Incident electromagnetic radiation pulses maintain bolometer's sensitive element in state of resonant oscillation and correlate amplitude of oscillation with incident radiation power. Bolometer can be employed with other devices for detecting motion of ribbon

Dimeff, J.

English

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1972	 Brief 72-10170 
NASA TECH BRIEF 
Ames Research Center	 -4r 
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 National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Vibrating Ribbon Bolometer - A Concept 
Devices for measuring the power of low-intensity 
electromagnetic radiations ordinarily are based on 
the principle that heat is developed when the radia-
tion strikes a sensitive element. In the devices which 
are called bolometers, radiant power is correlated 
with a change in the value of an electrical element 
(resistance, capacitance, or inductance); other de-
vices, such as a Golay cell, correlate the pressure of 
a fixed volume of gas as it is altered by energy ab-
sorbed from the incident radiation. The sensitivity 
of these transducers is limited by electrical noise, 
stability, of structure, inadequate thermal insulation, 
etc. A new type of bolometer, based on a thermo-
mechanical principle, is described in the following 
paragraphs. It differs from other bolometers in that 
a sensitive element is maintained in a state of res-
onant oscillation by incident pulses of electromag-
netic radiation, and the amplitude of the oscillation 
is correlated with the power of the incident radiation. 
As indicated in the diagram, electromagnetic radia-
tion is focused by a lens onto a very thin, conducting 
ribbon; the ribbon is mounted under tension between 
opposed arms of a rigid support. The beam of radia-
tion is periodically interrupted by a light-chopping 
disc driven by a servo motor. A fixed, elongated 
electrode is aligned with the ribbon but maintained 
at a small distance so that the electrode and ribbon 
become the elements of a capacitor which is to be 
used as a sensor. The fixed electrode is attached to 
an insulated conductor; the ribbon support structure 
serves as the contact for the ribbon. The capacitance 
of the sensor varies as a function of the spacing 
between these members, and since the periodically 
interrupted radiation causes a rhythmic heating and 
cooling of the ribbon, the ribbon expands and con-
tracts; as the ribbon moves toward or away from the 
fixed electrode, there is a periodic variation in capac-
itance. The amplitude of the periodically varying 
capacitance is, of course, relatable to the intensity 
MM W
of the incident radiation and serves as a sensitive 
indicator of the power level of the incident radiation. 
For maximum sensitivity, the beam of incident 
energy must be interrupted at the natural resonance 
frequency of vibration of the ribbon or at a suitable 
(continued overleäf( 
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harmonic of this frequency. To accomplish this, the 
capacitor formed by the ribbon and the fixed elec-
trode is connected to the input of an AC power 
amplifier and the output of the amplifier is fed to 
a phase-shift network which is modulated by a local 
oscillator tuned to the fundamental resonant fre-
quency of the ribbon (or a harmonic); the alternating 
phase output of the phase-shift network is then com-
pared in a phase-sensitive detector with a reference 
signal from the local oscillator and converted to a 
DC signal. 
If the ribbon is vibrating at the resonance fre-
quency (or a harmonic), maximum system response 
is attained, and the alternating phase shift introduced 
by the modulated phase shifter causes equal attenua-
tion on both half-cycles of the modulation. As a 
result, the phase detector does not generate an out-
put. If the frequency of vibration of the ribbon begins 
to drift away from resonance, an output is generated 
by the phase detector with a polarity which depends 
on the direction of drift. Accordingly, the output of 
the phase detector is used to vary the speed of the 
disc so that the output is reduced to zero, that is, 
the rotation of the disc is stabilized at the speed for 
which the beam is interrupted at the resonance fre-
quency of the ribbon or some harmonic thereof. The 
output of the AC power amplifier is also fed to a 
detector which generates a signal that varies directly 
with the input capacitance and is therefore used to 
provide indication of the power in the incident 
radiation.
Notes: 
1. As is typical of all bolometers, maximum sensitivi-
ties are obtained when the ribbon assembly is in 
a good vacuum and the entire device is kept at 
a very low temperature. 
2. The new bolometer can be employed with other 
devices for detecting the motion of the ribbon, 
e.g., optical strain gages or inductive devices. 
8. Requests for further information may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 72-10170 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,508,053) and royalty-free license rights 
will be granted for its commercial development. 
Inquiries about obtaining a license should be ad-
dressed to: 
Patent Counsel 
Mail Code 200-1A 
Ames Research Center 
Moffett Field, California 94035 
Source: John Dimeff 
Ames Research Center 
(XAC-10768) 
Brief 72-10170	 Category 03


Vibrating ribbon bolometer: A concept

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