SUN OPTICAL LIMITATION ILLUMINATION DETECTOR (SOLID)

PHOTODETECTEUR (SOLIDE) AVEC LIMITATION GRACE A UN DISPOSITIF OPTIQUE DE L'ECLAIRAGE SOLAIRE

English Abstract

An imaging system for obtaining an image of a field-of-view where high levels of light from an extraneous source may be present at one area and low levels of light present at other areas in the field-of-view. The imaging system has a lens to focus an image of the field-of-view onto a surface of a photodetector and an optical limiter is located between the lens and the photodetector. The optical limiter limits transmission of the high levels of light at areas onto which those are focussed by the lens but remains transmissive to the low levels of light at other areas. The imaging system is particularly intended for use in laser warning receivers where sunlight may be focussed on the optical limiter and blocked at the particularly area on which it is focussed, the optical limiter remaining transmissive to laser beams, even if intense, at locations other than where the sunlight is focussed allowing laser beams to be detected by the photodetector.

French Abstract

Système d'imagerie pour obtenir une image d'un champ de vision où des niveaux élevés de lumière provenant d'une source extérieure peuvent être présents dans une zone et de faibles niveaux de lumière présents dans d'autres zones du champ de vision. Le système d'imagerie est doté d'un objectif permettant de focaliser l'image du champ de vision sur la surface d'un photodétecteur et un limiteur optique est situé entre l'objectif et le photodétecteur. Le limiteur optique limite la transmission des niveaux élevés de lumière dans les zones que l'objectif focalise mais permettant la transmission des faibles niveaux de lumière d'autres zones. Le système d'imagerie est particulièrement destiné à des récepteurs d'alerte laser où la lumière du soleil peut être focalisée sur le limiteur optique et bloquée dans la zone spécifique sur laquelle il se focalise, le limiteur optique permettant la transmission des rayons laser, même si ceux-ci sont intenses, dans des endroits autres que ceux sur lesquels la lumière du soleil se focalise permettant aux rayons laser d'être détectés par le photodétecteur.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A light detection system for detecting laser beams where
high levels of light from an extraneous source may be present
at one area in the system's field-of-view, the light detection
system comprising means to focus an image of the field-of-view
onto a surface of a photodetector, an optical limiter being
located between the means to focus and the photodetector, the
optical limiter having means to limit transmission of said high
levels of light received from the extraneous source at a
particular area where the high levels are directed onto the
optical limiter by the means to focus, the optical limiter
remaining transmissive to light received from areas in the
field-of-view outside of said particular area and allowing any
laser beam received from the field-of-view outside of the
particular area to be focussed onto the photodetector, said
photodetector comprising a position sensing photodetector from
which the position of any laser beam focused onto the
photodetector can be determined and an angle-of-arrival of such
a laser beam can be determined from said position, and said
optical limiter comprising a liquid crystal display (LCD) and
said system further including means to determine the location
on the LCD of an area onto which the high levels of light are
directed by the means to focus, the system further including
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means to activate the LCD to place the LCD in an opaque mode at
the area where said high levels of light are directed.
2. A light detection system as defined in Claim 1, wherein an
optical filter transmissive to laser beams to be detected is
located between the optical limiter and position sensing
photodetector.
3. A light detection system as defined in Claim 2, wherein
the position sensing photodetector is a charge coupled device
imaging sensor that generates a signal at the position on its
surface where any laser beam is focussed.
4. A light detection system as defined in Claim 1, wherein
the means to focus comprises a first lens and the means to
determine the location on the LCD of said area comprises a
second lens having a similar field-of-view as the first lens,
the second lens focussing said similar field-of-view onto a
second position sensing photodetector where the location at
which said high levels of light is focussed can be determined,
the imaging system further including means to place the LCD
into an opaque mode at an area corresponding to the location
determined by the second position sensing photodetector.
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5. A light detection system as defined in Claim 4, wherein an
optical filter opaque to laser beams to be detected is located
between the second lens and the second position sensing
photodetector, the optical filter being transmissive to
radiation from the sun.
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Description

CA 02310262 2000-OS-29
FIELD OF THE INVENTION
The present invention relates, in general, to
imaging systems for low level radiation sources where high
levels of extraneous wide-band light may be present in the
field-of-view and, in particular, to Laser Warning Receivers
(LWR) for protecting military platforms against laser guided
weapons by detecting and locating very low power laser sources
associated with those weapons when light from other sources,
such as the sun, may be present in the same field-of-view.
BACKGROUND OF THE INVENTION
Many research laboratories and manufacturers around
the world are presently involved in the development of Laser
Warning Receivers (LWR) for the purpose of protecting military
platforms against laser guided weapons by detecting,
identifying and locating laser sources associated with those
weapons. One type of LWR is described in US Patent 5,428,215
by J. Dubois et al. The detection threshold of most LWRs are
limited, however, by extraneous light from other sources and
particularly by the high level of background radiation
generated by the sun. The sun is almost always the limiting
factor for highly sensitive light detection systems required
to detect and locate very low power laser sources such as
associated with a Laser Beam Rider (LBR) where the laser is
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aimed at a target to guide a missile towards that target. The
detectors for these types of very low power laser sources
still have a limited capability because the signal they
produce is often buried in the background radiation. It is
possible to reject part of the sun's radiation through optical
filtering but this is at the expense of a narrower optical
bandwidth covered by that LWR. Even when using optical
filters, the ultimate limit in the detection threshold is the
residual sun radiation.
The sun is also a problem for other imaging systems,
such as video cameras when the sun is in the field-of-view of
the image sensor. This results in it being very difficult to
balance the brightness of the image and a lot of information
in the image is lost because of the very bright spot of light
generated by the sun at one location in that image.
Various techniques already exist for the detection
of laser sources, some of which use narrow-band optical
filters, multiple narrow field-of-view (FOV) detectors or
multiple lenses combined with a detector array to handle the
problem of radiation from the sun during the detection of such
sources. LWRs based on these types of approaches succeed in
reducing the effects of the sun's radiation but, most of the
time, this leads to very costly and bulky devices. One
disadvantage of using multiple lenses combined with a detector
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array is that a large number of detection channels result.
The use of filters has limitations due to the large number of
them required for a successful rejection of radiation from the
sun.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide
a light detection system capable of discriminating between
high levels of radiation from bright extraneous sources and
light from other sources and of attenuating the high levels to
allow the detection system to detect the light from other
sources.
An imaging system for a field-of-view where high
levels of light from an extraneous source may be present at
one area in the field-of-view, according to one embodiment of
the present invention, has means to focus an image of the
field-of-view onto a surface of a photodetector, an optical
limiter being located between the means to focus and the
photodetector, the optical limiter having means to limit
transmission of said high levels of wide-band light received
from the extraneous source at a particular area where the high
levels of light are directed onto the optical limiter by the
means to focus, the optical limiter remaining transmissive to
light received from areas in the field-of-view outside of said
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particular area allowing light received from outside of said
one area to be focused onto the photodetector.
A light detection system for detecting laser beams
where high levels of wide-band light from an extraneous source
may be present at one area in the system's field-of-view,
according to a further embodiment of the invention, has means
to focus an image of the field-of-view onto a surface of a
photodetector, an optical limiter being located between the
means to focus and the photodetector, the optical limiter
having means to limit transmission of said high levels of
wide-band light received from the extraneous source at a
particular area where the high levels are directed onto the
optical limiter by the means to focus, the optical limiter
remaining transmissive to light received from areas in the
field-of-view outside of said particular area allowing any
laser beam received from the field-of-view to be focussed onto
the photodetector.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail
with reference to the accompanying drawings, in which:
Figure I illustrates a Sun Optical Limitation
Illumination Detector (SOLID) according to one embodiment of
the present invention;
Figure 2 illustrates a detector (SOLID) according to
another embodiment of the present invention; and
Figure 3 illustrates a still further embodiment of a
(SOLID) detector according to the present invention.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to an imaging
system capable of discriminating between high levels of
radiation from bright extraneous sources and low levels of
radiation from sources to be detected and of attenuating the
high levels at the image detector to allow detection of the
low level sources. The present invention is, in particular,
directed to Sun Optical Limitation Illumination Detectors
(SOLID) which allow Laser Warning Receivers (LWRs) to detect
very low level laser sources, those associated with Laser Beam
Rider (LBR) weapons, even when bright wide-band light from the
sun is present in the detectors field-of-view (FOV).
A basic concept of a SOLID detection module
according to the present invention for a LWR detector is
illustrated in the embodiments shown in Figure 1. In
Figure 1, the module consists of a photodetector l, or any
other type of light sensor, on which is deposited an optical
limiter 2 and a lens 3 is used to focus light from sources in
the field-of-view of the lens and direct that light onto the
limn er 2. Assuming that the module illustrated in Figure 1
is part of a LWR mounted on a military platform, such as an
aircraft, it would operate by laser radiation L from a laser
source, such as a LBR, being collected by lens 3 and focussed
as a small light spot 4 on the optical limn er surface 2 when
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a laser source associated with a LBR is aimed at the aircraft
to guide a missile towards it. The optical limiter 2 is
located just above the detector's 1 surface and the optical
limiter 2 for this particular module is one selected to react
to sunlight, which contains a full spectrum of light, but will
not react to the wavelength generated from a narrow band laser
source that is to be detected. The laser light that is
focussed on spot 4 will, as a result, be transmitted through
optical limiter 2 with very little attenuation and produce a
laser light spot 6 on the surface of photodetector 1. If this
type of module is used in the presence of the sun, the
sunlight S will also be focussed as a light spot 5 on the
optical limiter's 2 surface. The spot 5 on which light from
the sun is focussed would normally be at a different location
on the surface of limner 2 from that on which the laser
spot 4 appears since the sunlight and light from the laser
would generally arrive at different angles. The focussing of
the sunlight onto spot 5 of the optical limiter's 2 surface
will cause the limiter 2 to react and to strongly attenuate
the radiation from the sun locally but will not react to the
laser's wavelength or where the laser is focussed on the
limn er's 2 surface. This phenomenon is possible because the
sun contains a full spectrum of optical radiation whereas
lasers normally emit in a single narrow optical band.

CA 02310262 2000-OS-29
Suitably choosing the wavelength at which the optical
limiter 2 will react results in it being possible to
specifically attenuate radiation received from the sun and not
radiation from lasers which emits in a certain, narrow optical
band.
A typical example of an optical limn er 2 which may
be used is the photochromic process used in prescription
glasses where the UV content of sunlight is used to trigger
the optical attenuation phenomenon of the glasses in the
l0 visible range of the spectrum. The attenuation process is
relatively slow to initiate for prescription glasses, but, the
reaction time can be dramatically reduced by using the lens 3
to focus the sunlight S onto spot 5.
In a further embodiment of the invention, an optical
filter 7, as illustrated in Figure 2, is added between the
photodetector and the optical limiter 2 layers to further
reduce the residual sun background radiation. In the Figure 2
embodiment, the photodetector 1 of Figure 1 has been replaced
by a position sensing device 8 such as a CCD detector. A
20 position sensing device 8 is one that generates a signal at
the position of a light spot on its surface measured along the
X and Y axis. Therefore, it is possible to measure the angle
of arrival of the radiation through a calibration of the laser
spot 4 displacement as a function of the focal length of the
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CA 02310262 2000-OS-29
lens. The laser radiation, in this embodiment, is again
focussed on a spot 4 of the optical limiter where it is
transmitted through the optical limiter 2 and the optical
filter 7 to produce a laser spot 6 on the position sensing
photodetector 8. The angle-of-arrival of radiation L from a
laser source (not shown) can then be determined from the
position of the laser spot 6 on the position sensing
photodetector 8.
Figure 3 illustrates a further embodiment of a SOLID
detector according to the present invention wherein the
optical limiter 2 shown in Figure 2 has been replaced by a
liquid crystal display (LCD) 10 onto which the laser radiation
is focussed by lens 3 at laser spot 6'. A position sensing
photodetector 8' is located next to and below LCD 10 and that
laser spot (4 in Figure 2) is transmitted through the LCD onto
the position sensing photodetector 8' to a laser spot 6'. The
position sensing detector, i.e. a CCD image sensor, can then
detect the laser beam and the angle-of-arrival of the laser
radiation determined from the position of that laser spot 6'
on detector 8'. In this embodiment, a second parallel lens 30
(similar to lens 3) focuses the received laser radiation onto
a spot 34 of a second optical filter 37 which blocks the laser
radiation wavelength but which has an optical bandwidth to
transmit radiation from the sun S. The radiation S from the
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CA 02310262 2000-OS-29
sun is then focussed by lens 30 onto a second position sensing
photodetector 38. The position of the sun spot 35 which
lens 30 produces on the position sensing photodetector 38 is
then determined and a dark (opaque) spot 11 is generated at
the corresponding location on the LCD 10 where that sunlight
would be focussed by lens 3. The opaque spot 11 then locally
attenuates the sunlight transmitted through LCD 10 and blocks
it from reaching the position sensing photodetector 8'. The
position sensing photodetector 8' can then determine the
position of laser spot 6' and the angle-of-arrival of the
laser radiation L without the detection of laser radiation L
being lost in the background radiation from the sun. Although
lens 30 was similar to lens 3 in the Figure 3 embodiment,
other types of optical arrangements could be used to determine
the position of spot 5, where the sunlight S is focussed by
lens 3 on the LCD, and used to darken that area of the LCD 10
so as to prevent, or at least limit, the sunlight S from
reaching the position sensing detector 8'. This optical
arrangement can also be used in video cameras to specifically
attenuate the effects of the sun on a charge-coupled device
(CCD) image sensor where a picture from the camera's lens is
focussed, the sun being in the same field-of-view of the lens.
Various modifications may be made to the described
embodiments without departing from the spirit and scope of the
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invention as defined in the appended claims. The described
embodiments, for instance, have been directed to detection of
a low level laser beam but by selecting an optical limiter to
react to wavelengths outside of an optical bandwidth of
interest, a narrow bandwidth light source of interest from a
laser will be transmitted through the optical limiter and
detected even when it is an intense source. The performance
of the High Angular Resolution Laser Irradiation Detector
described in US Patent 5,428,215 by J. Dubois et al, for
example, could be substantially increased by adding a
cylindrical lens on top of that type of detector and combine
it with an optical limiter, as described above, positioned
over the photodetector array. This would result in a much
more sensitive detection unit through specific local
attenuation of the sun's radiation.
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Representative Drawing