Radiometer measures microsecond pulses of radiant flux in the presence of electromagnetic noise. It consists of a charged capacitor that delivers a voltage pulse proportional to the thermally induced depolarization of its polymeric dielectric

Beam, B. H.

Russell, L. D.

English

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Incident 
Radiation 
Pulse 
Opaque 
Dialectric 
.
	
Shield
June 1970	 Brief 70-10056 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the clearinghouse for Federal Scientific and Technical 
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be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Thermodielectric Radiometer 
Conjugate Polymer Film Capacitors

(Film Thickness Exaggerated) 
Conjugate Capacitor Configuration with Integral Preamplifiers 
The problem: 
To develop a radiometer with a wide and rapid 
spectral response for measuring microsecond pulses 
of radiant flux in the presence of the electromagnetic 
noise characteristically generated by spark gaps or 
shock tubes.  
The solution: 
A detector consisting of a charged capacitor that 
delivers a voltage pulse proportional to the thermally 
induced depolarization of its polymeric dielectric. 
How it's done: 
A conjugate capacitor configuration with integral 
preamplifiers is shown in the figure. The common 
grounded electrode of the polymer-film dual capacitor 
(C a , C b) is formed by evaporating a thin black 
gold deposit on an aluminum base that has been evap-
orated on a quarter-mill (6-micrometer) polymer film
(e.g., Mylar). The microporous gold deposit has an 
absorption of at least 0.90 from the ultraviolet region 
to about 15 micrometers, giving the detector a wide-
band sensitivity to radiant energy. The aluminum film 
is grounded by contact with the metal case, which acts 
as an electrostatic shield for the assembly. The indi-
vidual electrodes on the other side of the polymer 
film are also of deposited alunfinüm metal and form 
two equal-valued capacitors of about 100 pF, sharing 
the same dielectric and ground electrode. Contact with 
the capacitors is made by a copper pellet at con-
venient points. The dual capacitor assembly is 
charged through current-limiting resistors (from a 
voltage source V. ) to a gradient not exceeding the 
dielectric strength of the film, but generally of the 
order of about 100 volts/ micrometer. Capacitors C' 
are used to couple signals to high-input impedance 
(continued overleaf) 
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preamplifiers externally connected to a differential-
input oscilloscope amplifier. 
Capaitor Ca receives the incident radiation pulse 
and prevalent noise (e.g., electromagnetic noise ac-
companying a shock wave). Capacitor C b receives 
only the noise because the radiation pulse is blocked 
by an opaque dielectric shield. The signal from each 
capacitor is amplified by its preamplifier and trans-
ferred to the input terminals of the differential 
oscilloscope amplifier. If the two capacitors, C a and 
C b , and their response to common-mode noise are 
identical, and if the preamplifiers are balanced, only 
the radiant energy signal is transferred by the oscillo-
scope amplifier. 
Calibration can be effected with a 50 mW, cw laser 
chopped at I kHz/sec; the laser is referenced to a 
commercial, low-frequency radiometer. Typically, 
the radiometers have a sensitivity of 20 V/joule/cm2 
for a 6-micrometer film charged from a 500-V source. 
Response times are a function of the thermal response 
of the polymer film, the electrical response of the 
detector's equivalent circuit, and the external read-
out circuit; response times of the order of several 
microseconds are readily achieved. 
A radiometer utilizing only one radiation-sensitive 
capacitor can be constructed for use in a noise-free
ambient environment. It can be completely potted with 
a dielectric compound if it is to be used at low am-
bient pressures where shorting caused by ionization 
can occur. 
Notes: 
1. Additional information is contained in the paper: 
Russell, L. D. and Beam, B. H.: Thermodielectric 
Radiometer, 68-403 presented at AIAA Third 
Aerodynamic Testing Conference, San Francisco, 
Calif., April 8-10, 1968. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 95014 
Reference: B70-10056 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention may be made to National Aeronau-
tics and Space Administration,Code GP, Washington, 
D.C. 20546 
Source: Benjamin H. Beam and Larry D. Russell 
Ames Research Center
(ARC-10138)
is 
. 
1 
Brief 7040056
	
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Thermodielectric radiometer

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