FLIR-1005
`FLIR Systems, Inc. vs CANVS Corporation
`Page 1 of 7
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`FLIR-1005 / Page 3 of 7
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`1
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`SPECIFICATION
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`Low light viewing apparatus
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`5 This invention relates to low light viewing apparatus,
`and is more particularly but not exclusively con—
`cerned with the use of such apparatus as a night
`sight.
`Low light viewing apparatus based on image
`10 intensifiers is well known, as is its use as a night
`sight. Although night sights based on image intensi-
`fiers work reasonably well when used to view
`relatively high contract targets under relatively clear
`low light conditions, they are less satisfactory when
`15 used to view camouflaged or low contrast targets,
`particularly when mist and/or smoke are present.
`This is a serious drawback, since some or all of these
`latter conditions are commonly encountered in
`battlefield and like situations, where the use of a
`20 night sight is particularly desirable.
`Vision under these conditions can be achieved
`using thermal imaging systems based on photovol:
`talc technology. However, such systems require
`thermal stabilisation or cooling, and are therefore
`25 relatively large and heavy and consume consider-
`able power. As a result, they cannot be considered to
`be readily portable.
`It is therefore an object of the present invention to
`provide low light viewing apparatus which incorpo-
`rates thermal enhancement of its image, but which is
`nevertheless sufficiently light in weight and compact
`to serve as a night sight for a hand held weapon or
`as a readily portable observation device.
`According to the present invention, there is pro-
`vided low light viewing apparatus comprising image
`intensifier means, a thermal imaging system
`arranged to produce a thermal image ofa selected
`area in the field of view of the image intensifier
`means, means for combining said thermal image
`with the image produced by the image intensifier
`means, and an eyepiece for viewing the combined
`images, the thermal imaging system comprising:
`pyroelectric detector means comprising an array
`of pyroelectric detector elements;
`means operative to repeatedly apply an image of
`said selected area in the field of view ofthe image
`intensifier means to the pyroelectric detector means;
`electronic circuit means synchronised with the
`image applying means for sampling the outputs of
`the detector elements and for digitising and storing
`said sampled outputs; and
`means responsive to the digitised outputs stored
`by the electronic circuit means to form said thermal
`image.
`The invention will now be described, by way of
`example only, with reference to the accompanying
`drawings, of which:
`Figure 7 is a somewhat schematic representation
`of a night sight with thermal imaging in accordance
`with the present invention; and
`Figure 2 is a schematic circuit diagram of the
`electronic circuitry associated with the thermal im-
`aging in the night sight of Figure 1.
`The night sight shown in Figure 1 is indicated
`65 generally at 10, and comprises a small, lightweight,
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`optical night sight section 12, which is based on an
`image intensifier 14. The image intensifier 14, which
`is typically a Mullard XX1500 second generation
`image intensifier, has a photocathode 16, and is
`preceded by input optics 18 including an objective
`lens (not shown) forfocussing an image of the field
`ofview ofthe night sight section 12 onto this
`photocathode. The image intensifier 14 also has a
`phosphor screen 20, on which it produces a much
`intensified version ofthe image focussed onto its
`photocathode 16. The electrical power for operating
`the image intensifier 14 is derived from a recharge-
`able battery 21.
`The optical night sight section 12 described thus
`far is relatively conventional: in particular, its mode
`of operation is well known, and will therefore not be
`described in any further detail.
`The intensified image produced on the screen 20
`ofthe image intensifier 14 is viewed by the user of
`the night sight via a magnifier 22 having an eyecup
`24. Incorporated in the magnifier 22 is a beam
`combiner cube 26, whose purpose will become
`apparent hereinafter. The beam combiner cube 26
`has a diagonal beam combining surface 28, which is
`coated with a dichroic material selected to preferen-
`tiallytransmit the green light ofthe intensified image
`on the screen 20 into the eyepiece 24.
`As already indicated, although the night sight
`section 12 works well under relatively clear low light
`conditions, it works less well in the presence of mist
`and smoke and when viewing camouflaged and
`other low contrast targets. However, underthese
`latter conditions, thermal imaging can provide good
`results. To take advantage of this, the nigh sight 10 is
`provided in accordance with the present invention
`with a thermal imaging system 30.
`The thermal imaging system 30 is mounted adja-
`cent the night sight section 12, with its optical axis
`parallel to that of the section 12, in order to view an
`area in the centre of the field of view of the section
`12. Thermal radiation from this central area is
`collected by an afocal telescope 32, comprising two
`germanium lens elements (not shown). The radia-
`tion thus collected then passes through an optical
`scanning system 34, and is focussed by a two-
`element germanium focussing lens 35 onto a linear
`pyroelectric detector array 36.
`The pyroelectric detector array 36 is a 32 element
`compensated array of a kind available from Plessey
`Optoelectronics and Microwave Ltd, and comprises
`32 pairs of adjacent detector cells arranged in a line.
`One cell of each pair is blackened so that it will
`absorb incident infra-red radiation, while the other is
`gold plated to prevent this. The cells of each pair are
`then connected in opposition, which means that
`changes in the individual cell outputs in each pair
`due to changes in the temperature ofthe common
`substrate tend to cancel out.
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`The cells are responsive to the rate of change of
`incident infra-red radiation, ratherthan to the abso-
`lute level of this radiation, which is why it is
`necessary to provide the scanning system 34 to
`move the image viewed by the array 36 across the
`cells. However, the array 36 has the advantage that it
`does not require cooling for its operation, as do
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`some other forms ofinfra-red radiation detector.
`The scanning system 34 comprises two coaxial,
`contra-rotatable, germanium wedges 40, each of 1°
`wedge angle, driven in contra—rotation, typically at
`5 600 R.P.M., by an electric motor 42. To permit this,
`the wedges 40 are mounted in respective coaxial
`annular carriers 44, each having a respective 45°
`bevel gear 46 formed around its periphery. The
`gears 46 face each other, and are both driven by a
`10 single 45“ bevel gear 48, whose axis is perpendicular
`to the common axis of the gears 46 and which is
`driven by the motor 42.
`The scanning system 34 operates, in conjunction
`with the telescope 32 and the focussing lens 38, to
`15 sweep an image of the aforementioned central area
`ofthe field of view of the night sight section 12 back
`and forth across the pyroelectric detector array 36.
`This image moves in a straight line perpendicularto
`the length of the array 36, and its rate of movement
`20 varies sinusoidally.
`Secured coaxially to the rear face of the carrier 44
`nearerto the focussing lens 38 is an annular optical
`encoder 50. The encoder 50 has a slotted rim 52,
`which is disposed between a solid state light source
`25 54 and a solid state light detector 56, to alternately
`permit and interrupt the transmission of light from
`the former to the latter.
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`The outputs from the pyroelectric detector array
`36, and the output from the light detector 56, are
`30 connected to electronic signal processing circuitry
`60, which will be described in more detail in relation
`to Figure 2. The circuitry 60, whose operation is
`synchronised with that of the scanning system 34 by
`the output signals from the light detector 56, proces-
`35 ses the output signals from the pyroelectric detector
`array 36 as will hereinafter be described, and uses
`the processed signals to drive a 32 x 32 square array
`62 of light-emitting diodes, such that the array 62
`creates a thermal image, typically red in colour, of
`40 the aforementioned area scanned by the scanning
`system 34. The array 62 is positioned adjacent the
`beam combiner cube 26, such that the red light of
`the thermal image produced by the array passes into
`the cube via a lens 64, and is reflected by the dichroic
`45 material on the beam combining surface 28 into the
`eyepiece 24.
`The user looking into the eyepiece 24 therefore
`sees, superimposed on the central area ofthe
`intensified image of the field of view of the night
`50 sight section 12, a thermal image of any object or
`objects in this area, eg people or vehicles,whose
`temperatures are different from, e.g. higher than, the
`average for the area. It will be appreciated that these
`objects, if motionless or camouflaged, might other-
`55 wise not be visible in the intensified image produced
`by the image intensifier 14 alone.
`The electric motor 42 and electronic circuitry 60
`are powered by the battery 21, which is mounted
`between the night sight section 12 and the thermal
`60 imaging system 30. Controls forthe electronic
`circuitry 60, eg ON/OFF and reset switches and an
`overall thermal image brightness control, are indi-
`cated generally at 65.
`The electronic circuitry 60 is shown in Figure 2
`65 connected to receive the 32 compensated outputs
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`from the 32 pairs of cells making up the pyroelectric
`array 36. As can be seen in Figure 2, the 32 outputs of
`the array 36 are connected via respective buffer
`amplifiers and differentiating circuits 66 to respec-
`tive inputs of a 32: 1 multiplexer 68. The output of
`the multiplexer 68 is connected via a buffer amplifier
`70 to the analogue input of an eight bit successive
`approximation analogue—to—digital converter 72,
`whose digital outputs are connected to a microp-
`rocessor 74.
`The converter 72 has a control input 76, on which it
`receives a command signal from the microprocessor
`74 each time it is required to sample the signal
`currently being applied to it via the multiplexer 68.
`The operation ofthe multiplexer is also controlled by
`the microprocessor 74, via a control line 80. The
`microprocessor 74, which is mask-prog rammed,
`produces the'command signals for controlling the
`multiplexer 68 and the converter 72 in response to
`the timing or synchronising signals produced by the
`light detector 56 associated with the scanning sys-
`tem 34.
`These command signals are arranged to cause the
`converter 72 to successively sample the differenti-
`ated outputs from the pyroelectric detector array 36,
`and to cyclically repeat this successive sampling, but
`only during the two portions of each scanning cycle
`of scanning system 34 which are centred on the
`maximum rate of image movement and which each
`cover about 1/3 of the cycle. Thus although the
`overall rate of image movement varies sinusoidally,
`by confining the sampling to the abovementioned
`portions of each scanning cycle, the rate of image
`movement can be regarded as varying approximate-
`ly linearly (to within about 10%).
`The sampling rate is preferably such that each
`output of the detector array 36 is sampled 32 times
`per approximately linear scan portion, so that the
`microprocessor 74 receives 32 x 32 digital signals in
`each such scan portion, defining the respective
`temperature at each of a matrix of 32 X 32 points
`uniformly distributed over the area viewed by the
`thermal imaging system 30. Although these digital
`signals are eight bit signals, only the four most
`significant bits of each are retained, and are routed
`by the microprocessor 74 into a random access
`memory (RAM) 82.
`The digital signals stored in the RAM 82 are then
`read out sequentially, underthe control ofthe
`microprocessor 74, to control the level of illumina-
`tion of respective ones of the light emitting diodes in
`the diode array 62.
`To this end, the microprocessor 74 operates, via
`an output display logic control circuit 84, to index an
`output display address counter 86, which addresses
`the RAM 82 so as to sequentially read successive
`ones ofthe digital signals stored in it into an output
`pulse time counter 88. The counter 88 is supplied by
`the microprocessor 74, again via the circuit 84, with
`high frequency clock pulses, and is arranged to be
`counted down bythese clock pulses so as to produce
`as its output an output pulse whose duration is
`dependent upon the magnitude represented by the
`digital signal read into it from the RAM 82. This
`output pulse is applied to an addressable XY switch-
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`ing (or routing) buffer circuit 90, which is addressed
`by the same outputs of the address counter 86 which
`are addressing the RAM 82, in order to route the
`output pulse to the diode of the array 62 correspond-
`5 ing to the addressed memory location in the RAM.
`Since the level of illumination of each diode is
`proportional to the duration of the energising pulse
`applied to it, it will be appreciated that the diode
`array 62 produces a 32 X 32 thermal image, having a
`10 16 level grey scale, of the area viewed by the thermal
`imaging system 30. Although the electrical signals
`defining the image produced by the diode array 62
`are updated twice per scan cycle ofthe scanning
`system 34, they are outputted to the array 34 at a
`15 higher rate than this, typically at least twice as fast,
`in orderto keep the thermal image substantially
`flicker free.
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`in order to overcome potential problems due to
`mismatching ofthe diodes ofthe array 62, a digital
`20 signal corresponding to the radiating power of each
`diode can be generated and stored in an EPROM 81
`during an initial calibration or setting up operation,
`and then the appropriate stored signal can be used
`to suitably modify each digital signal read from the
`25 RAM 82 into the counter 88. Alternatively, an
`appropriate 32 X 32 filter matrix, produced photo-
`graphically from the array 62 while driving all its
`diodes with the same current, can be interposed
`between the array 62 and the lens 64.
`30 Many modifications can be made to the described
`embodiment ofthe invention. For example, a mov-
`ing mirror scanning system can be used in place of
`the contra-rotating wedge scanning system 30.
`Alternatively, the scanning system can be replaced
`35 by means for repetitively applying an image of the
`central area of the field of view of the night sight
`section 12 to the array 36, for example a shutter
`mechanism which periodically opens and closes. in
`this case, the linear detector array 36 could advan-
`40 tageously be replaced by a matrix-type array. More
`importantly, the diode array 62 can be replaced by
`several other output devices for outputting the
`thermal image for injection into the eyepiece 24. In
`particular, other possible output devices includes a
`45 back-lit liquid crystal display of the kind currently
`under consideration for use in small flat televisions,
`a single column of light emitting diodes in combina-
`tion with a scanning system synchronised with the
`scanning system 30, or a small cathode ray tube.
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`50
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`CLAlMS
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`1. Low light viewing apparatus comprising im-
`age intensifier means, a thermal imaging system
`55 arranged to produce a thermal image ofa selected
`area in the field of view of the image intensifier
`' means, means for combining said thermal image
`with the image produced by the image intensifier
`means, and an eyepiece for viewing the combined
`60 images, the thermal imaging system comprising:
`pyroelectric detector means comprising an array
`of pyroelectric detector elements;
`means operative to repeatedly apply an image of
`said selected area in the field of view of the image
`65 intensifier means to the pyroelectric detector means;
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`electronic circuit means synchronised with the
`image applying means for sampling the outputs of
`the detector elements and for digitising and storing
`said sampled outputs; and
`means responsive to the digitised outputs stored
`by the electronic circuit means to form said thermal
`image.
`2. Low light viewing apparatus as claimed in
`claim 1, wherein said array of detector elements
`comprises a linear array, said image applying means
`comprises scanning means operative to move an
`image of said selected area in the field of view ofthe
`image intensifier means over said linear array, and
`said electronic circuit means is arranged to sample
`the outputs of the detector means at a plurality of
`selected points in each cycle of operation of the
`scanning means.
`3. Low light viewing apparatus as claimed in
`claim 2, wherein the scanning means comprises a
`pair of contra-rotatable wedges and an electric
`motor connected to drive said wedges in contra-
`rotation.
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`4. Low light viewing apparatus as claimed in
`claim 3, wherein said wedges are made of germa-
`nium.
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`5. Low light viewing apparatus as claimed in
`claim 3 or claim 4, wherein said wedges have angle
`of about 1°.
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`6. Low light viewing apparatus as claimed in any
`one of claims 3 to 5, wherein the electric motor is
`further connected to drive a rotational encoder,
`which is arranged to produce synchronising signals
`for synchronising the operation of the electronic
`circuit with the scanning means.
`7. Low light viewing apparatus as claimed in
`claim 6, wherein the rotational encoder comprises a
`disc connected to be driven by the electric motor, a
`light source, and a light detector arranged to receive
`light from the source via the disc at predetermined
`angular positions ofthe disc.
`8. Low light viewing apparatus as claimed in any
`one of claims 2 to 7, wherein the scanning means
`produces an approximately sinusoidal scan, and
`wherein the electronic circuit means is arranged to
`effect said sampling only during the approximately
`linear portions of each scanning cycle centred upon
`' the maximum rate of image movement.
`9. Low light viewing apparatus as claimed in any
`preceding claim, wherein the thermal image forming
`means comprises an array of light emitting diodes.
`10. Low light viewing apparatus as claimed in
`claim 2 and claim 9, wherein said array of diodes is a
`rectangular array containing N x N diodes, where N
`is the number of detector elements in said linear
`array of detector elements.
`11. Low light viewing apparatus as claimed in
`claim 9 or claim 10, wherein the electronic circuit
`means includes a control circuit for energising said
`diodes at a plurality of at least three different levels
`of illumination.
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`12. Low light viewing apparatus as claimed in
`claim 11, wherein said control circuit includes means
`for supplying pulses to said diodes to energise them,
`and means for varying the duration of said pulses to
`vary the level of illumination thereof.
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`GBZ 143 397 A
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`13. Low light viewing apparatus as claimed in
`any preceding claim, wherein the electronic circuit
`means comprises an analogue-to-digital converter,
`and a multiplexerfor sequentially connecting the
`5 respective output of each detector element of the
`pyroelectric detector means to the input of said
`converter for conversion into a corresponding digital
`signal.
`14. Low light viewing apparatus as claimed in
`10 any preceding claim, wherein the combining means
`comprises a dichroic beam combiner.
`15. Low light viewing apparatus substantially as
`herein described with reference to the accompany—
`ing drawings.
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`Printed in the UKfor HMSO, 08818935, 12/84, 7102.
`Published byThe Patent Office, 25 Southampton Buildings, London,
`WCZA 1AY, from which copies may be obtained.
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