NATIONAL BUREAU OF STANDARDS REPORT

8327

PROCEDURES FOR PRECISE DETERMINATION

OF THERMAL RADIATION PROPERTIES

PROGRESS REPORT NO. 21

November 1, 1963 - January 31, 1964

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

Task No. 738103

AIR FORCE MATERIALS LABORATORY

Research and Technology Division

United States Air Force

Wright-Patterson Air Force Base, Ohio

to

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1009-11-10491 8327

PROCEDURES FOR PRECISE DETERMINATION

OF THERMAL RADIATION PROPERTIES

PROGRESS REPORT NO. 21

November 1, 1963 - January 31 s 1964

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

Task No. 738103

to

AIR FORCE MATERIALS LABORATORY

Research and Technology Division

United States Air Force

Wright-Patterson Air Force Base, Ohio

NATIONAL BUREAU OF STA

for use within the Government. E and review. For this reason, the whole or in part, is not authorh Bureau of Standards, Washingtoi

the Report has been specifically f

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

i accounting documents intended

ubjected to additional evaluation

listing of this Report, either in Office of the Director, National he Government agency for which

pies for its own use.

II. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

I. SUMMARY

Malfunctions in the correction circuits of the data-process ing attachment to the normal spectral emittance equipment have been corrected. The

remaining malfunctions are all concerned with the paper tape recording of

the corrected emittance data.

New thermopile detectors have been received for use with the ellipsoidal mirror ref lectometer . Working drawings of the laser integrating

sphere reflectomer have been completed, and the parts are now being fabricated .

II. NORMAL SPECTRAL EMITTANCE EQUIPMENT

A new vacuum thermocouple detector was received. Tests indicate that

it has about 2.5 times the sensitivity of the standard vacuum thermocouple

previously used with the Perkin-Elmer equipment. Use of this new detector

will be particularly advantageous in the 15 to 35 micron range; , where the

energy is very low. There is also a possibility that it may be usable in

the 0.3 to l.Op range.

The light-tight cover for the portion of the optical path between the

chopper and combining optics has been constructed and installed.

III. DATA PROCESSING ATTACHMENT

The manufacturer of the data-process ing attachment sent an engineer to

the Bureau late in the last report period to correct all remaining malfunctions in the equipment. All of the malfunctions were supposedly corrected

at the time of his departure. However, a short time later, series of

measurements were made to test the reliability of the equipment . The meas- urements included recording on punched paper tape the "100% line" and

"zero line" calibrations as well as the uncorrected and the corrected specimen lines. The results were then analyzed by the NBS SEAC computer.

The computer corrected the data on the basis of the recorded calibration lines, and it also compared these results to the data obtained by use

of the automatic correction feature of the data-process ing attachment.

This comparison showed that the errors remaining after automatic correction

of the data were unacceptably large in those ranges where large calibration

corrections were necessary. This was believed to be due to amplifier drift

and rechecking of the static correction circuit alignment revealed that

serious drift had occurred. A faulty component was located and replaced.

After this replacement, drift in the correction circuits was almost com- pletely eliminated. The data-process ing attachment now consistently corrects the recorded data to an accuracy of 0.5% or better.

A second source of trouble developed during the check of the dataprocessing attachment, which was manifested as incorrectly punched tapes.

Two kinds of faults appeared: (1) duplicate punching of some characters,

and (2) extra punches in some characters.
2

The data are punched on eight-channel tape in words of eight characters.

The data are recorded in digital notation. The first four channels are used

to record the digits 1 to 9 in binary code. The fifth channel is used for

the parity bit, the sixth channel for zero, the seventh is not used, and the

eighth is a carriage return. Odd parity is used, and a hole is punched in

the fifth channel as required to make the total number of holes in the character odd.

The first four characters in a word represent the position of the wavelength drive drum, in units of 0.001 turn. The next three characters represent the pen position in percent of full scale, in units of 0.1 percent.

The automatic data processing equipment cannot be considered to operate

successfully until the malfunctions responsible for incorrect punching of the

tape are corrected. Methods for doing this are being investigated. In the

meantime, any errors in the tape must be corrected before it can be used for

direct input to a computer. The errors must be identified before they can be

corrected. Consideration is therefore being given to methods of checking the

tape either during or shortly after punching. One method of checking for extra punches in the characters is to incorporate a parity checker operating

directly from the levers that punch the holes. This check would generate a signal and/or mark the tape whenever an even number of holes in a character

appeared. Because of the odd parity built into the recording system, there

will always be an odd number of holes when a character is recorded correctly.

The presence of duplicate characters will cause a word to consist of more

than eight characters. A system designed to generate a signal and/or mark

the tape when this occurs can also be designed. Another way to check for

extra characters is to transcribe the tape with a rape-reading typewriter,

and examine the typewritten output for words of more than seven characters.

IV. SPECTRAL REFLECTANCE

A. Ellipsoidal Mirror Ref lectometer

The design of the ellipsoidal mirror ref lectometer was described in detail in the last annual summary report. The specimen, located at the first

focus of the ellipsoidal mirror, is illuminated near normal by a slightly

converging beam of monochromatic radiation which passes through a small opening in the mirror. The ellipsoidal mirror collects the radiation reflected

by the specimen into a solid angle of 165° (that reflected at angles greater

than 82.5° from the normal is lost) and focuses it onto the detector, which

is located at the second focus of the ellipsoid. The linear magnification

factor of the ellipso-id is about 5.

The intensity of the incident beam is measured by moving the detector

to the specimen position. The reflectance of the specimen, at any one wavelength, is taken as the ratio of the reflected energy to the incident energy,

after correction for mirror losses.
3

A Golay cell has been used as the detector for the ref lectometer

Because of the large magnification factor, a cell with a sensitive area one centimeter in diameter was used. This cell proved to be highly microphonic

particularly when it was used with the sensitive diaphragm placed horizontally, which is the normal position for use with the ellipsoidal reflectometer. Satisfactory s ignal-to-noise ratios have not been achieved with

this detector, in spite of elaborate precautions to isolate the detector

from noise and building vibrations.

During the last quarter, two newly-developed thermopiles with a viewing

area of one square centimeter were procured for use with the equipment.

Tests for signal-to-noise ratio and linearity in the 1 to 15 micron wavelength range indicated that they will be satisfactory for the purpose.

Preliminary tests with flowers of sulfur with one of the new detectors

at 5 p gave an uncorrected reflectance of about 50 percent, well below

what has been reported by other investigators. This would indicate that

mirror losses are more nearly 45% than the 5 to 10% that had been expected.

There was reason to suspect that the losses were caused by aberrations of

the ellipsoidal mirror, which resulted in some of the energy reflected by

the specimen being focused outside the sensitive area of the detector.

Photographs were taken of (1) the beam incident on the specimen, which is

about 2 mm square, and (2) the beam incident on the detector. By careful

adjustment of the positions of the specimen and detector relative to the

mirror, it was possible to obtain an image at the detector of the illuminated spot on the specimen that fell entirely within a square one centimeter on a side. However, this careful adjustment did not significantly . reduce the mirror losses.

Further investigation revealed that the detector varied significantly

in sensitivity over its sensitive area. It is now believed that what

appeared to be mirror losses are caused by a difference in sensitivity

of the detector for the small beam (2x2 mm) when the detector is at the

specimen position (first focus) and the larger (10 x 10 mm) beam when the

detector is at the second focus when it is measuring the reflected flux.

To quantitatively establish the magnitude of the variation in sensitivity, cooperative tests with the Photometry and Colorimetry Section of

NBS have been initiated. Both the Golay Cell and thermopile detectors will

be examined in this investigation.

Consideration is being given to the design of a diffusing screen to be

placed over the detector window. This screen will help to distribute the

incident flux over the entire surface of the detector, and hence reduce

the effect of the localized variations in detector sensitivity.
- 4 -

B. Integrating Sphere Ref lectometer

The advantages of using an integrating sphere ref lectometer with a continuous “wave gas laser as a source were discussed in the last Annual

Summary Report (NBS Report 8174). Briefly, the method shows promise of

considerably greater precision than can be achieved with direct measurements of emitted flux, particularly at short wavelengths and very high

temperatures

It appears logical for the development program to proceed in six

phases, as follows: (1) Design and construction of an integrating

sphere ref lectometer, and evaluation of its performance at room temperature in an air atmosphere and at 0.632 and 1.15 microns. (2) Design of a specimen heater to permit reflectance measurements to be made in air at

temperatures up to 1400 K. (3) Modification of the equipment to permit

operation in vacuum or controlled atmosphere. (4) Extension of the operation capabilities of the equipment to 3.39 microns. (5) Extension of the

measurements to additional wavelengths, particularly to longer wavelengths,

by use of other lasers as sources and (6) Increase in the maximum temperature of operation to 2500 °K (4000°F).

While it is contemplated that the phases enumerated above will be

completed in the order listed, they will be carried on simultaneously to

the extent that it is practicable to do so,

a. Sphere Coatings : Magnesium oxide has been used for many years for

coating integrating spheres. Its high diffuse reflectance make it the

preferred coating for use in the 0.3 - 2.5 micron wavelength range in spite

of its extreme fragility and somewhat laborious application procedures.

Information is not readily available on use of MgO coatings in vacuum.

Its high surface area may make de-gassing a problem, and because of its

fragility it may not stand pumping or release of vacuum without rupturing.

These properties will be investigated. The reflectance of MgO coatings de- creases rapidly beyond about 2.5 microns, so that in any case another coating must be developed for use at longer wavelengths.

b. Ref lectometer Design : The preliminary design of the ref lectometer

is shown In figure 1. The integrating sphere is of stainless steel, 14 in.

in diameter, and has been designed so that it can be operated under vacuum

or positive pressure. The specimen mount accommodates a specimen disc which

can be heated by induction. The detector for use at 0.632 micron is an RCA 7102 photomultiplier. The source is a helium-neon continuous -wave

gas laser with accessories for operation at 0.632 and 1.15 microns. A

3/4-inch diameter detector port in the sphere wall is fitted with a flashed

opal glass cover, from which a light pipe directs the light to the detector.

Provision is made for filters to be inserted between the detector and

sphere, to absorb energy emitted by the hot specimen. The amplifier is a Perkin-Elmer Model 107, operated in conjunction with a synchronous rectifying chopping assembly.
5

As of the end of the report period all of the equipment required

for Phase 1 above is on hand or has been ordered. Fabrication of the

sphere and related components is nearly completed.

PRECISION CONTROL

DIAGONAL MIRROR MOVED J INTO OPTIC AXIS FOR L OPTICAL PYROMETER VlEWINGJX

LASER a ACHROMAT TO PROVIDE

F/48 BEAM. PLANE OF POLARIZATION

AT VARIOUS INCLINATIONS

sphere for reflectometer , with

schematic diagram of accessory

equipment