Conical cavity-type absolute radiometer measures the intensity of radiant energy to an accuracy of one to two percent in a vacuum of ten to the minus fifth torr or lower. There is a uniform response over the ultraviolet, visible, and infrared range, and it requires no calibration or comparison with a radiation standard

Kendall, J. M., Sr.

Plamondon, J. A., Jr.

English

NASA Technical Reports Server

G u arc 
ThermocouplE 
Silver Foil Cone 
Cone Resisto
1.25 in. 
December 1967	 Brief 67-10557 
NASA TECH BRIEF 
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. 
Improved Cavity-Type Absolute Total-Radiation Radiometer 
Guard Heater Coil
	
-	 Cover 
Thermal Guard 
An improved conical cavity-type absolute radiome-
ter, that has been developed, is capable of measuring' 
the intensity of radiant energy to an accuracy of '
 1 to 
2 percent when operated in a vacuum oft' i' torr or 
lower.' The radiometer exhibits a uniform response 
over the ultraviolet, visible, and infrared range. Since 
it is an absolute radiometer, it requires no calibration 
or comparison with a radiation standard. To measure 
a radiation intensity requires only an accurate knowl-
edge of the aperture area, the temperature of the 
conical cavity, and the electrical power needed to 
maintain the cavity at a preset constant temperature. 
The basic elements of this radiometer are not new. 
The improvements are due to refinements in design 
details, selection of the best materials, and the use of 
modern electronics.
Glass Support 
Physical description. The measuring element of the 
radiometer is a receptor made by winding enameled 
platinum wire (1.5 mils in diameter) over the outside 
surface of the hollow cone.. The cone is made of 
0.005-inch-thick silver foil which is rolled to shape 
and silver-soldered at the edges. The high thermal 
, conductivity of the silver foil minimizes thermal gra-
dients in the cone and also ensures rapid temperature 
equalization after any thermal change. The platinum 
wire (cone resistor) serves both as a sensitive resistance 
thermometer and .as an electrical heating element for 
the cone. The internal surface of the cone is coated 
with Parson's black lacquer or with gold or platinum 
black. The cavity formed by the cone (23.5 deg total 
angle) is estimated to act as a perfect blackbody to 
better than 99.5 percent for all wavelengths between 
0.2 and 40 microns.
(continued overleafl 
	
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The cone, supported by a glass fiber and glass-
tipped centering screws, is surrounded by a massive 
copper thermal guard which is gold-plated on its in-
side and outside surfaces. The guard minimizes tem-
perature gradients and radiant heat exchange with 
the receptor cone. Located in good thermal contact 
with the guard is a second platinum wire winding 
(guard resistor) which has the same electrical resist-
ance as the cone resistor. The guard resistor serves 
as a resistance thermometer for accurate comparison 
of the guard and cone temperatures. A heater coil 
on the external surface of the guard is used to main-
tain the guard at a desired constant temperature. A 
thermocouple placed in the inner portion of the guard 
provides an absolute measure of guard temperature, 
and by means of an electronic control the guard is 
maintained at the desired temperature. 
The electronic circuit devised for the radiometer 
maintains the thermal guard at a known constant 
preset temperature; supplies precisely enough electri-
cal heating power to the cone (equal to the difference 
between the thermal energy radiated from the cone 
aperture and the thermal energy received by it from
the radiant source to be measured); and accurately 
measures the electrical heating power to the cone. 
Notes: 
1. As presently constructed, the radiometer has a re-
sponse time of approximately 2 seconds and is 
capable of measuring a maximum radiation inten-
sity of approximately 0.3 watt/cm 2
 (0.25 Btu/ft2/ 
sec), which is a little more than double solar in-
tensity at 1 astronomical unit. 
2. Inquiries concerning details of the radiometer and 
electronic circuitry may be directed to: 
Technology Utilization Officer. 
Jet Propulsion Laboratory 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B67-10557 
Patent status: 
Inquiries about obtaining rights for the commer-
cial use of this invention may be made to NASA, 
Code GP, Washington, D.C. 20546. 
Source: J. M. Kendall, Sr. 
and J. A. Plamondon, Jr. 
(JPL-807) 
Brief 67-10557	 Category 01


Improved cavity-type absolute total-radiation radiometer

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