Method and apparatus for producing enhanced images of curved thermal objects

Curved thermal objects emit infrared energy with different ratios of ordiy-to-extraordinary polarizations compared to flat thermal objects. The invention takes advantage of this by taking the sum or difference of or between thermal images for two different polarization images of a scene. The curved objects are enhanced with respect to the remainder of the scene on a viewer.

BACKGROUND OF INVENTION 
This invention is in the field of viewers for infrared radiation emitted by 
thermal bodies. there are various known types of infrared detectors which 
provide visible images, such as photovoltaic and photoresistive diodes and 
transistors, infrared vidicons, charged coupled devices, etc. These 
detectors have different complexities and uses, but all have difficulty in 
providing a recognizable image of a thermal object in defilade or clutter. 
One manner by which this difficulty is countered is the use of polarizers 
for object radiation incident on the detectors. It is a fact that thermal 
objects of different shapes emit (or reflect) thermal energy at different 
amounts for different polarizations. By choosing a particular polarization 
of incoming thermal energy, it is possible to increase the contrast 
between differently shaped bodies in a viewer. Unfortunately, using only a 
particular polarization decreases the total amount of infrared energy 
reaching the detector, for a particular infrared scene. Obviously, this 
degrades image quality. Moreover, background and clutter are not 
suppressed, whereas the instant invention does suppress such background or 
clutter while at the same time enhancing the images of curved objects such 
as tank gun barrels or turrets. U.S. Pat. No. 4,333,008 of Jun. 1, 1982 
shows a system which uses alternate output pulses, having orthogonal 
polarizations, from a laser to illuminate a scene. Man-made specular 
objects reflect both polarizations, whereas natural objects act as diffuse 
reflectors; in any event, the reflected illumination is fed to two 
detectors, each responding to a different polarization. The differential 
sum of the detector outputs is high for specular objects and low for 
natural (non-specular) objects. This system does not give an overall image 
of the scene, but does indicate the presence of specular objects as the 
beam is manually swept over the scene. 
SUMMARY OF THE INVENTION 
The invention is a method for producing enhanced images of curved thermal 
objects in a field of other thermal objects, and an apparatus for 
practicing the method. The method consists of: obtaining thermal images of 
the field for different polarizations of the radiation therefrom, adding 
or subtracting pairs of images for different polarizations (preferably 
orthogonal) to produce an other image, and displaying this other image for 
a viewer. The apparatus includes means for performing these various 
functions or steps.

DETAILED DESCRIPTION OF INVENTION 
This invention may be best understood when this description is taken in 
conjunction with the drawings. The method proceeds as indicated in the 
flow chart of FIG. 1 and begins with detecting and storing a polarized 
field from a infrared scene. The next step is to detect another field of 
the same scene, but at a different polarization (preferably orthogonal to 
the previous polarization). The third step consists of adding or 
subtracting (pixel for pixel) the two stored fields to produce a sum or 
difference field. Finally, this last-mentioned field is displayed as a 
visible image to a viewer. If a summation is used, a curved surface is 
enhanced in the visible image; if subtraction is used, the edges of a 
curved surface are enhanced. 
FIG. 2 schematically shows an apparatus for performing the method of the 
invention, and includes polarizer 10 which passes one polarization of 
infrared energy omitted by a curved object 11 emitting infrared energy and 
directed onto 10 by lens 12. Although not shown, it should be understood 
that a cluttered background may surround or partly obscure object 11. 
Polarizer 10, for infrared, may be a CdTe crystal surface from which the 
radiation of 11 is reflected at the Brewster angle. In any event, the 
polarized radiation from 10 passes through shutter 13 to detector 14. This 
detector may take any one of several different forms, such as a photodiode 
scanned over the scene containing the object, a FLIR, or an infrared 
vidicon. The output of 14 passes to storage and processing device. 15. For 
a detector with a single photodiode, the storage may be performed by 
nothing more complex than a shift register for each polarization field, 
and the processing may be done by a comparator for the shift registers. 
For a television type of detector a magnetic tape or disc may be used, 
with the two fields recorded on separate tracks (or segments of tracks) 
and with heads for simultaneously reading out both tracks. The output of 
15 passes to display 16, which may be a cathode ray tube or other display 
device. All of 10, 13, 14, 15, and 16 are synchronized in their operation 
by synchronizer 17. For the case of an infrared television camera for 
detector 14, the synchronizer will be the normal television 
synchronization generator. Obviously, shutter 13 is not needed for a 
television system. Polarizer 10 may be a crystal rotated by a stepping 
motor, or may be an electro-optical device such as a Kerr cell. 
Although this disclosure is directed to a method and apparatus for infrared 
radiation, images of bodies which omit visible radiation could be enhanced 
in like manner. An example of such a body is a red-hot gun barrel.