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JUN 2 7 1968

^ecknlcciL ^^lote 279

FLUX AVERAGING DEVICES FOR THE INFRARED

S. THOMAS DUNN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

u
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

Technical Note 279

ISSUED DECEMBER 9, 1965

FLUX AVERAGING DEVICES FOR THE INFRARED

S. Thomas Dunn

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, Government Printins; Office

Washington, D.C., 20402 - Price 30 -cents
CONTENTS

PAGE

Table of Contents » . « . . . . li

Illustrations •• iii

Abstract j_y Introduction ..... 1

Flux Averaging Devices 1

Diffusing Screens 2

Diffusing Elbows ..... 3

Averaging Spheres ...... 3

Summary 9

Appendix A » . « 10

Appendix B 12

References „ ,...., 14-

Flgvares 15

li
FIGURE ILLUSTRATIONS

1 Results of Scans Across the Sensitive Area of the Thermopile Detector in

the A-A' and B-B' Directions, With Chopped Tungsten Incident Flux.

2 Results of Scans Across the Sensitive Area of the Thermopile Detector in

the A-A' and B-B' directions. With Unchopped Tungsten Incident Flux.

3 Results of Scan Across the Sensitive Area of the Golay Cell Detector in

the A-A' Direction . With Incident Flux in Three Different Wavelength

Regions as Indicated

4. Results of Scan Across the Sensitive Area of the Golay Cell Detector in

the B-B' Direction . With Incident Flux in Three Different Wavelength

Regions as Indicated

5 Spatial and Angular Sensitivity Test Apparatus

6 Angular Sensitivity of Reeder Thermopile Across the Two Rows of

Thermocouples

7 Angular Sensitivity of Reeder Thermopile Across the Five Rows of

Thermocouples

8 Results of Spatial Sensitivity Test for NaCl Diffusing Screen

9 Diffusing Elbow .

10 Angular Sensitivity of Diffusing Elbow

11 Angular Sensitivity of Diffusing Elbow (b)

12 4.5° Diffusing Elbow

13 Gonlophotometrlc Data of Spherical Impression Surface

L4. Spatial Sensitivity of 45° Diffusing Elbow

15 Area Sensitivity Test of 45° Diffusing Elbow

16 Reflectance of Crystex Sulfur

17 Results of Area Sensitivity Test for Various Sphere Coatings

IS Shield Configurations for Averaging Spheres

19 Area Sensitivity Test Results for Internal Shield with the Sulfur-Qoated

Sphere

20 Results of Area Sensitivity Test of Sulfur-Coated Sphere With External

Shield

21 Results of Spatial Sensitivity Test of Sulfur-Coated ^here With the

Internal Shield

22 Results of Spatial Sensitivity Test for the Sulfur-Coated Sphere With

the Exterjial Shield

23 Model for Sphere Angular Sensitivity Test

24 Sphere Test for Gold-Roughened Sphere

25 Sphere Test for Sulfur-Coated Sphere

111
FLUX AVERAGING DEVICES FOR THE INFRARED*

by

S. Thomas Dunnt

National Bureau of Standards

Washington, D. G.

ABSTRACT

The spatial and angular sensitivity of infrared detectors recently has been investigated (l, 2). In order to eliminate this effect and obtain acciorate measurements in the

infrared, it is necessary to distribute the flux as uniformly as possible over the entire

sensitive area of the detector. A solution to this problem is presented in the form of

several averaging devices developed at the National Bureau of Standards. Among the devices

investigated are roughened NaCl windows, diffusing light ducts, and spheres with different

coatings. Each device was subjected to a series of tests to establish its averaging capability and useful wavelength range. Results of these tests indicate that the use of a small

sulfur-coated hollow sphere over the detector increased the accuracy of most types of infrared reflectance measurements and, at the same time, decreased the requirement for precise

optical alignment of the detector in the wavelength range of 1.5 to 8 microns. The use of

the sulfur-coated sphere over a thermopile extends the usefulness of the multiple-reflection

specular reflectometer to about 10 microns. (Key Words: Detector, Averaging Device, Spatial Sensitivity, Angular Sensitivity, Sulfur, and Integrating Sphere).

*The work described in this paper was done under NASA Contract No. R-09-022-032.

tMember of Photometry and Colorimetry Section, Metrology Division, Institute for

Basic Standards.

It
INTRODUCTION

From the turn of the century, large area thermopiles have been used as Infrared

radiation detectors for large area beams. It is astounding that for all this time the general practice has been to use these detectors as though they were spatially insensitive .i/

Only recently have experimental measurements been reported that confirm the fallacy of

this assumption (l) . The spatial sensitivity for a ten-junction thermopile is shovm in

figures 1 and 2. Appendix A describes the thermopile concerned and the method of measurement.

Spatial sensitivity is not a unique property of thermopiles. The spatial sensitivity

of the Golay cell (fig. 3 and 4), the lead sulfide cell (l) , and the photomultlplier tube (l)

have all been illustrated. M. Finkel2/ has found the large area InSb detector to be similarly spatially sensitive. Thus, it is apparent that if a large area detector is to be

used to accurately compare a small area beam with a large area beam (2), or to compare two beams of similar size (but smaller than the detector), it will be necessajry to use some flux averaging device. Flgvires 6 and 7 illustrate that the angular sen3itivity3/ of a tenJunction thermopile (appendix A) is such that to accurately compare beams incident on the

detector from different directions, with different solid angles, or with marginal rays exceeding an angle of 25° from the normal to the sensitive area of the detector, it will be

necessary to use an averaging -device with the detector. Furthermore, the comparison of

beams or images whose irradiance varies with position ijill require the use of an averaging

device over the detector (3). In general, problems of angular and spatial sensitivity

occur in all focusing optical systems.

FLUX AVERAGING DEVICES

The data obtained on the spatial and angular sensitivity of detectors indicate that

a flux-averaging device would be required for use with any of the available large-area

infrared detectors. The function of such a device is to distribute the available flux

uniformly over the sensitive area of the detector, regardless of image size, shape, or intensity distribution. Any averaging device will, of course, reduce the efficiency of

a detector system, because some of the incident flux is absorbed by the diffuser, and some is scattered away from the sensitive area and is lost. In general, the losses in a

diff\iser tend to increase with an Increase in its effectiveness as a diffuser.

The literature provides several references to flux-averaging devices. One is the

work of Bennett and Koehler (4) » who used a small integrating sphere to average the incident radiation over a photomultlplier detector. Another is the work of Ronzhin (5), who

tried light ducts and Integrating spheres to average radiation over the sensitive area of

a photomultlplier. However, these references offer solutions only in the ultraviolet,

visible, and near infrared portions of the spectrum, where good integrating sphere coatings are available. In the infrared, no one has yet shown that satisfactory integrating

sphere coatings exist for use beyond 4 microns. Reference (l) also Illustrates the use of several similar averaging devices.

1/ Spatial sensitivity is defined as variation in response of the detector with chajige in

the irradiated portion of the sensitive area.

2/ Photometry and Colorimetry Section, Metrology Division, Institute for Basic Standards,

National Bureau of Standards. "^ Angtilar sensitivity is defined as the variation in response of the detector with angle

of incidence (with respect to the sensitivity area of the detector) of the measured flux.
Three different types of diffusing devices were investigated. They are listed in

estimated order of increasing degree of diffusion as: l) a diffusing screen placed directly over the detector, 2) a light duct with diffusing walls or a diffusing surface in the

system, and 3) an averaging spherei/ coated with a material having high reflectance in the

infrared and sufficient diffusion to permit it to be used as an averaging device.

To establish the usefiolness of the various averaging devices, three tests were devised

as follow:

1. Test A - Spatial Sensitivity: This test was designed to illustrate the required

precision of incident image placement for comparing beams of nearly the same image area.

The general optical system for this and the following tests is shown schematically in

figure 5. A 6-inch-diameter spherical mirror of 4.9-inch radius was used to form an image

of the monochromatic source on the entrance port of the averaging device. The averaging

device, with the detector, was mounted in a milling head, so that it could be moved 8 inches

in the x and y directions, and rotated 360 In the x-y plane. The x direction is aligned

with the optical axis

In the spatial sensitivity test, the averaging device was mounted at the center of the

milling head, with its plane of entrance perpendicular to the incident beam from the spherical mirror. The entrance port of the device was then moved across the incident beam (the

size of this beam was 3 nmi by 3 mm) and the detector response was recorded as a function of

beajn position.

2. Test B - Area Sensitivity: This test was designed to evaluate the variation in

detection response with the size of the irradiated area on the entrance to the averaging

device, when the total flux is held constant. The detector and averaging device were mounted on the milling head (figure 5) with the axial ray of the incident beam (3 mm by

3 mm) centered on and normal to the entrance port of the averaging device. The port was moved along the axial ray of the incident beam, and the detector response was recorded as a function of the entrance port position. Since the incident beam is diverging from an image,

the size of the irradiated area could be varied from- a minimum when the beam was imaged on

the entrance port, to a maximum when the marginal rays fell just inside the port.

3. Test C - Angular Sensitivity: In this test, the averaging device was placed on

the milling head and the image from the 4-9-inch radius of curvature mirror was placed on

the entrance to the averaging device. The output of the detector was recorded as a function of incident angle as the milling head was rotated (appendix A). The recorded data were then normalized to a specified direction.

When results of any one of the three tests indicated a particular averaging device to be unsuitable, no further tests were made,

DIFFUSING SCREENS

The first device tested was a roughened sodium chloride vri.ndow. The data in figure S were obtained in the same manner as those reported for figures 1, 2, 3, and 4, except that

a diffusing screen holder is now placed against the detector entrance window. Diffusing screen No, 2 was 5 mm thick, and one surface had been ground with a 9.5-micron abrasive.

1/ A distinction is made between an integrating and an averaging sphere. In the case of the

averaging sphere, the main requirement is that the distribution and fraction of the incident fl\ix on the detector must be independent of the area of the sphere wall irradiated

for a certain specified area on the sphere, while the integrating sphere assimes uniform

diffusion of flux over the entire sphere wall (with the exception of the directly

irradiated portion)
Screen No. 3 was 2.5 nim thick, and one surface had been ground with a 50-micron abrasive. ' In figure 8, it can be seen that diffusing screen No. 3 had only a slight effect in

smoothing out the peaks, but diffusing screen No. 2 was more effective, and produced a relatively uniform response across the sensitive area of the detector in both the a-c and d-c

s cans

.

It is apparent from the reduced spatial sensitivity shown in figure 8, (compared to figures 1 and 2) that the roughened window would be useful for reasonably accurate comparison of small beamsi/ (with respect to the detector area) that were accurately positioned in

the center of the detector sensing area. This device would not be good for use In measuring

beams of about the same size as the detector area, since much of the flux would be scattered

away from the sensitive area of the detector, and possibly, in some thermopiles onto the

cold junctions, thus yielding false indications.

DIFFUSING ELBOWS

The second device tested was a diffusing elbow, shown in figure 9» This elbow greatly

reduced the spatial sensitivity when the flux was incident on one end of the elbow, and the

detector was placed at the other end. However, figure 10 indicates that the elbow-detector

combination had high angular sensitivity in the plane of incidence across the diffusing surfaces, while figure 11 shows relatively little angular sensitivity in the plane of incidence perpendicular to the mirror surfaces. Figure 12 is aji improved design, in which the

walls are all mirrors, except for the diffusing 4-5° surface used to reflect the incoming

radiation toward the detector. The diffusely reflecting surface used with this device consists of a series of spherical depressions in aluminum, each of 1/16-lnch radius, spaced

0.088 Inch apart in a hexagonal, close pack, array. A goniophotometric reflectance curve for this surface for white light incident at 45° is shown in figure 13 » This surface has

since been liquid honed, and then gold plated. The liquid honing gives a diffusing surface

of small roughness, which, in combination with the large roughness of the spherical depressions, should reduce the height of the specular peak at 45° in figure 13.

Figure 14 illustrates the spatial sensitivity (test A) of this elbow as a small image

is traversed across the entrance port. The signal from a small area beam can be reproduced

by positioning the elbow to yield a maximum detector response. Figure 15 represents the area sensitivity test for the case when, for A/(A minimum) = 1, the elbow is moved until a maximum reading is obtained (figure 14 ) and then the area sensitivity of the elbow is measured (test B) . Figure 15 Indicates that the decrease In signal for the large areas can be

calibrated; that is, the device senses the largest area signal about 20 percent lower than

the smallest area signal of the same flux content. Thus, careful calibration and measuring

techniques woiild allow use of a device of this nature for reducing spatial sensitivity for

large area detectors.

However, even with careful calibration of this device, errors of up to 5 percent may

be present in comparisons between beams of flux of different cross-sectional areas.

AVERAGING SPHERES

Since none of the devices described so far provided the desired spatial insensltlvlty, an averaging sphere was tried.

Preliminary results, using a 2-lnch-diameter sphere mounted over the detector, with a 3M white velvet diffusing paint coating. Indicated that this approach seemed feasible, at

1/ Neglecting the case""where roughened windows are used over detectors to enable them to view an entire sphere.
least at short wavelengths where known diffusers are available (6) . It is known from the

theory of integrating spheres (V) that for sphere efficiency to be high, it is necessary

for l) the wall reflectance to be close to unity, 2) the diameter of the sphere to be a minimum, and 3) the area of the entrance and exit ports to be a minimum. In addition, the

detector should also view the entire sphere. Further, it is important that the sphere wall

be a diffusing surface if a constant irradiance across the detector port is to be attained.

High sphere efficiency is required in this application, beca\:ise the amount of flux

available for measurement in the infrared is near the lower limit of the useful range of

the available detectors, partic\ilarly at the longer wavelengths. Certain white paints, MgO,

and BaSO/ are good, sphere coatings in the visible and near infrared, but they have low reflectance beyond 4 or 5 microns and are not suitable for use at longer wavelengths.

Birkebak (8) showed that sulfur is both a good diffuser and reflector at 2 microns and

4 microns, and assumed that it is usable to 10 microns. However, he did not mention the

specific form of sulfur that was used for his measurements, or his method of applying it

to the sphere wall, Kronstein, et al (9) reported that mu sulfur and flowers of siilfur are good reflectors out to 15 microns, and gave spectral reflectance curves, but did not use sulfur as a sphere coating. Agnew and McQuistan (lO) showed that sulfiu- is a diffuse re- flector to the flux from a Globar with wavelengths shorter than and longer than 4 microns.

Data on the reflectance of mu sulfur is given in figure 16 (2),

Polished metals have high reflectance at all wavelengths from the near infrared to the

far infrared, but they are not suitable for use in integrating spheres, since they reflect

specularly. Roughening the surface of a polished metal, however, will on reflection diffuse the incident beam. Hence, it may be possible to produce a usable sphere coating by

first contouring a metal siirface and then applying a vacuum-deposited metal coating to increase the surface reflectance. In the present work, two general types of surfaces were

considered for use as an averaging sphere coating in the infrared: l) a rovighened goldplated surface, and 2) a sulfiir coating. ,

Many spheres were built and coated. The following is a partial list of those

tested:

1. A 4-inch-diameter aluminum sphere coated with smoked MgO. The entrance and detector port areas were 0,188 in. 2 and 0.875 in, 2, respectively,

2. A 2-inch-diameter sphere roughened by *ro to-blasting'" with spherical glass shots ,=/

The sphere was then vapor plated with an opaque coating of gold. Entrance and exit port

areas for all the 2-inch spheres utilized in this paper are 0,444 in, 2, and 0,515 in. 2,

respectively.

3. A 2-inch"-dlameter sphere coated with sulfur2/. The sulfur was handpressed onto a roughened sphere wall.

4» The roughened, gold-plated sphere wall of sphere 2., above, was overcoated id.th a very thin coat of sulfur. The sulfur was suspended in alcohol and sprayed with an ordinary

paint sprayer.

1/ The Roto-Blast procesa is a trade name used by Pangbom Corp, to d-escribe the blasting

of surfaces, in this case with spherical shot. Both glass and steel spherical shot are available from this company. The roughened spheres used in this investigation were Roto-Blasted by Mr, Mann of Pangborn Corp,, Hagerstown, Maryland,

2/ The sulfur used' in this investigation was Crystex brand sulfur and was supplied by

Mr. A, Blackwell, Manager, Technical Service Department, Stauffer Chemical Company,

380 Ma,dison Avenue, New York, N, I, The analysis given by the supplier is 99.5/6

Elemental Sulfur, 9056 mu (insoluble) sulfur, 0,10^6 ash, and the acidity is 0.05$6, Mu

(insoluble) sulfvir comprises 9056 of elemental siLLfur,
5, A 2-inch-dlameter sphere was coated with a 1/8-inch-thick coating of sulfur, which

had been sprayed from a suspension in alcohol.

6. A 2-inch-diameter sphere was coated with a 1/8-inch-thick coating of sulfur, which

was sprayed from a suspension in benzene.

Appendix B describes the methods of coating or preparing the sphere wall.

Test Bi Area Sensitivityj:^

In order to establish the ability of the averaging sphere-detector combination to compare beams of flux of various sizes, the previously described area sensitivity test was performed. See figure 5 for detector viewing configuration.

The measiored sphere position was experimentally correlated to the area of sphere wall

irradiated by the incident beam, and each area was divided by the cross-sectional area of

the beam at the focal plane of the spherical mirror to obtain the area ratio for each position. The detector response at each position was divided by the response at the position

where the flux was focused on the sphere wall, to obtain the response ratio R/Rq» Response

ratio was then plotted as a function of area ratio to obtain the curves shown in figures 17,

19, and 20.

This test simulates the conditions that exist when the detector is used to compare

beams of flux of small and large areas . The maximvmi area ratio attainable with the

described experimental arrangement was 12.25 to 1 for the 2-inch-diameter sphere, and

about 2.36 to 1 for the 4-inch-diameter sphere.

The area sensitivity test was applied to all the spheres considered for use as averaging devices. The data at the top of figure 17 represent the results for MgO at 1.5 microns,

where it is a known good diffuser, R/Rq varied by 0.8 percent for an area ratio 2,36 to 1.

Since the sphere was 4 in. in diameter, the optics of the test system limited the area

changes of the image on the sphere wall to a smaller ratio than for the 2-inch-diameter

spheres used for the other materials in the test. The results indicate that the sphere

does indeed reduce the area sensitivity of the detector. However, MgO is not suitable for

general use as a coating in the infrared.

The second curve in figure 17 represents the results for a roiighened sphere, which

had been vapor-plated with gold; the roughness of the sphere wall was of the order of

25 ^^,-in. rms. The change in R/Rq was 2 percent, indicating poorer diffuseness than for

the MgO. Further, the efficiency of this sphere is almost identical to that of the other

spheres tested, despite the very high reflectance of gold. In this design there is a large

specular component cif flux that passes out the entrance port of the sphere on the first reflection from the sphere wall. Thus, to increase the efficiency of a sphere of this design,

the specular component of the first reflection must be kept in, the sphere) on the other

hand, it must be kept away from the detector's sensitive area, since slight variations in

image placement would yield large changes in detector response.

Figure 17 shows the data for Crystex brand sulfur, which was hand pressed onto the

sphere wall. These data have a spread of 0,6 percent in R/Rq and ill\istrate the usefulness

of sulfur for an averaging sphere coating} however, the application. technique yielded a s\irface that was extremely fragile and whose reflectance probably varied significantly from

point to point over the sphere wall. Thus, other methods were tried to obtain a more uniform and mechanically durable surface. First, the gold sphere referred to above was coated

with a very thin layer of Crystex sulfur. The sulfur was suspended in alcohol and applied

with a paint spray gun. The results of the eirea sensitivity test indicate a variation in

R/Rq of 1,1 percent. Further, the efficiency of this sphere was nearly the same as that of

1/ The order of the tests on this device has been inverted for .sake of clarity.
the hand-pressed sulfur sphere. Since the efficiency was the same, and R/Rq showed a greater change than that of the hand-pressed sulfur sphere, it was decided to try spraying an optically opaque 1/S-inch-thick coat of sulfur. This sphere exhibited the same change

in R/Rq ^^ ^^^ hand-pressed sphere and the coating was less fragile.

To fvirther reduce changes in R/Rqj two different methods of shielding the detector

viewing area were tried. Shielding is useful because the detector does not view the entire

sphere equally well, as is illustrated by its angular sensitivity in figures 6 and 7. The

primary fvmction of a shield is to prevent the detector from viewing the directly irradiated

area on the sphere wall for all image configurations. The first shield, which is illustrated in figure 18a, was a 0.15 -inch-thick disk placed over the detector with a 1/2-inchdiameter hole centered over the detector sensing area. The sides of this hole were coated

with Parson's black paint, and thus restricted the detector's field of view. The results

are presented in the second to last graph in figure 17, and indicate an over-all range in

R/Rq of 0,6 percent for an area ratio spread twice as large as for the hand-pressed sphere.

The second shield tested is shown in figxire 18b. This shield was tried because it yields -a higher detector efficiency, since it only restricts the detector viewing field in the direction of the image on the sphere wall. The shield was constructed of 0,005-inch-thick polished platinum. The data for this sphere are plotted in the last graph of figure 17 and

show a 0,4. percent variation in R/Rq. Thus, these tests indicate that either of the

spheres with detector shields are iisable at 2,4 microns.

These two spheres were tested at other wavelengths in the range 1.5p. to 7,0^jl. The results for the platinum shield (shield 2) are given in figure 18. Note that the scale of

the graphs for the longer wavelengths is smaller. This figure shows that at the longer

wavelengths, where sulfur' s reflectance is lower, R/Rq decreases with an increase in

A/(Amin.) as much as 2,8 percent. This could be caused l) by the incident flux becoming

trapped between the platinum shield and the sulfur wall (this woiild be more pronounced at

the longer wavelengths, because the reflectance of the sulfur wall is lower), or 2) by

atmospheric absorption in the increased path length due to water and CO2 in the atmosphere.

Such atmospheric absorption is not probable, since the wavelengths used were between the

absorption bands (the results in figure 20 for the sphere with the circular disk shield

substantiate this conclusion)

Since the change in R/Rq for the sphere with the platinum shield was quite large at

the longer wavelengths, the sphere configuration using the circixlar disk was also tested

at these wavelengths .i/ The results of the tests for variation of response with image size are given in figure 20. These results show an increase in detector sensitivity with image

size, indicating that part of the flux is still reaching the detector on the first reflection for large images. However, the change in R/Rq ^s limited to 0,9 percent for the long--

est wavelength. The reason that the change of R/Rq varies with wavelength is that the re- flectance of sulfur varies with wavelength. With low sphere wall reflectance (i.e., long

wavelengths for sulfur), the flux from the first reflection is a major portion of the flux

in the sphere; and if the detector views even a very small amount of this flux (which is

the case for large images on the spheref wall), there is a significant increase in detector

response (2). This error can be eliminated by increasing the thickness of the shield

shoTrfn in figure 18j however, this will reduce the efficiency of the sphere, which is intolerable, because the system is already energy limited in the 7-10 micron region when a thermopile detector is used.

The conclusions from this series of test are that a sulfur sphere with the circular

disk shield provides a better averaging device for signals of different image sizes. It should be noted that sulfur has several absorption bands in this range, which may be

1/ Since the previous tests, this sphere had been recoated with sulfur sprayed from a benzene. suspension, which yielded a coating that was more stable mechanically than

that sprayed from an alcohol suspension.
detrimental for certain applications depending on high spectral resolutions.

Test A; Spatial Sensitivity

This test was designed to Illustrate the required precision of Incident Image placement for comparing signals of nearly equal image area. The sphere entrance port was traversed across the Incident beam, which was focused on the entrance port and had a 3 mm by

3 mm area.

The results for the sulfur sphere with the Internal platinum shield are presented in

figure 21. The data are arbitrarily normalized to one of the central readings and plotted

as function of position on the entrance port as measured from one edge. These. data show

variations exceeding 2 percent at the longer wavelengths

.

Results for the sphere with the circular disk shield show variations of less than

0.4 percent for the wavelengths below 5.5 microns, and variations of about 0.8 percent for

the longer wavelengths (figiu-e 22), This again Illustrates the effect of the first reflected flux, since at the long wavelengths, where the reflectance of sulfiir is lower, the' detector signal is higher for images between positions 0.4 and 0,6 on the entrance port of

the sphere, which is where more of the once reflected flux could reach the detector (lefthand side of sphere opening in figure 18).

The results of these tests indicate that for short wavelengths the position of the

incident flux on the entrance port is not very critical, while at longer wavelengths more

care must be taken in positioning the incident beam.

Test Ct Angular Sensitivity

The general optical system for the angular sensitivity test is shown schematically In

figure 5, In this test, the sphere was positioned with its entrance port at the center of

rotation of the milling head, and the incident beam was centered on the entrance port. The

sphere was ten rotated, and the response of the detector was recorded as a function of the

angular position of the sphere measured as the angle between the axial ray of the incident

beam and the normal to the sphere entrance port.

If a perfect integrating sphere were tested in this way, and the detector viewed only

a portion of the sphere wall, as illustrated In figure 23, then the signal from the detector

would change as the Irradiated spot moves around the sphere, by an amount proportional to the difference in radiance of areas on the sphere wall that are and are not directly irradiated by the incident flux. If the area irradiated by the incident flux is not viewed

by the detector, no flux that has been reflected only once will be received by the detector.

Thus, for a surface that approaches an ideal integrating sphere coating, the curve of

response as a function of angle should show two ranges of nearly constant response with a smooth monotonic transition between the two ranges. The lower range would represent those

angles at which the detector views none of the irradiated area, and the higher level would

represent those angles at which it views the entire irradiated area, and the transition

would represent those angles at which the detector views an increasing fraction of the

irradiated area. Reference 2 gives equation (l) as the quantitative description of this

change for the perfect dlffuser as the ratio (R) of the reading when the detector does not

view the directly irradiated area to the reading when it views the directly irradiated

area.

(fa/fi)[As- PsAshJAs

^^^ 1 + —=

Ps (Ash)(Aov- A^s)

where Aj Is the total area of the sphere (Aj = 4nR^) , k^„ = Aj - A^i - A^g (A^^^ and Ahs

are the areas of the exit and entrance ports, respectively), Apy is the area of the sphere

fully viewed by the detector, (A^y - k^^^)/A^ is the proportion of the twice-reflected flux

in the area viewed by the detector, f^ is the diffuse configuration factor from area

(A[jy - Ang) to the sensing element of the detector, fg is the configuration factor from the
area .Irradiated by the beam to the detector sensing element, and Ps is the reflectance of

the sphere wall.

Figvire 24 shows results obtained with the gold-plated 3-4-60 shot, 2-in, -diameter sphere,

at wavelengths of 2.2, 5, and 8 microns <, In this case, the detector port is in the plane

of incidence, and is diametrically opposite the entrance port. The curves indicate that

there is a large specular component of the reflected flux reaching the detector when the

angle is about 50°, and that the reflectance characteristics do not change appreciably

with wavelength. The S-460 shot surface has a roughness of about 150|j, in. rms; hence, no effect of wavelength would be expected in this range » This figure illustrates that the

position of the incident beam on the inside of the sphere is quite critical and indicates

the necessity of keeping the first reflection away from the detector port in a sphere wit*! imperfectly diffuse walls .1/

The sulfur sphere coating outlined earlier was tested in this manner. Figure 25 illustrates the results for sulfur at 1.5, 2.2, 5.0, and 10.0 microns. Each of these curves illustrates two flat regions with a smooth montonic transition between the regions. These

results gave a qualitative indication of the utility of sulfur surface as an averaging

sphere coating. Further, the flatness of the flat regions suggests that the placement of

a beaia inside the sphere is not as critical as with the roughened metal sphere walls.

In addition, the ratios of the heights of the flat portions of these curves (figure 25)

agree with the trends indicated in equation (l) . However, the ratios calculated from the

data in figure 25 were consistently lower by a factor of 2-3 than would be predicted by

theory [equation (l)]. Apparently, the experimental set-up did not entirely fit the theoretical model. Several possible soiirces of error arei

1. The flvix from the irradiated area when it is not directly viewed by the detector

(i.e., when the incident flux is in area c-d in figure 23) could reach the detector by

paths other than by being multiply reflected from the d-a-b-c area viewed by the detector,

by (a) hitting the lip of the detector port and being diffused to the detector by scratches on the CsBr window. The net effect would be to increase the height of the low flat portion

of the curves in figure 25.

2. The radiation in the a-b area of figure 23 is incident at near grazing angles,

where even the best diffusers tend to become somewhat specular. Thus, some flux is reflected

around the sphere wall into the area c-d, which is not viewed by the detector, instead of

being diffusely reflected to the detector. The net effect would be to reduce the height of

the high flat portion of the cuarves in figure 25.

3« Using the wrong value for K\i in equation (l).

4. Using the 'wrong value for the reflectance of the sulfur coating.

5 o Improper evaluation of fi and f2

The first two effects are largely responsible for the low ratio of the two signals,

as compared to the ratio computed from equation (l)

From the results established in this section, it can be stated that the use of an averaging sphere can be extended at least to 7 microns by use of sulfur as a sphere wall

coating. Further,' the inherent advantage of this approach Is the ability to accept Images

of varying size by use of a large entrance port and to measure accurately the total flux

contained in vairious Incident 'beams. The major disadvantage is the reduction (by about

90 percent) of the flux that reaches the detector.

1/ In addition, this test Illustrates that roughened surfaces do not and caimot follow the

integrating sphere model in any respect} thus, it does not appear promising as a true integrating sphere coating, as has been proposed by several investigators.
SUMMARY

The summary of this work Is presented In four parts J (a) the sulfur-coated averaging

sphere, (b) suggested improTements to the gold-roughened sphere, (c) other averaging devices

or techniques, and (d) present use of averaging spheres In the Infrared.

(a) The Sulfur-Coated Averaging Sphere . The data presented Indicate that, of the

dlffusers or averaging devices tested, the sulfirr-coated sphere (with a shield restricting

the viewing field of the detector) provides highest accuracy in comparing beams of different

geometry. Additional advantages accrued through the use of an averaging sphere are l) practically any detector geometry ceui be used to view the sphere, regardless of sensitive area

or type, 2) the use of an averaging sphere greatly reduces the problem of optical alignment,

since minor variations in beam placement do not affect the signal output of the detector,

and 3) the careful use of the averaging sphere with the detector will provide the capability

to measure flux very accurately [more accurately than can be read from the commonly used

10-inch strip chart recorder (additional accuracy can be obtained by use of a digital

readout)]

.

The major disadvantages of the averaging sphere are the low efficiency of the sphere

(the order of 10% for those spheres tested) and the fact that the sphere reflects flux back

out the opening. The efficiency of the sulfur-coated averaging sphere decreases significantly at wavelengths beyond about 10 microns, because of the decrease in wall reflectance.

However, the tests on the sulfur sphere reported in this paper, combined with the data in

references 2 and 8, indicate that sulfur is usable as an averaging sphere coating out to at

least 10 microns.

(b) Suggested Improvements to the Gold-Roughened Sphere . Since a prime reason that

the sulfur-coated sphere cannot be used beyond 10 microns is sulfiu-'s low reflectance between 10 and 15 microns (9), it is desirable to improve the performance of the goldroughened sphere^, The following two methods are proposed to accomplish thisi l) design

the sphere so that the first three specular reflections of the flux do not strike the detector or entrance port- of the sphere (especially, keep them away from the detector viewing

port), or 2) place an optically opaque coating of sulfur (or some other body scatterer for

wavelengths longer than 20 microns, such as ground-up CsBr) over the directly irradiated

area of the sphere to diffuse the flux on the first reflection. For best results, care should be taken to prevent the detector from viewing any of the diffusing material.

(c) Other Averaging Devices or Techniques . Several methods of averaging various

beams of flux that were not experimentally studied are l) the use of lenses over the detector, 2) the use of condensing cones (which have inherent angular and spatial sensitivity),

3) the viewing of diffusing blocks which have high spatial sensitivity and low angular

sensitivity, and 4.) the use of statistical methods to compare various detector signals by

traversing the sensitive area of the detector for each beam and cross-correlating the resulting sensitivity curves. This last approach is being Investigated at this time by

Mitchell Finkel.1/

(d) Present Use of the Averaging Sphere in the Infrared . The author (2) has success- fully used the sulfur-coated averaging sphere to improve the accuracy of the elipsoidal

mirror reflectometer and to construct a simple, but acciirate, multiple-reflection, infrared,

specular reflectometer. In both cases, an Increased accuracy accompanied a decrease in the

required precision of optical alignment. Reference (3) illiistrates the use of the goldroughened sphere to average flux from varioiis sources over the entrance slits to a monochromator.

The author would like to express his appreciation to John A. ¥iebelt=/ and Joseph Go Richmondi/ for helpful suggestions, and to John To Perone, Jr.l/'', for taking much of the

experimental data, ^

1/ Photometry & Colorimetry Section, National Bureau of Standards.

2/ Associate Professor of Mechanical Engineering at Oklahoma State University

»
APPENDIX A

MEASUREMENT OF DETECTOR SPATIAL AND ANGULAR SENSITIVITY

Spatial Sensitivity

The spatial sensitivity (figures 1 through 4-) was measured by W. Schneider,

Photometry and Colorimetry Section, National Bureau of Standards (l)«

The thermopile was mounted on an automatically driven micrometer head that covild move

It horizontally In the plane of the sensitive area at a rate of about 0,08 Inch per minute.

A stationary aperture stop, having a circular opening 1/16 Inch In diameter, was mounted

directly in front of the detectoro The detector was Irradiated through the aperture stop

by radiation from a tungsten lamp.

The thermopile consists of ten receivers, each approximately 2 mm by 5 Dua in size,

arranged in two columns of five rows each to form a sensitive area 1 cm square. A thermocouple was attached to the back of each receiver, and the ten thermocouples were connected

in series to form the thermopile. The scans in the A-A' direction were made across the

center of the sensitive area, along the line between the two columns. The scans in the

B-B' direction were across the center of the sensitive area along the long axis of the two receivers in the third row. In each case, scans were made with a) the Incident radiation

chopped at 13 cycles per second and amplified with a synchronous amplifier, and b) with unchopped radiation with a d-c amplifier. Resvilts are shown in figure 1 for the a-c scans,

and in figure 2 for the d-c s cans

In figure 1 for the A-A' scans, it can be seen that there are three distinct peaks,

with some indication of two others, corresponding to the positions of the five rows of

plates. In the B-B' scan, it is apparent that the plate on the right had greater sensitivity than that on the left.

In figixre 2, the d-c scans in the B-B' direction are somewhat similar to the equivalent

a-c scans} the scans in the A-A' direction show five distinct peaks.

At this same time, Mro Schneider also measured the spatial sensitivity of the Golay

cell in a similar manner. In this case, a Globar-filter arrangement provided the different

wavelengths . The results are shown in figiu-es 3 and 4.

Angular Sensitivity

To measure the angular sensitivity of the thermopile, the detector was mounted on a milling head (figure 5) with its sensitive area in a vertical plane and with the two columns

of thermocouple receivers vertical, in a position such that the center line of the sensitive area coincided with the vertical axis of the milling head„ An image of the exit slit of the

monochromator, 3 nim by 3 nun in size, was focused on the center of the sensitive area from a direction normal to it, by means of a 6-inch-dlameter spherical mirror having a 49-inch

radius of curvature. The cone of rays th\is had a half-angle width of about 3-2 °» The monochromator was adjusted to give a band of radiation centered at 2.2 microns. The response of

the detector was recorded as R^^^ when the axial ray of the incident beam was normal to the

sensitive a'rea. The milling head was then rotated to give incident angles of 5, 10, 15, 20,

25, 30, 4-0, 50, 60, and 70° to the normal, and the response of the detector was recorded at each setting as Rq, 9 being the angle of Incidence. The data were normaJLlzed by the reading

at normal incidence, and plotted as a function of angle of incidence to produce the curve shown in figure 6. Similar measiirements were made with a cover plate 0.15 inch in thickness

with a 1-cm by 1-cm hole mounted over the sensitive area. The entire procedure was repeated

with a band of radiation centered at 8 microns.

The experimental curves are compared in figure 6 with two computed theoretical curves.

10
The top curve, in which R^/Rg = 1 at all angles, would be obtained if the detector were equally sensitive to flux striking it at all angles, and if all of the incident flux struck

the sensitive area. The lower curve, in which B^/B.q = cosg, would be obtained if the detector were eq\ially sensitive to flux striking it at all angles and were completely filled

at normal incidence.

From the experimental curves in figure 6 it can be seen that the sensitivity increases

slightly from normal to 20°, then decreases. The Increase is undoubtedly due to the fact

that as the illuminated area of the detector increases, the more sensitive areas, as shown

in figures 1 and 2, become irradiated » The sharp drop in response beginning at about 30°

is due to some of the flux being lost; either not admitted thro\igh the vrindow or not striking the sensitive area if admitted. The presence of the cover plate increases the rate of

fall-off in this range, as might be expected.

Similar tests were made with the detector mounted with the five rows of plates vertical. The results shown in figure 7 are similar to those in figure 6, except that the increase in signal from 0° to 20° was not observed,, This is due to the fact that as the

angle was increased, more plates were illuminated, but in the same relative areasj hence

the signal remained constant.

The cuorves plotted in figures 6 and 7 show no significant differences for fluxes of

2.2 and 8 microns, respectively. The fact that the curves are nearly flat from 0° to 25°

indicates that variation in angular sensitivity will be a problem only when the marginal

rays of an incident beam exceed 2$°,

11
APPENDIX B

SPHERE COATINGS

Several types of sphere coatings were prepared for use as described in the body of this

paper. This section briefly outlines the methods used in preparation of these surfaces. In each case, two hemispheres were coated and then joined to form the sphere.

Sulfur

All sulfur used in this work was Crystex brand sulfur} however, reference (9) indicates

that ordinary flowers of sulfur has about the same reflectance as Crystex brand sulfur

throughout the infrared. No effort was made to establish the usefillness of forms of sulfur

other than Crystex brand sulfur. It should also be noted that Stauffer Chemical Company

also supplies Crystex brand sulfur which contains 20 percent by weight of oil and which has

excellent mechanical properties. However, information on the reflectance is not available.

Several different techniques were used to apply sulfur to the sphere walls, as follows:

Hand Pressed ! Initially, sulfur was applied to the sphere over a thin coat of rubber

cement by hand pressing (with the fingers) «. The sulfur was built up to a thickness of about

1/S inch, and contoured to roughly conform to the outline of the two hemispheres. The surface was then smoothed with an artist's brush. This surface had a fairly uniform appearance, but the coating was extremely fragile.

Sulfixr-Alcohol Slurry : To increase the uniformity of the coating over the surface of

the sphere and the reproducibility from one sphere to another, a spray application technique

was investigated. One part sulfur was mixed with about two parts alcohol by volume to form

a slurry which was sprayed from a vibrator-activated spray gun. The sliirry was sprayed so that most of the alcohol evaporated before the spray hit the roughened (approximately 50^- in. rms) sphere wall. To insiire rapid evaporation of the remaining alcohol, the hemispheres

were heated to 170° F before spraying. About ten spray applications were necessary to obtain a 1/8-inch coating. The hemispheres were reheated to 170° F and the sulfur surface was smoothed with an artist's brush between application. The resulting coating was very unifor^a in appearance, but it tended to crack with time and its adherence to the metal hemisphere

was pooro The sulfvir itself formed a comparatively hard surface.

Benzene-Sulfur Slijrry : To alleviate the problems experienced with the sulfur-alcohol

slurry, the alcohol was replaced by benzene. This slurry was applied to the roughened

hemisphere wall over a thin coat of a benzene-soluble contact cement. During the first

few seconds of spraying, the spray gun was held very close to the surface so that the benzene dissolved the contact cement, which migrated slightly into the sulfur coating. The

thin coating was then dried, leaving the sulfur bonded to the sphere wall. For the subsequent spraying operation, the spray gun was moved further away from the sphere wall, the

temperature of which was maintained at about 150° F by heat from two infrared lamps. This,

and the fact that benzene is more volatile than alcohol, permitted the slurry to be sprayed

continuously until a coating thickness of about 1/S inch was obtained. The surface was then

smoothed with an artist's brusho The surface produced by this technique was very uniform

in appearance and mechanically strong enough to withstand normal laboratory handling. The

surface hardened considerably with age.

BaS04, Surfaces

A BaS04-benzene sliirry was sprayed in the same manner as the sulfur-benzene slurry to coat spheres with BaSO*.

Gold-RoTjLghened Surfaces

Several spheres were roughened with glass and steel spherical shot by the Pangborn

12
Corp., using a Roto-Blast process. The glass shot (Pangbom No. L) are -20CM-325 mesh SAE

and yield a surface roughness on the aluminum sphere of about 25 IJ- in. rms. The steel shot

(Pangborn No. S-46O) £ire 10 mesh SAE and yield a siurface roughness on the aluminum spheres

of about I5OM. in. rms. After the hemdspheres had been uniformly roughened with one of the

above shots, they were cleaned and gold was vapor-deposited onto the surface.

Certain commercial materials are identified in this paper in order to adequately

specify the materials employed. In no case does such identification imply that the

material identified is necessajrily the best available for the purpose

»

13
REFERENCES

(1) a« Stair, Ro and Schneider, W. E«, ''Standards, Sources, and Detectors in Radiation

Measiarements." Symposixim on Thermal Radiation of Solids . Editors Dr. Samuel

Katzoff. NASA SP-55. To be published in 1965.

bo Stair, R.} Schneider, W. E.j Water, W. R.; and Jackson, J, K., "Some Factors

Affecting the Sensitivity and Spectral Response of Thermoelectric (Radiometric)

Detectors, Applied Optics V. 4, No. 6, June 1965, pages 703-710.

c. Coblentz, W, W., "The Radiation Sensitivity as a Function of Area Exposed."

Bulletin of the Bureau of Standards . V. 11, 1915, page 14.2.

(2) Dunn, S. T„, "Design and Analysis of Ellipsoidal Mirror Reflectometer." Ph.D, Thesis,

Oklahoma State University, May, 1965.

(3) Stair, R.} Schneider, W. Eoj Waters, Wo R.; Jackson, J, J.; and Brovn, R. E., "Some

Developments in Improved Methods for the Measurement of the Spectral Irradiances of

Solar Simulators .° NASA Contract Report CR-201, April, 1965.

(4) Bennett, H. E„ and Koehler, W. F., "Precision Measurement of Absolute Secular

Reflectance with Minimized Systematic Errors." Journal of the Optical Society of

America . V. 50, I960, pages 1-6.

(5) Ronzhin, V. V., "A Photomultiplier for Measiu-ing Radiant Flux in Lighted Models with

an Absorbing Medium." Iz vestis Yysshikh Uchebnykh Zavedeniy . Priborostroyeniye,

No. 1, 1961, pages 94-98.

(6) Richmond, J. C.j Dvmn, S. T.; DeWitt, D. P.; and Hayes, W. D., "'Procedures for

Precise Measurement of Thermal Radiation Properties." November, 1963 - October,

1964., National Bureau of Standards Technical Note to be published in 1965.

(7) Jacques, J. A. and Kuppenheim, H. F., "Theory of the Integrating Sphere."^ Journal

of the Optical Society of America . Vo 45, 1955, pages 46O-4.7O.

(8) Birkebak, R. Co, "Monochromatic Directional Distribution of Reflected Thermal

Radiation from Roughened Surfaces 0°^ Ph.D. Thesis, University of Minnesota, 1962.

(9) Kronsteln, M .} Kraushaas , R. J. and Deacle, R. E., "Sulfur as a Standard of Reflectance

in the Infrared." Jovirnal of the Optical Society of America, -Vo 53, 1963, pages

4.58-465.

(10) Agnew, J. To and McQylstan, R. B., "Experiments Concerning Infrared Diffiise Reflectance Standards in the Range ,8 to 20.0 Microns." Journal of the Optical Society

of America . V. 43, 1953, pages 999-1007.

14
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DIFFUSING SCREEN NO. 2

DIFFUSING SCREEN NO. 2

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Results of scans across the sensitive area of the thermopile detector in the

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Results of Area Sensitivity Test for Various Sphere Coatings

Figure 17
/-INCIDENT FLUX

.ENTRANCE PORT

^2" DIAMETER SPHERE

A-B IS FIELD OF VIEW FOR THE DETECTOR

® BLACKENED .15" SHIELD

a) EXTERNAL SHIELD

CS Br. WINDOW

Icmx Icm

THERMOPILE

DETECTOR

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^INCIDENT FLUX

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^2" DIAMETER SPHERE

CS Br. WINDOW

Icm X Icm

THERMOPILE

^DETECTOR

HOUSING

A-B IS FIELD OF VIEW FOR THE DETECTOR

POLISHED PLATINUM SHIELD

b) INTERNAL SHIELD

Shield ConTigurations for Averaging Spheres

Figure 18
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NORMALIZED IMAGE AREA

Area Sensitivity Test Results for Internal

Shield With the. Sulfur-Coated Sphere

.5u

Figure 19
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NORMALIZED IMAGE AREA

Results of Area Sensitivity Test of Sulfur-Coated

Sphere With External Shield

Figure 20
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POSITION ON ENTRANCE SLIT TO SPHERE

Results of Spatial Sensitivity Test of Sulfur-Coated

Sphere With the Internal Shield

Figure 21
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POSITION ON ENTRANCE SLIT OF SPHERE

Results of Spatial Sensitivity Test for the

S^J-fur-Coated Sphere With the External Shield

Figure 22
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