Filter radiometer utilizing photomultiplier circuit, in which a direct-coupled amplifier varies dynode voltage to maintain constant anode current, measures rapid variations of temperature of white-hot charred body at 2000 K to 3000 K

Liston, E. M.

English

NASA Technical Reports Server

December 1971	 B71-10498 
NASA TECH BRIEF 
Ames Research Center	 £ 
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. 
Wide-Range Logarithmic Radiometer for

Measuring High Temperatures 
.
The problem: 
Measuring rapid variations of the temperature of 
a white-hot charred body at 2000° to 3000°K. 
The solution: 
A filter radiometer utilizing a photomultiplier cir-
cuit in which a direct-coupled amplifier varies dynode 
voltage so as to maintain constant anode current; 
the logarithm of the voltage applied to the photo-
multiplier is a nearly linear function of the reciprocal 
temperature of the white-hot source. 
How it's done: 
Examination of a plot of the radiant energy 
emitted in the near ultraviolet by a nearly black body 
obeying Wien's law reveals that there is approxi-
mately a four-order-of magnitude change of radiant 
energy over the interval 2000° to 3000°K and that 
it is nearly constant over the range of emissivities 
commonly associated with charred bodies. In com-
parison, the emission of radiant energy in the 
infrared is a more sensitive function of emissivity. Ac-
cordingly, the radiometric measurement of tempera-
ture is accomplished with the logarithmic photometer 
depicted in the figure by passing radiation emitted 
from the white-hot body through a filter which trans-
mits largely in the 320 nm region before it falls onto 
the sensitive surface of the photomultiplier. 
The output current of the photomultiplier anode 
generates a voltage across Ri; this voltage is directly 
coupled to the grid of Vi. The plate voltage of Vi 
is direct-coupled to V2 via Zener diode Di, and thus 
is differentially compared with the Zener potential 
across D2. The output plate voltage of V2 in turn
is coupled directly to a series-control tube, V3, and 
its cathode bias, Zener diode D3. The circuit ele-
ments are adjusted so that the output current through 
Ri is of the order of 1 microampere; any variation 
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in the voltage drop across Ri is greatly amplified and 
applied to the control grid of V3. As a result, the 
total potential applied to the photomultiplier dynode 
resistor string is varied and the initial conditions at 
Ri are re-established. The logarithm of the voltage 
that must be supplied to the photomultiplier to main-
tain design potential across Ri is nearly a linear 
function of the logarithm of light intensity. The cir-
cuit can change the dynode voltage from 25 to 
(continued overleaf) 
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125 volts per stage, corresponding to a total change 
of photomultiplier gains of a million to one. The 
drop across resistor R2 (less than 1 volt) is measured 
and taken as an indication of the total photomulti-
plier supply voltage. 
The radiometer can be calibrated with a disap-
pearing-filament pyrometer when both instruments 
are viewing a flat graphite surface maintained at 
white heat. The logarithm of the photomultiplier 
voltage bears a closely linear (±0.5%) relationship 
to the reciprocal of the absolute temperature, 
log V = K(1/T) + C 
over the range of 2000° to 3000'K with a 320-nm 
filter at a 7-nm hall-bandwidth. 
Notes: 
1. The response of the radiometer was checked with
a pulsed ultraviolet glow lamp and found to be 
of the order of 50 microseconds. 
2. Requests for additional information may be 
directed to: 
Technology Utilization Office 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP- 71-10498 
Patent status: 
No patent action is contemplated by NASA. 
Source: Edward M. Liston of 
Stanford Research Institute 
under contract to
Ames Research Center 
(ARC-10254)
S 
B71-10498	 Category 01


Wide-range logarithmic radiometer for measuring high temperatures

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