GB2143397A - Low light viewing apparatus - Google Patents

Abstract

A thermally-enhanced night sight (10) comprises an image intensifier (14), and a separate thermal imaging system (30) arranged to superimpose a thermal image of a selected area of the field of view of the image intensifier on the image produced by the image intensifier. The thermal imaging system (30) comprises an array (36) of pyroelectric detectors, and a rotating prism system (34) for repeatedly sweeping an image of the selected area over the detector array. A microprocessor (74) processes the outputs of the detector array (36) to drive a 32 x 32 LED array (62) which produces the thermal image. <IMAGE>

Description

SPECIFICATION Low light viewing apparatus This invention relates to low light viewing apparatus, and is more particularly but not exclusively concerned with the use of such apparatus as a night sight.

Low light viewing apparatus based on image intensifiers is well known, as is its use as a night sight. Although night sights based on image intensifiers work reasonably well when used to view relatively high contract targets under relatively clear low light conditions, they are less satisfactory when 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 night sight is particularly desirable.

Vision under these conditions can be achieved using thermal imaging systems based on photovol taictechnology. However, such systems require thermal stabilisation or cooling, and are therefore relatively large and heavy and consume considerable 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 incorporates 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 provided low light viewing apparatus comprising image intensifier means, a thermal imaging system arranged to produce a thermal image of a 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 of the 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 1 is a somewhat schematic representation of a night sight with thermal imaging in accordance with the present invention; and Figure2 is a schematic circuit diagram of the electronic circuitry associated with the thermal imaging in the night sight of Figure 1.

The night sight shown in Figure 1 is indicated generally at 10, and comprises a small, lightweight, optical night sight section 12, which is based on an image intensifier 14. The image intensifier 14, which is typically a Mullard Xxi 500 second generation image intensifier, has a photocathode 16, and is preceded by input optics 18 including an objective lens (not shown) for focussing an image of the field of view of the 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 of the image focussed onto its photocathode 16. The electrical power for operating the image intensifier 14 is derived from a rechargeable 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 of the image intensifier 14 is viewed by the user of the night sight via a magnifier 22 having an eyecup 24. Incorporated in the magnifier22 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 preferentially transmit the green light of the 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, under these 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 adjacent 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 radiation thus collected then passes through an optical scanning system 34, and is focussed by a twoelement 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 of the common substrate tend to cancel out.

The cells are responsive to the rate of change of incident infra-red radiation, rather than to the absolute 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 some other forms of infra-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 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 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 sweep an image of the aforementioned central area of the 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 perpendicular to the length of the array 36, and its rate of movement varies sinusoidally.

Secured coaxiallyto the rear face of the carrier 44 nearer to 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 54 and a solid state light detector 56, to alternately permit and interrupt the transmission of light from the former to the latter.

The outputs from the pyroelectric detector array 36, and the output from the light detector 56, are 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, processes 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 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 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 of the intensified image of the field of view of the night sight section 12, a thermal image of any object or objects in this area, eg people orvehicles,whose temperatures are different from, e.g. higher than, the average for the area. Itwill be appreciated that these objects, if motionless or camouflaged, might otherwise 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 imaging system 30. Controls for the electronic circuitry 60, eg ON/OFF and reset switches and an overall thermal image brightness control, are indicated generally at 65.

The electronic circuitry 60 is shown in Figure 2 connected to receive the 32 compensated outputs 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 respective 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 of the multiplexer is also controlled by the microprocessor 74, via a control line 80. The microprocessor 74, which is mask-programmed, 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 system 34.

These command signals are arranged to cause the converter 72 to successively sample the differentiated 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 approximately 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, under the control of the microprocessor 74, to control the level of illumination 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 of the 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 by these 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 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 corresponding 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 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 of the scanning system 34, they are outputted to the array 34 at a higher rate than this, typically at least twice as fast, in orderto keep the thermal image substantially flicker free.

In order to overcome potential problems due to mismatching of the diodes of the array 62, a digital 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 RAM 82 into the counter 88. Alternatively, an appropriate 32 x 32 filter matrix, produced photographically from the array 62 while driving all its diodes with the same current, can be interposed between the array 62 and the lens 64.

Many modifications can be made to the described embodiment of the invention. For example, a moving mirror scanning system can be used in place of the contra-rotating wedge scanning system 30.

Alternatively, the scanning system can be replaced 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 advantageously 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 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 combination with a scanning system synchronised with the scanning system 30, our a small cathode raytube.

Claims (15)

1. Low light viewing apparatus comprising image intensifier means, a thermal imaging system arranged to produce a thermal image of a 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 of the 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.

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 of the 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 contrarotation.

4. Low light viewing apparatus as claimed in claim 3, wherein said wedges are made of germanium.

5. Low light viewing apparatus as claimed in claim 3 or claim 4, wherein said wedges have angle of about 10.

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 of the 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 circuitforenergising said diodes at a plurality of at least three different levels of illumination.

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.

13. Low light viewing apparatus as claimed in any preceding claim, wherein the electronic circuit means comprises an analogue-to-digital converter, anda multiplexerforsequentially connecting the 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 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 accompanying drawings.