A method and apparatus for measuring the absorptivity of a radiation detector by making the detector an integral part of a cavity radiometer are described. By substituting the detector for the surface of the cavity upon which the radiation first impinges a comparison is made between the quantity of radiation incident upon the detector and the quantity reflected from the detector. The difference between the two is a measurement of the amount of radiation absorbed by the detector

Sheets, R. E.

English

NASA Technical Reports Server

United States Patent [191 
Fletcher et al. 
[ 1 1  I 3,971,940 
[451 July 27, 1976 
DETECTOR ABSORPTIVITY MEASURING 
METHOD AND APPARATUS 
Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administraton, with respect to an 
invention of; Ronald E. Sheets, 
Westminster, Calif. 
Filed: Mar. 19, 1975 
Appl. No.: 559,845 
U.S. CI .................................. 250/340; 250/353 
Int. CL2 ............................................ GOlJ 1/00 
Field of Search ........... 250/338, 340, 347, 351, 
2501353, 252, 349; 731355 R, 355 EM 
References Cited 
UNITED STATES PATENTS 
3,350,562 10/1967 Flint .................................... 2501347 
3,392,282 7/1968 Astheimer ........................... 250/35 I 
3,794,838 2/1974 Weiss et a1 .......................... 250/351 
Primary Examiner-Davis L. Willis 
Attorney, Agent, or Firm-Howard J .  Osborn; Wallace 
J .  Nelson; John R. Manning 
[571 ABSTRACT 
A method and apparatus for measuring the absorptiv- 
ity of a radiation detector by making the detector an 
integral part of a cavity radiometer. By substituting 
the detector for the surface of the cavity upon which 
the radiation first impinges a comparison is made be- 
tween the quantity of radiation incident upon the de- 
tector and the quantity reflected from the detector. 
The difference between the two is a measurement of 
the amount of radiation absorbed by the detector. 
10 Claims, 2 Drawing Figures 
I I 
11 
https://ntrs.nasa.gov/search.jsp?R=19760022463 2020-03-20T02:22:32+00:00Z
U.S. Patent JU~Y 27,1976 3,971,940 
IO 
FIG. 1 
111 
FIG. 2 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 
85-568 (72 Stat. 435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
This invention relates generally to radiation detectors 
and specifically to an apparatus and method for mea- 
suring the amount of impinging radiation which is re- 
flected from the detector surface. 
The absolute measurement of the radiation striking a 
detector depends upon what portion of that radiation is 
absorbed. If no radiation were reflected, the detector 
3,97 1,940 
1 2 
the radiant energy, and the detector which is being 
calibrated for absorptivity. Any optical system may be 
used which accurately directs the radiant energy onto 
the detector so that no energy entering its cavity aper- 
5 ture impinges on any surface other than the detector. 
One example of such a system consists of a xenon arc 
lamp radiation source focused on a selectable aperture 
disc by an aluminized mirror. A condensor lens assem- 
bly with a magnification ratio of one images the energy 
10 passing through the aperture onto the active area of the 
detector being calibrated. The aperture discs of this 
optical system are changed to vary the size of the image 
falling upon the detector in order that the active area of 
the detector may be fully illuminated with energy, 
15 while no energy strikes beyond the active area of the 
detector, regardless of the size of the detector being 
tested. 
The energy absorbing cavity radiometer is specially 
constructed to permit the substitution of the detector 
DETECTOR ABSORPTIVITY MEASURING 
METHOD AND APPARATUS 
would react to the entire impinging radiation and its 20 being calibrated or a conventional energy absorbing 
measurement would be an indication of the total radi- surface for what would otherwise be the integral inter- 
ant energy. However, all detectors cause some reflec- nal surface of the cavity upon which radiant energy first 
tions and, to the extent that they do, the readings they impinges. When the radiant energy entering the cavity 
yield are inaccurate. Since the reflectance and absorp- strikes the detector surface which has been substituted 
tivity of any material varies with the type of radiation to 25 for the cavity internal surface, most of the energy will 
which it is subjected, it is imperative for accurate mea- be absorbed by the detector to perform the detection 
surements to evaluate the absorptivity of such radiation function. However, since no detector is ideal, some 
detectors in regard to the particular radiation of inter- energy will be reflected from the surface of the detec- 
est. tor. This reflected energy will impinge on  the other 
The requirement is for a convenient system for Cali- 30 surfaces of the cavity radiometer, be converted to heat, 
bration of radiation detectors to an accepted secondary and measured in the conventional manner by the func- 
standard so that measurements taken with such detec- tioning of the cavity radiometer. 
tors may be adjusted to account for the radiation which Upon completion of the measurement of the re- 
is reflected from the surface of the detector and which flected energy, the detector is removed from its posi- 
has no effect on the reading given by the detector. Such 35 tion as one surface of the cavity radiometer and a thcr- 
a calibration must be applicable to the particular radia- mally conducting energy absorbing cover surface is 
tion for which the detector is used but need not be an substituted for the detector, thus forming a complete 
absolute measurement. The specific requirement is that conventional cavity radiometer. The light source and 
the calibration system yield a reading of the relation- optical system are operated in the same manner as 
ship of reflected or absorbed radiation to the total 40 before, so that this radiometer is then used to measure 
applied radiation which may then be applied to modify the total incident energy that enters the radiometer. 
the output readings of the detector. The difference between the total energy entering the 
The present invention provides an accurate and con- radiometer and the energy reflected from the detector 
venient apparatus and method for the calibration of the is a measure of the energy the detector absorbs, and 
absorptivity of radiation detectors which may be used 45 may be used to correct the readings of the detector to 
for any type of radiation and with all detectors. account for the energy not measured because of reflec- 
tion. 
SUMMARY OF THE INVENTION 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic cross-section of the apparatus 
of the invention during measurement of the energy 
reflected from the detector; 
FIG. 2 is a schematic cross-section of the apparatus 
of the invention showing the cover substituted for the 
The present invention operates essentially on a sub- 
stitution principle by which the total radiation to which 5 0  
the detector is subjected and the radiation which is 
reflected from the detector are compared to establish 
the portion of radiation actually absorbed by the detec- 
tor and contributing to its reading. This comparison is 
accomplished by the use of a specially constructed 55 detector. 
cavity radiometer which acts as a standard of compari- 
son to both the total incident radiation reflected from DETAlLED DESCRIPTION OF THE INVENTION 
the detector. The preferred embodiment of the invention selected 
The particular advantage of the invention is the capa- for illustration is shown in FIGS. 1 and 2 in which the 
bility of calibrating detectors at the same conditions 60 same parts are assigned the same designations in both 
that they will ultimately be used. The invention oper- figures. The absorptivity measuring apparatus consists 
ates with the same radiation source and power levels of an optical system shown generally at 10 and a cavity 
that are used for later measurements, therefore elimi- radiometer shown generally at 12, held in rigid aline- 
nating any need for adapting of the results to different ment by base 11. 
radiation frequencies or power levels. Optical system 10 includes a xenon lamp 14 attached 
The invention consists essentially of an optical sys- to and partially enclosed by reflector 16. Reflector 16 
tem which focuses the radiant energy upon the detec- is held rigid from base 11 by support 13. Ray 15 and 17 
tor, an energy absorbing cavity radiometer to measure generated by xenon lamp 14 are focused by mirror 18, 
65 
3.97 1,940 
3 
held by support 19 onto apcrturc 20 within disc 22. 
Disc 22 may bc rcplaccd from within its support 21 by 
other discs with apertures of various sizcs to vary thc 
size of the radiant imagc applied to radiomctcr 12. 
The image of aperture 20 is focused by condensing 
lcns assembly 24, kept in alincmcnt by support 23, onto 
a surfacc within cavity 26, which is alincd by support 
25. In FIG. 1, radiation detector 28 is positioned and 
supported by block 27 so as to be thc first surfacc upon 
which radiation entering cavity 26 impinges. Detector 
28 is held tightly against cavity 26 at radiation tight seal 
34 by straps 30 and screws 32 to prevent any radiation 
loss. Radiation which is not absorbcd by detector 18 is 
rcflcctcd, as shown by rays 15 and 17, at such an anglc 
that it subsequently impinges on the blackened surfaces 
of cavity 26. Each subsequent surface absorbs sizable 
portions of the radiation so that all thc energy entering 
cavity 26 is finally absorbed eithcr by detector 28 or 
thc surfaces of cavity 26. Since all the radiation re- 
flected from detector 28 is absorbed by cavity 26, the 
rise in temperature of cavity 26 is caused by the im- 
pinging radiation is a measure of energy reflected from 
detector 28. The thcrmal energy absorbcd by cavity 26 
is transferred to heat sink 36 via constantan rods 38. 
Constantan rods 38 arc bonded to copper heat sink 36 
and at thc cavity end to copper platc 40 forming a 
differential copper-constantan thermocouple. The one 
dimensional flow of thermal energy produces a temper- 
ature difference at each junction thus producing an 
electrical voltage between leads 42 and 44 which is 
measurcd by millivoltmeter 46. This first reading of 
millivoltmetcr 46 is proportional to the energy re- 
flected from detector 28 and received by the surfaces 
of cavity 26. 
In order to determine the total radiant energy to 
which detcctor 26 is subjected, thermally conducting 
cap 48 is substituted in the position of detector 28, and, 
supported by block 27, is held tightly against cavity 26 
by straps 30 and screws 32 as shown in FIG. 2 and 
xenon lamp 14 is operated at exactly the same energy 
level as before. In this orientation radiant energy enter- 
ing cavity 26 strikes conducting cap 48 rather than, as 
in the previous setup, detector 28. Some of the radiant 
energy is absorbed by cap 48 which causes heating of 
cap 48. This heat is conducted to the structure of cavity 
26 with which cap 48 is in intimate thermal contact, 
through heat conductive and radiation tight joint 34 
and ultimately, the heat raises the temperature of cav- 
ity 26. The energy reflected from cap 48 is absorbed by 
the surfaces of cavity 26 just as was the energy re- 
flected from detector 28. Therefore, both the energy 
reflected and absorbed by cap 48 causes an increase in 
temperature of cavity 26 which is then converted to an 
electrical signal by the constantan copper junctions and 
read by millivoltmeter 46. The ratio of the millivoltme- 
tcr readings when detector 28 is in the apparatus to the 
millivoltmeter reading when cap 48 is in the apparatus 
is a measure of the reflectivity of the detector. The 
reflectivity ratio subtracted from one, yields a measure 
of the absorptivity of the detector. 
Typical absorptivities secured by this method are: 
0.85 for acetylene black on copper, 0.80 for oxidized 
copper, 0.33 for polished stainless steel, and 0.38 for 
polished copper. 
It is understood that the form of the invention herein 
shown is merely a preferred embodiment. Various 
changes may be made in shape, size, and the arrange- 
ment of parts; equivalent means may be substituted for 
4 
thosc illustrated and dcscribed; and ccrtain fcaturcs 
may bc used indcpendcntly from other featurcs without 
departing from the spirit and scope of the invention as 
defined in the following claims. For example, the cavity 
may be of a diffcrcnt configuration or the method of 
encrgy measurement may be by calorimeter rather than 
thcrmocouple. Also, a diffcrcnt configuration of opti- 
cal system may be used so long as the illumination of 
the detector meets the aforementioned criteria. 
What is claimed as new and desired to be secured by 
Letters Patcnt of the Unitcd States is: 
1. An apparatus for measuring thc absorptivity of 
radiation detectors comprising: 
cavity radiometcr means which converts the radiant 
energy into thermal energy, one inner surface of 
said cavity radiometer being a thermally conduc- 
tive detachable cap, said detachable cap being that 
surface upon which radiant energy entering said 
cavity radiometer first impinges; 
heat conducting joint means attached to said cavity 
radiometer, joining said cavity radiometer with said 
detachable cap and preventing any leakage of radi- 
ant energy; 
25 attachment means holding said detachable cap 
tightly against said cavity radiometer and permit- 
ting the replacement of said detachable cap with a 
radiation detector to be measured for absorptivity; 
thermal energy measurement means attached to said 
cavity radiometer indicating the increase in ther- 
mal energy of said cavity radiometer due to ab- 
sorbed radiation; and 
optical means for generating the radiant energy ap- 
plied to said cavity radiometer. 
2. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 1 wherein the thermal 
energy measurement means is a thermocouple means 
connected to a voltage sensing means. 
3. An apparatus for measuring the absorptivity of 
40 radiation detectors as in claim 2 wherein the thermo- 
couple means is a differential copper-constantan ther- 
mocouple which generates an electrical voltage read by 
a millivoltmeter electrically connected to said copper- 
constantan thcrmocouple. 
4. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 1 wherein the internal 
surfaces of the cavity radiometer means are blackened 
to accomplish the conversion of radiant energy to ther- 
mal energy. 
5. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 1 wherein the optical 
means includes an aperture in the light path restricting 
the image of the radiant energy impinging upon the 
radiation detector to the size of the active area of the 
6. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 5 wherein the aperture is 
within a replaceable disc whereby the disc may be 
exchanged and the aperture size may be varied 
60 whereby the image of the radiant energy impinging 
upon the radiation detector may be adjusted to match 
the size of the active area of various detectors. 
7. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 1 wherein the optical 
8. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 7 wherein the lamp is a 
xenon lamp. 
I" 
l 5  
2" 
3O 
35 
45 
50 
>S radiation detector. 
65 means includes a radiation source which is a lamp. 
3,97 1,940 
5 6 
9. An apparatus for measuring the absorptivity of 
radiation detectors as in claim 1 wherein the optical 
means includes a focusing means comprising a reflector 
attached to and partially enclosing the source means 
which directs the radiant energy into a narrow beam; 
supplying focused radiant energy to said cavity radi- 
ometer so that it impinges only on said removable 
inner surface whereby said thermal measuring 
means indicates the total amount of radiation inci- 
dent uDon said removable inner surface; 5 _ _  
a focusing mirror redirecting the radiant energy and 
further focusing the energy; and 
a condensing lens assembly finally focusing the en- 
ergy onto the active surface of the radiation detcc- 
tor. 
10. A method of measuring the absorptivity of a 
providing a cavity radiometer which absorbs radiant 
energy and converts it into thermal energy, one 
inner surface of which is removable; 
providing thermal measuring means which indicates 
the amount of radiant energy converted to thermal 
energy by said cavity radiometer; 
radiation detector which comprises: 
1 
removing said removable inner surface and replacing 
it with a radiation detector which is to be mcasured 
for absorptivity; 
supplying the exact amount of focused radiant energy 
as previously to said cavity radiometer, so that it 
impinges only on the active area of the radiation 
detector substituted for said removable inner sur- 
face, whereby said thermal measuring means indi- 
cates the amount of radiation reflected from the 
radiation detector and together with the mcasure- 
ment of total incident radiant energy furnishes 
sufficient information to determine the absorptivity 
of thc radiation detector. 
0 
5 
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Detector absorptivity measuring method and apparatus

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