A dewar cooled piezo electric activated beam splitter permits a filtered dimensional multispectral multidetector staring imager to operate as a target acquisition and recognition device as well as a detector and classifier of unknown chemical vapors or other targets with spectral fingerprint.

BACKGROUND OF THE INVENTION 
The use of chemicals as a potential threat in modern warfare has generated 
a need to detect the presence of these materials as quickly as possible so 
that military personnel can take necessary precautionary measures. It is 
known that all threat and most pollutant chemical vapors have absorption 
features in the 8-12 micron region. Since prior art standard Thermal 
Imagers (TIs) view this whole wavelength region at once, a vapor signature 
would represent only a small amount of energy and be difficult to detect. 
In addition, all vapors that absorb on the 8-12 micron wavelength band 
would yield the same type of image data making it very difficult to 
differentiate one vapor from another. In order to discriminate chemical 
species one must divide the 8-12 micron band into a large number of small 
regions so that these may then be analyzed relative to one another. 
Presently the Thermal Imagers that exist in the armed forces are used for 
tactical target acquisition, tracking and fire control and are known as 
Forward Looking Infrared (FLIRs). It would be a great financial and 
logistical advantage if these prior art FLIRs could be used as an adjunct 
chemical vapor detection sensor. 
The problem with prior art Thermal Imagers is division of the 8-12 micron 
band into smaller parts can only be done with filters or a dispersion 
optic. Both of these approaches are not satisfactory because they both 
have transmission losses. In the case of filters, the pass band may be as 
narrow as 1/2 micron and still yield 80% transmission. Filters much 
narrower than 1/2 micron quickly degrade in peak transmission. Under these 
conditions the filter would decrease the total energy incident on the 
array detector, thus lowering overall sensitivity. 
There are two problems with using standard band pass filters. Firstly, in 
order to divide the 8-12 micron band fully into 1/2 micron wide segments 
would require 8 individual filters These individual filters need to be 
mechanically rotated into the field of view sequentially to obtain 
spectral data, which is difficult to do, or there would have to be 8 to 10 
single band filtered detectors and some method of scanning the field of 
view over each. The second problem with using standard band pass filters 
is that the prior art detector now views a "hot" filter element which is 
opaque over much of the sensitivity range of the detector. This is a 
problem particularly if the scene background is colder than the filter, it 
would result in considerable loss of sensitivity. The problem specific to 
tactical military FLIRs is the requirement for excellent spatial 
resolution for target acquisition and recognition. It is very important 
that the image quality and operational availability of the tactical sensor 
not be comprised in any way by the addition of further missions or 
hardware. 
SUMMARY OF THE INVENTION 
The present invention relates to a focal plane filtered multispectral 
multidetector imager which can be used for target acquisition and 
recognition and for the ability to detect and classify chemical vapors or 
any target with a spectral signature. The invention uses two array 
detectors, which are two-dimensional N.times.N pixel focal plane array 
detectors capable of instantaneously detecting the entire image on the 
image plane. The image plane is alternated between the two array detectors 
by a piezo-electrically driven beam switcher, each array detector 
producing an image which is transformed into a video output. The first 
array detector is unfiltered, providing a standard thermal image. The 
second array detector is filtered at the focal plane by a matrix or mosaic 
filter and provides the multispectral image. 
An object of the present invention is to permit the modification of a 
Thermal Imager to deliver both its standard image and a filtered image 
simultaneously. 
Another object of the present invention is to permit the modification of a 
Thermal Imager which does not degrade the standard image in any way. 
Another object of the present invention is to provide for a Thermal Imager 
wherein the filtered image will have the highest sensitivity obtainable 
for the given detector array and filter bands. 
A further object of the present invention is to provide a focal plane 
filtered multispectral multidetector imager wherein the use of both 
filtered and unfiltered images will allow sufficient spectral 
characterization of the viewed scene to detect and classic chemical vapor 
clouds or any target with a spectral signature. 
For a better understanding of the present invention, together with other 
and further objects thereof, reference is made to the following 
description taken in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 the imager comprises a first focal plane array 
detector 10 which is operatively disposed on top of a first detector 
substrate 12. A second focal plane array detector 14 is operatively 
disposed on top of a second detector substrate 16. The detector substrates 
12 and 16 serve as mounting structure and as an electrical interconnection 
path for the detector arrays 10 and 14. The first array detector 10 is 
used for normal target acquisition, recognition, and detection. The second 
array detector 14 is used for chemical detection. The first array detector 
10 and the second array detector 14 are positioned at right angles with 
respect to each other and located in a dewar flask, not shown, which has a 
dewar will 18 with a dewar window 20 therein. Filter mounting elements 22 
and 22' support a two dimensional spectral filter array 24 which will be 
described in more detail hereinafter. A beam splitter comprising a first 
prism 26 and a second prism 28 can direct the optical path shown by arrow 
30 to either detector 10 or 14 depending upon their physical arrangement. 
A piezo electric: transducer 32 adjusts the air gap 34 to produce either 
reflection or transmission at the prism diagonal surfaces 36 and 38 of 
prisms 26 and 28 respectively. Since there is no scanning necessary to 
produce an image, from the outputs of focal plane detectors 10 and 14, 
there is sufficient dwell time for each array to develop a signal at a 
normal 30 Hertz video frame rate. 
Referring now to FIG. 2, if a lens 40 is placed in front of dewar window 
20, the piezo electric transducer 32' and beam switchers 26' and 28' can 
be made physically quite small by placing them at an intermediate focus. 
Transducer extension member 42 connects piezo electric transducer 32' 
motion to the small first prism 26'. 
Referring now to FIG. 3, the filter array may have a rear illumination 
filter layout. For applications involving infrared wavelength, the process 
of depositing filters on the detector elements themselves may affect the 
detectors at the high temperatures involved. To avoid this problem patches 
44, 46 and 48 having different pass bands are deposited on the rear 
surface 50 of a transparent detector substrate 52. Detector elements not 
shown, in other wavelengths, not as temperature sensitive in the dopant 
migration rates (e.g. silicon) may be front surface 56 coated. The rear 
surface 50 coating will be successful due to the insulating effect of the 
transparent substrate 52 on the detector elements 54. The substrate 52 may 
be bonded to a mesh substrate (detector face down) for electrical 
connections and mechanical mounting. Detector array 54, 54', and 54" as 
shown is illuminated from the rear, permitting the filter array 44, 46, 
and 48 to be deposited directly onto the back surface 50 of the 
transparent substrate 52 without affecting the characteristics of the 
detector elements 54, 54', 54" during the high temperature vapor 
deposition process. The detector elements 54, 54' and 54" may be either 
etched, grown, or bumped onto the substrate, by methods well known in the 
art, and are separated from each other by doped channels 58. 
Referring now to FIGS. 4 and 5, the two dimensional filter mosaics show two 
possible arrangements of four distinct filter pass bands 60, 62, 64, 66 
and 68, 70, 72 and 74 respectively in a two dimension array which provides 
adequate spectral coverage. In each case every pass band has the other 
three pass bands adjacent to it and no filter patches of the same pass 
band are adjacent. An image has a high degree of spatial correlation. 
Based upon the statistical history of the correlation for a particular 
family of scenes, adjacent different pass band signals may be compared for 
digression from the norms. Digression would reveal objects with spectral 
features in that portion of the image. 
There exist optimal covering patterns for any number of discrete filter 
pass bands. The complexity of the spectral features being sought will 
determine the number of filter bands required. Filter patches as small as 
50 by 50 microns have been fabricated and arranged in a two dimensional 
array using processes standard in the semiconductor fabrication industry. 
However, this is the first known use of semiconductor masking with vapor 
deposition of optical materials. 
In operation referring again to the two detector two dimensional focal 
plane array imaging system of FIG. 1. Each of the detector arrays have 
paths which are optically identical. One is the standard detector array 
10, the second detector array 14 is similar to the first array but has an 
optical filter array such as shown in either FIG. 4 or 5 in front of it. 
The purpose and use of the filtered detector array is that of a staring 
imager, with no scanning of the field of view required to form an image. 
Key to this arrangement is the piezo electrically driven beam switcher 26. 
To form two image streams at the 30 HZ video rate, the image beam must be 
switched quickly enough to permit each detector array 10 and 14 enough 
dwell time to generate a signal of full sensitivity. Charges are generated 
within the detector elements 10 and 14 during the time the array views the 
scene. When the beam is switched, charges are latched and then swept out 
to form an analog image signal during the dark time when the other 
detector is exposed to the scene. Additionally, the beam switcher must 
function within the cooled detector dewar and not generate heat or 
appreciable vibration. The beam switcher works on the principle of 
frustrated total internal reflection. Prism 26 moves reciprocally in the 
direction indicated by arrows 76 and prism 28 is fixed. Prism 28 acts as 
in a normal fashion to reflect the beam 30 from its diagonal face 38 to 
the filtered detector 14. As the beam reflects from the diagonal face 38 
of prism 28, it creates an evanescent wave which extends several 
wavelengths beyond the crystal-air interface. As long as this wave sees 
only air, the reflection process is total. With smooth surface 
(.lambda./20 or better surface roughness) and good alignment, the piezo 
actuator 32 can move prism 26 to close the diagonal air gap 34 to a width 
of .lambda./5 or smaller. At a depth of .lambda./5 the evanescent wave 
from prism 28 extends strongly into prism 26 and generates an image beam 
there which propagates as if prisms 26 and 28 were a cube. Thus the 
reflection within prism 28 is frustrated. 
Piezo actuators can function at the speed and temperature required and 
since the total movement will be no more than 100 microns, no appreciable 
vibration will be created. Additionally, the heat generated by the piezo 
actuator 32 is small and within the capacity of the dewar coolers on the 
market. The interior of the dewar wall 18 is evacuated and, operates at 80 
degrees Kelvin. These conditions will not optically alter the operation of 
the beam switcher. The aforementioned operational comments apply equally 
to the FIG. 2 beam switcher which operates at or near intermediate focus. 
While a specific embodiment of the invention has been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles. The invention may 
generally be applied to the detection of targets with spectral signatures. 
Examples of targets with spectral signature are well known and include 
environmental vapor hazards and vapor leaks at industrial sites. Although 
the specific embodiment describes the application to chemical vapor 
detection, it is intended that the invention cover all alternatives, 
modifications, and equivalents as may be included within the spirit and 
scope of the invention defined in the appended claims.