The sensing head of a two-color band ratioing pyrometer of a known type using a fiber optic cable to couple radiation to dual detector photodiodes is improved to have high spatial resolution by focusing the radiation received through an objective lens (i.e., by focusing the image of a target area) onto an opaque sheet spaced in front of the input end of the fiber optic cable. A two-mil hole in that sheet then passes radiation to the input end of the cable. The detector has two channels, one for each color band, with an electronic-chopper stabilized current amplifier as the input stage followed by an electronic-chopper stabilized voltage amplifier

Bickler, Donald B.

Henry, Paul K.

LoGiurato, D. Daniel

NASA Technical Reports Server

United States Patent 
Bickler et al. 
[54] HIGH RESOLUTION COLOR BAND 
[75] Inventors: Donald B. Bickler, Temple City; Paul 
PYROMETER RATIOING 
IC. Henry, Altadena; D. Daniel 
LoGiurato, Covina, all of Calif. 
[73] Assignee: California Institute of Technology, 
Pasadena, Calif. 
[21] Appl. No.: 80,980 
[22] Filed Aug. 3,1987 
[52] U.S. Cl. ........................................ 356/43; 356/45; 
374/131 
[58] Field of Search ........., ............. 356/44, 43, 45, 49; 
374/130, 131 
[561 References Cited 
[51] Int. Cl.4 ............................ GOlJ 5/08; GOlJ 5/32 
U.S. PATENT DOCUMENTS 
4,650,318 3/1987 Pointer et al. ........................ 356/43 
OTHER PUBLICATIONS 
Bristol-Velotron Two-Color (Ratio) Radiation Pyrorn- 
[ill Patent Number: 4,815,841 
[45] Date of Patent: Mar. 28, 1989 
eter, Bulletin P-1299, Sep. 1965, The Bristol Co. Water- 
bury, Corn. 
Primary Examiner-Vincent P. McGraw 
Attorney, Agent, or Firm-Freilich, Hombaker, Rosen & 
Femandez 
t571 ABSTRACT 
The sensing head of a two-color band ratioing pyrome- 
ter of a known type using a fiber optic cable to couple 
radiation to dual detector photodiodes is improved to 
have high spatial resolution by focusing the radiation 
received through an objective lens (i.e., by focusing the 
image of a target area) onto an opaque sheet spaced in 
front of the input end of the fiber optic cable. A two-mil 
hole in that sheet then passes radiation to the input end 
of the cable. The detector has two channels, one for 
each color band, with an electronic-chopper stabilized 
current amplifier as the input stage followed by an elec- 
tronic-chopper stabilized voltage amplifier. 
5 Claims, 2 Drawing Sheets 
' 16 (FIBER-OPTIC CABLE) 
https://ntrs.nasa.gov/search.jsp?R=20080012366 2019-08-30T03:56:30+00:00Z
U S  Patent ~ m .  28,1989 Sheet 1 of 2 4,815,841 
f FOCUSING 
BARREL 
FIBER-OPTIC 
CABLE' TO DETECTOR PHOTODIODES 
J 
nrr 
II  L I Lr ILWL 
L F l E L D  LENS 
WITH RETICLE 
FIG. I 
(PRIOR ART) 
TIC 
' 16 (FIBER-OPTIC CABLE) 
FIG. 2 
US. Patent MW. 28,1989 Sheet 2 of 2 4,815,841 
-q0+ a 
1 
1 
1 
4,8 1 5,84 1 
CI 
HIGH RESOLUTION COLOR BAND PYROMETER 
RATIOING 
ORIGIN OF 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 Public Law 96-517 (35 USC 
202) in which the Contractor has elected to retain title. 
BACKGROUND OF THE INVENTION 
This invention relates to optical pyrometers and more 
particularly to a two-color band ratioing pyrometer 
having high spatial resolution. 
In attempting to measure the temperature of certain 
critical elements in the hot zone of a silicon crystal- 
growth furnace with thermocouples, three problems 
were encountered: measurements were not repeatable 
to the accuracy required, the thermal distribution was 
disturbed, and the thermocouples could not be used to 
measure the temperature of the molten silicon. It there- 
fore became necessary to use an optical pyrometer illus- 
trated in FIG. 1, which avoided those problems and 
allowed valid temperature measurements, even under 
conditions that would produce misleading results with a 
single-color optical pyrometer, such as variation in 
emissivity for a particular temperature (if the variation 
is equal in the two color bands), and viewing the target 
through a furnace window (if the window has equal 
transmissivity in the two wavelengths). 
A two-color pyrometer measures temperature by 
calculating the ratio of the radiation at two different 
wavelength (color) bands. By using a ratio, the mea- 
surement becomes much less sensitive to absolute radia- 
tion falling on the detector than is the case for pyrome- 
ters using a single wavelength band. Thus, a two-color 
pyrometer is desirable in many applications because 
such instruments are valid over a wide range of sensor 
illumination. 
Referring to FIG. 1, which shows the sensing head 
for the prior-art pyrometer, an objective lens at the end 
of a focusing barrel is adjusted so that the focal point of 
a telescope comprised of the object lens and a field lens 
with a reticle for sighting the target. The objective lens 
is an achromatic lens to prevent color distortion. 
The radiation from the objective lens passes through 
the aperture of the housing for the sensing head, and is 
reflected by a plane mirror that is front coated to reflect 
the color bands of interest, but allow visible radiation to 
pass through the mirror to the field lens. An eyepiece is 
focused on the reticle of the field lens so that it will be 
visible to the viewer superimposed on the image of the 
target. 
The mirror reflects the incoming radiation in the 
color bands of interest to a first end of a fiber-optic 
cable used to guide the radiation to a pair of detector 
photodiodes, each designed for optimum response to 
the different one of the two color bands of interest. As 
noted above, the fiber-optic cable is made up of a bundle 
of fibers. The result is a cable having a 0.032 inch diame- 
ter. The fibers are randomly arranged at the input end 
relative to the other end and serve to distribute the 
radiation uniformly over the detector, a factor that 
contributes to lower detector noise. 
L 
input end of the cable. As a consequence, the diameter 
of the fiber-optic cable determines the sensing field of 
view and therefore the spatial resolution of the optical 
pyrometer. 
A problem with the prior-art two-color ratioing py- 
rometer was the inability to spatially resolve small tar- 
gets and simultaneously maintain the validity of the 
temperature measurement. If the spatial resolution were 
restricted to the size of interest (in this case, targets less 
10 than 0.030 inch diameter), then the radiation reaching 
the detectors, and the detector output signal, would be 
reduced proportionally to the ratio of the squares of the 
target diameters. When driven by this vastly reduced 
detector output, detector signal processing electronics 
l5 lacked the sensitivity to yield valid measurements. This 
problem was solved by designing and fabricating an 
entirely new detector signal processing electronics 
package as described herein. 
Another problem was encountered in the optical 
2o design of prior-art pyrometers when the spatial resolu- 
tion was restricted by a simple optical stop at the input 
end of the optical fiber cable used in prior art to define 
the spatial resolution. The optical stop restricted the 
number of optical fibers transmitting radiation to the 
25 detector, thus resulting in non-uniform detector illumi- 
nation and higher detector noise generation. To solve 
this problem, the prior-art optical design was modified 
as described herein to assure uniform detector illumina- 
5 
3o tion while maintaining the high spatial resolution. 
OBJECTS AND SUMMARY OF THE 
INVENTION 
An object of this invention is to increase the spatial 
A further object of the invention is to increase the 
sensitivity of prior-art pyrometers so that valid mea- 
surements are assured with a field of view substantially 
reduced for high spatial resolution. 
In accordance with the present invention, the optics 
of the, pyrometer are arranged to focus the image of a 
target onto an opaque sheet in front of the input end of 
a fiber-optic cable that transmits radiation from the 
target to dual photodiodes responsive to two color 
45 bands used for band ratioing. A two-mil hole is pro- 
vided in the sheet to pass a small portion of the target 
field of view to the input end of the cable spaced from 
the hole a distance sufficient for that portion to defocus 
and illuminate all of the input end of the cable. In that 
50 way, spatial resolution of the instrument is increased 
while still providing for uniform illumination of the dual 
detector photodiodes. 
The detector includes two separate amplifying chan- 
nels, one for each color band detected by the separate 
55 photodiodes. Each channel includes an electronic-chop- 
per stabilized current amplifier, an electronic-chopper 
stabilized voltage amplifier, and a scaling amplifier. The 
first two amplifiers are preferably in the sensing head of 
the pyrometer which houses all of the optics. The out- 
60 put signals from the amplifiers are transmitted by elec- 
trical cable to the scaling amplifiers which drive an 
analog divider housed in a display unit which computes 
the ratio of the short wavelength radiation detected to 
the long wavelength radiation detected. The output of 
35 resolution of prior-art pyrometers. 
40 
A fiber-optic mount holds the face of the input end of 65 the divider is coupled by an RC filter and voltage fol- 
lower to a display device, and optionally to a recorder. 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
the fiber-optic cable perpendicular to the optical axis of 
the reflected radiation. When the target is focused at the 
reticle of the field lens, it will also be focused on the 
4,8 1 5,84 1 
appended claims. The invention will best be understood 
from the following description when read in connection 
with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a prior-art two-color band ratioing 
FIG. 2 illustrates an improvement in the optics of the 
FIG. 3 is a circuit diagram of a detector for an optical 10 
pyrometer. 
prior-art pyrometer illustrated in FIG. 1. 
pyrometer improved in the manner illustrated in FIG. 2. 
DESCRIPTION OF PREFERRED 
EMBODIMENTS 
Referring to FIG. 2, a modification of the fiber-optic 15 
mount of the prior-art pyrometer of FIG. 1 is shown to 
consist of a thin brass sheet 10 having a 0.002 inch diam- 
eter aperture 12 spaced away from the end 14 of a 0.032 
inch diameter fiber-optic cable 16. That one end is pol- 
ished even with the sloped face 18 of the fiber-optic 20 
mount, which is normal to the optical axis of the radia- 
tion from the target reflected by the mirror (not shown 
in FIG. 2). Aperture sheet mounts 20 and 21 space the 
0.002 inch diameter aperture 0.1 inch above the end of 
the 0.032 in diameter fiber-optic cable. The image of the 25 
object focused on the field lens within the reticle, 
shown in FIG. 1, is focused on the plane of the aperture 
pinhole 12. That portion of the image passing through 
the aperture then falls, defocused, over much of the 
surface of the one end of the fiber-optic cable that is 30 
flush with the fiber-optic mount, thereby assuring more 
widespread illumination of the fiber-optic cable which 
in turn assures more complete illumination of photodi- 
odes D1 through D2 shown in FIG. 2. In that manner, 
measurements of various high temperature elements in a 35 
complex crystal growth furnace, for example, can be 
made with high spatial resolution while still avoiding 
"hot spots" on the photodiodes. 
Normally the diameter of the fiber-optic cable deter- 
mines the sensing field-of-view when the image of an 40 
element is focused on the plane of the fiber-optic mount 
containing the flush end of the cable. By focusing the 
image of the element on the pinhole sheet 10, the field of 
view is limited to the diameter of the aperture 12, which 
has been laser drilled to a diameter chosen to be 0.002", 
and by spacing that plate away from the end of the 
fiber-optic cable, more widespread illumination of all 
fibers in the bundle is maintained while focusing on a 
smaller spot on the target for higher resolution. 
It should be noted that the diameter of the aperture 12 
is to be chosen for the resolution desired in a particular 
application, and that the spacing of the aperture plate 
from the end of the fiber-optic cable is a direct linear 
function of the diameter of the aperture. Consequently, 
it is to be understood that the dimensions 0.002" and 
0.1'' for a 0.032" diameter cable are given by way of 
example, and not limitation. The aperture may be laser 
drilled to virtually any size pinhole required. 
In the example given, resolution is improved over the 
same prior-art pyrometer by restricting radiation mea- 
sured from a portion of the target with a diameter that 
is about 1/800 of a distance to the target, as compared to 
a portion of the target with a diameter that is about 
1/100 of that distance using the prior-art pyrometer 
optics. As a consequence, the radiation density for the 
same diameter of fiber-optic cable is decreased propor- 
tionally. That requires improved sensitivity in the de- 
tector circuit, which will now be described. 
45 
50 
55 
60 
65 
4 
Referring to FIG. 3, the photodiodes Di and D2 
positioned proximate the exit end of the fiber-optic 
cable are designed for short and long wavelength sensi- 
tivity. In practice, the two photodiodes employed in the 
prior-art pyrometer are fabricated in a semiconductor 
(silicon) with three layers doped P, N and P, in that 
sequence. The top (P) layer absorbs the short wave- 
length light. The longer wavelength passes through the 
center (w layer connected to circuit ground and is 
absorbed by the bottom (P) layer. The two (P) layers 
function as anodes and the center (N) layer functions as 
a cathode. Consequently, the photodiodes are shown 
schematically in FIG. 3 as two back to back photodi- 
odes D1 and D2. 
Separate but identical channels are provided for am- 
plifying the currents generated by the photodiodes. 
Referring to the short-wavelength channel, a high gain, 
high stability electronic chopper-stabilized current am- 
plifier 30 converts the current signal into a voltage 
signal, and a voltage amplifier 32 amplifies the voltage 
signal sufficiently to enable it to be transmitted over a 
shielded connecting electrical cable to a ratioing display 
unit. The amplifiers 31 and 33 in the long wavelength 
channel function in the same way. 
Scaling amplifiers, 34 and 35 scale the voltage signal 
in each channel to a range that is appropriate for the 
operation of a ratioing circuit 36, which divides the 
short wavelength signal by the long wavelength signal. 
Such a ratioing circuit may be an analog integrated 
circuit (IC) divider 4214BP working with voltage sig- 
nals from 308AN scaling amplifiers, which receive volt- 
age signals from 760IJD chopper stabilized amplifiers. 
The current amplifiers may also be 760IJD chopper 
stabilized amplifiers. However, it should be recognized 
that the ratioing signal may be obtained by any linear 
divider and could even be implemented using digital 
techniques, such as with a programmed microprocessor 
or a specially designed digital IC chip. 
The output of the analog ratioing circuit 36 is filtered 
by an RC circuit having a time constant sufficient to 
stabilize the output for display using a digital voltmeter 
38. A voltage follower 40, comprised of a lOlAH inte- 
grated circuit, couples the output of the ratioing circuit 
to the digital voltmeter and to an output terminal for use 
with an external display or recording device. 
What is claimed is: 
1. In the sensing head of a two-color band ratioing 
pyrometer using a fiber optic cable to couple radiation 
to dual detector photodiodes, where the radiation from 
a target is split by a beam splitter into two beams, one 
beam to a field lens with a reticle and an eyepiece for 
aiming the beam onto a target and an objective lens for 
focusing the image of the target at said field lens, and 
one beam reflected by said beam splitter to said fiber 
optic cable coupling radiation onto said dual detector 
photodiodes adapted for detection of two different 
color bands, an improvement comprising an opaque 
sheet having a pinhole positioned in the focal plane of 
said one beam reflected by said beam splitter to said 
fiber optic cable, said opaque sheet being spaced in front 
of the input end of said fiber optic cable, and a position- 
ing of said opaque sheet with the axis of said pinhole 
aligned approximately with the axis of said fiber optic 
cable, said spacing of said opaque sheet in front of said 
cable being sufficient to pass a small portion of the 
radiation reflected to said opaque sheet and focused 
onto said pinhole, and to allow the focused beam pass- 
4,s 15,841 
5 6 
ing through said pinhole to defocus over a substantial 
part of the end of said fiber optic cable. 
2. A two-color band ratioing pyrometer as defined in 
4. A two-color band ratioing pyrometer as defined in 
claim 3 comprising a voltage follower for producing an 
output signal for display Or recording. 
5. In a radiation pyrometer having means for focusing 
lowed to defocus over a substantial part of the end of 5 a target onto the end of a fiber optic cable which cou- 
said fiber optic cable, the two ples radiation onto photo-detection means, apparatus 
separate amplifying channels, one for each color band for improving spatial resolution comprising means for focusing said target onto a focal plane, an opaque sheet detected by dual detection photodiodes, comprising for having an aperture placed in said focal plane, and a 
1 where the focused beam in said pinhole is al- 
each an electronic-chopper stabilized current 10 spacing between said one end of said fiber optic cable 
and said opaque sheet sufficient for the portion of said 
target focused onto said focal plane that passes through 
an electroNc-chopper stabilized 
plifier3 and a in cascade, and a sing1e 
divider connected to the outputs of the two channels for 
producing a band ratioed signal. 
claim 2 where the output of said divider is filtered by an 
RC circuit having a time constant sufficient to stabilize 
the output for display, and using a digital voltmeter for 
said aperture in said opaque sheet to defocus and illumi- 
nate substantially all of said one end of said fiber optic 
3. A two-color band ratioing Pyrometer as defined in 15 cable thereby providing a means for limiting the field of 
view of said pyrometer for increased spatial resolution 
without limiting the area of said photodetection means 
receiving radiation through said optical cable. 
display. * * * * *  
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40 
45 
50 
55 
60 
65 


High resolution color band pyrometer ratioing

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