The responsiveness of a foil radiometer is increased and its time constant is simultaneously decreased by isolating the foil in a controlled environment. Using an optical system, it is coupled to the media to be measured, and the resulting concentration of energy permits the thermocouple junction temperature to respond quickly

Schumacher, P. E.

English

NASA Technical Reports Server

November 1967	 Brief 67-10448 
NASA TECH BRIEF 
5po	 .: 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Foil Radiometer Accessory Improves Measurements 
The problem: 
To increase the responsiveness and to simultane-
ously decrease the time constant of a foil radiometer 
used to measure radiation from a high-energy environ-
ment. These two characteristic changes cannot usually 
be made simultaneously. For example, a decreased 
time constant could be accomplished by reducing the 
foil area, but this would decrease the responsiveness 
of the instrument. 
The solution: 
Isolate the foil in a controlled environment, and by 
means of an optical system, couple it to the media to 
be measured. The optical system focuses the radiant 
flux on a very small area at the center of a tantalum 
foil—the area which carries the thermocouple junc-
tion. This concentration of energy at the junction 
permits the junction temperature to respond more 
quickly than if the energy is distributed over the 
entire area of the foil. 
How it's done: 
The radiometer, comprised of an optical system and 
thin foil thermocouple detector, is enclosed in a 
sealed metal housing covered with thermal insulation. 
The housing is purged at a low flowrate with nitrogen 
gas which is exhausted around a sapphire window that 
serves as the entry aperture for the radiant flux. 
In operation, the optical system establishes the field 
of view and focuses the radiant flux at the center of 
the foil. Thisresults in a higher differential temperature 
between the center of the foil and foil support surface 
around its face, and upgrades the sensitivity of the 
instrument. Because the incoming radiant energy is 
concentrated at one.small area, the area heats more 
rapidly and results in a decreased time constant for the 
instrument.
The controlled field of view of the optical system 
permits a high degree of selection in the portion of 
the source that is being observed, and provides critical 
measurement capability. Optical systems of prototype 
models provide a field of view of 6.160. 
Notes: 
1. In test, output signal stability was within 0.3 
percent over a 10-minute interval when exposed to 
a blackbody source radiating at a level of 250 
watts/cm 2 . When subjected to step changes of 50 
watts/cm 2 in the radiation level, the signal consist-
ently reached 63.2 percent of its final value in less 
than 200 milliseconds. Control of the spectral 
range being measured was accomplished by con-
ventional filtering. 
2. A further in-depth study has been made to deter-
mine how to increase the signal output (in milli-
volts) of a thermal foil radiometer for a fixed-
exposure time interval within severe dimensional 
restraints. Consideration has been given to con-
figuration geometry, thermal characteristics, and 
thermoelectric properties of available materials, 
Results indicate that an increase of >60 percent 
signal output is potentially available. 
The radiometer is fabricated from 0.0002-inch 
constantan foil, a relatively massive copper support 
that functions as a heat sink, and 0.001-inch copper 
wire. The instrument delivers a signal that is 
nearly linear with irradiance. The preferred foil 
geometry is a strip having a center-to-end width 
ratio of 4 to 1. This represents a 30 percent im-
provement over a rectangular strip, which has been 
found to be 62 percent better than a circular foil 
with full peripheral support as used in conventional 
radiometers.
(continued overleaf( 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19670000448 2020-03-11T21:13:33+00:00Z
3. Inquiries concerning this innovation may be di- 	 Patent status: 
rected to:
	
No patent action is contemplated by NASA. 
Technology Utilization Officer	 Source: P. E. Schumacher 
Marshall Space Flight Center 	 of North American Aviation, Inc. 
Huntsville, Alabama 35812	 under contract to 
Reference: B67-10448
	
	
Marshall Space Flight Center
(M FS- 12684& 12717) 
Brief 67-10448	 Category 01


Foil radiometer accessory improves measurements

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