at the beam splitter) shows that an additional energy increment amounting to 12% of the

calibrating energy can reach the monitor if this procedure is not observed.

When the transfer comparison between the calorimeter and the phototubes was carried

out, a discrepancy of about 9% was observed. The calorimeter indicated lower energies than

the phototubes.

7. Discussion of Possible Sources of Error

In view of the discrepancies observed between data on laser energies obtained with

the phototubes and with the calorimeter, consideration has been given to possible systematic grrors in the calibration of the photoelectric receivers.

The estimated uncertainty of the E^ values is 3% in the spectral region of interest,

650 to 750 run. Since the laser wavelength, 694.3 nm, lies in the visible region a check

on the irradiance values of the standard lamps was possible through a measurement of

luminous intensity and color temperature. [4] The irradiance values thus obtained checked

those obtained directly within l/27» and 3% for the two lamps.

The transmittance values, t (\) f of the interference filters used to approximate the

monochromaticity of the laser beam were measured at NBS on a Cary 14 recording spectrophotometer. Since filters having narrow pass bands centering approximately on the laser

wavelength were used, the calibration procedure is spectrally a substitution method. The narrower of the two interference filters employed has a half-band width of 1.1 nm. The

spectrophotometer has a maximum resolution of 0.2 nm and thus will not completely resolve

this filter spectrally. Instead the readout will be an average transmittance value for a 0.2-nm bandwidth.
THERMOPILE

\

CHOC BREAKER

AMPLIFII

AAAr

01 ft

CHART

RECORDER

±- 1.5 V.

300 K ft

lOOKft

G>

/

MICROAMMETER

Figure 5. Block Circuit Diagram of the Thermopile Measuring Circuit

MONITOR

LASER

7

BEAM SPLITTER

CALORIMETER

Figure 6. Experimental Setup for Measuring Laser Energy with a Calorimeter

10
Although the measured spectral transmit tance values at any given wavelength will be in error, the area under the transmission curve should be constant for a wide range of resolutions. Further, since E^ and R(\) are slowly varying functions across the pass band of

the filter, the energy reaching the phototube as computed in the integral in Equation 2b

should be very nearly correct because of the self-compensating nature of this error in the

t (\) values. Hickey's experiments confirm this directly. [5]

The relative spectral response, R(\), of the phototube-diffusing-glass combinations

was measured on a detector-spectrum recorder. This instrument consists of an electronically-controlled incandescent lamp which, together with a single-grating monochromator,

gives a constant spectral radiant flux with a band width of approximately 10 nm. The

instrument is similar to the one described by Mitsuhashi and Nakayama. [6] Measurements

made on this instrument are believed to be uncertain by no more than ±5% for the slope of

the R(\) curve in the spectral region of interest. Note that since the laser wavelength

lies within the narrow band passed by the interference filter it is the slope of R(\)

within the pass band of the interference filter that affects the accuracy of the calibration rather than the values of R(\).

The narrow pass band interference filters were used to insure that the shape of the

T (\) curve would be the dominant factor in the integral in Equation 2b since t (\) can be

measured with higher accuracy than R(\) or E, . Both the E, and the R(\) functions are slowly varying in the narrow spectral region of the pass band of the interference filters.

Analytic studies show the dominance of T (\) over E-. and R(\) on the calibration accuracy.

For example, a 5% shift in the slope of the R(\) curve makes only a 1/2% change in the

value of the integral.

Systematic errors ini-(\), R(X) and the exposure times would have been detected

immediately in the course of the analysis of the factorial calibration experiment. Systematic errors in the reflectance factor^ g (Equation 1), or the irradiance, E^ (Equation 2b)

could still be present and would not necessarily have been so detected. As indicated

above, experience and direct measurement indicate that E is known to within 3%. Past

experience indicates that reflectance factor "measurements should be in error by no more

than 1%. As indicated previously interreflections can be a source of error. However, the

magnitude of the residual interreflections, after correction for the positional effect, is

believed to be much too small to account for the difference between the phototube and

calorimeter calibrations. Lack of experimental precision is not likely to be the cause. The 3-sigma limit for the agreement of two independent and simultaneous measurements of

laser output based on the difference column of Table II is 4.5%. Since the average of 12 sets of 3 measurements each with the photoelectric receivers was used in the comparison

with the calorimeter, the 3-sigma limit for the mean of the photoelectric measurements is

insignificant in comparison to the possible systematic errors. Thus the discrepancy

between the phototubes and calorimeter is difficult to explain.

8. Conclusions

The output of low energy, ruby lasers has been measured using photoelectric radiometers. Attenuation of the laser beam was accomplished by intercepting the entire laser

beam on a diffusely reflecting surface and viewing this surface from a distance with the

photoelectric receiver. Measurements made with different photoelectric receivers agree to about 1.57o. Total uncertainty is estimated to be about 5%. Measurements made using this

radiometric method have been compared with calorimetric methods and a discrepancy of 9%

was observed.

Future plans call for continued research to eliminate the discrepancy between the

calorimeter and phototube measuring systems. Comparisons with a thermopile calibration

and measuring system are also planned. The results reported above have been obtained with

a relatively low-powered, conventional ruby laser. Present plans call for applying these

measurement techniques to lasers of higher energy, to Q-switched lasers, and to lasers

operating at wavelengths other than 694.3 nm. Once agreement is established among the

various measuring systems, transfer calibrations similar to the one described for the

monitor used in the calorimeter work can be used to calibrate any number of additional

measuring systems.

11
The authors wish to express their thanks to Mr. R. P. Teele, [7] whose preliminary

studies led to the present work, and to Mr. Esien Ku of the NBS Statistical Engineering

Section for his help in analyzing the results of the calibration and laser-measuring

experiments. In addition, the authors wish to thank their other colleagues, too numerous

to mention individually, for their valuable assistance.

9. References

[l] R. Stair, W. E. Schneider, W. R. Waters, and J. K. Jackson, Some Factors

Affecting the Sensitivity and Spectral Response of Thermoelectric (Radiometric)

Detectors, Applied Optics, Vol. 4, pp. 703-710 (June 1965).

[2] R. Stair, W. E. Schneider, W. R. Waters, J. K. Jackson, and R. E. Brown, Some

Developments in Improved Methods for the Measurement of the Spectral

Irradiances of Solar Simulators, NASA CR-201.

[3] D. A. Jennings, Calorimetric Measurement of Pulsed Laser Output Energy,

IEEE Transactions on Instrumentation and Measurement, Vol. IM-15 , No. 4,

pp. 161-164 (Dec. 1966).

[4] L. E. Barbrow, Memorandum on a Procedure for Obtaining Spectral Radiant

Intensities of Tungsten-Filament Lamps, 400-700 mm Journal of the Optical

Society of America, Vol. 49, p. 1122 (Nov. 1959).

[5] J. R. Hickey, Correlation of Monochromator and Filter Radiometry Determinations

of the Spectral Distribution in Large Solar Simulators (The Eppley Laboratory, Inc., Reprint Series No. 26), Institute of Environmental Sciences and

American Society for Testing and Materials International Symposium on Solar

Radiation Proceedings, pp. 95-105.

[6] H. Mitsuhashi and T. Nakayama, Apparatus for the Study of Spectral Response of

Photoelectric Surface, Journal of the Physical Society of Japan, Vol. 11 , pp. 308-311 (March 1956).

[7] R. P. Teele, Measurement of Pulsed Laser Output, presented at the 1965 Spring

Meetings, Optical Society of America, Dallas, Texas, Paper TE12,

31 March - 2 April 1965.

* U. S. GOVERNMENT PRINTING OFFICE: 1967 —301-020/1*7

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