Sensor including an array of sensor elements and circuitry for individually adapting the sensor elements

An infrared sensor includes a linear array of plural photoconductive sensor elements, and a similarly linear multiplexing circuit arranged in side-by-side relation with the linear array of sensor elements. The multiplexing circuit includes a matching plurality of signal input ports, and a wirebond of plural conductors substantially in parallel conducts signals from the plural sensor elements of the array to the respective signal input ports of the multiplexer circuit. The multiplexer circuit includes a matching plurality of adaptation circuits or cells interposed between the respective sensor elements of the array and the input ports of the multiplexing circuit. Each adaptation cell includes a multi-position switch which by its position varies the transfer function of a feed back circuit of the adaptation cell. Plural modes of signal processing for each individual sensor element of the array are provided according to the positions of the multi-position switches of the plural adaptation cells. The sensor includes a microprocessor which allows each sensor element of the array to be operated in a selected one of the plural modes of operation according to the position of the multi-position switch commanded by the microprocessor.

CROSS REFERENCE TO RELATED APPLICATIONS 
The invention of this application is related to technology presented in 
U.S. application Ser. Nos. 08/283,314, filed 29 Jul. 1994, and 08/190,671, 
filed 31 Jan. 1994, now U.S. Pat. No. 5,453,618, both of which are also 
assigned to the same assignee as this application. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is in the field of sensors. That is, the present 
invention relates to sensors which provide an electrical output signal in 
response to an input of another type. The input may be in the form of 
light, or other electromagnetic radiation. The electrical output response 
of the sensor may be used to provide an indication of the existence of an 
input source, of the direction of the source relative to the sensor, or to 
image the source, for example. 
Such a sensor according to the invention further includes a self-adapting 
sensing circuit. A feature of the self-adapting sensing circuit allows the 
sensor to operate in any one of several modes. That is, in the operation 
of such sensors, it is frequently desirable on the one hand to compare the 
signal provided by a sensor element of the sensor to either an average of 
the signals provided by the sensor as a whole, or by the particular sensor 
element over a period of time. Thus, the output signal provided by the 
sensor is "time averaged", and features of the input source which change 
with a time interval in a range shorter than a certain maximum interval 
and longer than a determined minimum interval, each associated with a 
certain minimum and maximum response frequency for the sensor, are 
detected. That is, input source features which are changing with time 
within a selected range of frequencies or time intervals of change, are 
detected by the sensor. 
On the other hand, it may be desirable to compare the output signal of a 
sensor element to its own immediately preceding output signal in order to 
provide an output signal indicative of the difference in these signals. 
This comparison and output signal provision effectively provides a 
pseudo-radiometric output for a sensor responding to photons of light. The 
output signal of the sensor is indicative of changes in the input source 
without reference to the frequency, time interval, or time rate of change 
of the level or value of the input source, but with reference to a 
predetermined (known or unknown) value. 
Alternatively, it may be desirable to provide such a sensor with an output 
which is referenced to an externally-provided value. That is, in the case 
of sensors responding to infrared light, it may be desirable to reference 
the output of the sensor to a level of infrared photons, such as from a 
black body source at a set temperature, for example. Such a sensor 
provides a calibrated output which is radiometric in the sense that the 
output signal from the sensor is proportionate to the incident infrared 
light flux according to a proportionality based on the temperature of the 
reference black body source. 
Still more particularly, the present invention relates to such a sensor 
which is photoconductively responsive to infrared light, and which 
includes an array of individual sensor elements which are individually 
adaptive. The sensor may be either of a linear-array or of a 
two-dimensional array type, and may be a fully staring sensor which does 
not require a chopper or any other moving parts, for example. 
2. Related Technology 
Conventional photoconductive sensors generally include an array of sensor 
elements, each of which provides its own individual response to the 
particular portion of the input infrared light which falls upon a 
particular sensor element. The individual electrical outputs of the sensor 
elements are conducted outwardly of the sensor in order to provide an 
indication of existence, direction, or image information for the source of 
infrared light, for example. The sensor may include either a linear array 
of such sensor elements, or a two-dimensional array of sensor elements. 
When a linear array of sensor elements is used, the usual sensor system 
includes a scanner or other such device to scan various parts of the input 
across the linear array so that all parts of the "scene" (or field of 
interest within which input sources may be found), scan across the linear 
array of sensor elements. Alternatively, relative movement of the sensor 
or source may be used to effect relative movement of portions of the 
"scene" across the sensor. 
Further, conventional focal plane array imaging devices (both for visible 
light, and for other portions of the electromagnetic spectrum, such as for 
the invisible infrared portion of the spectrum) have been known for some 
time. These devices are generally of the charge-coupled type or of the 
direct-injection type. For purposes of convenience and simplicity in 
description hereinafter, the term "light" or "light-responsive", and other 
such terms, should be understood to refer to the electromagnetic spectrum 
in general, and may include both infrared, and ultra-violet radiations, 
and other wavelengths in addition to visible light. The known conventional 
focal plane array imaging devices currently are fabricated as arrays of 
light-responsive elements, or pixels, in the form of thin-film devices 
generally in a rectangular array of photo-responsive receptors on the face 
of a semiconductor substrate. The devices are fabricated using 
conventional CMOS, thin-film, and other currently-known semiconductor 
fabrication techniques. 
Other conventional infrared or thermal imagers use "room temperature" or 
near room temperature, ferrielectric sensors, which may be fabricated of 
barium strontium titanate (BST), for example. Another conventional 
infrared sensor uses a thin-film bolometer fabricated as a current-mode 
monolithic array. Such BST or bolometer sensors are considerably more 
expensive to make than are photoconductive infrared sensors. However, 
prior to the present invention, photoconductive infrared sensors could not 
generally provide a level of sensitivity and performance favorably 
comparable to sensors using the more expensive technologies. Circuit 
techniques to improve the performance of the photoconductive sensors could 
not be implemented at a small enough size to be packaged with the sensor 
in a thermal enclosure. If the performance enhancing circuits were 
implemented outside of the thermal enclosure, a large number of conductors 
were required to penetrate the thermal enclosure between ambient and the 
chilled sensor. Each of these electrical conductors also represented a 
thermal conductor which allowed ambient heat to leak into the thermal 
enclosure. The cooling requirements of such a sensor could either rule out 
the possibility of properly cooling the sensor to its optimum response 
temperature, or would require a prohibitively large expenditure of power 
to achieve this level of cooling despite the comparatively large heat 
leakage into the thermal enclosure along the multitude of conductors. 
Importantly, known conventional imaging devices of the focal plane array 
type are based on an architecture which requires the pixels of the device 
to be accessed in serial order. That is, the image signal from the pixels 
is fed out of the imaging device as an analogue or digital data stream 
representing light levels incident on the pixels individually in a 
row-by-row scan of the array. Generally this scan starts at one corner of 
the rectangular array and proceeds across the row of pixels individually, 
preceding subsequently across the next or adjacent row of pixels. Of 
course, scanning every other row of the array with the scan rate being 
such that two such partial scans of alternate rows are completed in the 
same time as would be required for a complete scan of adjacent rows is 
also known to reduce the flicker of a video image (interlacing). With 
either type of scanning, this type of serial image output signal 
indicative of a pixel scan is long-familiar from the television 
technology. 
Unfortunately, when it is desired to foveate, or to concentrate attention 
on a stationary or moving image which resides in a particular part of the 
image array and occupies a comparatively small portion of the array, a 
large part of the serial information in the signal stream is of little or 
no interest. That is, after the last portion of the serial signal stream 
which includes information about an image of interest is received, almost 
the entire remaining portion of the array scan (or scan of interlacing 
alternate rows) must be completed before the scan will return to the area 
of the array which is of particular interest. Thus, time is lost in 
acquiring image information from the part of the array which is of most 
interest. This time loss is the case even is signal acquisition circuitry 
is employed to acquire and concentrate attention on (i.e., create a window 
of image out of) the array signal stream. 
Similarly, when it is desired to acquire an image of an image source which 
is fast-moving, for example, then the time lost in scanning the entire 
array, including those areas of the array where image information of 
little or no interest is located, is a great detriment. This time loss can 
result, for example, in loss of the image source from the field of view of 
the imaging system, in confusion of background noise sources for the image 
source of interest, or both. 
One conventional expedient is to simply increase the scanning rate at which 
the pixels of an imaging device are accessed. This increase of scanning 
rate results in the scan returning to the area of the array which is of 
interest more quickly and with less loss of time between scans of the 
interesting area of the sensor array. However, when the image to be 
generated is digitized, the analogue-to-digital converters (digitizers) 
themselves have a finite settling time which limits the rate at which the 
array can be scanned. The conventional solution to this lack of speed in 
array scanning is to use plural digitizers in parallel. In this 
architecture, each of the plural digitizers in sequence is supplied with a 
portion of the analogue data stream, and is then interrogated for its 
portion of the resulting digital signal after the digitizer has had time 
to settle. With multiple digitizers sharing the load, doubling the 
scanning speed requires double the number of digitizers. Of course, it is 
easily seen that this conventional expedient itself has limitations with 
respect to the cost and complexity of the overall imaging system. As the 
rate of pixel scanning increases, the number of digitizers required 
becomes prohibitive. 
Another conventional expedient is known in accord with U.S. Pat. No. 
5,262,871, issued 16 November 1993, to Joseph Wilder, et al. The '871 
patent is believed to disclose a two-dimensional focal plane array sensor 
in which individual pixels or super pixels (groups of individual pixels) 
of the array can be accessed individually for their output response. This 
teaching allows the output signals of pixels on the array which are of 
interest to be obtained, which skipping the outputs of pixels having 
output (image) information which is of lesser or of no interest at a 
particular time. Understandably, the time required to complete a scan of 
the array is significantly shorter when only part of the pixels are 
interrogated for their output information, when pixels are grouped into 
super pixels for which averaged output information is obtained, or both. 
The '871 patent illustrates another aspect of conventional technology, and 
an aspect which limits the utility of this technology. In order to provide 
the electrical output signals from an array of sensor elements to external 
signal processing circuitry, the sensor taught by Wilder uses a "signal 
read out section" (or multiplexer) to provide a serial stream of output 
signals to the external circuitry for further processing. The pixels of 
the array use a photodiode type of photon sensor which is actually the 
sensor element of each pixel. These pixels are accessed individually or in 
groups in order to obtain their individual or a group-average output 
signal. However, the output signals themselves are received by the "signal 
read out section", or output multiplexer via an input port or connection 
which receives a serial stream of output signals from the selected 
sequential ones or from all of the pixels in a particular row of the 
sensor sequentially, for example. The input port does not discriminate 
between individual pixels of the multiple pixels served by a particular 
signal input port, other than to attribute the input signal received to a 
particular pixel whose address has been interrogated. So far as adapting 
the signal input port for individual differences in the pixels of the 
array, the '871 device does not have this capability. Also, the device of 
the '871 patent does not provide for individually different processing of 
input signals from the pixels served by an input port and which are 
received sequentially. Accordingly, all of the pixels from which an output 
signal is provided into the output multiplexer via a particular port or 
connection, and in fact, all of the pixels of the sensor, are treated 
equally and as equals so far as signal processing (i.e., application of a 
bias current level or voltage, for example) is concerned. Accordingly, the 
sensor of the '871 patent cannot implement the alternative modes of signal 
processing outlined above. 
SUMMARY OF THE INVENTION 
In view of the above, the present invention has as a primary object the 
provision of a sensor which allows all individual sensor elements of the 
sensor to be treated as individuals in the receipt of, and in the 
application of an adapting bias or offset voltage or current signal to, an 
output signal from the individual sensor elements of the sensor. 
Another object of the present invention is to provide an infrared sensor 
which employs a sensor element array. 
A further objective of the present invention is to provide such an infrared 
sensor which employs a photo-conductive sensor element. 
Such a sensor may be provided according to another object of the present 
invention by the use of a lead selenide (PbSe) photoconductive sensor 
element. 
Still another object of the present invention is to provide such a sensor 
which includes an output multiplexer with plural signal input ports and 
with such an individual adaptation circuit serving each input port of the 
multiplexer and allowing adaptation to be effected for each individual 
sensor element of the sensor, while providing a compact and efficient 
arrangement of the sensor. 
Yet another object of the present invention is to provide such a sensor in 
which the adaptation of the individual output signals of the various 
sensor elements of the sensor allows any one of plural signal processing 
modes to be implemented at any one of the sensor elements. 
An additional object for the present invention is to provide such a 
multiplexer with individual input port adaptation circuits to be 
implemented as a fine-dimension scale comparable with the fine-dimension 
scale of the sensor elements of the sensor. 
Still further, an object for this invention is to provide such a sensor 
with a fine-dimension multiplexer having individual adaptation circuits at 
the multiple input ports of the multiplexer, and in which the combination 
of sensor and multiplexer are of such fine-dimension scale that both can 
be housed in a thermal housing of sufficiently small size that the sensor 
can be cooled with a small energy expenditure. 
Additionally, an object for this invention is to provide such a sensor in 
which the number of electrical (and thermal) conductors penetrating the 
thermal enclosure is minimal. 
Accordingly, the present invention provides according to one aspect thereof 
an infrared sensor comprising a photoconductive infrared sensor element 
varying in conductivity in response both to temperature (thermic 
variation) and to incident infrared light (photonic variation); an 
adaptation circuit conducting a variable bias current from a source 
thereof through the sensor element, the adaptation circuit including a 
resistive element in parallel with a variable-conductance element. The 
variable-conductance element varying in conductivity in response to a feed 
back signal to provide a portion of the bias current flow resulting form 
thermic variation of conductivity of the sensor element, and the resistive 
element providing a portion of the bias current resulting from photonic 
variation of conductivity of the sensor element. The resistive element 
also providing a first feed back signal in response to voltage drop across 
the resistive element. 
The above and additional objects and advantages of the present imaging 
device will appear from a reading of the following description of a 
particularly preferred exemplary embodiment of the invention taken in 
conjunction with the following drawing Figures. 
Advantages of the present invention derive from its use of thermoelectric 
cooling, as opposed to using a more complex and limited cryostat, 
cryocooler, cryoprobe, or cryogenic liquid cooling; from its simplified 
construction having an infrared sensor, and multiplexer for signal output 
both carried on a thermoelectric cooler, all within a compact thermal 
housing. Additional advantages of the present sensor follow from its use 
of comparatively low-cost lead-selenide (PbSe) sensor elements. These 
sensor elements are formed of photo-conductive material, which 
conventionally has imposed performance limitations upon sensors heretofore 
using this sensor material. However, the present invention substantially 
overcomes all of the limitations in performance of conventional sensors. 
The present invention thus provides an economical infrared sensor of low 
cost, rugged construction, and good performance (especially when 
considered on a performance versus cost basis), which is suitable for use 
a variety of industrial and air-borne environments.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Considering now FIGS. 1-4 in conjunction, with attention first to FIGS. 1 
and 2, a sensor module 10 is shown in perspective and in elevational cross 
section. The sensor module 10 includes a thermally-insulative housing 12, 
which upwardly defines an opening 14 into which is sealingly received a 
window member 16. This window member is transparent to infrared radiation, 
but may or may not be opaque to visible light. For purposes of 
illustration, the window 16 is depicted as though it were transparent to 
visible light. In an actual embodiment of the sensor module 10, a person 
may not be able to view the internal structures of the module 10 through 
the window 16, dependent upon whether this window is transparent to 
visible light. Downwardly from the housing 14 depends a heat sink mass 18. 
For example, the mass 18 may be formed of beryllium oxide. 
Circumferentially around the housing 12, the sensor module 10 includes an 
outwardly extending flange portion 20, on which is carried plural 
depending electrical contact pins 22. The flange portion 20 sets upon a 
circuit board 24, and the thermal mass 18 is received into a recess or 
opening 26 of this circuit board. The pins 22 provide electrical interface 
of the sensor module 10 with the circuit board 24. 
The housing 12 sealingly encloses an evacuated chamber 28 within which is 
received upon a three-stage thermoelectric cooler 30, both a 
photo-conductive lead selenide (PbSe) sensor-array chip 32 and a pair of 
multiplexing circuit chips 34. The multiplexing circuit chips 34 flank the 
sensor-array chip 32. A pair of respective wire bonds 36, each of multiple 
conductors, extends laterally between the sensor-array chip 32 and each of 
the multiplexing chips 34. Also, a plurality of conductors 38 extend from 
the multiplexing circuit chips outwardly to an annular ceramic 
feed-through portion 40 of the housing 12, viewing FIG. 3. Inwardly of the 
chamber 28, the ceramic feed-through portion 40 defines plural metallic 
electrical contact pads 42, to which the conductors 38 are respectively 
connected individually. Outwardly of the housing 12, the ceramic 
feed-through portion 40 of the housing defines the flange 20, and carries 
the depending contact pins 22 in electrical connection individually with 
the contact pads 42. Within the ceramic feed-through portion 40, the 
contact pads 42 are individually connected electrically to respective ones 
of the contact pins 22. 
The thermoelectric cooler 30 includes three cascaded or series arranged 
stages of reversed Peltier-effect cooling semiconductor junctions, which 
move heat from the upper end of the cooler 30 toward the lower end of this 
cooler when an appropriate voltage and current flow is provided in the 
cooler. Consequently, the upper surface 44 of the cooler 30 becomes very 
cold, and heat is moved to the lower surface 46 of the cooler, warming 
this lower surface. The lower surface 46 of the cooler 30 is thermally 
connected to the heat sink mass 18 through the intervening lower wall 48 
of the housing 12. Heat sink mass 18 is exposed to the ambient environment 
around the sensor module 10 (on the side of the circuit board 24 which is 
opposite to the window 16), and is cooled by air convection and radiation 
to the environment. Accordingly, heat from the heat sink thermal mass 18 
does not warm surrounding structures by convection or radiation to produce 
unwanted sources of infrared radiation within view of the window 16. At 
the upper end of the cooler 30, the upper surface 44 is thermally 
connected to the sensor-array chip 32 via an intervening 
thermally-conductive synthetic sapphire upper mother board member 50. This 
upper mother board member 50 also carries the multiplexer circuit chips 
34, so that these circuit chips are cooled and operate at a low 
temperature. 
FIG. 4 shows an enlarged fragmentary plan view of a small portion of the 
sensor-array circuit chip 32 seen in FIG. 3. As is shown on FIG. 4, the 
sensor-array chip 32 includes a bi-linear array of fine-dimension PbSe 
photo-conductive sensor elements 52, arrayed on opposite sides of a 
central conductive metallic trace 54 in two staggered rows 56. This 
conductive trace 54 has a width of about 0.007 inches, while the sensor 
elements 52 are about 0.0022 inches square on 0.004 inch centers. These 
sensor elements 52 are effectively pixels or image elements when the 
sensor module 10 is used to provide image information about a source of 
infrared light. Because the sensor elements 52 have a dimension larger 
than their center-to-center spacing, there is an overlap of about 20 
percent between the two rows of these image (sensor) elements 52. In other 
words, with the two cooperative rows of sensor elements 52, all of the 
image information which is delivered to the sensor-array chip 32 by an 
optical system will pass across at least one of the sensor elements 52, 
and will have an opportunity to produce an electrical photo-conductive 
response. 
Preferably, the sensor-array chip seen in FIG. 4 includes 256 sensor 
elements 52 arrayed in two closely spaced apart rows 56, each of 128 
sensor elements. The sensor-array chip 32 provides about 20 percent image 
capture redundancy because of overlap of these sensor elements 52. 
Extending in respectively opposite directions from each of the sensor 
elements 52 in each of the two sensor rows 56, are respective conductive 
traces 58 leading individually to respective contact pads 60. The wire 
bond conductors 36 connect individually at their inner ends to the contact 
pads 60, and extend to similar contact pads 62 at the multiplexing circuit 
chips 34. These contact pads 62 at the multiplexing circuit chips 34 are 
best seen in FIG. 3. The length of the multiplexing chips 34 is favorably 
comparable to that of the sensor array chip 32. Accordingly, hereafter it 
will be recognized that the circuit elements of the multiplexer chips 34 
are implemented at a fine-dimension scale comparable in size and in pitch 
(repeat distance along the length of the multiplexer chips 34) similar to 
the dimensions set out for the array chip itself. 
Those ordinarily skilled in the pertinent arts will recognize that a 
scanner or other such device optically scans a "scene", "view", or field 
in which an infrared source of interest may be located across the sensor 
elements 52. This scanning may be achieved, for example, by relatively 
moving the scene, the sensor 10, or both. Alternatively, neither the scene 
or the sensor need be relatively moving. An optical scanner device may be 
used to scan portions of the scene successively across the sensor elements 
52 of the sensor 10. On the sensor array chip 32, the successive small 
viewed portion of the scene changes at the sensor elements 52 in a 
direction perpendicular to the length of the sensor rows 56 as the 
scanning is performed. The effect is similar to an observer staring out of 
a very narrow window of a moving rail road car at the scenery passing by. 
Such a very narrow window would not provide the observer with a very 
satisfying view of the scenery. However, if the observer were to take a 
rapid sequence of narrow photographs, and then were to fit these photos 
together in sequence, a complete mosaic image of the scenery which had 
passed the narrow window could be assembled. 
Diagrammatically, this scan of an image across sensor-array chip 32 is 
represented by a plurality of long, narrow sequential image portions (each 
illustrated with a dashed-line box 64) approaching the sensor elements 52 
in FIG. 4, as is represented by the arrow 66. As can be easily envisioned, 
the image lines 64 adjacent to one another form a mosaic image of a scene, 
At the sensor elements 52, a bias voltage is applied between the trace 54 
and the connector pads 60 so that a bias current always flows through the 
sensor elements 52. These photoconductive PbSe photoconductive elements 52 
becomes more or less conductive both in response to temperature and in 
response to infrared radiation which is incident upon them in the scene 
portions 64. The part of the bias current which flows through a sensor 
element 52 because of temperature will be at least four times the photonic 
bias current flow. The bias current level through the sensor elements 52 
does change significantly in response to the infrared radiation from the 
scene. However, this photonic change in conductivity is superimposed upon 
a much greater thermic conductivity, as will be seen. The image 
information, then, consists of the differences in the levels of current 
flow between trace 54 and pads 60 as the infrared radiation from the scene 
sweeps across the sensors 52 in response to movement of the scanning 
platform 26. However, the total bias current level through a particular 
sensor element 52 will be at least two orders of magnitude (20) times 
greater than the signal level. In some instances, the signal to noise 
ratio may be as low as 1/200,000! These electrical current signals from 
the sensor elements 52 are passed directly by the wire bond conductors 36 
to the associated contact pads of the multiplexing circuit chips 34. 
At the multiplexing circuit chips 34, the signals conducted from the 
individual sensor elements 52 of the sensor-array chip 32 are biased, 
filtered, and amplified. The voltage at the output of each channel of the 
multiplexer circuit then represents an electrical analog of the infrared 
image information contained in a particular image line 64. The voltage 
levels are then serially transferred by the multiplexer 34 to a common 
output line (on of the conductors 38) on each of the multiplexing circuit 
chips 34 by use of a shift register of the multiplexing circuit. This 
transfer of voltage-level image information from the multiplexer chips 34 
takes place at a high speed, and the channels of these multiplexer 
circuits are prepared to receive and temporarily store image information 
from the next succeeding image line 64 as the scanning proceeds across the 
sensor elements 52. 
Each of the multiplexing circuit chips 34 then provides a respective serial 
digital signal containing the image information for the image lines 64. 
That is, the multiplexers 34 also effect an analog to digital conversion 
of the image signals. These multiplexing circuit chips 34 are synchronized 
in their operation so that they serially handle and output the image 
information from the two rows 56 of sensor elements 52. For example, the 
multiplexing circuit chips 34 can alternate in their operation so that all 
of the image information from the sensor elements 52 of one of the rows 56 
of sensor elements 52 is fed out serially by the associated one of the 
multiplexing circuit chips 34, and is followed then by the image 
information from all of the sensor elements 52 of the other row 56 from 
the other multiplexing circuit chip 34. This alternating of serial bit 
streams from each of the two rows of sensor elements 52 would be repeated 
for each succeeding image line, with a line-synchronizing signal 
indicating the start or end of each image line's bit stream. 
Alternatively, the multiplexing circuit chips 34 can alternate in 
sequentially providing serial portions of the output signal, which serial 
portions each represent image information from one of the sensor elements, 
to be followed by image information from the adjacent sensor on the other 
side of the central conductive trace 54, and so on back and forth in 
stair-step fashion across the trace 54 and along the length of the 
bi-linear sensor array chip 32. 
It will be recalled that the image falling on the detector elements 52 
causes certain ones of these elements to become more conductive in accord 
with the level of infrared radiation flux falling on these detector 
elements. A bias voltage maintained between the conductive trace 54 and 
the contact pads 60 ensures that a certain level of bias current always 
flows through the detector elements 52. As these detector elements 52 
receive varying levels of infrared radiation from the image volume 22, the 
level of current flow varies. These current flow levels are representative 
of the image information, and become the genesis for the various forms of 
image signals to be produces from these currents. Viewing now FIG. 5, it 
is seen that each one of the multiplexer chips 34 includes plural signal 
input ports, which are indicated with the arrows 68. FIG. 5 shows only one 
of the multiplexer chips 34. Both multiplexer chips 34 have the same 
interface structure with the sensor array chip 32. FIG. 5 also indicates 
that the input ports are actually an individual conductor 68' extending to 
the multiplexer 34 from an individual one of a like plurality of 
fine-dimension adaptive signal input cells 70. That is, each signal input 
port 68 of the multiplexer chips 34 has associated with it a typical 
adaptive signal input cell 70. These cells are implemented at a 
fine-dimension scale like that of the sensor elements 52, and at a like 
pitch along the length of the multiplexer chips 34. 
FIG. 5 shows that the signal input cells 70 each include a respective 
three-position switch 72. This switch is not a mechanical switch with a 
moving switch element, but will be implemented using CMOS technology, and 
will include several MOSFET transistors. Accordingly, the switch 72 also 
has an open switch position in which none of the switch positions are 
closed. The switches 72 of all of the adaptation cells 70 are individually 
under the control of a microprocessor 74 via an interconnection, which is 
generally indicated with the dashed line and numeral 76. Cells 70 each 
also include a low pass filter (LPF) 78. In the "A" position of the switch 
72, the low pass filter 78 is interposed in a feed back loop formed as 
part of the electrical connection of input signals from the cell 70 to the 
respective input port 68. In the "B" position of the switch 72, the LPF 78 
is bypassed so that the feed back loop is not influenced by the frequency 
of the transmitted output signal, and connection of the output signals 
directly from the particular sensor element 52 and adaptation cell 70 to 
the particular input port 68 of the multiplexer 34 is effected. Finally, 
FIG. 5 also shows that in the "C" position of switch 72, the 
microprocessor 74 may individually provide a signal to the individual 
adaptation cells 70. 
Those ordinarily skilled in the pertinent arts will recognize that 
photoconductive elements, like the elements 52 (whether fabricated of 
PbSe, or other photoconductive material), are semiconductors which depend 
upon generation and recombination (G-R) of electrons and electron holes 
for conductivity. That is, in G-R electrons are excited from the valence 
band to the conduction band by either thermal (thermic) or optical 
(photonic) excitation. Thermal excitation occurs when an electron absorbs 
an internal thermal photon, while optical excitation occurs when an 
electron absorbs an incident external (i.e., infrared) photon. The 
electrical conductivity of a photoconductor material is proportional to 
the density of electrons in the conduction band. For most photoconductors, 
the density of thermally excited electrons in the conduction band is up to 
three orders of magnitude larger than the density of optically excited 
electrons. A photoconductive sensor is operated by applying a constant 
direct-current bias voltage to the sensor and measuring the resulting 
current flow. Consequently, a measurement of the incident optical flux 
requires extraction of the much larger thermal current from the much 
smaller optical current flow. The current flow resulting from the optical 
excitation of the photoconductive material is then amplified and processed 
to extract useful information. 
Further considering FIG. 5, it is seen that the individual adaptation cells 
70 each have a branch connection with a conductor 80 carrying a selected 
voltage level "V.sub.DD. From the branch connection with conductor 80, 
both a resistor "R.sub.1, and a transistor T.sub.1 (referenced with 
numerals 82 and 84, respectively), in parallel provide direct current 
connection between the voltage level V.sub.DD, and an input control 
transistor 86. The transistor 86 has a gate connection to a constant bias 
voltage level PC.sub.bias, via a conductor 88. The photoconductive sensor 
element 52 and transistor 86 in series with resistor 82 and transistor 84 
(the latter two elements in parallel) are connected between the voltage 
source V.sub.DD and the ground potential at trace 54 (indicated on FIG. 5 
with the ground symbol). The gate of transistor 84 has connection to the 
conductor 80 via a capacitor C.sub.1 (referenced with the numeral 90). The 
switch 72 in both of its positions "A", and "B" closes a feed back loop 
from the drain of transistor 84 to the gate of this transistor. Transistor 
84 also provides a parasitic capacitance C.sub.2 to ground potential, 
which parasitic capacitance (not a physical capacitor), is indicated with 
the dashed-line capacitor symbol and the numeral 92. Each adaptation cell 
70 also includes a gain element 94. The gain elements 94 operate to best 
advantage with a particular level or range of input signals, which is 
effected by the feedback loop of the adaptation cell. Also, the gain 
elements 94 effect an amplification of about twenty for the input signals 
from each sensor element 52 and cell 70 as these signals are supplied via 
the gain elements 94 as output signals to the multiplexer 34. 
In practice, resistor R.sub.1 is provided by a drain-gate connected MOSFET 
transistor. C.sub.2 represents parasitic capacitance on the output signal 
node (i.e., on conductor 68). In theory, summing the currents at the 
output node (conductor 68) provides: 
EQU I.sub.PC =I.sub.b +V.sub.o(s) (1/R.sub.1 +sC.sub.2) 
Where I.sub.pc is the current flow through the photoconductive sensor 
element 52, I.sub.b is the bias current flow through the transistor 84, 
and V.sub.o(s) is the voltage level at conductor 68 (i.e., the output 
signal voltage level supplied to multiplexer 34). 
The bias current flow I.sub.b through the transistor 84 as a function of 
gate voltage applied to transistor 84 is given by: 
EQU I.sub.b =V.sub.g .multidot.g.sub.m 
Where g.sub.m is the MOSFET transconductance parameter for this transistor, 
and V.sub.g is the gate voltage applied to the transistor. The gate 
voltage V.sub.g for transistor 84 is given by: 
EQU V.sub.g =F.sub.(s) .multidot.V.sub.o(s) 
Where F.sub.(s) is the transfer function of the low-pass filter 78. An 
example of this transfer function may be: 
##EQU1## 
Combining these equations gives: 
##EQU2## 
Approximating this equation and evaluating for low frequency, 
mid-frequency, and high frequency operation of the sensor gives the 
following results: At low frequencies, sR.sub.1 C.sub.2 is very much less 
than 1, and F.sub.(s) equals 1. Thus: 
##EQU3## 
At mid-frequencies, sR.sub.1 C.sub.2 is very much less than 1, and 
F.sub.(s) is equal to or approaches zero. Thus: 
EQU V.sub.o =I.sub.pc R.sub.1 
At high frequencies, sR.sub.1 C.sub.2 approaches infinity, and F.sub.(s) 
approaches zero, so the V.sub.o approaches zero. 
Thus, the output response of the adaptation cells 70 (and of the sensor 10) 
will peak at mid-frequencies, and will fall off as required at both low 
and high frequencies in the "A" position of the switch 72. This mode of 
operation is provided when the microprocessor 74 causes switch 72 of a 
particular cell 70 to be in its "A" position so that the LPF 78 is 
influencing the gate voltage applied to transistor 84 in the feedback 
loop. This mode is referred to as an "Average", or "AVG", mode of 
operation because it provides a weighted averaging of the last several 
scans of the sensor element 52 with successive image portions 64, 
recalling FIG. 4. 
By switching control of the bias current flow through the transistor 84 
directly to the V.sub.(s) signal (i.e., directly to conductor 68 in switch 
position "B") during one line scan (i.e., during one image portion 64); 
and by storing this V.sub.(s) result capacitively by subsequently opening 
switch 72; then all following line scans [i.e., the V.sub.(s) output 
signal results for following image portions 64], will effectively be 
subtracted from or will represent a difference value over the stored image 
signal value. That is, the following image portions will be compared to 
the stored image portion by differencing of the V.sub.(s) line scan values 
so that the output signals provided are functions of the differences 
between image portions. This mode of operation is referred to as "Last", 
or "LST", because it uses as a reference for future image portions the 
last scan of an image portion during which the switch 72 was commanded to 
a closed position (i.e., position "B"). 
Further to the above, it will be recognized that due to variations in the 
process of manufacture, and in the resulting physical structures of the 
multiplexers 34, adaptation cells 70, and sensor elements 52, the control 
voltages applied to each transistor 84 of the various adaptation cells 70 
are slightly different. In both the "AVG", and "LST", modes of operation, 
these differences are accommodated adaptively. By commanding switch 72 to 
the "C" position, the microprocessor 74 may supply a reference calibration 
voltage level to each individual transistor 84 of each of the various 
adaptation cells 70. This reference voltage may be different for each of 
the cells 70 according to the differences among the cells. With each line 
scan (portion of image 64), the reference control voltage is updated or 
refreshed to the appropriate value for each adaptation cell. This mode is 
referred to as "External", or "EXT" mode because it uses as a reference 
for the sensor elements 52 of sensor 10 an external reference voltage 
source (i.e., a bias voltage level and bias current flow for each 
adaptation cell provided by reference to a memory facility of the 
microprocessor 74). 
The particular reference voltage levels required by the sensor elements 52 
of sensor 10 can be determined by exposing the sensor to a reference image 
source (i.e., to a black body radiator at a known temperature) and with 
switch 72 in position "C", and then causing microprocessor 74 to memorize 
the resulting V.sub.(s) values from each sensor element 52. Thereafter, 
the memorized voltage level can be used to establish an external precise 
reference for each individual sensor element 52. In the "EXT" mode of 
operation, true direct-current radiometric operation of the sensor 10 is 
possible. That is, the V.sub.(s) of the sensor will be proportional to the 
ratio between the number of photons incident on the sensor at a particular 
time from the scene being viewed by the sensor and the number of photons 
provided by the reference source used earlier to establish the individual 
reference bias levels for the sensor. Those ordinarily skilled in the 
pertinent arts will recognize that for such radiometric operation, a 
temperature compensation function will be necessary to compensate for 
thermal drift. One way to effect this temperature compensation is to 
provide extra photoconductors 52 on the sensor array chip 32, which 
elements are shielded from incident photon radiation (and are therefore 
"blind"). These extra photoconductive elements will also vary in their 
conductivity in response to thermal drift of the sensor elements, and will 
provide a temperature compensation mechanism. That is, these extra "blind" 
photoconductive sensor elements will still be subject to the same level of 
thermic excitation as the sensor elements which are exposed to the 
incident light flux. The signal level from these extra sensor elements by 
its variation provides an indication of temperature variations in the 
sensor array chip 32, which can then be compensated for electronically 
within the multiplexer chips 34. 
While the present invention has been depicted, described, and is defined 
with reference to a particularly preferred exemplary embodiment of the 
invention, such reference does not imply a limitation on the invention, 
and no such limitation is to be inferred. The invention is capable of 
considerable modification and variation, as will suggest themselves to 
those ordinarily skilled it the pertinent arts. The invention is intended 
to be limited only by the spirit and scope of the appended claims, giving 
full cognizance to equivalents in all respects.