U.S. patent number 3,742,124 [Application Number 05/171,838] was granted by the patent office on 1973-06-26 for color infrared detecting set.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Melvin J. Borel, Charles B. Weaver, Jesse C. Wilson.
United States Patent |
3,742,124 |
Wilson , et al. |
June 26, 1973 |
COLOR INFRARED DETECTING SET
Abstract
A color infrared detecting set is provided which utilizes the
variation of emissivity of objects to add the effect of color to
the sensing and presentation format. With color added to brightness
a large number of shade combinations are possible and a large
visible distinction between objects is realized. Color image
signals are obtained by infrared energy passing through filters of
different wavelengths (for a range of detection interest) to
corresponding detectors which convert the infrared energy to
electrical signals. The electrical signals are amplified, weighted
and summed in one channel to produce a brightness signal, and
processed in another channel to produce a plurality of
color-difference signals. The brightness signal drives a plurality
of light modulators, and each light modulator receives a color
perturbation signal derived from the color-difference signals to
obtain a color signal representative of one of the tristimulus
values. The color signals are combined to produce an image of
brightness equal to a black and white image, but having an apparent
hue.
Inventors: |
Wilson; Jesse C. (Richardson,
TX), Borel; Melvin J. (Carrollton, TX), Weaver; Charles
B. (Dallas, TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
22625340 |
Appl.
No.: |
05/171,838 |
Filed: |
August 16, 1971 |
Current U.S.
Class: |
348/33;
348/E9.028; 348/164 |
Current CPC
Class: |
H04N
9/43 (20130101) |
Current International
Class: |
H04N
9/00 (20060101); H04N 9/43 (20060101); H04n
009/02 () |
Field of
Search: |
;178/5.4R,5.2R,DIG.2
;250/83.3H |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Primary Examiner: Murray; Richard
Claims
What is claimed is:
1. A color infrared detecting set comprising:
a. an infrared receiver for receiving radiating energy from a
source thereof and producing a plurality of electrical signals
responsive to selected wavelengths;
b. a video signal processing and enhancement control means
responsive to the plurality of electrical signals of the infrared
receiver for selectively combining the plurality of electrical
signals to produce a plurality of enhanced color signals; and
c. display means responsive to the enhanced color signals for
producing a color image.
2. An apparatus according to claim 1 wherein the infrared receiver
comprises a plurality of infrared filters defining regions for
passing radiant energy of selected wavelengths, and a plurality of
infrared detectors responsive to radiant energy passing the
plurality of infrared filters for producing a corresponding
plurality of electrical signals.
3. An apparatus according to claim 2, wherein the said filters
define regions among the 8 to 14 micrometer wavelengths.
4. An apparatus according to claim 2, wherein said filters define
three regions having wavelengths of about 8-10 micrometers, 10-12
micrometers, and 12-14 micrometers.
5. An apparatus according to claim 1, wherein the infrared
detectors are low-energy-gap, high mobility materials of the group
consisting of lead tin telluride, lead sulfide, indium antimonide,
mercury cadmium telluride, and mercury doped germanium.
6. An apparatus according to claim 1, wherein said display means
comprises a system for producing an image in real time responsive
to the enhanced color signals of the signal processing and
enhancement control means.
7. An apparatus according to claim 6, wherein said display means
includes a color picture producing plate.
8. An apparatus according to claim 1, wherein said display means
comprises a system for producing a continuing color image.
9. An apparatus according to claim 8, wherein said display means
comprises a plurality of light source means responsive to the
enhanced color signals of the signal and enhancement control means
for producing primary colors, and light mixing means responsive to
the light from the plurality of light source means to combine the
light for establishing a match with a given color to produce a
color image.
10. An apparatus according to claim 9, wherein said plurality of
light source means comprises color light emitting diodes responsive
to the enhanced color signal output of the signal and enhancement
control means.
11. An apparatus according to claim 9, wherein said plurality of
light source means comprises a plurality of glow-tubes, and a
corresponding plurality of visible color filters, each filter
responsive to the light emitted by its corresponding glow-tube for
producing a primary color.
12. An apparatus according to claim 11, wherein said light mixing
means further comprises a light focusing means and a light
sensitive display means, said light focusing means responsive to
the combined light to focus the combined light on the light
sensitive recording means, and said display means responsive to the
focused mixed light for producing a color image.
13. A method of detecting infrared energy comprising the steps
of:
a. receiving infrared energy from a source thereof;
b. converting the infrared energy to a plurality of electrical
signals representative of selected wavelengths of the infrared
energy;
c. processing by selectively combining the plurality of electrical
signals to produce enhanced color signals from said plurality of
electrical signals representative of selected wavelengths of the
infrared energy, and
d. producing from the enhanced color signals a color image of the
source of infrared energy.
14. A method according to claim 13, wherein the step of receiving
infrared energy from a source thereof comprises scanning the source
of infrared energy with an optical system for collecting and
focusing radiant energy.
15. A method according to claim 13, wherein the step of converting
infrared energy to a plurality of electrical signals representative
of selected wavelengths of the infrared energy comprises passing
the radiant energy received from the source thereof through a
plurality of radiant energy filters to a plurality of radiant
energy detectors.
16. A method according to claim 15, wherein the radiant energy is
passed sequentially through the plurality of radiant energy
filters.
17. A method according to claim 13, wherein the step of processing
enhanced color signals comprises amplifying the electrical signals
of the detectors, summing the plurality of electrical signals for
each of said plurality of radiant energy detectors to produce a
signal indicative of the brightness of the source, selectively
combining the plurality of signals to produce a plurality of color
signals, and summing the signals to produce a plurality of enhanced
color of signals.
18. A method according to claim 13, wherein the step of producing a
color image of the source of infrared energy comprises converting
the enhanced color signals to a plurality of visible colors and
combining the colors to produce an image having a hue
representative of the source of radiant energy.
19. A method according to claim 18 wherein the image is recorded on
color film.
20. An apparatus according to claim 1, wherein said video signal
processing and enhancement control system comprises a first signal
processing channel operative to produce from the electrical signal
output of the infrared receiver a signal to control the total
brightness in the display means, and a second signal processing
channel operative to produce, from the electrical signal output of
the infrared receiver, color-difference signals, said first and
second signal processing channels operatively coupled to produce
the plurality of enhanced color signals for the display means.
21. An apparatus according to claim 20 wherein said video signal
processing and enhancement control means comprises a plurality of
preamplifiers having input terminals coupled to the detector
outputs of the infrared receiver to amplify and balance the output
signals of the plurality of detectors to produce signals and for
alleviating the introduction of a hue signal with any change in
temperature, and output terminals coupled to the first and second
signal processing channels.
22. An apparatus according to claim 21 further comprising a delay
means selectively coupled to preamplifiers of the plurality of
preamplifiers for producing time coincident signals as the output
signals of the plurality of preamplifiers.
23. An apparatus according to claim 20, wherein the first signal
processing channel includes a summing amplifier, said summing
amplifier responsive to the detector outputs of the infrared
receiver for producing a signal indicative of the brightness of the
source of energy.
24. An apparatus according to claim 20, wherein the second signal
processing channel includes a plurality of identical circuits
coupled to the detector outputs of the receiver, the first circuit
of the plurality of circuits including a first signal inverting and
signal noninverting means coupled to a first detector output of the
receiver for producing an inverted signal and noninverted signal
from the detector output; a first signal summing means coupled to
the signal noninverting output of the first signal inverting and
noninverting means and coupled to the signal inverting output
terminal of the first signal inverting and signal noninverting
means of a second one of said plurality of circuits for combining
the noninverted output signal of the first detector with the
inverted output of a second detector; a second signal inverting and
signal noninverting means coupled to the output of the first
summing means for producing inverted and noninverted signals from
the output of the first summing means; a second signal summing
means having an input terminal coupled to the output of the first
signal processing channel, an input terminal coupled to the
noninverted output of the second signal inverting and noninverting
means of the first circuit, and an input terminal coupled to the
inverted ouput of a second signal inverting and noninverting means
of a third circuit for selectively combining the total brightness
signal of the first channel with the noninverted signal of the
second signal inverting and noninverting means of the first
circuit, and the inverted signal of the second signal inverting and
noninverting means of the third circuit, to produce an enhanced
color difference signal; and a light source driver having one input
terminal coupled to the output of the second signal summing means
for receiving a color-difference signal, and a second input
terminal for connection to a pedestal signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a radiant energy detecting system and
more particularly to a color infrared detecting system.
2. Description of the Prior Art
In the past, infrared detecting sets have comprised a receiver,
recorder, and associated electronics sensitive only to total signal
magnitudes, hereinafter referred to as black and white infrared
detecting sets. A typical system employs as the receiver a time
scanning unit having a set of reflective or refractive optics, a
rotating scan mirror, and a detector assembly in which the rotating
mirror scans the object and the set of reflective or refractive
optics focuses the infrared energy from the scanned object onto the
detector assembly. If a rectangular mirror having four surfaces is
used each rotation of the mirror produces four scans. The detector
converts the infrared energy to an electrical signal which is
processed by the associated electronics to a video signal. The
video signal is used to modulate a glow tube which applies light to
a set of rotating microscopes. Each microscope corresponds to one
side of the scan mirror and is rotated in phase with it. Thus, as
the scan mirror sweeps the object, the microscopes focus the light
of the glow tube to a fine point for exposure of a roll of
photographic film. Black and white film only has been used and by
varying the amplification of the signal for the glow tube the
shades of gray can be controlled from white to black.
As infrared energy is not visible to the eye, no color has been
associated with it and although three major areas for improvement
exist the development of scanners, the development of target and
background signatures at infrared wavelengths, and the development
of signal processing and recording - the scanning technology has
developed far ahead of the other two areas. Highly sophisticated
scanners exist today which provide systems, such as mapping
systems, with resolution and velocity-to-height ratios suitable for
almost any requirement. Despite various new scanning and recording
systems, the video signal information has remained largely
unchanged, that is, until the present invention.
In black and white infrared detecting sets used as mapping devices,
several problems exist. One such problem is referred to as the
"cross over" effect. The term "cross over" effect describes the
situation occurring when, for example, during exposure to the sun
the earth is warmer than a body of water, and during nonexposure
the temperature of the earth becomes equal to and then "crosses
over" to become cooler than the body of water. Another problem is
the effect of reflected ground radiance from clouds above the black
and white infrared mapping device. This cloud reflected energy
tends to produce a blackbody or no contrast effect.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved
infrared detecting set.
Another object is to provide an infrared mapping apparatus which
utilizes the unique emissivity variations of target objects.
A further object of the invention is to provide an apparatus which
adds color intelligence to the brightness information of the
typical black and white system.
Still another object of the invention is to provide an infrared
mapping apparatus which utilizes the variation of emissivity of
objects to add the effect of color to the sensing and presentation
format.
Still a further object of the present invention is to provide an
infrared detecting set for identifying an object from its spectral
signature.
Yet another object of the invention is to eliminate cross over
effects normally associated with infrared collection and
analysis.
Yet a further object of the invention is to provide an infrared
detecting set which combines spectral emissivity information with
the normal thermal information for more detailed analyses of
imagery.
Still yet another object of the invention is to provide an infrared
detecting set which alleviates the effect of reflected radiance
such as ground radiance reflected by clouds.
Briefly stated the invention is to impose color information on
virtually the same black and white information of prior infrared
detector systems. According to the following equation infrared
energy is dependent on two properties; namely, the temperature and
emissivity of the object being analyzed.
I = .sigma. ET.sup.4
where
I = infrared power emitted from an object
.sigma. = Stefan-Boltzmann constant
E = emissivity
T = temperature of object in degrees Kelvin
Thus, the present invention utilizes the variation of emissivity of
objects to add the effect of color to the sensing and presentation
format. With color added to brightness a large number of shade
combinations are possible, and a large visible distinction between
objects is therefore realizable. Brightness is defined as the
characteristic of light that gives a visual sensation of more or
less light, and emissivity is defined as the ratio of the radiation
emitted by a surface to the radiation emitted by a perfect
blackbody radiator at the same temperature.
In its simplest form, the color infrared detecting set constituting
the subject matter of this invention includes a color infrared
electronic processing scheme which utilizes a receiver operating in
three different and distinct spectral regions representative of the
red, blue and green color spectrum used to produce information on
color film.
Each region is defined by a detector and a filter which allows only
energy of the power wavelength to pass. The detected signal due to
the energy passing each filter is processed through a video
processing and enhancement control system to provide a signal
indicative of the intensity or brightness of the source,
hereinafter referred to as either an intensity or brightness
signal, which drives light modulators that make up one channel and
color-difference signals which are introduced as a perturbation on
the intensity signal at each of the light modulators and therefore
are referred to hereinafter as color perturbation signals. The
intensity signal is adjusted by varying the amplification (gain) to
provide the full range of grays. The color difference signals are
adjusted to zero while scanning two blackbody targets of different
temperatures so that the color information is primarily dependent
on the emissivity of an object and not its temperature. Thus,
functionally, the video signal of the color system is a composite
signal from two channels -- one channel carrying the intensity
signal, the other carrying three color-difference signals used to
yield an enhanced color signal having different tristimulus values.
The combined three color signals do not effect the total brightness
of the combined light sources but do effect their apparent hue.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the color infrared detecting set
constituting the subject matter of the invention;
FIG. 2 is a sectional view showing the arrangement of the receiver
optics for the color infrared detecting set invention;
FIG. 3 is a graph of peak wavelengths as a function of
temperature;
FIG. 4 is a graph defining the detectors filter bandpass
regions;
FIG. 5 is a block diagram of associated electronics for the
receiver and recorder of the color infrared detecting set
invention;
FIG. 6 is a chromaticity diagram;
FIG. 7 is a graph of the average tristimulus values for the eye of
an observer;
FIG. 8 is a graph of the variance of detection with wavelengths of
three elements in a detector array;
FIG. 9 is a tridimensional plot; and
FIG. 10 is an exploded view showing in perspective the recorder for
the color infrared detector system invention.
A detailed description of a preferred embodiment of this invention
follows with reference being made to the drawings wherein like
parts have been given like reference numerals for clarity and
understanding of the elements and features of the invention.
Referring to FIG. 1, the Color Infrared Detecting Set 10
construction of the present invention comprises a receiver system
12, a video signal processing and enhancement control system 14,
and a display or recorder system 16.
The receiver system 12 may be constructed in many ways, however, a
suitable receiver includes a scanner 20 (FIG. 2) having a housing
22 housing a rotatable optical scanning mirror 24 adjacent a
housing aperture 26 which admits infrared energy of an object being
scanned. The mirror 24 which is made of a suitable material such as
quartz, Pyrex, or metal, is rotatably mounted upon a scan mirror
shaft 28 (FIG. 10) connected to a motor not shown for rotation
about its axis of symmetry. The mirror 24 (FIG. 2) may have two or
more reflecting surfaces, but as shown has four surfaces which
produce four scan lines of an object with each rotation of the
mirror shaft. In this configuration the optical scanning mirror 24
provides two optical mirrored faces rotating through a scan of
180.degree., thereby enabling the scanner 20 to collect radiant
energy impinging thereon throughout an effective scan of
120.degree.. Assuming a clockwise rotation of the four sided mirror
24, the radiant energy striking a first face or surface of the
mirror is reflected towards a flat surface reflecting mirror 30 and
another portion of the radiant energy impinging on a second face of
the mirror 24 is reflected onto a flat surface reflecting mirror
32. Hence, there is obtained a split image from the effective spot
area from which radiant energy waves are to be collected. As the
four-sided mirror rotates clockwise about its axis of symmetry, the
amount of radiant energy reflected by the first face or surface of
mirror 24 towards mirror 30 and impinging thereon varies from a
maximum to a minimum; and, radiant energy impinging on the second
face of the scanning mirror 24 is reflected toward mirror 32 and
the radiant energy impinging on mirror 32 goes from a minimum to a
maximum. Consequently, a constant optical aperture is maintained
within the effective scan of 120.degree..
The radiant energy impinging on flat surface reflecting mirror 30
is directed to mirror 34 and the radiant energy impinging on mirror
32 is directed towards mirror 36. Mirror 34 and mirror 36 direct
respective portions of the split bundle of radiant energy collected
by rotating scanning mirror 24 towards paraboloidal mirror 38 which
collects and focuses the bundle of radiant energy and directs it to
a convex mirror 40 which directs the converging bundle of radiant
energy toward a mirror 42 having a surface positioned to direct the
bundle of radiant energy through a filter 44 having three
wavelength regions X, Y and Z to infrared detectors 46, 47, and 48.
The image produced is a moving picture of the object, which would
be the terrain in a mapping system, passing through the three
regions X, Y and Z of the filter 44. The infrared detectors may be
constructed of any suitable material such as lead tin telluride,
indium antimonide, lead sulfide, mercury-doped germanium, or
mercury cadmium telluride having a proper sensitivity.
Referring to FIG. 3 the range of temperatures of objects and the
types of materials to be detected determine the peak wavelengths of
the three regions X, Y and Z of the filter 44. For example, for an
infrared color terrain mapping system, which will be used in
describing this invention, most objects of any significances are
within the +1000.degree. Farenheit to -60.degree. Farenheit which
have peak wavelengths of from 3.0 to 14.0 micrometers (.mu.m). For
ambient temperatures, i.e., 110.degree. Farenheit to -60.degree.
Farenheit the peak wavelengths are from 8.0 to 14.0 .mu.m. Further
it has been found that atmospheric attenuation due to moisture
covers the 6.0 to 8.0 .mu.m region. Hence, for a color terrain
mapping system an 8.0 to 14.0 .mu.m spectral band is preferred. The
spectral band is divided into a plurality of regions of any
suitable band width, which may be, for example, with each Region X,
- 8.0 to 10.0 .mu.m, Region Y, - 10.0 to 12.0 .mu.m, and Region Z,
- 12.0 to 14.0 .mu.m (FIG. 4). The filter 44 may be, for example, a
typical germanium substrate with a multilayer bandpass coating of
magnesium oxide.
Before proceeding with the description of the video signal
processing and enhancement control system 14 the function of the
detectors as to the emissivity and detectivity of the set is
discussed. Infrared system sensitivity is generally expressed as
either the necessary power received at the system aperture and
detected by the infrared detector to provide a unity
signal-to-noise ratio or the temperature difference between two
bodies necessary to provide a unity system signal-to-noise ratio.
The development of a sensitivity equation depends usually only on
the detector-preamplifier assembly, except when detectors are
summed to yield a higher detectivity.
In the infrared color system of this invention, the sensitivity is
modified as the spectral signals are processed, and must be
considered if any meaningful sensitivity equation is to be derived.
The complete electronics package of the signal processor is treated
as a blackbox, and suitable coefficients that modify the
detectivity of the individual detector are included to take account
of the added noise introduced by the signal processor.
The responsitivity is defined as the signal level per watt of
incident radiant power of the detector; the responsivity depends
upon spectral distribution of this power as well as upon the
detector.
R.sub.v = (rms output voltage)/(rms power incident upon the
detector) (I)
where R.sub.v is measured in rms volts/rms watts and is
semiconstant for a detector and its preamplifier.
In a color system, two detection assemblies are used basically, and
their difference signal or system-response signal may be looked
upon as the output signal resulting from the difference in the
radiation flux of two bandpasses of the detection assemblies.
Noise levels for operating detectors determine the detector
threshold signal levels. The noise equivalent power (NEP) of a
detector system for a source is the amount of spectral radiant
power difference from the source which will produce a signal level
equal to the rms-noise level. Therefore,
NEP = (rms internal noise level)/(R.sub.v) (II)
the internal noise level is made up of the noise levels of the
individual detectors which are basically background-noise limited.
For the two-unit detector assembly used as an example, the noise
contributes a factor of .sqroot.2 of the noise level of one
detector because of the lack of correlation in the noise
signals.
The detectivity is defined as
D = R.sub.v /(rms noise level) (III)
and for this system the detectivity would have the form
D.sub.sys = (R.sub.v /(rms noise level of one detector unit
.sqroot.2) IV
or
D.sub.sys = D/.sqroot.2 V
where D is the detectivity of the detector.
In this light, the detectivity of the complete color system is
related to the detectivity of one detector unit.
In this color system, the final mathematical function for the color
signal has the form
2A - B - C VI
if A were to increase by .DELTA.A above B and C, then there would
be an increase in signal level by 2.DELTA.A.
The noise level of this system would increase above the noise level
of a single-channel system as follows. The A signal is summed with
itself; thus, there is a direct correlation in the noise signals,
and the noise level is doubled. A, B, and C are independent noise
sources. If .mu..sub.i is the standard deviation (rms noise value)
of each distribution, then combining the noise source results
in
.mu..sub.t.sup.2 = .PSI..sup.2 + .mu..sub.B.sup.2 +
.mu..sub.C.sup.2 VII
where .mu..sub.t.sup.2 is the variance (power noise level) of the
noise resulting from the addition of several noise sources. The
term .PSI. is the variance of the noise resulting from the addition
of A to itself, thus
.PSI..sup.2 = 4 .mu..sub.A.sup.2 = (2 .mu..sub.A).sup.2 VII
since .mu..sub.A, .mu..sub.B, and .mu..sub.C are the same order of
magnitude in this type of system
.mu..sub.A .congruent. .mu..sub.B .congruent. .mu..sub.C (IX)
then
.mu..sub.t.sup.2 = 6 .mu..sub.avg.sup.2 (X)
where .mu..sub.avg.sup.2 is the average of the variance of the
detectors.
Thus, the signal increases by a factor of 2 and the noise increases
by a factor of .sqroot.6. This results in a detectivity
corresponding to a change of the detectivity of a one-detector
system 2/.sqroot.6 = 0.816.
The Stefan-Boltzmann relation for a graybody is ##SPC1##
where
W = C.sub.1 /(.lambda..sup.5 [exp(C.sub.2 / T) - 1])
where
W = total radiant emittance
C.sub.1 = Planck's first constant, 3.74 .times. 10.sup.-.sup.12
C.sub.2 = Planck's second constant, 1.44 cm-degree
.lambda. = wavelength in .mu.m
T = temperature in K
.epsilon. = emissivity
The response of the detectors is adjusted, as hereinafter
described, so that for a given change in temperature the detector
preamplifier assembly yields the same response in one bandpass as
another, and the power difference between two bandpasses created by
a change in emissivity between them is considered as a change in
power in one bandpass if the detectivity is altered. Or,
##SPC2##
considering ##SPC3##
then ##SPC4##
is the incremental power input representative of a change in
emissivity between the two optical bandpasses. The term .DELTA.W is
in watts/cm.sup.2. The power of incident radiation per solid angle
is given by ##SPC5## where .DELTA. .epsilon. = .epsilon..sub.1 -
.epsilon..sub.2 and .DELTA. W/.pi. is in watts/cm.sup.2 steradian
##SPC6##
Then the noise equivalent power is ##SPC7##
where
.OMEGA. = A.sub.1 /R.sup.2
where
R = the range to target
A = the area of the aperture
A.sub.T /R.sup.2 = A.sub.d /f.sup.2 ##SPC8##
where A.sub.T = (R.sup.. .DELTA. .THETA.).sup.2 ##SPC9##
Detectivity is defined as
D = 1/NEP (XX)
a special kind of detectivity (D*) has unit noise bandwidth and
detector area:
D* = (A.sub.d .DELTA. f).sup.1/2 D (XXI)
d* = [(a.sub.d .DELTA.f).sup.1/2 ]/NEP (XXII)
where D* is in cm-(Hz).sup.1/2 per watt
A.sub.d = detector cell area
.DELTA. f = noise equivalent bandwidth
Note: d* is a function of field of view and preamplifier used.
NEP = (A.sub.d .DELTA. f).sup.1/2 /0.82 D* (XXIII)
where the coefficient 0.82 is the degradation factor for the system
signal processing explained earlier. Therefore, if
.DELTA..epsilon.= NEE (noise equivalent emissivity) ##SPC10##
The noise equivalent emissivity is then ##SPC11##
A.sub.d = (.DELTA..THETA.f).sup.2
where
f = focal length
.DELTA..THETA. = resolution ##SPC12##
accounting for optical transmission, .UPSILON.o and system
response, .UPSILON.a ##SPC13##
Proceeding now with the description of the video signal processing
and enhancement control system 14, the detectors 46, 47, and 48
produce three electrical signals each of which is an input to a
preamplifier 50, 52, 54 (FIG. 5). The amplifiers 50, 52, and 54 may
be any standard amplifier having a gain (.alpha.) of 1000. The
preamplifiers provide the main signal gain for the system, while
maintaining a high impedance load for the load detectors and a low
output impedance to drive the cable to the associated
electronics.
As the rate of change of the photon flux in each of the spectral
bands is different, the gain of each succeeding preamplifier must
ba varied to balance the signals of the preamplifiers for use in
the color system. Failure to balance the signals introduces a hue
signal with any change in the intensity signal caused by
temperature in an unbalanced system. The largest change occurs in
the 8- to 10- .mu.m range.
The emitted photon flux between a temperature range of 280.degree.K
(7.degree. C) to 320.degree. K (47.degree. C) is tabulated in Table
I for the 8--10 .mu.m, 10--12 .mu.m, and the 12--14 .mu.m spectral
bands. Since the change in the photon flux in the 12- to 14- .mu.m
bandpass is the smallest, it will of necessity require the largest
gain if the apparent electronic signals are to remain equal. Since
the responsivity of the detectors is a linear function, the
required gains can be calculated from the rate of change of the
photon flux. Choosing 300.degree. K (27.degree. C) on an arbitrary
means, the rate of change of the photon flux at this temperature
is
(.DELTA.P/.DELTA.T) = 5.16 .times. 10.sup.15 (XXVIII)
8--10 .mu.m
(.DELTA.P/.DELTA.T) = 5.078 .times. 10.sup.15 (XXIX)
10--12 .mu.m
P/T = 4.39 .times. 10.sup.15 (XXX)
12-14 .mu.m
Using the signal from the 8--10 .mu.m spectral band X as a
reference, the gain in the remaining two spectral bands Y and Z
must be increased by a factor equal to the ratio of their rate of
change of the photon flux and the rate of change in the reference
band. Then,
[(P/T) 8-10 .mu.m]/(P/T) 10-12 .mu.m] = 1.0162 (XXXI)
[(p/t) 8-10 .mu.m]/(P/T) 12-14 .mu.m] =1.175 (XXXII)
are the respective preamplifier gains required over the unity gain
of the 8-10 .mu.m band X for the 10-12 and the 12-14 .mu.m spectral
bands Y and Z. The change in the photon flux is representative of
the ac signal in the system. The change in the photon flux times
the gain factor for each spectral band yields a value
representative of the balanced signal in the system. Values for
these weighted changes are listed under .DELTA. in Table I for
their respective optical passbands. ##SPC14##
The problem is complicated because the changes of the photon flux
with respect to temperature in the bands are not linear, but the
change in photon flux of the incident radiation can be considered
linear over a small change in temperature. As a result of
experimenting with the operational gains on systems and their
respective signal levels for temperature differences obtained in
the lab, it has been determined that for an average mapping scene,
50 percent of the targets lie within .+-. 4 degrees K. Assuming a
normal distribution for the video signal, the standard deviation
would be equal to 5.39. If the distribution was normal, 1.4 percent
of the targets would lie outside of .+-. 12 degrees K. The systems
are usually set up so that the video saturates at .+-. 12 degrees K
which yields a fairly high contrast image. At .+-. 12 degrees K,
the assumption of linearity introduces an error of
approximately
100 .times. (9.44 .times. 10.sup.15 photons/sec - 9.02 .times.
10.sup.15 photons/sec/9.02 .times. 10.sup.15 photons/sec) = 2.5
percent XXXIII ##SPC15##
the dashed curve in FIG. 6 is the locus of points that represents
the blackbody temperature perturbations on the system tristimulus
values created when the linear assumption is invalid. Targets much
hotter than the ambient drift into the x region of the system unit
plane and those much cooler drift into the y. Since the system is
set up so that the image is virtually saturated at .+-.12K.degree.
(that is, either completely black or completely white), this effect
is then virtually nonexistent. However, this does manifest itself
when the intensity signal A(x + y + z), hereinafter described, is
reduced by lowering its gain A by a significant amount so that the
color portion of the signal is predominant in the recorded
image.
It will be apparent to those skilled in the art that in the system
disclosed, as the image of the object (terrain) passes through the
three regions X, Y and Z of the filter 44 (FIG. 4) in sequence
there is a time delay in the generation of the three signals for
the same object; i.e., a signal for an object will appear for
region Z before it appears for Region Y, and for regions Z and Y
before it appears at region X. To properly sum the signals and
thereby reconstruct the image a delaying device such as, for
example, a delay line 56 (FIG. 5) is connected to the output of
preamplifier 52 to delay the signal of region Y until the
corresponding signal appears at region X, and a delay line 58 is
connected to the output of region Z to delay the signal of region Z
until the corresponding signal appears at region X. It will be
readily apparent that other optical arrangements can be used to
generate the three time coincident signals; such arrangements can
eliminate the necessity for the delaying means.
The function of the three signals will now be discussed
mathematically. In a black and white system all that is required
for an information match is that two patches of information appear
equally intense or bright. The information match between two
patches of spectral radiance, N.sub.1 (.lambda., T.sub.1) and
N.sub.2 (.lambda., T.sub.2) is ##SPC16##
where S(.lambda.) is the relative spectral sensitivity of the
detector-filter combination.
The spectral radiance is characterized by the equation
N.sub.1 (.lambda., T.sub.1) = [.epsilon.(.lambda.)2.pi.c.sup.2
h]/(.lambda..sup.5 [exp(ch/k T )-1]) (XXXV)
where
.epsilon.(.lambda.) = the spectral emissivity
c = velocity of light
h = Planck's constant
k = Boltzmann's constant
T = temperature
.lambda. = wavelength
The condition met by Equation XXXIV is that of a color-blind human
and characterizes essentially a system with one degree of
freedom.
Most human observers can distinguish two objects even though the
radiant energy flowing from them toward the observer is so adjusted
as to make them appear equally bright; that is, the objects may
also differ in the red-green sense and in the yellow-blue sense.
Thus, for an observer of normal color vision, the two objects must
simultaneously satisfy three conditions if one is to be
indistinguishable from the other, ##SPC17##
where, for that observer, x, y, and z are tristimulus values of a
spectrum of unit radiance per unit wavelength. The tristimulus
value is equivalent to the relative spectral sensitivity of the
detector-filter combination. The average values for an observer may
be seen in FIG. 7, and the relative spectral sensitivity of three
detector-filter combinations may be seen in FIG. 8.
Given this formulation for the condition necessary for a color
match between objects for normal color vision, it has been found
that a similar condition holds for spectral variation in the
infrared. Thus, the system by incorporating this requirement for a
match between objects adds two more degrees of freedom to the
information obtained from a target.
Choosing the spectral response of a given sensor, it is possible to
computationally determine the variation in its response as a
function of the variation of the parameters of a source. The
responses x, y, and z for a three-sensor system are the tristimulus
values of the source. ##SPC18##
The tristimulus values may be considered to form the tristimulus
vector
A = Xe.sub.x + Y e.sub.y + Z e.sub.z XXXVII
by defining the coordinate values in an orthogonal coordinate
system. Therefore, given a source, a unique tristimulus vector A is
obtained which defines the parameters of its spectral
emittance.
If the coordinates x, y, and z of an arbitrary infrared source are
determined (perhaps indirectly through mathematical conversion),
the coordinates in a system based on sources other than in the
infrared (for example, in the visible range) can be found. The
method in mathematics is called affine transformation.
Color photography is based on a trichromatic principle. Thus, it
lends itself to the problem of converting spectral information in
the infrared into the storage or memory medium from which the
infrared information sought may be quickly obtained.
In color film the tristimulus value for one pigment is
##SPC19##
where
N(.lambda.,.OMEGA..sub.i) = the unit spectral radiance of the light
source
S.sub.1,i (.lambda.) = the spectral sensitivity of the filter
S.sub.2,i (.lambda.) = the spectral sensitivity of the pigment
.OMEGA..sub.i = a variable which depends on the signal driving the
light source
Thus, for a given system, the integral in Equation XXXVIII may be
considered a constant, and the tristimulus value for the pigment is
then
x.sub.i = C.sub.i .OMEGA..sub.i XXXIX
the term .OMEGA..sub.i in a system represents the mathematical
transform of the electronics. Thus, for a given target and a given
.OMEGA..sub.i, the resulting tristimulus value of the pigments of
the color film can be determined.
In this preferred embodiment, the electronic transformation is
built-up as follows. An intensity signal is formed by the addition
of the ac signals from the three detectors 46, 47, and 48, and a
gain (.alpha.) is added to this signal such that the full range of
grays on the film is utilized. If x, y, and z are the tristimulus
values from the respective preamplifiers, the intensity signal fed
to each light source (glow-tube), hereinafter described, is
represented by A(x + y + z).
The color signal is introduced as a perturbation on the intensity
signal at each glow tube driver. The net effect of the three
respective perturbations, however, is that there is no change in
the apparent brightness of the combined light sources; that is, the
total intensity of the combined glow tubes of the recorder 16 does
not change with respect to a change in hue only. These three
perturbations may be represented by
B(2 x - y - z) XLa
B(2 y - x - z) XLb
B(2 z - x - z) XLc
where B is the weighting or gain factor of the color signals.
Summing these three equations (which is what is done when looking
at the intensity signal) gives a value of zero, and as stated
earlier there is no apparent change in brightness. However, if
different values are used for the signals of the three light source
or glow tube drivers, each equation will have a unique value, and
the combination of the three colors represented by these equations
will represent unique hues. The combined intensity and color
perturbation signals can be represented by
A(x + y + z) + B (2 x - y - z) XLIa
A(x + y + z) + B (2 y - x - z) XLIb
A(x + Y + z) + B (2 z - x - y) XLIc
Each represents the ac video signal of a color channel which drives
a light source of the recorder 16. Each signal can demonstrate both
positive and negative values, and in order to realize the full
value of the signal on color film, it must be introduced into its
video driver, hereinafter described, of the recorder 16 with a dc
signal that sets the pedestal or background density and hue. The
complete relation for the recorder light source driver is:
C.sub.R + A(x + y + z) + B(2 x - y - z) XLIIa
C.sub.B + A(x + y + z) + 2 2 y - x - z) XLIIb
C.sub.G + A(x + y + z) + B(2 z - x - y) XLIIc
where C.sub.R, C.sub.B and C.sub.G are dc-level signals. Light
sources vary in response to a voltage signal for a number of
reasons, such as age and construction. Thus the values of C.sub.R,
C.sub.B, and C.sub.G will not be equal in most real instances. The
dc-level signals are characteristic of the response of the light
emitter, and the ac component must be varied proportionally such
that the actual emitted signal between the color channels is
balanced. The ac signal must be weighted then by the
proportionality factor C.sub.i .GAMMA., or
C.sub.R + C.sub.R .GAMMA. [ A(x + y + z) + B(2x - y - z)]
XLIIIa
C.sub.B + C.sub.B .GAMMA. [ A(x + y + z) + B(2y - x - z)]
XLIIIb
C.sub.G + C.sub.G .GAMMA. [ A(x + y + z) + B(2z - x - y)]
XLIIIc
For emitters that are fairly uniform,
.GAMMA. .congruent. 1/C.sub.i
The last three expressions are actually the transform Equations
.OMEGA..sub.i.
The two different weighting factors A and B are required by the
unique spectral variation in the 8- to 14 .mu.m optical band.
Unlike the large spectral variation of reflectance found in the
0.3- to 0.8- .mu.m and the 1.0- to 6.0- .mu.m optical bands (where
it is not uncommon for the value of the spectral reflectance to
vary from nearly zero to one through the band); the spectral
variation of the value of the emissivity varies in the 8- to 14-
.mu.m band on the order of only 0.02 to 0.08 from the mean value of
the emissivity throughout the band for most materials. In the 0.3-
to 0.8- .mu.m and the 1.0- to 6.0- .mu.m optical bands (because of
the large variation of the spectral reflectance) the value of the
ratio B/A would be nearly unity if this transformation were used.
But in the 8.0- to 14- .mu.m band, the ratio has a value on the
order of 10:1 since the variation of the tristimulus values of the
detected signals does not vary as greatly as does the radiation
intensity signal by about this magnitude.
If a value of unity were used for the ratio B/A in the 8- to 14-
.mu.m band and the gains on the signals were set so that the full
dynamic range of the film is used to record the radiation intensity
signal, the full range of color on the film would not be used. This
can be corrected electronically by changing the ratio B/A so that
the intensity and hue of signal is unchanged, but the level of
saturation is increased to achieve the widest variance of
color.
This correction, or what would be more properly called the addition
of greater color sensitivity, can be more easily seen from the
following analysis. Color data can be graphically represented by a
tridimensional plot (FIG. 9). Each set of tristimulus values (x, y,
z) determines a vector, V, in three-dimensional space starting from
the origin of an orthogonal coordinate system.
The magnitude of the vector V is the value of the total combined
signal. The surface which defines the set of vectors that have the
same magnitude as V in this coordinate system represents a total
radiance level.
Thus, any other vector with a terminus at this surface will have
the same magnitude as V and represents the same value for the total
radiance. Hue and saturation are defined by the unit plane
intersection of the vector V with the plane passing through unit
amounts on each coordinate axis. The direction of a vector that is
drawn from the point in which the tristimulus values are equal to
the intersection point of V, indicates the chromaticity (i.e., hue
of the color). The magnitude of the excursion from the point where
the tristimulus values are equal to the intersection point of the
radiation vector V in the unit plane indicates the saturation of
the color. If the radiation signal is such that the full range of
the saturation of the recording media is not used, as in this case,
the tristimulus values can be adjusted so that the magnitude of V
and the hue of the signal are unchanged, but the amount of
saturation is increased. Thus a greater sensitivity to the amount
of color would result in the recorder 16.
This is not a true representation of color in the infrared;
however, the use of color in this invention is just to sense and
display the spectral differences. The variations of the infrared
coordinates (although more than enough to detect) are not great
enough in a straight one-to-one transformation to generate the
variations of coordinates which the color film is capable of
recording. With the addition of the electronic modification of the
tristimulus values in the transformation, a condition is created
analogous to the shunting of an ammeter. The scale is changed so
that smaller variations may be seen more easily. In a color system,
deeper colors will result in the recording media, thus making
differences more discernible.
Normalizing the transform relations
C.sub.R + C.sub.R .GAMMA. [ A(x + y + z) + B(2x - y - z)] XLIVa
C.sub.B + C.sub.B .GAMMA. [ A(x + y + z) + B(2z - x - y)] XLIVb
C.sub.G + C.sub.G .GAMMA. [ A(x + y + z) + B(2z - x - y)] XLIVc
with respect to the intensity, all the spectral data may be
projected into a plane.
The normalized transform equations are for .GAMMA. = 1/C.sub.i,
C.sub.R = C.sub.G = C.sub.B,
[ C.sub.R + A(x + y + z) + B(2 x - y - z) ]/(3[ A(x + y + z) +
C.sub.R ] ) = 1/3 + [B(2x - y - z) ]/(3[ A(x + y + z) + C.sub.R ] )
XLVa
[ C.sub.G + A(x + y + z) + B(2 y - x - z) ]/(3[ A(x + y + z) +
C.sub.G ] ) = 1/3 + [ B(2 y - x - z) ]/(3[ A(x + y + z) + C.sub.G ]
) XLVb
[ C.sub.B + A(x + y + z) + B(2 z - x - y) ] /(3[ A(x + y + z) +
C.sub.B ] ) = 1/3 + [ B(2 z - x - y) ] /(3[ A(x + y + z) + C.sub.B
] ) XLVc
which give the plane coordinates.
It is evident from the chromaticity coordinate given in the set of
Equations XLVa, b, and c, that since B/A>1, it is possible for
one of the coordinates to exceed unity and the other two to become
negative. When this conditions occurs, the light elements of the
recorder system 16 that are represented by negative numbers will be
in an OFF condition and the intensity of the one having a value
that exceeds unity will be higher than the intensity signal of the
source would indicate. Since this condition is rare, little
information is lost in this distortion of the color scale because
the event itself will distinguish the material from its
surroundings.
Returning now to the construction of the electronic system to
perform the mathematics, the transform Equation XLIII in the video
electronics is implemented through summation and subtraction of the
signals from the three filtered detectors 46, 47 and 48. If x, y,
and z are assumed to be the electronic signals from the three
detector-preamplifier combinations, then the seven basic component
signals required by the system to build the transform equation
are
x - y XLVIa
y - z XLVIb
z - x XLVIc
-x + y XLVId
-y + z XLVIe
-z + x XLVIf
x + y + z XLVIg
The signal represented by Equation XLVIg is the intensity or
brightness signal and, with some amplification (A), drives three
light modulators or summing amplifiers 92, 94, and 96 that make up
one channel. The color perturbation signal is formed by summing two
of the remaining color-difference signal equations. The signal 2x -
y - z is obtained by summing Equations XLVIa and XLVIf. The signals
2y - x - z and 2z - x - y are formed by summing Equations XLVIb and
XLVd and XLVIc and XLVIe, respectively. After some amplification
(B) these perturbation signals are then summed with the intensity
signal at their respective light modulators 92, 94 and 96 and the
final transform equations are obtained:
A(x + y + z) + B(2x - y -z) XLVII
a(x + y + z) + B(2y - x - z) XLVIII
a(x + y + z) + B(2z - x - y) XLIX
returning to FIG. 5 the time coincident signals x, y, and z form
the input respectively, to three post amplifiers 74, 76, and 78.
The post amplifiers 74, 76, and 78 buffer the ac signal outputs of
the preamplifiers 50, 52, and 54 and provide both noninverted and
inverted signals x and -x, y and -y, and z and -z to facilitate
consequent summing processes. The post amplifiers and differential
amplifiers may be, for example, Motorola designated MC 1519
amplifiers.
The noninverted ac signals x, y and z are fed to a summing
amplifier 72 where they are summed unweighted and the output level
adjusted by amplification (A) to produce the A(x + y + z)
brightness signal of equation XLVIg. The value of A is set, for
example, by a variable potentiometer, not shown, which is varied by
an operator to control the shades of gray produced from white to
black. The summing amplifier 72 may be, for example, a Texas
Instruments designated SN 52741 amplifier.
The noninverted ac signals x, y and z are also fed, respectively,
to three summing amplifiers 80, 82 and 84 where they are summed
with the inverted outputs -x, -y, and -z of the post amplifiers 74,
76, and 78 as follows: the -x output is connected to the summing
amplifier 84, the -y output is connected to the summing amplifier
80, and the -z output is connected to summing amplifier 82. The
amplification or gain B of the summing amplifiers 80, 82, and 84 is
controlled simultaneously, for example, by a three decked
potentiometer (not shown); thus, the gain B is set by one control.
The output signals for summing amplifiers 80, 82 and 84 are
respectively the B(x - y), B(y - z) and B(z - x) of equations
XLVIa, b, and c. These ac output signals are coupled, respectively,
to another set of differential amplifiers 86, 88, and 90 which
produce noninverted and inverted ac signals, i.e., differential
amplifier 86 provides B(x - y) and B(y - x) signals, differential
amplifier 88 provides B(y - z) and B(z - y) signals, and
differential amplifier 90 provides B(z - x) and B(x - z) signals.
These ac signals are the color-difference signals and are dependent
primarily on the emissivity of the object (terrain in a mapping
system) and not the temperature of the object.
Light modulators or summing amplifiers 92, 94, and 96 are each
connected to the output of summing amplifier 72 to receive as one
input the intensity signal A(x + y + z). In addition, summing
amplifier 92 is connected to the output of summing amplifier 86 to
receive as a second input the B(x - y) signal, and to the output of
differential amplifier 90 to receive as a third input the B(x - z)
signal. The summing amplifier sums and amplifies these signals to
produce an output signal equivalent to C.GAMMA. [ A(x + y + z) +
B(2x - y -z)]. Similarly, summing amplifier 94 is connected to
differential amplifiers 86 and 88 to sum and amplify the B(y - x)
and B(y - z) signals with the A(x + y + z) signal to produce at its
output a signal equivalent to D.GAMMA. [ A(x + y + z) + B(2y - x -
z)]. And summing amplifier 96 is connected to differential
amplifiers 88 and 90 to sum and amplify the B(z - y) and B(z - x)
signals with the A(x + y + z) signal to produce at its output a
signal equivalent to E.GAMMA. [ A(x + y + z) + B(2z - x - y)]. The
amplification or gains C, D, and E are fixed precentages relative
to one another so as to give a gray shade image of an object or
image on color film when the emissivity is the same in all three
filter regions X, Y and Z.
The ac output signals of the light modulators or summing amplifiers
92, 94, and 96 are coupled as one input, respectively to glow tube
drivers 98, 100, and 102. Also coupled to the glow tube drivers 98,
100, 102, respectively, are dc signals which set the pedestal or
background density and hue. The glow tube drivers 98, 100 and 102
provide the input signals to the recorder system 16.
The display or recorder system 16 (FIG. 1) includes red, blue and
green light sources which may be, for example, light emitting
diodes or a red glow tube 104 connected to the output of driver 98,
(FIG. 5) a blue glow tube 106 connected to the output of driver
100, and a green glow tube 108 connected to the output of driver
102. Each of the glow tubes 104, Ektachrome film 106, and 108 has a
filter 110 (FIG. 10) for transmitting the desired red, blue, or
green wavelengths of the visible spectrums to a collimating lens
112 arranged about a rotating drum 114 which is attached to the
scan mirror rotating shaft 28. The drum 114 contains a stationary
fiber optics 116 for receiving the light from the glow tubes 104,
106, and 108, mixing it and transporting it to a set of four
microscopes 118 carried by the rotating drum for focusing the
colored light to a fine point on the color film 120. Although any
color film can be used, a suitable film is that sold under the
trademark Ektachrome by Eastman Kodak Co. Each microscope 118
corresponds to one side of the scan mirrors 24 and rotates in phase
with a face of the mirror 24. The film 120 is carried by film
holding spool 122 having a suitable brake 124 for preventing back
lash of the film and take up spool 126 driven by a servo motor 128
at a proper speed for recording the continuous scan of the scan
mirror 24 on the film 120. A film shaping platen, not shown, is
positioned above the path of the rotating microscopes 118 for
curving the film 120 in a circular arc corresponding to the
circular path of the rotating microscopes. In this position the
film shaping platen maintains the film equidistant from the
rotating microscopes. Although, a film type recorder is described,
it will be readily apparent to one skilled in the art that a real
time display may be employed without departing from the scope of
this invention.
To analyze the resulting exposed film it is necessary to determine
the tristimulus values and the chromaticity coordinates in the
Commission Internationale de l'Eclairage (CIE) system for various
materials. The values of the integrals in Equation XXXVa-c are
designated as tristimulus values x.sub.i. The amounts of N.sub.1,
N.sub.2, and N.sub.3 of any three emitters of radiant flux required
to produce by additive mixture any color, xyz, may be computed from
Equation XXXVa-c. Let a unit amount of the first emitter
combination (e.g., the red glow tube with its associated filter and
film response) have tristimulus values x.sub.r, y.sub.r, z.sub.r ;
the second (the green combination) x.sub.g, y.sub.g, z.sub.g ; and
the third (the blue combination) x.sub.b, y.sub.b, z.sub.b. It
follows from XXXVa-c that the tristimulus values x, y, of the color
produced by adding N.sub.1 units of the red emitter and N.sub.2
units of the green to N.sub.3 units of the blue are:
x = x.sub.r N.sub.1 + x.sub.g N.sub.2 + x.sub.b N.sub.3 La
y = y.sub.r N.sub.1 + y.sub.g N.sub.2 + y.sub.b N.sub.3 Lb
z = z.sub.r N.sub.1 + z.sub.g N.sub.2 + z.sub.b N.sub.3 Lc
The CIE color system provides a Standard Observer to complete the
description of the color of an object. The 1931 CIE Standard
Observer is a numerical description of the response to color of the
normal eye and is expressed by color-matching functions x, y, and
z. The recording film is high-speed Ektachrome. The chromaticity
coordinates for unit amounts of radiant flux or system-signal
levels in the recorder are obtained through Equation Equation LI.
This procedure is tabulated in TABLE 2. The dye in the CIE system
has the following coordinates:
Blue Green Red x + 0.1511 x = 0.3136 x = 0.618 y = 0.0425 y = 0.623
y = 0.351 z = 0.8063 z = 0.0632 z = 0.0308
and for any tristimulus value from the video processor N.sub.1,
N.sub.2, N.sub.3, the color coordinates are
x = 0.618 N.sub.1 + 0.1511 N.sub.2 + 0.3136 N.sub.3 LI
y = 0.315 N.sub.1 + 0.0425 N.sub.2 + 0.623 N.sub.3
z = 0.031 N.sub.1 + 0.8063 N.sub.2 + 0.0632N.sub.3
TABLE 2
FILM CIE COORDINATES
Wave Length N Nx Ny Nz Chromaticity .mu.m coord. for unit BLUE
signals 340 360 7 380 30 0.066 0.195 x = 0.1511 400 51 0.729 0.02
3.462 y = 0.0425 420 50 6.72 0.2 32.28 z = 0.8063 440 35 12.19
0.805 61.145 460 22 6.40 1,32 36.72 480 14 1.34 1.95 11.38 500 7
0.034 2.261 1.9 520 1.7 0.11 1.21 0.1329 Total 27.59 7.776
147.2
GREEN
380 1.7 0.0024 0.011 400 3.5 0.5 0.0014 0.2376 x = 0.3136 420 2.7
0.363 0.011 1.743 y = 0.623 440 1.8 0.627 0.041 3.145 z = 0.0632
460 2.0 0.582 0.12 3.338 480 3.5 0.335 0.487 2.845
GREEN
500 11.0 0.054 3.553 2.992 520 33 2.09 23.43 2.581 540 65 18.88
62.01 1.320 560 65 38.64 64.67 0.2535 580 32 29.32 27.84 0.0544 600
1 1.0622 0.631 0.0008 Total 92 182.8 18.52
A mathematical model of the system can be constructed by use of
Equation L and the transform Equation LI; this will yield a close
approximation to what should be expected as far as color content by
a known spectral emissivity. Remaining in units of photon flux
since the responsivity acts as a constant that would eventially
drop out, Equation L can be used to obtain the tristimulus values
for the video processing using the weighted differences in the
photon flux obtained from the product of the emissivity and the
total flux. Setting C = 6 .times. 10.sup.16 photons/sec cm.sup.2,
which is the photon flux equivalent to .+-.12.degree.K limits on
the saturation of the system, .GAMMA. = I/C and the ratio B/A set
at a value of 10, the tristimulus values and the chromaticity
coordinates in the CIE system for two materials are tabulated in
Table 3.
TABLE 3
THEORETICAL CHROMATICITY COORDINATES IN THE CIE SYSTEM FOR
DIFFERENT MATERIALS AT 300.degree.K
Wave Length Emissivity System Chromaticity Tristimulus Coordinates
(.lambda.) (.epsilon.) (N.sub.i) (x.sub.i) 8-10 .mu.m 0.988 0.610
0.526 Paint 10-12 .mu.m 0.921 .173 .109 Zinc oxide 12-14 .mu.m
0.940 0.217 0.365 base 8-10 .mu.m 0.911 0.163 0.288 Silt Loam 10-12
.mu.m 0.975 0.462 .401 Tennessee 12-14 .mu.m 0.967 0.526 0.311
Only two are shown but are representative of the data used.
The same process was used with a prototype three-color infrared
film imaging system to develop the chromaticity coordinates for
objects as they appeared on the film. This took into account the
spectral response of the filters used in the densitometer. What
appeared to be a painted roof top on a metal building and a field
with bare soil were used as representative data points. Their
chromativity coordinates are presented in Table 4.
TABLE 4
CHROMATICITY COORDINATES IN THE CIE COORDINATES SYSTEM FOR TARGETS
ON FILM
Wave System Film System Film Chromaticity Color Length transmission
Tristimulus Coordinates (.lambda.) (T.sub.i) Values (x.sub.i)
(x.sub.i) Red 8-10 .mu.m 0.087 0.1427 0.3054 Blue 10-12 .mu.m 0.275
0.4653 0.3880 Soil Green 12-14 .mu.m 0.229 0.3875 0.3066 Red 8-10
.mu.m 0.089 0.519 0.492 Painted Blue 10-12 .mu.m 0.042 0.238 0.199
Roof Green 12-14 .mu.m 0.041 0.243 0.309
These two sets of chromaticity coordinates of the two data points
(soil and paint), are representative of the data obtained from the
test flights and tabulated emissivity values published by the
University of Michigan. The difference between the theoretical and
experimental coordinates is due to the rough way the data was
handled and would be minimized by obtaining more representative
spectral data of the light sources used in the system recorder and
the densitometer employed to obtain spectral data from the
film.
Although only a single embodiment of this invention has been
described herein, it will be apparent to a person skilled in the
art that various modifications to the details of construction shown
and described may be made without departing from the scope of this
invention.
* * * * *