GB2052907A - Thermal object coordinate determination - Google Patents

Abstract

The positional accuracy obtained when observing an airspace over a wide angular range using a plurality of optical channels arranged at an angle relative to each other to determine the coordinates of a thermal object may be low because the individual channels are relatively large and hence limited in number. An arrangement is provided which comprises a number of optical channels, which scan adjacent solid angles and, via a moving modulation grating 3 which operates as a spatial filter, image each optical channel on a matrix 8 of IR detectors by means of an optical system 5, 6, 7. When an IR detector supplies a signal corresponding to a thermal object to be detected, the coordinate values of the thermal object relative to the relevant optical channel can be derived from the position of said IR detector within the matrix. When the individual IR detector signals are scanned by means of a multiplexer 13 controlled by a counter 14 the counter output 16 may directly represent the binary coded coordinate values. <IMAGE>

Description

SPECIFICATION Thermal object coordinate determination The invention relates to an arrangement for determining the coordinates of a thermal object when observing an airspace over a wide angular range, the arrangement comprising a plurality of optical channels arranged at an angle relative to each other, which channels each have an objective, a moving grating for modulating the image fields of the optical channels and, disposed in a common imagefield plane, an assembly of IR detectors associated with the optical channels.

An arrangement of this type, known from the German patent application 25 11 016, employs image-field modulation by means of a regular so-termed random grating, so that point-shaped objects can be filtered out of a large-area background radiation. This results in a high sensitivity and thus a wide range, but the positional accuracy is not very high, because the individual optical channels have comparatively large mechanical dimensions and their number is consequently limited. For a given solid angle to be observed each optical channel therefore covers a comparatively large part of the solid angle, which determines the resolution and the positional accuracy respectively.

Also known are arrangements which employ a grating with a specific pattern. By analyzing the variation of the detector signal as a function of time it is then possible to assign a space point to the thermal object. The accuracy with which the coordinates are determined then depends on the bandwidth of the elements which amplify and analyze the detector signal and this accuracy decreases very strongly if, in order to suppress spurious signals or noise, the detector signal is integrated over a long time so as to increase the sensitivity.

It is the object of the present invention to provide an arrangement of the type mentioned in the preambie which combines a high positional accuracy with a high sensitivity.

According to the invention the arrangement is characterized in that in each optical channel a matrix of IR detectors is arranged and in that the outputs of said detectors are connected to an analyzing device which generates the coordinate values of a thermal object out of the detector signals. The positional accuracy, i.e. the accuracy of the generated coordinate values, is increased in accordance with the number of IR detectors in a matrix.

The arrangement in accordance with the invention has the advantage that the number of channels with a complex construction comprising many mechanical precision-components can be reduced, whilst the accuracy obtained can be increased substantially in comparison with the known arrangement.

Moreover, the sensitivity is improved by increasing the signal-to-noise ratio, because the thermal object imaged on the detector matrix is substantially larger in proportion to the total area of an IR detector. The increase in signalto-noise ratio results in particular from the fact that a large-area background radiation, which is hardly modulated by the grating, is divided over many detector elements, so that the noise power is reduced, whilst the signal power remains unchanged.

Embodiments of the invention will be described in more detail by way of example with reference to the drawings. In the drawing Figure 1 schematically represents an optical channel of an arrangement according to the invention.

Figure 2 represents a detector matrix on a greatly enlarged scale.

Figure 3 represents an optical transformation system.

Figure 4 represents an analyzing device comprising a counter.

As is shown in Fig. 1 an optical channel, a plurality of which have been provided in the arrangement according to the invention comprises an objective lens 1, a deflection mirror 2, two transformation lenses 5 and 6 and one immersion lens 7. In the image plane of the objective lens 1 there is disposed a modulation grating disc 3, which by means of a drive system 4 is rotated through the image field and for example has a modulation grating as is described in the published German patent application 25 10 301, without the trapezoidal interruptions, but with a uniform continuous grating pattern.

Via the transformation lenses 5 and 6 and, as the case may be, an immersion lens 7, the moving grating 3 is imaged onto a matrix 8 of IR detector elements, as is shown on an enlarged scale in Fig. 3. Fig 2 is an elevation of the immersion lens 7 viewed from the side of the matrix 8, where the IR detectors 8' are arranged in rows and columns. This detector matrix, which may comprise several hundred of individual detectors 8', is directly arranged on the immersion lens 7, if such a lens is used. However, if the immersion lens 7 is dispensed with, it may be accommodated on a separate substrate.

For the IR detectors 8', which may for example be epitaxial detectors of PbTe on Bay2, there is provided a thermo-electric cooler 9 as shown in Fig. 1, and the detector matrix 8 with the cooler 9 is accommodated in a housing 10, which also accommodates the detector matrices of the other optical channels. The one lens 6 at the same time constitutes the window of said detector housing 10.

The grating of the disc 3 consists of the checkerboard arrangement of radiation-transmitting and opaque portions, which suitably are rhomboid-shaped, the bounding lines of the rhombs being disposed at an angle of for example 30 to 60 relative to the direction of movement. The dimensions of said rhombs are of the order of magnitude of the resolution of the optical arrangement, i.e. of the order of magnitude of the circle of confusion. In this way an optimum separation of the pointshaped thermal objects from the background radiation is obtained. The individual IR detectors, however, may be selected to be substantially larger than the grating pitch, because their size determines the positional accuracy.

If it is for example assumed that an optical channel covers a solid angle of 100 mR x 100 mR, i.e. a solid angle of approximately 5" x 5 , and the matrix comprises 1 0 x 10 elements, a positional accuracy of +0,25' can be obtained.

Each IR detector 8' of the matrix is connected to a corresponding amplifier 11, as can be seen in Fig. 4. The outputs of said amplifiers 11 are applied to an analyzing device 12, whose outputs are connected to a fire-control system, for example. In the simplest case the analyzing device 1 2 comprises only direct interconnections, so that the firecontrol device receives the coordinate values, i.e. the azimuth angle and the elevation angle, as a 1-out-of-n representation and derives the corresponding angle values therefrom. However, it should then also be taken into account that each optical channel comprises such a matrix of IR detectors, whose signals are then all applied to the fire-control device.In a practical case a signal from IR detector no. 28 in channel 1 2 for example could then correspond to an azimuth angle of 67 and an elevation angle of 4,5 .

If the IR detector signals are applied directly, or merely amplified, to the fire-control deivice, a very great number of connections are necessary. In order to reduce this number of connections the analysing device may convert the coordinate values to be applied to the fire-control device into an encoded form, for example in the form of two binary numbers for the two angle values. This may be effected by means of a known encoding circuit, which consists of diodes or gate circuits. Such a circuit may ten be provided for each channel, the number of the channel then being contained in the code. In the case of a large number of IR detectors per matrix and the use of many channels, however, the arrangement may become very intricate.

Fig. 4 represents a practical embodiment of the analyzing device 1 2. It comprises a multiplexer 13, which consecutively scans its inputs which are connected to the outputs of the amplifier 11 and transfers said input signals to its output. Said multiplexer 1 3 is controlled by a counter 14, which receives a clock signal CL on its count input 1 7 and thus sequentially steps through all counter positions. The outputs of the counting stages of the counter 14 are connected in parallel with the control inputs of the multiplexer 13, which is indicated by the double connection line, and each counter position corresponds to the scanning of one specific input.

The output of the multiplexer is connected to a stage 18, which specifically comprises a threshold value detector, but which may be preceded by an analog amplifier, so that the amplifiers 11 need only have a low pre-gain as well as the known intergrating properties.

The multiplexer 1 3 of course comprises analog switches. As soon as the stage 1 8 detects a signal which exceeds the threshold value, which corresponds to a substantially certain object detection, the stage 1 8 supplies an output signal to the multiple AND gate 1 5 and thus transfers the outputs of the counting stages of the counter 14, which outputs are applied via said gate 1 5 to the output 16, which is connected to the fire-control device.

If for each IR matrix of the individual optical channel such a device in accordance with Fig.

4 is used, the number of the channel may be taken into account by means of an additional multiple input N of the gate 15, or the counter 14 begins to count from a position corresponding to the channel number. However, it is also possible to employ the analyzing device 1 2 for several or all channels, in which case the multiplexer 1 3 should have a corresponding number of inputs, because the scanning sequence of the individual IR detectors 8' or their associated amplifier is only slow, whilst the scanning speed of the multiplexer 1 3 can be substantially higher. In the case of a suitable choice of the matrix size, i.e. the number of IR detectors per matrix, and the channel number in combination with the counting code employed in the counter 14, the binary number applied to the line 1 6 may directly represent the binary-coded coordinate values subdivided in two digits.

The stage 1 8 may also comprise a plurality of threshold-value detectors or one threshold value detector with a plurality of stages, in which case a signal indicating that the threshold of the individual stages is exceeded is applied to the line 19, from which an indication about the distance of the thermal object can be derived.

Claims (8)

1. An arrangement for determining the coordinates of a thermal object when observing an air space over a wide angular range, the arrangement comprising a plurality of optical channels arranged at an angle relative to each other, which channels each have an objective, a moving grating for modulating the image fields of the optical channels, and, disposed in a common image-field plane, an assembly of IR detectors associated with the optical channels characterized in that in each optical channel a matrix of IR detectors is arranged and in that the outputs of said detectors are connected to an analyzing device which generates the coordinate values of a thermal object out of he detector signals.

2. An arrangement as claimed in Claim 1, characterised in that the analyzing device is an electronic encoder.

3. An arrangement as claimed in Claim 1, characterised in that the analyzing device sequentially scans the outputs of the IR detectors and from the interval between the beginning of scanning and the scanning of the signal from an IR detector which corresponds to a thermal object, derives the coordinate values of said object.

4. An arrangement as claimed in Claim 3, characterised in that a counter in the analyzing device controls the scanning of the IR detectors by means of a multiplexer and that an output signal of the multiplexer transfers the counter position to the output of the analyzing device as coordinate values.

5. An arrangement as claimed in any of Claims 1-4 characterised in that the IR detectors are a multiple larger than the grating pitch of the moving grating.

6. An arrangement as claimed in any of Claims 1-5 characterised in that the detector matrix is arranged on an immersion lens, which constitutes an element of an image-field transformation optic in the optical channel.

7. An arrangement as claimed in Claim 1 or any of the following Claims, characterised in hat the IR detectors are epitaxial detectors of PbTe on Ba F2.

8. An arrangement for determining the coordinates of a thermal target substantially as described with reference to the accompanying drawings.