Method and apparatus for scanning thermal images

An imaging sensor for scanning an image in an object space is disclosed. The imaging sensor comprises a first and second plurality of infrared detectors. Also provided is a reflecting means for reflecting a predetermined portion of the object space on the first plurality of infrared detectors during first portion of the scanning cycle. The reflecting means is further able to reflect the predetermined portion of the object space on the second plurality of infrared detectors during the second portion of the scanning cycle. In a further embodiment, the orientation of the detectors permits each of the detectors to be calibrated with respect to another of the detectors by a sequential comparison of the outputs of pairs of the detectors whose fields-of-view overlap.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of infrared sensing, and more 
particularly, to a method and apparatus for scanning thermal images. 
2. Description of Related Art 
Elemental infrared detectors are often used in conjunction with missiles 
and night vision systems to sense the presence of electromagnetic 
radiation having a wavelength of 1-15 .mu.m. These detectors often operate 
on the principle of photoconductivity, in which infrared radiation changes 
the electrical conductivity of the material upon which the radiation is 
incident. Such detectors are often fabricated from 
mercury-cadmium-telluride, though other materials such as CdTe and CdSe 
are also used. 
While an array of elemental infrared detectors may be used in an elemental 
system in which the detectors sense the average energy generated by an 
object space, they may also be used in thermal imaging systems. In one 
such imaging system using a charge coupled device ("CCD"), the elemental 
detectors produce free charge carriers which are then injected into the 
CCD structure and are processed by using time delay integration and 
parallel-to-serial scan conversion. In real time thermal imaging systems 
such as forward looking infrared ("FLIR") imaging sensors, moving mirrors 
are used to scan radiation emitted by the object space across an array of 
elemental detectors, the temporal outputs of which are a two-dimensional 
representation of the thermal emission from the object space. 
The optical system of an imaging sensor projects a real image of the scene 
(or object space) upon the plane (usually referred to as the focal plane) 
containing the detector array sensitive surface. The array may be two 
dimensional, with the corner elements viewing the corners of the desired 
image or sensor field-of-view ("FOV"). The array may be essentially one 
dimensional (usually referred to as a linear array, perhaps with multiple 
rows) where the end elements define two edges of the FOV but the narrow 
dimension of the array is much smaller than the other image dimension and 
the image must be moved (or scanned) in a direction normal to the long 
dimension of the array in order for the linear array to cover the desired 
FOV. The array may also be essentially a point in the sense that both 
dimensions of the detective array are much smaller than the desired image 
or FOV, and the image must be scanned in two directions across the 
detector in order for the detector to cover the desired FOV. The relative 
movement of a linear array from one edge of the FOV to the opposite edge, 
or of a point detector from one corner of the FOV to the opposite corner, 
generates a field of image infomation. The two dimensional detective 
array, used in what is referred to as a "staring sensor", generates a 
field of information without relative motion between the detective array 
and image. In all three cases, the individual elements of the detective 
array will have non-zero area and dimensions, and the detective array will 
cover some part of the total image area during each field. 
In general, there will be some space between individual elements, the area 
swept out by the detector elements in one field will be less than the 
total area of the image, and some image information may be lost. For this 
reason many sensors operate in the interlace mode. Consider, for example, 
a linear array with adjacent detector elements separated by spaces equal 
to the detector height, where height is the dimension parallel to the long 
dimension of the array. In one field this array would cover or sweep out 
one-half the image area. In the interlace mode of operation, the image 
would be shifted one element height in the direction parallel to the array 
length and a second field generated. The combination of two fields, which 
together cover the desired image, is generally called a frame. The same 
approach may also be needed by and applied to two dimensional arrays 
(staring sensors) and point arrays (used in what are generally referred to 
as serial scanners). In the example given, the interlace ratio is 2:1 
since it takes two fields to generate one complete image (or frame). 
Interlaced operation is also used to reduce signal band width. 
When each field covers exactly half the image, there is no overlapping of 
fields, and the sensor is said to have zero overscan (usually given as a 
percentage). Some overscan may be desireable. Returning to the previous 
example of 2:1 interlace, increasing the detector height while keeping 
everything else constant allows the fields to overlap. The increased 
detector size produces increased spatial filtering. As another example, 
keeping the detector geometry constant but doubling the interlace ratio 
produces 100% overscan and can reduce image artifacts due to aliasing. In 
both these cases, the centers of different detective elements do not 
sample the same image point (for a staring sensor) or image line (for a 
linear array). 
When used in conjunction with certain imaging systems, the output from each 
elemental detector is often coupled to the amplifying electronics through 
an A.C.-coupling circuit. Such A.C.-coupling circuits generally provide 
three advantages when used in imaging systems. The first of these 
advantages is that good contrast rendition of the object space requires 
background subtraction, which can generally be approximated by using an 
A.C.-coupling circuit. Secondly, the D.C. biasing potential supplied to an 
elemental detector can be removed by the coupling circuit so that the 
biasing potential will not influence the subsequent processing of the 
detector output. Finally, an A.C.-coupling circuit is able to minimize the 
effects of detector l/f noise on the processing electronics. 
Because the implementation of the A.C.-coupling circuit often requires an 
RC high-pass network, the circuit will generate a zero output voltage when 
a D.C. signal representing the average thermal intensity of the object 
space is produced by the elemental detectors. While the elemental 
detectors could therefore sense variations in thermal intensity of the 
object space, the average intensity could not be determined without some 
means for restoring the D.C. portion of the detector output. 
To restore this D.C. portion of the detector output after the output had 
passed through an A.C.-coupling circuit, the imaging sensor was often 
designed to scan a thermal reference source during an inactive portion of 
the scan cycle. The thermal reference source would often comprise a 
passive source such as a field stop or an active source such as a heated 
strip. When the thermal emission from the thermal reference source was 
received by a detector, the last coupling capacitor output was shorted to 
ground. By shorting the coupling capacitor in this manner, the capacitor 
would rapidly charge to a D.C. value equal to the signal produced by the 
detector upon receipt of the thermal emission of the thermal reference 
source. When the detector reached the active portion of the scan cycle, 
the circuit resumed normal operation allowing passage of the signal 
variation around the thermal reference signal voltage. 
In addition, to compensate for differences in responsivities (i.e., the rms 
signal voltage generated by a detector per unit rms radiant power incident 
upon the detector) between the detector channels (i.e., the detector 
together with its coupling and amplifying electronics), it was often 
necessary to use a second thermal reference source. At different times 
during the inactive portion of the scan cycle, each elemental detector 
would receive thermal emissions from each of the thermal reference 
sources. Because the thermal reference sources emitted different 
intensities of infrared radiation, the responsivities of the detectors 
could be measured by comparing the output of each detector when receiving 
radiation from each of the sources. The output signal from each of the 
detectors could then be adjusted to compensate for the variation in the 
responsivities among the various detectors. 
While the methods for providing D.C. restoration and responsivity 
equalization described above were somewhat effective, they required an 
imaging sensor to scan at least one thermal reference source during the 
inactive portion of its scan cycle. The imaging sensor therefore often had 
to be used in conjunction with relatively complex opto-mechanical 
mechanisms. Additional complications also existed with respect to 
maintaining the temperature of the thermal reference sources within the 
required operating limits. 
SUMMARY OF THE INVENTION 
According to the preferred embodiment of the present invention, an imaging 
sensor is disclosed for scanning an image in an object space. The imaging 
sensor comprises a first and second plurality of infrared detectors. A 
reflecting means is provided to reflect a predetermined portion of the 
object space on the first plurality of infrared detectors during a first 
portion of the scanning cycle. The reflector means is further able to 
reflect the predetermined portion of the object space on the second 
plurality of infrared detectors during the second portion of the scanning 
cycle. In a further embodiment, the orientation of the detectors permits 
each of the detectors to be calibrated with respect to another of the 
detectors by a sequential comparison of the outputs of pairs of the 
detectors whose fields-of-view overlap.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a detector array 10 is provided to detect a thermal 
image in the field-of-view of the array 10. The thermal image may be 
generated by the different intensities of thermal radiation emitted by a 
source 12 in the object space which is within the field-of-view of array 
10. The detector array 10 comprises a plurality of elemental detectors 
each able to scan a portion of the source 12 which lies within its 
field-of-view. For purposes of illustration, the detector D1 is able to 
scan a field-of-view 14. The field-of-view 14 includes four object space 
scan lines each corresponding to one of four fields which the array 10 
uses to scan the source 12 as discussed subsequently. To deliver the 
thermal image to the detector array 10, a scan mirror 16 is provided. The 
scan mirror 16 receives infrared radiation from the source 12 and directs 
the thermal image to the detector array 10. The scan mirror 16 is able to 
move about a scan axis 18 to allow the detector array 10 to horizontally 
scan the source 12, and is able to rotate about an interlace axis 20 to 
allow vertical interlace scanning. 
To process signals received from the source 12, the output from each 
detector to the array 10 connected to A.C.-coupling circuit. For purposes 
of illustration, the AC-coupling circuit for only the detector D1 is shown 
and includes a coupling capacitor 22 and a resistor 24. The capacitor 22 
and the resistor 24 are used to remove the D.C. bias potential supplied to 
the detectors forming the array 10. The output of the capacitor 22 is 
coupled to an amplifier 26 which is in turn coupled to a signal processor 
28. The output of the processor 28 is used to evaluate the thermal image 
received by the array 10. 
The method of scanning used with the detector array 10 can best be 
described by reference to FIG. 2. To detect a thermal image in an object 
space, a plurality of elemental detectors is provided. The detectors 
D1-D10 may be part of a forward looking infrared imaging sensor, though 
they may also be part of another suitble imaging system. For purposes of 
illustration, the object space viewed by the detectors D1-D10 is scanned 
in four fields: FIELD I, FIELD II, FIELD III and FIELD IV. Each field is 
divided into a series of object space scan lines 1-18 which are viewed by 
the detectors D1-D10 in the manner described subsequently. The output from 
the detectors D1-D10 form image scan lines 1-18 which are electrical 
representations corresponding to the object space scan lines 1-18. It is 
to be understood however that the object space may be scanned in a greater 
or smaller number of fields, and the object space may comprise a larger or 
smaller number of scan lines. 
To scan FIELD I, the detector D2 receives object space scan line 1 and 
generates image scan line 1. Also during the scanning of FIELD I, the 
detector D3 generates image scan line 3 upon receipt of object space scan 
line 3. Similarly, the detector D4 generates image scan line 5 upon 
receipt of object space scan line 5, and the detector D5 generates image 
scan line 7 in response to object space scan line 7. The detectors D6-D10 
also produce image scan lines 9, 11, 13, 15 and 17 from object space scan 
lines 9, 11, 13, 15 and 17 respectively. While viewing FIELD I, the 
detector D1 does not receive infrared radiation from the object space 
which is useful in subsequent processing. 
To scan FIELD II, the position of the elemental detectors D1-D10 is 
displaced with respect to the object space by approximately the distance 
between the edges of adjacent detectors. By providing such an interlace 
shift, the detector D2 is able to receive object space scan line 2 and 
generate image scan line 2. Similarly, the detectors D3-D10 are able to 
receive object space scan lines 4, 6, 8, 10, 12, 14, 16 and 18 and 
generate image scan lines 4, 6, 8, 10, 12, 14, 16 and 18. While viewing 
FIELD II, the detector D1 does not receive infrared radiation from the 
object space which is useful in subsequent processing. The interlace shift 
of the detectors D1-D10 with respect to the object space may be achieved 
by using horizontally and vertically rotating germanium prism mirrors 
oscillating in orthogonal directions, which typically may provide a 
displacement of 0.0002 inch. It is to be understood, however, that other 
scanning mechanisms which can provide a suitable interlace shift may be 
used. 
To scan FIELD III, the position of the detectors D1-D10 is again shifted 
with respect to the object space in the manner described above. In this 
orientation, the detectors D1-D9 are able to receive object space scan 
lines 1, 3, 5, 7, 9, 11, 13, 15 and 17 and generate image scan lines 1, 3, 
5, 7, 9, 11, 13, 15 and 17 respectively. While viewing FIELD III, the 
detector D10 does not receive infrared radiation from the object space 
which is used in subsequent processing. To scan FIELD IV, the orientation 
of the detectors D1-D10 is further shifted with respect to the object 
space. The detectors D1-D9 are therefore able to receive object space scan 
lines 2, 4, 6, 8, 10, 12, 14, 16 and 18 to generate image scan lines 2, 4, 
6, 8, 10, 12, 14, 16 and 18 respectively. While viewing FIELD IV, the 
detector D10 does not receive infrared radiation from the object space 
which is useful in subsequent processing. 
After viewing FIELD IV, the detectors D1-D10 are displaced with respect to 
the object space by approximately four times the distance between the 
edges of adjacent detectors, returning the orientation of the detectors 
D1-D10 to that which existed when the detectors D1-D10 viewed FIELD I. By 
displacing the detectors D1-D10 in this manner, the detectors D1-D10 are 
able to overlappingly scan the object space in a 4:1 vertical interlace 
pattern. 
To allow responsivity equalization, the output of adjacent pairs of 
detector channels (i.e., the detector together with its coupling and 
amplifying electronics) are compared when each of the detectors view the 
same object space scan line. The output of the detector channel which 
includes the detector D2 in FIELD I, for example, is compared with the 
output in FIELD III of the detector channel which includes detector D3. 
Because both the detectors D1 and D2 view object space scan line 1 when 
their outputs are compared, the difference between the output of their 
respective detector channels may be attributed to the responsivities 
differences. By comparing the differences in the outputs of the detector 
channels which correspond to the detectors D1 and D2, their relative 
responsivities can be calculated and appropriate signal compensation used 
when their outputs are processed. In a similar fashion, the output in 
FIELD II from the detector channel which includes the detector D3 is 
compared with the output in FIELD IV of the detector channel which 
includes the detector D2. Because their outputs are compared when both the 
detectors D2 and D3 view object space scan line 4, the differences in the 
outputs of the detector channels due to their relative responsivities can 
be calculated so that appropriate signal compensation can be implemented. 
As the outputs of each of the remaining detector channels are compared in 
the manner described above, their outputs can be adjusted to provide the 
desired equalization. 
To allow D.C. restoration of A.C.-coupled outputs from the detectors 
D1-D10, the image scan lines generated in overlapping fields are averaged 
after A.C.-coupling. When the pattern of FIG. 2 is used to scan the sample 
object space shown in FIG. 3, the detectors D1-D10 generate outputs 
indicative of the average thermal emission received when scanning FIELDS 
I-IV as shown in FIG. 4. It will be noted that each adjacent pair of 
detectors have individual fields of view which overlap by two object space 
scan lines. This output from the detectors D1-D10 is supplied to an 
A.C.-coupling circuit to produce the output shown in FIG. 5. The 
horizontal line segment 30 in FIG. 5 represents the average A.C.-coupled 
output from image scan lines 5 and 6 as generated by the detector D4. 
Similarly, the horizontal line segment 32 represents the average 
A.C.-coupled output from image scan lines 7 and 8 produced by the detector 
D4. With respect to detector D5, the horizontal line segment 34 represents 
the average A.C.-coupled output from image scan lines 7 and 8, while the 
horizontal line segment 36 represents the average A.C.-coupled output from 
image scan lines 9 and 10. The horizontal line segments 38, 40, 42 and 44 
represent similar averages corresponding to the average A.C.-coupled 
outputs from detectors D6-D7. The average A.C.-coupled output for the 
detectors D1, D2, D3, D8, D9 and D10 are zero as indicated. Though the 
average outputs for pairs of image scan lines are shown in FIG. 5, the 
averages for individual image scan lines could also be used. 
To reconstruct the D.C. signals from the detectors D1-D10 blocked by the 
A.C.-coupling circuit, the voltage difference between the horizontal line 
segments in FIG. 5, representing the average outputs of overlapping 
detector FOV's are calculated. These differences shown in FIG. 5 as the 
vertical arrows connecting the line segments representing common averages 
are then summed with the A.C.-coupled signal to produce a reconstructed 
output according to the following formula: 
##EQU1## 
Where: O(n)=reconstructed output signal 
I(n)=A.C.-coupled signal 
.DELTA..sub.(n, n-l) =difference between the average outputs of two 
detectors measuring the same output scan line. 
To illustrate the operation of the manner in which D.C. restoration is 
obtained using the present invention, a non-limiting example of the 
restoration technique will be presented. As is shown in FIG. 5, the value 
of .DELTA..sub.(2,1) and .DELTA..sub.(3,2) is zero as both the detectors 
D1, D2 and D3 sense the same intensity of infrared radiation (i.e., the 
sky shown in FIG. 3). The value of .DELTA..sub.(4,3) is approximately 1.5 
volts, while the value of .DELTA..sub.(5,4) is 3.5 volts. To reconstruct 
the signal for the detector D4, the value of the summation term in the 
previous equation is equal to .DELTA..sub.(2,1) +.DELTA..sub.(3,2) 
+.DELTA..sub.(4,3) +.DELTA..sub.(5,4) =+5 volts. Therefore, the 
reconstructed output signal O.sub.(5) is equal to I.sub.(5) +5 volts. 
Reconstructed outputs for the remaining detectors can be similarly 
calculated. 
It should be understood that the invention was described in connection with 
a particular example thereof. While the scanning pattern discussed above 
allows D.C. restoration as well as responsivity equalization of the 
detectors of a vertically interlaced imaging system, other scanning 
patterns may be used in which nonequivalent sets of detectors view the 
same region of the object space. The invention can be used in different 
applications which may employ other types of signal transducers, and may 
be used to calibrate transducers with respect to other types of electrical 
characteristics. Other modifications will become apparent to those skilled 
in the art after a study of the specification, drawings and following 
claims.