^

0F

*<*

W^ J

National Bureau of Standards

library, E-01 Admin. Bldg,

OCT 6 1981

131130

/CO NBS TECHNICAL NOTE 935

US. DEPARTMENT OF COMMERCE / National Bureau of Standards

Measure

Energy Irradianc

Single Pulse Sources
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of the Office of Measurement Services, the Office of Radiation Measurement and the following Center and divisions:

Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center

for Radiation Research: Nuclear Sciences; Applied Radiation — Laboratory Astrophysics 2 — Cryogenics " — Electromagnetics * — Time and Frequency \

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government, agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials, the Office of Air and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and

Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Insti- tute consists of the following divisions and Centers: Standards Application and Analysis — Electronic Technology — Center for Consumer

Product Technology: Product Systems Analysis; Product Engineering — Center for Building

Technology: Structures, Materials, and Life Safety; Building Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations — Office of International Standards.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

- Located at Boulder, Colorado 80302.-
ml Bureau of Stanflaru* MR l o J977 Measurement of Energy Irradiance from

Single Pulse Sources

t. :&J^Wj^kfi. |VJ5\a ^O/ qi>$-

A. R, Schaefer and E. F. Zalewski

Optical Physics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

>? °F >

**fAU Of

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

^NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued February 1977
National Bureau of Standards Technical Note 935

Nat. Bur. Stand. (U.S.), Tech. Note 935, 32 pages (Feb. 1977) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 75 cents

Stock No. 003-003-01741-8 There is a minimum charge of $1.00 for each mall order
TABLE OF CONTENTS

PAGE

1. INTRODUCTION 1

2. THEORY OF THE MEASUREMENT 2

3. DETAILED EXPERIMENTAL CONSIDERATIONS 3

3.1 The Shutter Mechanism and the Evaluation of the Time

Integral 3 3.2 Errors Inherent in a Photopic Measurement 5 3-3 Electronic Considerations 6

4. MEASUREMENT PROCEDURE ..... 7

5. DATA ANALYSIS 9 5.1 A Mathematical Model of the Illuminance from an Apertured

Cylindrical Lamp 9 5.2 Treatment of the Data 12

6. CURVE-FITTING 17

7. SUMMARY 21

8. REFERENCES 21

LIST OF FIGURES

1. Shuttered time response of detector illustrating integration of

total pulse. (A) Detector has same shape as sector disk.

(B) Circular detector 4 2. Experimental setup for measurement 8

3. Illustration of model for analysis of variation of illuminance

as a function of detector distance from flashlamp 10

4. Plot of illuminance-distance squared product versus distance of

a given flashlamp with detector 1 13

5. Plot of illuminance-distance squared product versus distance of

a given flashlamp with detector 2 14

6. Data of Fig. 4 with theoretical curve derived from Eq. 25 ... 15

7. Data of Fig. 5 with theoretical curve derived from Eq . 25 ... 16

8. Data of Fig. 4 with curve fitting of Eq. 17 18

111
List of Figures (continued)

PAGE

9. Data of Fig. 5 with curve fitting of Eq. 17 19

10. Data of Fig. 5 with three curve fitting equations. _. . is

Eq. 27. is Eq. 28. is Eq. 26 20

LIST OF TABLES

1. Illuminance of a Given Flashlamp as a Function of Distance as Measured by Detector 1 22

2. Illuminance of a Given Flashlamp as a Function of Distance as Measured by Detector 2 2k

IV
MEASUREMENT OF ENERGY IRRADIANCE FROM SINGLE PULSE SOURCES

A. R. Schaefer and E. F. Zalewski

A method of measuring the energy irradianoe from a single

pulsed source, such as a xenon flashtube, is presented. Details

of the approach used in this particular measurement are given, along

with a discussion of the data analysis. In particular, we con- sider some of the inherent problems encountered in curve fitting

which are also common to other radiometric and photometric measurements .

Key Words: Curve fitting; energy irradianoe; flashtube, pulsed

source; radiometry

.

**********

1. Introduction

In this paper we describe a method for measuring the amount of energy per unit area,

or energy irradianoe [l] 1

, propagating from a pulsed source such as a xenon flashtube at a given distance from the source. The basic measurement procedure is applicable when the

energy is either to be measured spectrally (per unit wavelength) or to be integrated over a spectral bandwidth, such as in the case of photopic (luminous) energy. The latter measurement was performed as a special calibration by NBS. While preparing for this calibration,

it was realized that little previous documentation existed concerning this type of measurement. Since there is an apparent need for such measurements, this paper is being written

to document the procedure we have used at NBS.

We will present a specific method of calibrating the energy per unit area of a pulsed

source using a continuous (non-pulsed) standard of either irradiance or illuminance (luminous

intensity) . The theoretical approach will be outlined, followed by a detailed discussion of

the factors contributing to the uncertainty of the measurement. As a specific example, we

will describe the particular measurement performed at NBS. Finally, we will conclude with

a discussion of the treatment of the data, the curve fitting and its inherent problems.

2. Theory of the Measurement

In measuring the energy irradiance output of a pulsed source, the first problem to arise comes from the fact that the only basic reference standards currently available are constant power sources, such as irradiance or illuminance standards, the latter being derived

to measure the radiant power incident on a given area within a known time interval. The

integral of power over this time interval is the total energy. That is,

E(t) dt (1)

where E(t) is the power irradiance as a function of time and W is the energy per unit area. A continuous source can be chopped by a rotating blade to provide a single pulse within a measurable time interval and the integration can be accomplished electrically. For our

measurements, a silicon photodiode and operational amplifier were used. The emitted current,

i(t), was proportional to the irradiance incident upon the detector:

i(t) = kx E(t) (2a)

Figures in brackets indicate the literature references at the end of this paper.

1
If we write Eq. (2a) in terms of the charge Q, then

|f = k'i E(t) , (2b)

We used an electrometer in its charge measuring mode, in which it behaves essentially like

a capacitor. It produces a voltage, V, proportional to the charge, so that,

V=§ . (3a)

Since Q and V change in time we can differentiate and obtain

dV _ 1 d£ ,,, . dT-cdt ' (3b)

Substituting into Eq. (3b) yields

cf-ki E(t) (4a)

or f =kE(t) . (4b)

Therefore, over a time interval t^ to t2

/.

t2 V = k | E(t) dt . (5)

tl

Equation (5) is the working relation of the measurement system: a power irradiance pulse

is received by the detector, and subsequently a voltage, proportional to the total energy

in the pulse, is read on the electrometer capacitor. The calibration is made absolute by

the use of a reference standard in the determination of k.

Determination of the system calibration constant, k, can be accomplished by measuring

the voltage Vg that results after applying a pulse of known power irradiance. The energy

irradiance integral can be evaluated in the following manner. First, a known constant power

irradiance, Eq, is produced at the detector by a suitable standard incandescent lamp. A

shutter is placed between the standard lamp and the detector. If one assumes that the

shutter instantaneously opens at time t\ and closes at t2> then a pulse of known power

irradiance is incident on the detector in the specified time interval. From Eq. (5) we

can obtain the value of k:

k= -=- : (6)

/.:

2 E dt

E 0<t 2 " M>
3. Detailed Experimental Considerations

We have outlined the essentials of the method whereby energy irradiance calibrations

can be made. We shall now go into several more detailed, practical aspects of the measurement which, if not given due attention, could have an adverse effect upon the results.

3.1 The Shutter Mechanism and the Evaluation of the Time Integral

The shutter that provides the time calibration information must have the following

characteristics. It must, of course, provide a stable, repeatable, and well-determined

open time. In addition, it should expose the entire detector surface to the source in a well defined manner. In this regard, one must be wary of accidentally using the shutter

in such a way that the detector will not be illuminated in the same manner by both the

shutter and standard lamp combination and the pulsed source to be calibrated. This would

be the case, for instance, if one used a shutter opening that was not larger than the

detector aperture.

The shutter used for this calibration was a single opening sector disc of 63°, rotating

once every half second. The circular detector, area about 1 cm2

, was placed on a radius

about 15 cm from the axis of the sector disc. Under these conditions the shutter opening

or closing can be approximated by a knife edge shadow moving across the detector in a period about one sixteenth that of the total open time. Let us assume for the purpose of

discussion that we have a detector that has the same shape as the opening in the sector disc

plus uniform and linear responsivity. The time dependent output of the detector will then

be a trapezoid, as outlined in Fig. la. Segments t^ to t 3 and t^ to tg will be straight

lines, since the area irradiated is changing linearly with time. In order to apply Eq. (6) one must know the area of this pulse from t\ to tg assuming height Eg. The area of the

trapezoid is given by A = 1/2^^ - t 3 + tg - t^) Eg. Since

t 3 " t 2 = t 2 - t l = t 6 - t 5 = t 5 - t k (8)

then

t«f " t 3 = t 5 - t 2 - 2(t 3 - t 2 ) (9)

and

t 6 - t l = t 5 - t 2 + 2(t 2 - t : ) . (10)

Therefore, the area of the trapezoid is simply

A = (t 5 - t 2 ) E . (11)

That is, the area of a trapezoidal pulse is equal to the area of a rectangular pulse drawn

through half-power points t 2 and t$ and of height Eg. This is a useful operational simplification.

In reality, the detector used is generally not rectangular in shape, but circular.

Hence the segments t^ to t 3 and t^ to tg are not straight lines, since the detector area irradiated will not be a linear function of time. They will have a shape similar to that

shown in Fig. lb.

One can calculate from the area of a circular segment, what the area of this pulse

shape will be. Let
E

1/2 E

t, t 2 t

3

(a)

U t

5 t

6 t

t, t

2 t

3 t

4

t

5 t

6

t

(b)

Fig. 1: Shuttered time response of detector illustrating integration of total

pulse. A) Detector has same shape as sector disk. B) Circular detector.
1 = 2~ - x V(E /tt) " x

2 + (E„/ir) sin_1

(x V*/E ) , (12a)

2 = J-+ U V(E /ir) ~ t

2 + (E /ir) sin_1

(t V^/E^) , (12b)

|x V(E /tt) " x

2 + (E /tt) sin_1 (x V*/E ) , (12c)

= 2~ - ft V(E /tt) - t

2 + (E /ir) sin_1

(t \N/E )| , (12d)

E l3= r +

I

where

then the area is

A =

E /tt - t ; (12e)

ft2 A3 An A5 /*t 6

I Iidt + I I 2 dt + / E dt +

f

I 3 dt + / Iitdt . (13)

'tl J to J to J t h J t*

Working through Eq. (13), and using the fact that, from symmetry, Eqs. 8, 9, and 10 are again true, one gets the result

A = E (t 5 - t 2 ) (14)

This is the same result obtained for the simpler case treated earlier.

From the foregoing discussion we see that in practice one utilizes Eq. 6 with the

following modification: instead of (t 2 - t^), one uses the time interval between half

power points, (ts - t 2 )

We should emphasize the importance of maintaining a constant measurable shutter motion.

Due to mechanical imperfections in the system, one should not assume that an ordinary

synchronous motor will drive a rotary shutter wheel at the precisely expected rate. The

importance of the accuracy of the time measurement to the overall calibration is obvious.

3.2 Errors Inherent in a PhotoPic Measurement

There are specific problems that arise from doing this calibration photometrically. In this case, the quantity of interest is the photopic[2] energy irradiance obtained from the

time integral of the photopic power irradiance (illuminance) rather than the spectral power

irradiance. The measurement is carried out exactly as described above, except that a photopic detector must be used. It must be calibrated with a photometric standard, such as luminous intensity. The uncertainty arises in answer to the question: How well does the

detector response actually correspond to the CIE defined V(X) curve?

The photometric standard that was used to calibrate the detector was an incandescent

source having a spectral output resembling that of a thermal radiator at 2856 K. This

calibrated detector was then used to measure the output of xenon flashtubes which have a very different spectrum. They have a much higher relative energy in the blue than the

incandescent standards. These spectral differences coupled with the detector response

deviations from the V(X) curve can lead to a sizeable measurement error.
In order to approximate the probable magnitude of this error, hypothetical spectral

response curves of several photopic detectors* were used in calculating a theoretical

calibration between a blackbody incandescent source at 2856 K and the spectral output of

a typical xenon flashlamp. We used the relation:

! 4 = K I ij mj,

760 nm

D.(X) E.(X) dX (15)

380 nm

x J

lere E-h represents the illuminance measured by a detector i from a source j, Ej (X) is the

>ectral irradiance from the j

1-*1 source, D^(X) is the spectral responsivity of the i"1 wh«

spe detector, and K^ is the maximum luminous efficacy. The value of Km need not be specified

since it cancels out in the comparison. For the perfect detector, D^(X) is the CIE V(X)

curve. The integrals were evaluated numerically using Simpson's Rule [3] with 10 nm wavelength intervals.

The hypothetical detector responses usually deviated from the defined V(X) curve by

one or two percent, and sometimes more in the extreme red and blue regions. The results of

these calculated calibrations differed from those obtained using the perfect photopic

detector by as much as 5%. This was in agreement with the results of actual experimental

calibrations. The same flashlamp and standard lamp were compared under identical conditions

with two different types of photopic detectors (silicon photodiodes from different manufacturers) . This experiment yielded results which disagreed by about 4%. The experimental

precision of the measurement in each case was about ±0.15%. It is obvious that accurate

photometric measurements of this type require precise characterization of the spectral

response of the detector being used.

3.3 Electronic Considerations

The following experimental considerations hold true for many types of measurements in

addition to the one under discussion. It is important not to overlook them so they will

be briefly mentioned at this time.

First, the detector and amplifier should have a response time much faster than the

rise times of the pulses being measured. This is to insure that the calibration pulse

from the shuttered lamp and the flash pulse are both integrated properly in spite of their

different waveforms. This can easily be checked with a variable high speed sector disc and

an oscilloscope to monitor the detector-amplifier output as a function of pulse frequency.

For this calibration, an FET operational amplifier with low capacitance in the feedback

circuit was used. This device proved to have an adequately short rise time. It is the

second version of the dc amplifier described in Technical Note 594-2. [4]

Another problem is detector linearity. Although one can attempt to set the energy

irradiance from the shuttered source approximately equal to that of the pulse, this is

difficult to do exactly. In addition, energy irradiance measurements may have to be made

at a number of different distances, and it is impractical to recalibrate the detector for

each distance from the flashlamp. Finally, the detector-amplifier obviously must respond

to a large variation in signal level during each pulse. It is, therefore, very important

to ascertain that the detector and amplifier respond linearly. For the detectors used in

these measurements the linearity was checked using the inverse square approximation with a luminous intensity standard lamp. They appeared to be acceptably linear; however, as we

will show in Section 5.2, this was not an entirely satisfactory test.

Additional details which must be considered are the accurate measurement of the

detector to source distance, the repeatability of the positioning of the lamp and detector,

standard lamp accuracy, flashlamp repeatability, noise pickup, and detector-amplifier

stability.

information obtained from the specification sheets of several manufacturers.

6
4. Measurement Procedure

The calibration of a flashlamp (with power supply included) was a two part operation:

calibration of the detector-amplifier system for photopic energy using a luminous intensity

standard lamp with a timing shutter, and subsequent calibration of the flashlamp using the

calibrated detector. It was found experimentally that the system drift during the course

of a flashlamp measurement was negligible, hence the calibrations of the detector-amplifier were actually done last, after a given flashlamp measurement. The flashlamp was placed in

front of the standard lamp as shown in Fig. 2. In this way, the flashlamp could be removed

after measurement without disturbing the detector and standard lamp arrangement.

After aligning the optical bench, detector, and standard lamp with a He-Ne laser to define the optic axis, the flashlamp was installed in the position shown. The flashlamp

assembly consisted of a thin vertical discharge tube behind a rectangular aperture and an absorbing glass filter. Using the laser, the flashlamp assembly was centered on the optic

axis and aligned by autocollimating the laser reflection from the filter glass. An end

gauge was used to measure the separation distances from the detector to the flashlamp

aperture. The detector was then placed at the smallest source-detector separation distance

to be measured in the experiment. Several flash readings were observed on the oscilloscope

to select the proper amplifier and electrometer gains, allowing maximum readings without

overload distortion. These settings were unchanged for the remainder of the calibration of

that flashlamp and for the subsequent detector calibration using the Incandescent lamp.

Several flashes were first fired on a "cold" tube to allow its output to stabilize before

recording readings. Prior to making a reading, the flashtube power supply was allowed ample

time to recharge after a previous flash. A gate switch was then opened to the electrometer,

and bucking current from the amplifier was set to null the electrometer output reading at zero. Then the lamp was flashed via a remote trigger switch, and the electrometer gate

closed, "locking in" a voltage reading on the electrometer capacitor. Several measurements were made at each distance position.

When these measurements were completed, the detector was left at the maximum distance

point and the flashlamp assembly removed as noted earlier. The shutter was installed in

front of the detector and the standard lamp turned on and warmed up. The shutter was then

started and its time interval determined with an oscilloscope. The oscilloscope time base

was calibrated with standard frequency signals. After the lamp warmed up, the detector

was momentarily shuttered in order to zero the background current on the detector amplifier.

Then a series of readings were made as follows. The pulses were spaced at least 1/2 second

apart so they would be easy to isolate with the electrometer gate switch. When the pulses were exactly the proper time width, the gate was momentarily closed, then opened, isolating

one pulse. The reading was recorded and the electrometer cleared for the next sample.

These readings were then averaged to yield a value for k using Eq. (6). As mentioned

earlier, the calibration lamp was a standard of luminous intensity. One must therefore

assume that the inverse square approximation is valid. Hence the form of Eq. (6) actually

used was

V (D + 6)2

k . (16)

Mt5 " t 2 )

Here I is the luminous intensity of the standard lamp, D is the measured distance, 6 is the

correction to the "light center" distance and (t 5 - t 2 ) is the time interval at the pulse

half-height. The value of 6 was determined separately by fitting* the measured distances

D-l (from about 1.5 to 3 meters) and amplifier readings R± (assumed proportional to the

illuminance) to the following equation

R.<5 2 + 2R.D J 6 + R.D. 2 - I = (17)

*Some of the difficulties encountered in curve fitting will be taken up in the next Section,

7
B

A N

,,/®

I

n

Fig. 2: Experimental setup for measurement. I. Measurement of flashlamp.

A) Position of shutter (not used in this part of measurement)

.

B) Detector. C) Flashlamp assembly. D) Lamp standard (in position

but not in use in this part of measurement). E) Detector. F) Shutter.

G) Position of flashlamp (now removed for this part of measurement).

H) Standard lamp.

8
Finally the flashlamp photopic energy irradiance values, W, were calculated from the

measured voltages using the relation.

W = V/k . (18)

5. Data Analysis

Once the illuminance versus distance data have been obtained, it is often desirable to fit a smooth curve through these points in order to allow interpolation or possibly extrapolation to other desired points. One may be tempted to use some form of the inverse square

approximation again, such as

I = ED2. (19)

However, this is not always the best function to describe the variation of illuminance with

distance. In fact, by simply assuming there is always a characteristic constant of the

source, i.e. the intensity, one can easily be mislead in attempting to interpret the data.

Let us first derive the functional form of the relationship that describes the variation

of illuminance from the flashlamp as a function of distance.

5.1 A Mathematical Model of the Illuminance from an Apertured

Cylindrical Lamp

The xenon flashtubes we measured consisted of a long thin cylinder masked by a rectangular limiting aperture, a fairly common configuration for such sources. In order

to make the model of this lamp analytically tractable, we shall assume the following.

Consider the narrow cylindrical tube to be a one dimensional, infinitely long vertical

linear source of uniform luminance L per unit length. Let us take this to be the x

direction. Also consider the source as apertured by an infinitely long horizontal slit

of width 2w, in a plane parallel to that of the source and a distance d from it. The axis

of the slit is then in the y direction. Finally, consider a rectangular detector, of

dimensions 2a in the y' direction and 2b in the x' direction, also in a plane parallel to that of the source and aperture. The detector is at a distance D from the aperture. Figure

3 illustrates this model.

We know that the radiant flux, d<f> a , reaching an element a(x',y') of area dx' dy' of the

detector surface from an element of source dx is related to the radiance in the following manner

,

,, L dx cos0 dx' dy' cosy ,««\ d*

a = —z

. (20)

Here 6 and y are the angles between the distance r and the normals to the source axis and

detector plane, respectively. At this point we shall make another approximation. Let us assume that 6=y> which is equivalent to saying that the detector is slightly curved with a radius of curvature equal to D + d, the center of curvature being coincident with the source axis. We can now calculate the illuminance at the point a on the detector which

comes from the entire source:

dd> /*X ; a I max cos'

:' dy'

= L

J tf

dd> „ a T

I max cqs±l

E = , , = L I —— dx

a dx dy ] min

I ma?

min

dx =L(D+d)2 | ^

[( D + d)2 + x' 2 + (x-y')2]2 (21)
±

a(x .V ) — --I

D

(a)

max.

(a)

mm.

Fl8 - 3: ^^t^AllTM1^

SJSS!- - —-

10
The limits of this integral are a function of the slit width and can be determined from the

geometry of Fig. 3. They are

= -w(D + d)

min D and

x

_ w(D + d) max D

The result of integrating Eq. 21, is then:

d<|> (D+d) 3 L (y'-w) D

dx 1 dy' 2[(D+d) 2 + x' 2

]

| [(D+d) 2 + x' 2

] D

2 + (y'-w) 2 (D+d) 2

(y'+w) D 1

[(D+d) 2 + x' 2

] D

2 + (y'+w) 2 (D+d) 2 2[(D+d) 2 + x' 2

] 3 / 2

tan

-(y'-w) (D+d)

D[(D+d) 2 + x' 2

] 1 /^

- tan

-(y'+w) (D+d)

D[(D+d) 2 + x' 2

] 1 / 2

;

(22)

To obtain the average illuminance over the entire detector, we must integrate this expression over x' and y' and divide by the detector area. If Eq. (22) is integrated first over

y' , many terms cancel, leaving

dcj> L(D+d) 2

dx' [(D+d) 2

- (a+w) tan

+d) 2

( / -(a-w) (D+d) \ — { (a-w) tan * ( — J + x' 2

] 3 / 2

( \D[(D+d) 2 + x' 2

] 1 / 2 /

/ -(a+w) (D+d) \^

\D[(D+d) 2 + x' 2

] 1 / 2 /)

(23)

At this point we shall make another approximation to aid in carrying out the analysis.

Since D is usually large compared with the detector dimensions,

(a ± w) (D + d) « D[(D + d) 2 + x' 2

] 1 / 2

Hence we can replace the arctan terms with the first two terms in the series expansion:

, "I - x

3 tan A x = x - —

We can now carry out the final integration over x' to give the total flux <j> intercepted by

the detector. After some rather tedious algebra and using the arctan approximation just

mentioned, we can write the average illuminance, the flux divided by 4ab , in the following

form:

11
2Lw

E = D(D+d)

£

(D+d) 2 b

2 a

2 + w

2

2[(D+d) 2 +b2

] 6(D+d) 2 4D2

(a2+w2

) (D+d) 2 (a2+w2) (D+d) 4 (a 2+w2

) b

2 - +

4D2 [(D+d) 2 + b

2

] 6D2 [(D+d) 2 + b

2

] 2 12D2 (D+d) 2

We can further simplify Eq. (24) by rewriting it as

(24)

2Lw

E = D(D+d)

*

a

2+w2

2 t 1+ (5+dF] 6(I»d):

! 2+W2

a

2+w2

4D^

(a 2+w2

) b

2 **"+<£>* °** + 7&*2 12D2<D+d>2

(24a)

Then, by dropping terms of order greater than

(D + d) 2 « 1

the average illuminance is approximately given by

E = 2Lw

D(D+d)

1 -

2b 2 _ 2(a2+w2

)

3 (D+d) 2 3D 2 (25)

This equation, rather than the inverse square approximation, is a much better model of the

relationship between distance and illuminance of the flashlamp sources we studied.

5.2 Treatment of the Data

In Tables 1 and 2 we have listed, and in Figs

ments made on the same flashlamp using two different

photopically corrected silicon photodiodes operated

bias plus a low input impedance amplifier) . The det

and, therefore, would be expected to have different

just the illuminance vs. distance for these two sets

product of illuminance and distance squared versus d refers to the measured distance from the aperture of

aperture.

4 and 5 we have plotted, some measuredetectors. Both detectors were

in the photovoltaic mode (zero reverse

ectors were from two different suppliers

characteristics. Instead of plotting

of measurements, we have plotted the

istance. The distance in these data

the lamp housing to the detector

One would expect from the inverse square approximation that at large distances the

quantity ED2 would approach a constant value. On this basis we would be inclined to reject

the data from detector 2 (Fig. 5). However, let us see how well each of these data sets

will fit the equation we have derived in the previous section. For the flashlamp we

measured, d was about 4 cm and w was about 0.5 cm. Both detectors were of the same size

with a = b % 0.5 cm. The only unknown in Eq. (25) is the luminance, L, which can be

obtained by using the value of the illuminance measured at one distance. If we arbitrarily

choose the points at 1.52 m, the average value of the illuminance measured by detector 1 is

0.281 lux and from detector 2 it is 0.286 lux. The luminance can now be computed and in turn we can compute a theoretical ED2 based on these single measurements of illuminance.

These calculated curves are drawn in Figs. 6 and 7 along with the experimental points. It

is obvious that the theoretical curve fits the data from detector 2 (Fig. 7) very well while

the curve obtained from the data of detector 1 (Fig. 6) does not. Here is a case where

12
~r

-H- -f- 1 1 + + + + 4-4-

4- 4- 4-

—

+4 4- +++

4- 44 + 4- + i i i —

— 1 1 I 44- 4- 4f 4-

—

— 44f

4f 4-

—

— 4H4-

4+4-44f

—

44f

444

— 444

4+4

—

— i 1 1 _L~"

cvi O

csi

lo

CD

CD

CD

in

CD

CD

CO

CO

LO

CO

CD

CO

CO

CD CD

O

CD

LO

CO

CD

CD

CD

CO

CD

<Uoa

R) u

to •HT3

CO3

CO

(-1 CU> 3 o3

-o LU o

)-l CD o- ^ cu < cfl 1— 3 • CO CD ^^TM cu o o cfl CU U 4J

CO cu •h -a

-dI .fi 0) *J O -H c £

cfl c a,

•h e g aj D <-i -H J=

iH CO •h rt -H

4-i y-i o C

4-> CU O >

iH -H

&< 00

-tf

00

•H

fe

13
csi

CD

cvi m

LOd

moc

«j

4-1 WH

T3

cn3w

V-i (U> ^—

4-1 £ o3

T3 ^^—

O

(J LU D. CD X) S< 3 • 1— CO 00 a) o O 4J CD cd 0) 4J 4-1 w a) -H T3

T31 XI

a) 4-i O -H a s

cOC D. H g 6 cd 3 vH rH x:

i-t cn •H nj H 4-1 4-1C

4J <U o >

CD

CD

CD

CO

inm

CO

CD

co

LO

CNI CO

CD CD CD

CMQ

CD

CD

CO

CD

CM 00

60 •H

14
csi

+ 4- +

CD

cvi

LO

4f 4-

CD

<

l—

CO

LO

CD

LO

cr w

eo

u

14-1

0)>

•rl

!-l

(1) 13

0)>

DO

uo

Q

CD

CZ)

LO

CO

CD

CO

CO

LO

co

CNIQ

CD

CO

CO

CD

ID

CO

cb

CD

CO

CO

•H

15
CD

CNJ

LO

CD<

I—

CO

CD

m

CD

m

CO

CD

CD

mCNI CO

CD

CD

T3>

•i-t n>oO

•rl

4-1 01

1-1 o

<a

4-1 Q

00

•H

CNJCD

16
intuition based on the inverse square approximation may lead to an incorrect interpretation

of the experimental results.

One might ask how well the inverse square approximation with a term to correct the

distance as in Eq. (17) would fit these two sets of data. The answer is given in Figs. 8 and 9. Here we have plotted the curves obtained by fitting all the data to a function of the

form given in Eq. (17). The fit appears to be reasonably good for detector 1 and fair for

detector 2. On the other hand, the distance correction term 6 computed from the data of

detector 1 is 0.23 cm and from detector 2 it is 2.18 cm—a difference of a factor of ten

for two sets of data obtained using the same lamp. The inverse square approximation itself

gives no indication of which detector is more linear. In fact, even after fitting all the

data to an equation with a correction for distance one may still be misled into believing

that detector 1 is better than detector 2. One must know in some detail how the illuminance

from a source varies with distance before beginning the data analysis.

6. Curve-Fitting

If one were to interpolate or possibly to extrapolate to large distances in order to determine the so-called "constant of the source", that is the intensity, one must fit the

experimental data to a smooth curve. Using Eq. (25) we may fit our data to several different

approximations of the same function. We may take as the best approximation an equation of

the form:

A l A2 A3 1= + + (26)

ED(D+d) ED(D+d) 3 ED 3 (D+d)

or we may assume that A2 and A3 will be very small and just fit to an equation of the form

1 = (27)

ED(D+d)

In both Eqs. (26) and (27) we would have to use the approximate values of d. Since the

lamp is placed behind a filter of unknown index of refraction and since it was difficult

to measure the exact separation between the lamp and the aperture, it may be well to include d in the unknown coefficients to be determined from the fitting. Equation (27) can

be rewritten as

ED2 + dED = B l (28a)

or

1 = C^ED 2 + C 2ED

Taking the data obtained from detector 2 we used a least square method to fit them to each of the above three equations. These curves are plotted in Fig. 10 along with the

original data. All three equations provide a reasonable fit to the data with Eqs. (26) and

(28) having a better fit. The difference between interpolated values of E at 0.6 m are: from Eq. (26), 1.774 lux; from Eq. (27), 1.764 lux; and Eq. (28), 1.774 lux. The extrapolated values at 0.4 m are: from Eq. (26), 3.920 lux; from Eq. (27), 3.849 lux; and from

Eq. (28), 3.900 lux. The "intensity" computed from each of these equations, that is, the

value of ED 2 at 10 m is: from Eq. (26), 0.671 cd; from Eq. (27), 0.675 cd; and from Eq. (28),

0.668 cd. The difference between the interpolations obtained using Eqs. (26) and (28) are negligible, however, the differences between both extrapolations are about 0.5%.

The last extrapolation to very large distances shows that the exact form of the equation

to which one chooses to fit the data affects the determination of the "intensity". We have

17
+

ii i i 44- 4- f +

+- 44

4-4- +

4f 4

-H4 4 4f

4- f 4

4f4

— 4-4-i -+-—

— 44

— "+ Ur

4H-l 44

V-H4 \ * — 4(4 \ — i 1 1 1 — o

lo

CD<

l—

CO

LO

CD

LD CD lo m ^" CM

CO co CO CO

CD o

60

60

•H

ft)

00

•H

CNJ

18
CZ>

cvi

LCD

C_D <

I—

COo

lo

CO

CD

co

LCD

CVI CO

CD CD CMo

CD

CO

a* W

M-lo

W)

•H

•Hm>3

t>0 HO

60 H

Pn

19
CD

LO

CJ> <

I—

CO

o

Lf3

CD

CO

CD

CD

CD

CNI CD

CD

CO

CD

tu>

3

O

a)

<u U

in a*

bo w

•H -H

60

•H

OJ

a

20
not attempted to weigh any of our data. Had we done so, to account for the increase in

noise at large distances for example, this would have affected our determination of the

"intensity" even more.

This exercise in data analysis and curve fitting alludes to a serious problem in

photometry: having intensity as the base unit and fundamental quantity. This is unfortunate because, as demonstrated in this work, the accurate determination of the intensity

of a source is subject to unquantifiable errors which are of necessity introduced by curvefitting.

7. Summary

We have presented a method which is useful in determining the energy irradiance or illuminance output of single flashes from a pulsed source. We have described a general

approach, discussed a specific technique employed, and presented in detail a general model

which can be used to describe the illuminance (and irradiance) as a function of distance

from an aperture-lamp configuration typical of flash sources. We have examined a number

of experimental details and the effects they have upon the results of these measurements.

Finally, we have indicated some of the difficulties encountered in curve fitting, and how

they could yield erroneous results. Although several of the techniques employed in this

measurement could obviously be refined and improved if desired, the basic approach should

be satisfactory to many needs.

***********

The authors gratefully acknowledge the contributions and stimulating discussions with

J. Geist , particularly concerning the models for data analysis.

8. References

1. Geist, J. and Zalewski, E. F. , Applied Optics 12, 435 (February 1973).

2. CLE. Publication No. 17: International Lighting Vocabulary. Third edition. Paris:

Bureau Central de la C.I.E. 1970.

3. Margenau, H. and Murphy, G. , The Mathematics of Physics and Chemistry , (D. Van Nostrand

Company, New Jersey) p 477, (1956).

4. Zalewski, E. F. ; Schaefer, A. R. ; Mohan, K. ; and McSparron, D. A.; Optical Radiation

Measurements : Photometric Instrumentation and Research (1970 to 1971) , NBS Technical

Note 594-2, (1972).

21
TABLE 1

Illuminance of a Given Flash I amp as a Function of Distance as Measured by Detector 1.

Distance

(m)

II ummance

(lux) ED*

.4141

,5141

,6141

,7141

,8141

.9141

1.0141

1.1141

1.2141

1.4141

1.5141

1.6141

1.7141

1.8141

1.9141

3.6841

3.6828

3.6815

2.4277

2.4316

2.4251

1.6843

1.6856

1.6856

1.2519

1.2489

1.2508

.96126

.96218

.96336

.76875

.76757

.76678

.62282

.62296

.62125

.52048

.51891

.52101

.43672

.43685

.43738

.32103

.32129

.31985

.28125

.28229

.28007

.24526

.24617

.24591

.22065

.21830

.21725

.19552

.19382

.19395

.17537

.17458

.17485

22

,63174

.63152

.63130

.64164

.64167

.64095

.63518

.63567

.63567

.63839

.63686

.63783

.63708

.63769

.63847

.64235

.64136

.64070

.64051

.64065

.63889

.64603

.64408

.64669

.64374

.64393

.64471

.64196

.64248

.63960

.64476

.64715

.64206

.63898

.64135

,64067

.64830

.64139

.63831

.64345

,63785

,63828

,64252

,63962

,64061
Table 1 (continued)

D Distance

(m)

1 1 I uminance

(lux) ED'

2.0141

2.1141

2.2141

2.3141

2.3641

.15796

.15914

.15940

.13912

.14082

.14003

.13218

.13179

.13362

.12197

.11988

.11883

. 11504

.11517

,11438

.64078

.64557

.64662

.62178

,62938

.62585

.64798

.64607

.65504

.65316

,64196

,63634

.64295

,64368

,63927

23
TABLE 2

Illuminance of a Given Flash I amp as a Function of Distance as Measured by Detector 2.

D

Distance

(m)

1 1 I uminance

(lux) ED'

,5152

,7652

1.0152

1.2152

1.5152

1.7652

2.3896

2.3861

2.3922

2.3913

2.3887

1.1017

1.1035

1.1026

1.1009

1.1026

.63157

.63243

.63243

.63139

.63209

.44104

.44174

.44070

.44209

.44287

.28609

.28626

.28635

.28617

.21235

.21313

.21217

.21226

.21200

.63427

,63334

,63496

,63472

,63403

,64508

.64613

.64561

.64461

.64561

.65091

.65180

,65180

,65073

,65145

,65129

,65232

,65079

,65284

,65400

,65681

.65720

,65741

,65700

,66167

,66410

,66111

,66139

,66058

24
U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-935

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

MEASUREMENT OF ENERGY IRRADIANCE FROM SINGLE PULSE SOURCES

5. Publication Date February 1977

6. Performing Organization Code

7. AUTHOR(S)

A. R. Schaefer and E. F. Zalewski

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period CoveredFinal

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A method of measuring the energy irradiance from a single pulsed source, such

as a xenon flashtube, is presented. Details of the approach used in this particular

measurement are given, along with a discussion of the data analysis. In particular, we consider some of the inherent problems encountered in curve fitting which are also common to other radiometric and photometric measurements.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Curve fitting; energy irradiance; flashtube; pulsed source; radiometry.

18. AVAILABILITY [£] Unlimited

I I For Official Distribution. Do Not Release to NTIS

I X] Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13«40:935

1 I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS, REPORT)

UNCLASSIFIED J

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

32

22. Price $ . 75

USCOMM-DC 29042-P74
BACK BY

POPULAR DEMAND

MORE ENERGY

AND DOLLAR

SAVING IDEAS

FROM

for you, the cost-conscious manager, in a new supplement to EPIC.

This first EPIC Supplement builds on the success

of the original Energy Conservation Program Guide

for Industry and Commerce, a comprehensive

handbook that has helped thousands of managers

establish or improve energy conservation pro- grams in their organizations.

In the 89-page EPIC Supplement, you will find:

• A reorganized and simplified explanation of how to implement an energy conservation pro- gram.

• More than 50 additional Energy Conservation

Opportunities to save energy in 13 different

functional and operating areas.

• Two dozen new case studies that let you esti- mate your potential for conserving energy and

reducing costs.

• New sources of assistance.

Industrial and commercial firms, hospitals, universities, research institutes, public utilities—ANY

organization—whatever its size—that uses energy

and has to pay the bills will want a copy of the

EPIC Supplement.

The EPIC Supplement is intended to be used with

the original EPIC guide. If you already have a copy

of EPIC, don't delay in ordering the Supplement.

Make EPIC part of your continuing plan to control

energy use and costs.

To order your copy of EPIC Supplement, send

$2.25 per copy (check, money order, or Superintendent of Documents coupons) to Superintendent

of Documents, U.S. Government Printing Office,

Washington, D.C. 20402. Use S.D. Catalog Number

C13.11:115/1. To order EPIC, send $2.90 a copy to Superintendent of Documents, S.D. Catalog Number C1 3.1 1:1 15. Discount of 25% on orders of 100

copies or more mailed to one address.

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards FEDERAL ENERGY ADMINISTRATION/Energy Conservation and Environment
SINGLE

CRYSTAL

DATA

REVISED! UPDATED!

In 1954, the first edition of CRYSTAL DATA (Determinative Tables

and Systematic Tables) was published as Memoir 60 of the Geological Society of America. In 1960,

the second edition of the Determinative Tables was issued as Monograph

5 of the American Crystallographic

Association, and in 1967, the Systematic Tables were issued as Monograph 6. These editions proved ex- tremely valuable to crystallographers

throughout the world. Recognizing the

need for updated crystallographic in- formation, the National Bureau of Standards Office of Standard Reference Data has sponsored the issuance of a new

edition.

This, the THIRD EDITION, should be of particular interest not only to crystallographers but also to chemists, mineralogists, physicists and individuals in related fields of study. The current edition,

which comprises two volumes, Organic and

Inorganic, is a thoroughly revised and up- dated work, containing over 25,000 entries. 1 The entries are listed, within each crystal sys- tem, according to increasing values of a determinative number: a/b ratio in trimetric systems, c/a ratio in dimetric systems, and

cubic cell edge a, in the isometric system. In addition, the following information is given:

'-•<>

^«"\coW "

INORGANIC VOLUME $50.00

ORGANIC VOLUME $30.00

axial ratio(s) and interaxial angles

not fixed by symmetry, cell dimensions, space group or diffraction aspect, number of formula units per unit cell, crystal structure, (whether determined), measured

density and x-ray calculated density. Also listed is the name of the compound and synonym(s),

chemical formula, literature ref- erence and transformation

matrix. When available, the crystal structure type, crystal habit,

[cleavages, twinning, color, optical

properties, indices of refraction,

optical orientation, melting point

and transition point are also

listed. THIS EDITION culminates years of

effort by J. D. H. Donnay, Johns

Hopkins University, Helen M. Ondik,

National Bureau of Standards, Sten Samson, California Institute of Technology, Quintin Johnson, Lawrence Radiation Laboratory,

Melvin H. Mueller, Argonne National

Laboratory, Gerard M. Wolten, Aero- space Corporation, Mary E. Mrose,

U.S. Geological Survey, Olga Ken- nard and David G. Watson, Cambridge University, England and

Murray Vernon King, Massachusetts General Hospital.

Plus shipping and handling

Shipments are made via insured parcel post. Additional charges for shipments by air or commercial carrier. TERMS: Domestic—30 days Foreign—prepayment required. Address all orders to: JOINT COMMITTEE ON POWDER DIFFRACTION STANDARDS 1601 Park Lane, Swarthmore, Pennsylvania 19081

Please accept my order for CRYSTAL DATA, DETERMINATIVE TABLES, Third Edition, Donnay/Ondik.

D Organic Volume

Inorganic Volume

Ship to:

Signature jCPDs
MORE ENERGY

AND DOLLAR

SAVING IDEAS

FROM

for you, the cost-conscious

EPIC

manager, in a new supplement

to EPIC.

This first EPIC Supplement builds on the success of the

original Energy Conservation Program Guide for Industry and

Commerce, a comprehensive handbook that has helped thou- sands of managers establish or improve energy conservation programs in their organizations.

In the EPIC Supplement, you will find: • A reorganized and simplified explanation of how to imple- ment an energy conservation program.

• More than 50 additional Energy Conservation Opportunities

to save energy in 13 different functional and operating areas.

• Two dozen new case studies that let you estimate your

potential for conserving energy and reducing costs.

• New sources of assistance. Any organization—whatever its size—that uses energy and

has to pay the bills, will want a copy of the EPIC supplement.

The EPIC Supplement is intended to be used with the

original EPIC guide. If you already have a copy of EPIC, don't delay in ordering the Supplement. To order your copy of EPIC Supplement, send $2.25 per copy (check, money order, or Superintendent of Documents

coupons) to Superintendent of Documents, U.S. Government

Printing Office, Washington, D.C. 20402. Use S.D. Catalog

Number C13.11:115/1. To order EPIC, send $2.90 a copy to Superintendent of Documents, S.D. Catalog Number

C1 3.1 1:1 15. Discount of 25% on orders of 100 copies or more

mailed to one address.

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards FEDERAL ENERGY ADMINISTRATION/Energy Conservation and Environment

6 U. S. GOVERNMENT PRINTING OFFICE :1977—240-848/25
NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. It is published in two

sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $17.00; Foreign, $21.25. • Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical

analysis, theoretical physics and chemistry, logical de- sign and programming of computers and computer sys- tems. Short numerical tables. Issued quarterly. Annual

subscription: Domestic, $9.00; Foreign, $11.25.

DIMENSIONS/NBS (formerly Technical News Bulletin)—This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. The magazine highlights and reviews

such issues as energy research, fire protection, building

technology, metric conversion, pollution abatement,

health and safety, and consumer product performance.

In addition, it reports the results of Bureau programs

in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements available from ACS, 1155 Sixteenth

St. N.W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental

functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under proce- dures published by the Department of Commerce in Part

10, Title 15, of the Code of Federal Regulations. The

purpose of the standards is to establish nationally rec- ognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series—Practical information,

based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent

of Documents, Government Printing Office, Washington,

D.C. 20402. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Springfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for

outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Buraau off Standard*

Washington. DC. 20234

OFFICIAL BUSINESS

Penalty for Private Use. S300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-21S

SPECIAL FOURTH-CLASS RATE BOOK