NATIONAL BUREAU OF STANDARDS REPORT

. O' O

8561

PROCEDURES FOR PRECISE DETERMINATION

OF THERMAL RADIATION PROPERTIES

to

Research and Technology Division

United States Air Force

Wright -Patterson Air Force Base, Ohio

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

ms PROJECT NBS REPORT

42108-11-4210481 8561

PROCEDURES FOR PRECISE DETERMINATION

OF THERMAL RADIATION PROPERTIES

PROGRESS REPORT No. 23

May 1, 1964 - July 31, 1964

Contract No. DO(33-615) 64-1005

Task No. 738103

Research and Technology Division

United States Air Force

Wright-Patterson Air Force Base, Ohio

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government, i and review. For this reason, the whole or in part, is not authorij Bureau of Standards, Washington

the Report has been specifically f

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

; accounting documents intended ubjeded to additional evaluation

listing of this Report, either in Office of the Director, National

the Government agency for which

Dies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

1 . SUMMARY

The integrating sphere of the laser re f lectometer was coated with 3M

Velvet white paint, and preliminary measurements were made at room temperature of the reflectance of two types of shallow cavities with several

different depth-to-radius ratios. Fair agreement with theory was obtained

even without making corrections for known errors. A lock-in amplifier

system was procured for use with the laser ref lactometer . A heater was constructed for operation to 800°K.

The angular sensitivity of the thermopile detector used with the ellipsoidal ref lectometer was evaluated and found not to be a problem.

Several different integrating devices were designed and constructed in an effort to overcome the errors due to the variation in spatial sensitivity

of the detector.

The theoretical relationships between electrical resistivity and emlsaivity of metals were reviewed, and curves are presented showing the agreement of the theoretical curves and experimental data obtained on the plati- num and platinum-137, rhodium working standards of normal spectral emlttance

The data obtained on these alloys and on oxidized Inconel are compared to

data on the same materials reported in the literature.

Two procedures for measurement of normal spectral emlttance in the 15

to 32.4 micron range were evaluated. One of the procedures was found to

give better precision and accuracy than the other. The emlttance of a platinum working standard of emlttance was measured out to 32.4 microns.

New specimens of platinum-137, rhodium alloy, Kanthal and Inconel have been

procured on Bureau funds and cut to size for use as working standards of

normal spectral emlttance.

2 . INTEGRATING SPHERE REFLECTOMETER

2.1 Background

Spectral emlttance measurements obtained by direct comparison of the

spectral radiant flux density of a hot specimen to that of a laboratory

blackbody furnace at the same temperature are subject to large errors not

only at wavelengths that are appreciably shorter than the peak of the

Planck distribution function at the test temperature but also at temperatures above those at which standard thermocouples can be used.

The errors at the short wavelengths arise from two sources: 1) The

energy available for measurement decreases rapidly with decreasing wavelength

at wavelengths below the peak, hence there is not enough energy to permit

measurements of the highest accuracy at the shortest wavelengths.

2) The temperature dependence of spectral flux density increases rapidly with

decreasing wavelength at wavelengths below the peak; therefore, a small

temperature difference between specimen and blackbody reference will cause

a much larger error in emlttance at wavelengths shorter than the peak than

at wavelengths longer than the peak.
At temperatures above those at which standard thermocouples can be used,

the temperature scale is based upon the optical pyrometer, and either an optical pyrometer or a radiation pyrometer is commonly used for temperature

measurements in this range. These instruments are accurate for measuring

the temperature of a laboratory blackbody furnace and they also measure the

brightness temperature or the radiance temperature of a specimen with ac- ceptable accuracy. Conversion of this brightness or radiance temperature

to true temperature, however, requires knowledge of the spectral emittance

of the specimen over the appropriate wavelength range. Thus one Is faced

with the necessity of knowing the emittance in order to measure the temperature, which is in turn required to evaluate the emittance.

Reflected flux, as contrasted to emitted flux, is nearly independent

of the temperature of the specimen, at least over temperature intervals that

do not exceed about 50e K. For opaque specimens, the emittance is equal to

one minus the reflectance, by Kirchhoff's law. The measurement of reflectance

involves only the comparison of incident and reflected flux. Thus, such

measurements would appear to offer the possibility of considerably increasing

the precision with which emittance can be determined, particularly in the

short wavelength and high temperature regions where direct measurements are subject to large errors. However, for reflectance to be equal to one minus

the normal spectral emittance, the reflectance must be measured under con- ditions of either normal illumination and hemispherical viewing, or diffuse

illumination and normal viewing.

Three general methods have been used to measure reflectance under these

conditions: (1) the integrating sphere, (2) the integrating hemisphere, and

(3) the hemispherical source instruments. The integrating sphere is generally preferred and is the most widely used at wavelengths for which suitable

sphere coatings are available. The principal problem in measuring reflectance

of a specimen at high temperatures is to distinguish between the flux re- flected by the specimen and that emitted by the hot specimen. This can be

done by chopping the beam of incident flux, so that an ac signal is produced

by the detector, which is synchronously amplified and recorded. The dc signal from the flux emitted by the hot specimen is not amplified, and appears

only as noise. The signal-to-noise ratio can be increased by using a source having a high flux density within the wavelength interval of interest. A

helium-neon continuous wave laser was selected for this purpose because its

flux density within the narrow wavelength band at which it lases is several

orders of magnitude larger than the flux emitted by any solid specimen at a temperature below its melting point, within the same wavelength interval.

Design and construction of the integral ing-sphere ref lactometer with a helium-neon laser source was described in a previous report.

2.2 Description of Integrating Sphere Ref iectometer

a. Basic Design . A cross sectional view of the integrating sphere

is shown in Fig. 1. The sphere coating used for the preliminary measurements was a wh ite primer with a thin top coat of 3M Velvet paint .A'

.1/ The primer was DuPont 65-3010 white, sprayed to a thickness of about

0.003 in. and air dried.

2
The reflectance* of the sphere coating relative to MgO Is shown in Fig, 2,

Preliminary gortiephot ©metric measurements indicate that the coating is a good approximation of a perfect diffuser,

The laser beam is nearly parallel, and is about 1/4 inch in diameter

at the specimen port. It strikes the specimen at an angle of 6° to the

normal *

b e Heater design . A heater, for heating specimens 1/2 inch in diameter to temperatures up to 800®K, has been built. A schematic drawing of

this heater is shown in Fig. 3. An important design feature is the provision of holders for specimens of thickness of 1/4 inch for nonmetallic

materials and 0 o 065 inch for metals,

c . Detection Eastern . A new detection system consisting of a lock-in

amplifier, preamplifier „ and lead sulfide cell has been designed. The

aaplifi&r and detector have been procured, and the preamplifier is being

•constructed in the HB§ Electronic Instrumentation Section.

2,3 Theory of the Integrating Sphere

The theory of the integrating sphere ref lactometer for a perfectly diffuse sphere wall and diffusely reflecting specimen® has been presented by

Jaequeas and Kuppenheim.— For measurements by the substitution method (in

which first th« standard and then the specimen is placed over the specimen

port and measured) the reflectance of the specimen, p flS relative to the

reflectance of the standard p@^s is expressed by

S 32 total area of sphere wall, 4rrR2

» 615.752 in.3

d total reflecting area of sphere wall -613.089 in.3

2/ Jacquez, John A » , and Kuppenheim, Hans F., Theory of the Integrating

Sphere, Jour. Optical Soe, of America, 45, pp 460-470 (1955).

3/ Computed for shallow cavity testa described later.

( 1 )

where

If “ brightness at detector port with specimen at specimen port.

B “ brightness at detector port with standard at specimen port,

13 cl p “ reflectance of sphere wall, 0.825 at .632 microns

a “ spherical area of entrance port, 0.994 in. 2

b « spherical area of detector port, 0.442 in,3

3 / c «= spherical area of specimen port, 1,227 in.2 —

3
The ratio Bs /B sfc is the ratio of the ref lactometer readings for the

specimen and standard, respectively, and E is the correction factor for

flux losses through the sphere ports. Substituting the values given in the

table above for the sphere shown in Pig. 1, we get

•1786 - 0.00199 p Bt 1.00 ( 2 )

For reasonable values of p and p ot, , the factor E varies from 1*00

S

ti L- by less than 1%, Hence this correction will be neglected in the preliminary

checking of the ref lactometer *

The theory developed above la for the "perfect sphere", where the

specimen and standard have a spherical surface of the same radius as the

sphere. It can be modified for the case of flat specimens. No theory is

available for the case of specularly reflecting specimens.

Integrating sphere ref lectometers can also be operated by the comparison method in which there is both a standard port and a specimen port.

In this case, both standard and specimen are always in position, but they

are illuminated alternately.

2.4 Calibration of Integrating Sphere

Theory . A calibration procedure has been developed, based on previous

studies of Kelly and Mooreit/ • In this study the Gouffe equation relating

the "emifctance" of a shallow cavity to its geometry and the emittance of its

walls was verified experimentally, by measuring with a General Electric re- cording spectrophotometer, the reflectance of shallow cavities of known

properties. Two types of cavities were used. The first was constructed so that only the light reflected from the bottom of the cavity was permitted

to leave the opening. A sketch of this cavity is shown in Fig. 4.

The depth of the cavity was adjustable to give L/R ratios from 0 to 2.0.

(L is depth and R is radius of the cavity opening.) If the bottom of the

cavity is a perfect diffuser, the fraction, f, of the energy escaping

through the opening is

f " ~T + (p

2 (3)

Equation 3 is based on the assumption that the cross-sectional area

of the incident beam is small compared to the area of the bottom of the

cavity, and strikes the bottom at its center. Such illumination is referred

to as center point illumination. The theory can be modified to accommodate

other conditions of illumination, as is described in the fererence

The second type of cavity had diffusely reflecting walls and bottom,

as is illustrated in Fig, 4. The experimental data agreed well with that

computed from the Gouffe'" equation for the reflectance of the cavity, p , In terms of the L/R ratio and the reflectance of the cavity walls, p w

: A/ Kelly, F.J. and Moore, D.G., A Test of Analytical Expressions for the

Thermal Emittance of Shallow Cylindrical Cavities. Symposium on Thermal Radiation of Solids, San Francisco, Gal., March 1964.

4
(i-p w

)[i+p w dr

~ f)3

l-Pw^ 1 " r )

( 4 )

where m 1 1+(k)

2

is the ratio of the area of the opening to the total area of the cavity,

opening included, and f is the fraction of the incident flux leaving the

cavity on the first reflection.)

Eq . 4, like eq . 3, is based on center point illumination, and the assumption that the walls and bottom of the cavity are perfectly diffuse reflectors.

A plot of the reflectance ratio , p c /pw , as a function of the ^ ra tio computed by eq. 4 is shown in Pig. 5, for different values of p w .

The shallow cavity tests are particularly suitable for calibration of

the integrating sphere ref lectometer for two reasons: (1) the Gouffe equations have been verified experimentally with an instrument of proven reliability, and (2) the geometric distribution of the flux reflected from the velvetlined cavity (Fig. 4a) varies from that of a perfectly diffusing reflector in

that at an L/R ratio of 2.0 practically no flux is reflected at angles greater

than about 30° to the normal.

b. Experimental . Preliminary measurements were made at the reflectance

of the two shallow cavities. In each case the bottom of the cavity (L/R = 0) was used as the standard, and the L/R ratio was varied up to 2.0.

All measurements were made with the laser operating at 0.632|a, and the

incident beam of approximately \ inch diameter was centered on the bottom of

the cavity at the L/R “ 0 position. Thus the incident beam approximated center point illumination.

The detector was mounted in the viewing port as is shown in Fig. 6.

According to theory, the detector should measure the flux density of an area

on the sphere wall. When the flashed opal glass covered the detector port,

the detector viewed the glass, and hence measured the flux density at the

detector port. When the opal glass was removed and a black sleeve inserted

into the detector port, the detector viewed an area on the sphere wall diametrically opposite the detector port, and hence measured the flux density

of that area.

Preliminary measurements of the p c /p w

ratio for the first reflection

cavity are plotted as a function of L/R ratio in Fig. 7. Similar data for

the lined cavity are shown in Fig. 8. In both figures the solid line represents the p c /p w ratio computed from the appropriate Gouffe equation for the

cavity lining of reflectance of 0.825 at 0.632p,, as reported by Kelly and

Moore .

5
No attempt hats been made as yet to correct the measured values for known

errors. These include (1) deviations from center point illumination (2) loss

of flux through sphere ports , and (3) flux from the first reflection that is

incident on the detector. Rigorous computation of the correction is possible

for (1) and (2) but for (3) a correction cannot be made without complete information on the geometric distribution of the flux reflected from the speci- men on the first reflection. Before obtaining such information, attempts will

first be made to eliminate this error by Internal baffling in the sphere.

2 .5 Future Plans

Work with the integrating sphere ref lectorneter during the next quarter

will be concentrated along the following lines:

a. Calibration . Complete the shallow cavity calibration at 0.632 and

1.15 microns with the present sphere coating. Coat the sphere with barium

sulfate and repeat calibrations at 0.632 and 1.15 microns, and calibrate at

3.39 microns.

b. Measurements , Bagin comparative reflectance measurements at 0.632

and 1.15 microns at temperatures up to 80Qe K.

c. Theoret ical s tudies ,

(1) Make necessary corrections to the shallow cavity calibration,

and/or modify the sphere design as necessary to reduce or eliminate

known errors

.

(2) Study use of an “appended sphere (instead of the shallow cavity)

for absolute reflectance measurements.

III. ELLIPSOIDAL MIRROR REFLECTOMETER

3.1 Background

In the ellipsoidal mirror reflectorneter a Perkin-Elmer model 83 monochromator supplies a beam of chopped monochromatic flux, which is directed to a large spherical mirror, which in turn focuses the beam through a small hole in

the ellipsoidal mirror onto the specimen at the first focus of the ellipsoid.

The axial ray of the Incident beam strikes the specimen at an angle of about

7° to the normal. The ellipsoidal mirror collects the flux reflected from the

specimen and focuses it onto the detector at the second focus of the ellipsoid

The detector can be positioned alternately at the first or second focus, to

measure the incident or reflected flux. The ellipsoidal mirror has a magnification factor of about 5. For this reason, a large area detector must be used

6
A Golay cell detector having a sensitive area one centimeter in diameter

was used in the earlier work with the ref lectometer . While it had the desired

sensitivity it was also highly microphonic , particularly when used with its

sensitive area in a horizontal position, as is the case with this equipment.

Extensive attempts to isolate the detector from building vibrations by use

of a seismic table mount, and from air-borne noise by acoustic insulation of

the ref lectometer enclosure, were not effective in reducing the noise to a reasonable level. Use of the Golay cell detector was abandoned as a result

of these experiments.

As a substitute detector a sensitive thermopile, having a sensitive area

of one square centimeter, was procured. It was found to be essentially free

from microphonics, but tests indicated that it varied significantly in sensitivity over its sensitive area. The sensitive area was made up of ten very

thin black-coated plates, each 2x5 mm in size. These plates were arranged

in two columns of five rows to produce the one square centimeter sensitive

area. One thermocouple was attached to each plate. When a fine-line image

about 12 mm long was moved across the sensitive area in a direction parallel

to the rows, two peaks in sensitivity were observed, roughly corresponding

to the centers of the two columns of plates . When the image was moved in a direction parallel to the columns, five peaks were observed, roughly corresponding to the centers of the five rows of plates.

3.2 Angular Sensitivity of Detector

Since the detector needed to be equally sensitive to radiation striking

at any solid angle from 0 to 24°, the angular sensitivity of the thermocouple

detector was measured. To make this measurement the detector was mounted on

a milling head with its sensitive area in a vertical plane and with the two

columns of plates vertical, in a position such that the center line of the

sensitive area coincided with the vertical axis of rotation of the milling

head. An image of the exit slit of the monochromator, 3 x 3 mm in size, was focused on the center of the sensitive area from a direction normal to it, by means of a 6-in. diameter spherical mirror having a 49-in. radius of curvature.

The cone of rays thus had a half-angle width of about 3%c

. The monochromator

was adjusted to give a band of flux centered at 2.2 microns. The response of

the detector was recorded as Rn when the axial ray of the incident beam was normal to the sensitive area. The milling head was then rotated to give

incident angles of 5, 10, 15, 20, 25, 30, 40, 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 normalized by dividing each reading by

the reading at normal incidence, and plotted as a function of angle of incidence to produce the curve shown in Fig. 9. Similar measurements were made

with a cover plate 0.15 in. in thickness with a 1 x 1 cm hole centered over

the sensitive area. The entire procedure was then repeated with flux of 8 microns wavelength.

The experimental curves are compared in Fig. 9 with two computed theoretical curves. The top curve, in which Rn /Rg ” 1 at all angles would be obtained

if the detector was 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

Rn/R0 * cos 9» would be obtained if the detector was equally sensitive to flux

striking it at all angles, and was completely filled at normal incidence.

7
from the experimental curves In Fig. 9 it can be Been 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 the previous report, become

illuminated. The sharp drop in response beginning at about 30° Is due to some of the flux being lost, either not admitted through the window 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 Fig. 10 are similar to those in Fig. 9

except that the Increase in signal from 0 to 20e was not observed. This is

undoubtedly due to the fact that as the angle was increased, more plates were

illuminated, but in the same relative areas, hence the signal remained constant .

The curves plotted in Figa. 9 and 10 show no significant differences for

flux 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 no problem with the ellipsoidal ref lactometer since with the present instrument

the marginal rays strike the detector at an angle of approximately 24®

3.3 Integrating Devices

The information on angular sensitivity in Section 3.2 above and the data

on spatial sensitivity in the previous report indicate the need for an integrating device to spread the incident flux uniformly over the detector sensitive

ar®a regardless of the image size or uniformity* or the angular distribution of

incident flux.

Results with & two-inch diameter integrating sphere coated with 3M Velvet

white paint were given in the previous report. The major problem with this

approach is the sphere wall coating. Date are presented in Fig. 11, showing

sphere efficiency as a function of reflectance of the sphere wall for spheres

of 1, 2, and 3 in. diameters . The curves were computed from equations given

by Jaequez and Ktippeaheims/ . These curves indicate that, for high efficiency,

high diffuse reflectance of sphere wall and small diameter of sphere are required. High sphere efficiency is essentia 1 at the longer wavelengths where

only a small amount of incident flux is available with the particular monochromator that is being used.

White paints in general have very low reflectance at wavelengths beyond

about three microns $ hence painted spheres are not suitable for use at long

wavelengths. Polished metal surfaces, on the other hand, have- high reflectance

at wavelengths from the visible into the far infrared. They are not normally

considered suitable for use In integrating spheres because they reflect specularly. However, since a suitably contoured metal surface might provide

reasonably good diffusing power, a specially contoured surface was prepared

for trial. The surface consisted of a series of spherical depressions, of

1/16 in. radius, spaced 0,088 in. on centers in a hexagonal close pack array.

Goniophotometric reflectance curves for this surface with white light are shown for 45® illumination In fig. 12, and for normal illumination in Fig. 13.

Footnote 2, p. 3

8
While the surface is far from a perfect diffuser, it does reflect some energy

in all possible directions, and thus it has promise for use in an integrating

device

3.4 Future Plans

Work during the next quarter will be along the following lines:

a. A satisfactory integrating device will be developed to overcome

the effect of variations in spatial sensitivity of the thermocouple detector.

b. When a satisfactory diffuser-detector combination has been obtained,

the ellipsoidal mirror ref lectometer will be calibrated for use over the

wavelength range of approximately 2 to 15 microns.

c. A theoretical study of the errors in the ellipsoidal mirror ref lectometer will be initiated.

4. COMPARISON OF SPECTRAL EMITTANCE DATA FOR PLATINUM WORKING STANDARDS WITH

VALUES PREDICTED BY THE HAGEN-RUBENS RELATION AND WITH LITERATURE VALUES

4.1 Background

One of the long range objectives of the thermal radiation work has been

to obtain a better understanding of the mechanisms involved in the interaction

between matter and electromagnetic radiation; the processes of reflection, absorption, transmission, and emission. Such an understanding can contribute

appreciably not only to the development of methods for evaluating the thermal

radiation properties of materials, but also to the design of equipment used in

such measurements as well as to the evaluation of errors.

The work that has been done on theory will be summarized in the next

annual summary report. As one of the last phases of this work, the data obtained earlier for the platinum and platinum-137# rhodium working standards

were compared with values predicted by the so-called Ha gen -Rubens 5_/ equation

(Drude-Zener ) . This equation relates normal spectral emissivity, e 0 (K),

to dc resistivity, p, and wavelength, \

Jx e o 0O 13 0.365(pA.) - 0.0464(p/\) (1)

Equation 1 is applicable at long wavelengths and/or at high temperatures.

It is derived from classical electromagnetic theory assuming a negligible

phase change of the electric field caused by the interaction of the wave with

the metal.

5/ Hagen, E. and Rubens, H., "Emissions-vermogen und electrische heitfahigkeit

der Metallegrerungen," Verh . Deut . Phys . Ges . _6, 128 (1904).

9
4,2 Agreement of Spectra 1 Data, with Hagen-Rubens Equation

Fig, 14 show® the type of agreement observed for platinum. The solid

lines represent the average measured emitt&nces of the six platinum working

standards. The vertical lines indicate the statistical uncertainty , expressed

as the total standard deviation* due to error of measurement and difference

in specimens. The two broken lines represent the values computed from ©question (1), using resistivity data given by Caldwell .Ji/

There are seme obvious areas of mismatch between the measured and compufced values. The maximum near 9 microns at ail three temperatures could be

caused by (a) a bound electron phenomenon, (b) an instrumentation error, or

(c) an impurity effect. No impurity was identified when a specimen was examined by electron diffraction, but further examinations are planned.

The rise in emittance values at wavelengths near 15 microns at the higher

temperature is believed to be an instrumentation error. It has not been

found in repeat measurements made on the same specimens after completing cer- tain modifications in the spectrometer.

The maxima between 1 and 2 microns have previously been attributed to

resonance of bound electrons, although data reported by RobertsX/ indicate

that no resonance effect is to be expected in this wavelength region. The

maxima may be due to instrumentation error, since they occur in wavelength

regioins where the energy is low.

At 1100& K a reasonable fit of the experimental data at wavelengths beyond

about 3.5 microns was obtained with the values computed from the resistivity

of platinum at SOO^K given by Caldwell^./. At 14009 K, however, a resistivity

higher than that given by Caldwell was required to produce even an approximate

fit, and at 800®K a resistivity lower than that given by Caldwell was required.

These results indicate that equation (1) cannot be used with published resistivities to extrapolate the measured emittance data to temperatures above 1400®

or below 800G K. Information regarding the most suitable methods for extrapolation of the spectral emittance data to other temperatures and waveletigths xtfill be included in the final summary report.

The properties of the plat inum-13% rhodium alloy are similar to those of

pure platinum. Normally the presence of an alloying constituent might be ex- pected to introduce changes in the properties. However, these effects are minor and appear as a slight increase in emittance and also in resistivity, as compared to pure platinum. The experimental data are compared in Fig. 15 with

values computed from equation (1). The agreement between computed and observed values is about as good as was the case for platinum. In general, however, application of the simplified theory to alloys should be done with cau- tion since an alloy, in most cases, can no longer be considered as a simple

me ta 1

6/ Caldwell, F.R, “‘Thermocouple Materials", NBS Monograph 40, March 1962 , V Roberts, S, "Interpretation of the Optical Properties of Metal Surfaces"

Rhys Rev 100, 1667 (1955),

10
The observed data show the maximum between 9 and 10 microns that was observed with platinum at each temperature, but the maximum between 1 and 2 microns appears only in the platinum-13% rhodium data at 800°K, and the rise in

emittance at wavelengths near 15 microns was not observed with the alloy

specimens

4.3 Comparison of Standards Data with Literature Values

Since the optical properties of a metal are markedly affected by the

conditions of the surface, differences in values reported by different investigators are to be expected. Recently the problem of surface characterization

of metals.§/has received increasing attention. The complexity of the problem

makes it extremely difficult to characterize completely a metal specimen so

that data obtained in one laboratory can be reproduced on other specimens of

the same material prepared and measured in a second laboratory. In spite of

these difficulties, comparison of data obtained on the working standards of

normal spectral emittance with data obtained in other laboratories on similar

specimens may serve a useful purpose.

Fig. 16 shows values for normal spectral emittance of platinum at 1400®K

obtained in three other laboratories, together with the data reported for the

working standards. The agreement is reasonably good, except that the increase

in emittance at 14 to 15 microns was not reported by other investigators.

Fig. 17 shows values for oxidized Inconel obtained in other laboratories

together with the data on the working standards. In general, when oxidation

times were equivalent, agreement is satisfactory.

A brief description of the measurements made in other laboratories is

given in Table I.

5. NORMAL SPECTRAL EMITTANCE

5.1 Background

Equipment and procedures were developed earlier for measuring the normal

spectral emittance, over the wavelength range of 1 to 15 microns, of metal

specimens that can be heated by passing a current through them, as described in

WADC TR 59-510 Pt IV. Recently the equipment was modified to operate in the

15-38 micron range by (1) enclosing it in an atmosphere of super-dry CC^ -free

air, (2) replacing the thermocouple detector with a new high-sensitivity detector with a cesium iodide window, (3) replacing the sodium chloride prism

with a cesium bromide prism, and (4) by addition of filters to reduce scattered

light in the monochromator. Wavelength calibration of the spectrometer with

the cesium bromide prism was completed.

8/ DeWitt, David P., "Surface Characterization of Real Metals," Symposium on Thermal Radiation of Solids, Session II Surface Effects ASD (USAF) NBS

NASA, San Francisco, March 1964.

11
Two test procedures were developed for use In the 15-38 micron range;

(A) a black polyethylene filter was placed over the entrance slit to the monochromator, and the spectrometer controls were set to permit scanning from 12,4

to 38.5 microns, and (B) the range was subdivided into three overlapping portions, 12,4 to 27.3, 26.4 to 32.4, and 32.4 to 38,5 microns, respectively.

The black polyethylene filter was used in the 12.4 ~ 27.3 micron portion, and

the spectrometer controls were set to permit scanning over this range. In the

two longer wavelength portions the black polyethylene filter was replaced with

a CaF8 restsfcrahlen plate, and the controls were set to permit scanning over

the respective portions of the range.

5.2

Evaluati on of Jes t Procedures

The test procedures were evaluated by using the combination of a 50% transmittance sector-dlsc attenuator and a laboratory blackbody furnace to simulate

a specimen having a normal spectral ernittance of 0.50 at all wavelengths. Four

testa were made at each of the three temperatures 800, 1100 and 1300e K, using

each of the two procedures. The data for the first two wavelength ranges were reduced at 23 wavelengths in the 12.4 to 26.8 micron range and 15 wavelengths

in the 26.4 to 32.6 micron range. The average standard deviations are shown in

Table II, and the overall average emittances are shown in Table III.

The data in Table II show that the overall scatter, measured as the average standard deviation at the 23 and 15 wavelengths, respectively, wag smaller

for procedure A than for procedure B in four of the six cases, and the data in

Table III show that the accuracy was better for procedure A than for procedure

B in all six cases. On the basis of these results, procedure A has been se- lected for use in future measurements in these wavelength ranges

5.3 Measurements_ on Platinum

The normal spectral ernittance of a platinum working standard of normal

spectral ernittance was measured over the wavelength ranges of 1 to 15 microns

and 13 to 28,5 microns , at ISOO^K, Three separate runs were made and the data were reduced as before. The average ernittance values and standard deviations

are plotted in Fig. 18. The curve for the average ernittance values shows that the

results with the sodium chloride prism (1 to 15p) and those with the cesium bromide prism (12.8 to 28p) check well within the known precision of measurement in

the range where the two curves overlap. The data on the standard deviations of

the measured values indicate that the precision of measurement is about the same

in the two ranges

The rise in ernittance in the 13 to 15 micron region that was found in the

original calibration was not confirmed in the most recent measurements with

either prism.

5,4

New Working Standards

The new working standards of normal spectral ernittance have been purchased

on Bureau funds, and are being prepared prior to calibration.

12
TABLE I

BRIEF DESCRIPTION OF METHODS OF MEASUREMENT AND

SAMPLE PREPARATION FOR CITED LITERATURE VALUES

Material Authors Method /Apparatus Specimen Description

P la t inum Schatz and McCandless [1] Double-beam recording

spectrophotometer with

transfer optics to

view separate specimen

and blackbody furnaces

Commercial foil, no additional

description given.

Platinum Maki, Stair 6c Johnson [2] Single-beam recording

spectrophotometer with

transfer optics and

traveling optical

bench to alternately

view blackbody furnace

and resistance heated

specimen

Commercial grade

strip (8% x % x §|"),

annealed at 1525 K

for one hour

Oxidized

Inconel

Maki, Stair 6c Johnson [2] Same as above Commercial Inconel

strip, sandblasted

and heated at elevated temperature

for six hours

1 Oxidized

Inconel

Blau, et al [3] Single-beam recording

spectrophotometer with

specimen imbedded in

wall of hollow silicon

carbide heating element

with cavity to serve as blackbody

No details given

other than oxidized

in air.

1 Plat Inum

l

Price [4] Single-beam recording

spectrophotometer with

cylindrical specimens

electrically heated in

vacuum with lateral

slit serving as blackbody.

Commercial 99.8%

purity sheet.

(Oxidized

Incone 1 Slemp 6c Wade [5] Single-beam recordingspectrophotometer views

specimen as it rotates

past viewing port of

blackbody cavity fur- nace .

Oxidized in air

for 2 0 minutes

[1] Schatz

ings ," cesses

, Elihu A. and McCandless, Lee C., "Research for High Emittance CoatTech. Doc. Rept. No. ASD TR 62-443, Directorate of Materials 6c Pro-

, ASD, A.F. Systems Command, WPAFBase , Ohio, May 1962.

13
TABLE I- cont’d.

1.2] Maki, Arthur G., Stair, Ralph and Johnston, Rugsell G., "Apparatus for

the Measurement of the Normal Spectral Emlsslvlty in the Infrared,"

J. of Res. NBS 64G , 99-102 (1960).

[3 | Blau, Henry H. Jr.,, Marsh, John B., Martin, William S., Jasperse, John R.,

and Chaffee, Eleanor, "Infrared Spectral Emittance Properties of Solid

Materials," Final Rept . AFCRL-TR-60-416 , Geophysics Research Directorate

of the Air Force Research Division, Cambridge, Mass., Oct. 1960.

F4] Price, Derek J., "The Eraissivity of Hot Metals in the Infrared," Proe.

Fhy. Soc. (London) 59, 118 (1947).

[5] Slemp, Wayne S. and Wade, William R., "A Method for Measuring the Spectral

Normal Emittance in Air of a Variety of Materials Having Stable Emittance

Characteristics," Proceedings of Symposium on Measurement of Thermal

Radiation Properties of Solids, Joseph C. Richmond, Editor, Page 443,

NASA SP-31, (1963)

TABLE II

TABLE OF AVERAGE STANDARD DEVIATIONS

Temperature Wavelength Range

12.40 - 27.30(p)

Procedure

26.40’ - 32 ,4 Op

Procedure

A ... B .JL B __

800e K 0.398 0.522 0.346 0.220

1100s K 0.360 0.299 0.274 0.800

1300e K 0.342 0.447 0.338 0.436

14
TABLE III

TABLE OF AVERAGE MEASUREMENTS

Temperature Wavelength Range

12.40 - 27.30(h) 26.40 - 32.40(h)

Procedure Procedure

A _B A _ B

800°K 52.00 52.20 51.99 52 .43

1100°K 50.53 51.77 50.52 53.21

1300® K 50.11 50.66 50.58 51.17

15
ENTRANCE PORT

Figure 1. Significant dimensions and location of ports for 14- inch integrating sphere.

16
REFLECTANCE

RELATIVE

TO

FRESHLY

SMOKED

MgO

WAVELENGTH, jj

Figure 2. Spectral reflectance, relative to MgO, of the 3M white Velvet coating

used on the integrating sphere, and of a BaS04 coating as reported by

Middleton and Sanders, Illiminating Engineering 48_, 254 (1953). The

data for the 3M coating, measured by the Metrology Division, NBS , are not necessarily typical, since the reflectance is very sensitive to

coating techniques

17
Integrating sphere (14" OD) & 6" OD flange, flange outer

flat surface is approx 1/2" below sphere at heater centerline

Heated silver core with cavities for 1/2" diameter

specimens of ,070" and .250" thicknesses, TO hole dia .099"

Three layers of GE asbestos paper, 5 ft 18 gauge (.050" D)

nichrome wire® .425 fi/ft, aluminum phosphate cement coating

Mica spacers & washers, clearance at 6 is .030"

cleranee at 7 is 3/32"

End plate, 1/2" thick; transite coat with § i,0-o =Na3 S 103

Tie down bolt, 1/8" dia nominal, 3 req'd at 120°, steel

Sphere wall filler disc, .065 thick, aluminum, coat with

Si(% , Na3 Si03

Alignment boss, aluminum

Cap screw 10=32, 3 req'd @ 120° with washer

Body assembly, brass

Cooling coils, 1/8" OD tubing

Cap screw 8=32, 3 req'd® 120e Insulation, Fiber f rax

Lower cap & cover, brass

1

2

3,4,5

6,7

8

9

10

11

12

13

14

15

16

17

Figure 3. Specimen heater designed to heat 1/2" diameter specimens

to 800J K. Specimens of metals (0.065 in. thick) and dielectrics (0,25" thick) can be accommodated in the cavities at

the respective ends of the silver core.

18
U

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19
RATIO

OF

CAVITY

TO

KALL

REFLECTANCE

p

c

/p

Figure 5, Ratio of cavity reflectance, p c to wall reflectance, p w , plotted as a function of depth-to-radius (L/R) ratio

for different values of pw » for center point illumination

as computed by Equation 4.

20
TRIAL

^17

ft

f9

OPAL

WINDOW

IS

REMOVED

AND A

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INSERTED

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Figure

6.

Sketch

of

detector

mount

and

detector

port

on

integrating

sphere,

with

opal

glass

window

in

place

the

detector

views

the

window.

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window

removed

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black

sleeve

in

place,

detector

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an

area

of

the

sphere

wall

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opposite

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H

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yj

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CAVITY DEPTH TO RADIUS RATIO* l/R

V TRIAL 16 WITH FLASHED OPAL CAP

O TRIAL 17 WITH NO VIEW CAP

Figure 7. Ratios of reflectance of cavity to reflectance of bottom (p c /p w ) as experimentally determined with the first reflection cavity, plotted

as a function of depth-to-radius (L/R) ratio. Two different detector viewing configurations were used, as shown in Fig. 6. The

solid curve is computed for center point illumination.
REFLECTANCE

RATIO

CAVITY DEPTH TO RADIUS RATIO l/R

TRIAL # 14 WITH OPAL CAP

v TRIAL # 19 NO CAP, BLACK SLEEVE

Figure 8. Ratios of reflectance of cavity to reflectance of walls

(pe/Pw) ail experimentally determined with the lined cavity

plotted as a function of depth-to-radius (L/R) ratio. Two

different detector viewing configurations were used, as shown in Fig. 6. The solid curve is computed for center

point illumination.

23
0001

24 0

(ANGLE

FROM 1

NORMAL

OF

DETECTOR)

Figure

9.

Angular

sensitivity

of

Reeder

Thermopile

at

2.2

microns

and 8

microns,

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and

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mount

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DETECTOR)
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THEORETICAL SPHERE EFFICIENCY

/^REFLECTIVITY OF SPHERE: WALL)

Figure 11. Efficiency of an integrating sphere as a function of diffuse

reflectance of the walls for different diameter spheres, and

different size entrance and exit ports.
RECORDER

SIGNAL

(R)

GQNIOPHOT OMETRIC DATA

OF SPHERICAL IMPRESSION SURFACE

1.0

.9 -

.8 -

.7 -

.6 -

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,4 -

.3 -

.a -

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0

Figure 12.

© (ANGLE FROM NORMAL TO SURFACE)

Goniophotometr ic data of spherical impression surface

for 45" incidence angle.
RECORDER

READING

(R)

GONIOPHOTOMETRIC DATA

OF SPHERICAL IMPRESSION SURFACE

a (ANGLE FROM NORMAL TO SURFACE)

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