Thermal imaging apparatus

Abstract: Thermal imaging apparatus including a non-linear optomechanical scanner (10,11) receiving light from a scene and directing it onto an image plane, the scanner (10,11) defining a non-linear scan velocity in the image plane, a SPRITE detector (14) disposed in the image plane and including first and second spaced contacts (20,22) and apparatus (24,31,16) for applying a bias voltage across said contacts (20,22), and apparatus for varying (24) the bias voltage across the contacts (20,22) as a function of the non-linear scan velocity, thereby to compensate for the non-linearity of the optomechanical scanner (10,11).

Description: FIELD OF THE INVENTION The present invention relates to thermal imaging apparatus generally and more particularly to thermal imaging apparatus employing non-linear scanning. BACKGROUND OF THE INVENTION Various types of thermal imaging devices are known in the art. These include staring arrays, parallel scan devices and serial scan devices. Serial scan devices which employ a plurality of detectors arranged in a linear array and interconnected to provide time delay and integration are described in the Laakmann Patent, Israel Patent IL-A-39,389. There is described in an article entitled "An Integrating Detector for Serial Scan Thermal Imaging, by C.T. Elliott, et al, Infrared Physics, Vol 22, pp 31 - 42, 1982, the use of a Mercury Cadmium Telluride "SPRITE" (Signal Processing In The Element) detector for thermal imaging. The SPRITE detector is operative to perform time delay and integration within the detector material itself. The SPRITE detector itself is described in U.K. Patents GB-A-2119508, published October 26, 1983, and GB-A-2127619 published on April 11, 1984 and in U.S. Patents US-A-4,572,953; US-A-4,679,063 and US-A-4,691,107. US-A-3 723 642 discloses a thermal imaging device including an opto-mechanical scanner and a linear detector array. Output signals from each detector element are delayed as a function of the scan rate. However, the scanner can only operate at a constant scan velocity. One disadvantage in the use of the SPRITE detector in a scanning thermal imaging device is that the scan velocity across the detector is non-linear. Accordingly, the time delay and integration provided by the SPRITE detector involves a corresponding inaccuracy. SUMMARY OF THE INVENTION The present invention seeks to provide improved thermal imaging apparatus employing a SPRITE detector. There is thus provided in accordance with a preferred embodiment of the present invention, thermal imaging apparatus including a non-linear optomechanical scanner receiving light from a scene and directing it onto an image plane, the scanner defining a non-linear scan velocity in the image plane, a SPRITE detector disposed in the image plane and including first and second spaced contacts and apparatus for applying a bias voltage across said contacts, and apparatus for varying the bias voltage across the contacts as a function of the non-linear scan velocity, thereby to compensate for the non-linearity of the optomechanical scanner. In accordance with a preferred embodiment of the present invention, the apparatus for varying comprises a feedback loop coupled to the scanner. Alternatively, the apparatus for varying may comprise a buffer memory of any suitable configuration, such as a ROM or look up table (LUT). Preferably the operation of such a buffer memory is synchronized with the motion of the scanner. In accordance with a preferred embodiment of the present invention, the apparatus is suitable for use for scanning in either of two opposite directions and includes means for selectably switching the polarity of the bias voltage across said contacts. In such a case, apparatus is provided for selectably obtaining an output signal at a selected end of said SPRITE detector in accordance with the desired direction of scanning. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a simplified partially block diagram, partially schematic illustration of thermal imaging apparatus constructed and operative in accordance with a preferred embodiment of the present invention; and Fig. 2 is a simplified partially block diagram, partially schematic illustration of thermal imaging apparatus constructed and operative in accordance with another preferred embodiment of the present invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Fig. 1, which illustrates a preferred embodiment of a scanner, such as that useful in a thermal imager, constructed and operative in accordance with the present invention. The apparatus of Fig. 1 constitutes a FLIR, it being appreciated that the present invention is not limited to FLIRs but is applicable to any suitable type of scanner, such as one employed, for example, in medical imaging or in a scanning thermal imager. An optomechanical vertical scanner 10 and an optomechanical horizontal scanner 11, such as is described in U.S. Patent US-A-4,347,530 to Stetson, entitled Scanning Mirror Arrangement, the disclosure of which is hereby incorporated by reference, view a scene and optically direct an image of the scene via suitable scanning optics 12 onto a SPRITE detector 14. The term "SPRITE detector" is used throughout to designate an elongate detector which performs time delay and integration along its length. Preferred SPRITE detectors are described in the publications set forth hereinabove in the Background of the Invention, which publications are hereby incorporated herein by reference. Suitable SPRITE detectors are commercially available from Mullard of the U.K. and are described inter alia in the above-mentioned article by Elliot et al, the disclosure of which is hereby incorporated by reference. As is well known, the time constant of the time delay and integration performed by the SPRITE detector is controlled by the bias voltage applied thereacross, which may be controlled by the output of a voltage amplifier 16, such as a Motorola 741, connected across opposite terminals 20 and 22 of the SPRITE detector 14. In accordance with a preferred embodiment of the present invention, means are provided for varying the bias voltage across terminals 20 and 22 of the SPRITE detector in a non-linear manner so as to match and thus compensate for the non-linearity of the scan velocity of the image across the detector. In the illustrated embodiment, a voltage variation controller 24 operates voltage amplifier 16 to vary the bias voltage across terminals 20 and 22 in response to control inputs from a vertical scanning velocity indicator 26. Typically the voltage variation controller 24 comprises an AID converter 27 which receives the output from scanning velocity indicator 26 via an amplifier 29. The output from converter 27 is supplied to control logic circuitry 31, typically including an Intel 8051 chip which is operative for every received velocity indication to draw from a ROM 33 a suitable compensation value which is supplied via a digital to analog converter, to voltage amplifier 16. Vertical scanning velocity indicator 26 may comprise an encoder, such as a series HEDS-5000 encoder manufactured and sold by Hewlett Packard of the U.S.A., which is coupled to a high speed scanning mirror and senses its position in real time. Alternatively indicator 26 may comprise a line CCD which senses a light source such as an LED mounted on the scanning mirror. As a further alternative, indicator 26 may comprise a current detector which senses the current in a galvanometer or motor which drives the scanning mirror. As a further alternative, a pre-measured indication of non-linearity of the scanning may be employed, in the form of a look up table or any other suitable medium. The output of detector 14 is supplied via an amplifier 28, such as an output amplifier, to a video processor 30. Video processor 30 comprises a multiplexer 32 which receives an input from amplifier 28 and also receives a vertical sync input from indicator 26 via an amplifier 33 and a horizontal sync input and clock bit from a sync generator 34. Sync generator 34 receives an oscillator input from a crystal oscillator, a horizontal sync input from horizontal scanner 11 via an amplifier 38 and a vertical sync input from indicator 26. The output of multiplexer 32 may be supplied as a RS 170 standard signal to a monitor 40 or other utilization apparatus. Reference is now made to Fig. 2, which illustrates another preferred embodiment of the invention which is suitable for scanning of the SPRITE detector in two opposite directions along its axis. The apparatus is essentially identical to that of Fig. 1 other than in that provision is made for applying the bias voltage accross the SPRITE detector alternatively in opposite directions. Here control logic circuitry 31 is operative to provide switching inputs to a two pole controllable switch 42, such as an AD 7512 DITQ 883B from Analog Devices, which selects from which end of SPRITE detector 14 an output is supplied to amplifer 28. The selection of which end of SPRITE detector 14 is employed for the output is coordinated with the vertical scan direction. Control logic circuitry 31 is operative also to switch the polarity of the bias voltage applied to the ends of the SPRITE detector 14 via the voltage amplifier 16 to correspond to the selection of the output of the SPRITE detector and the direction of scan. It is appreciated that the present invention is operative with both focussing and non-focussing optics. The present invention is operative to correct any measurable nonlinear motion accross the detector axis. The invention is applicable both to serial detector arrangements and to serialparallel detector arrangements, wherein a plurality of serial detectors are connected in paralllel. It is appreciated that the designations of vertical and horizontal are arbitrary and may be interchanged or used to refer to any arbitrary directions. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow:

Claims: Scanning apparatus comprising: a non-linear optomechanical scanner (10,11) receiving light from a scene and directing it onto an image plane, said scanner (10,11) defining a non-linear scan velocity in the image plane; a SPRITE detector (14) disposed in said image plane and including first and second spaced contacts (20,22) and means for applying a bias voltage (16,35,24) across said contacts (20,22); means for varying (24) the bias voltage across said contacts as a function of the non-linear scan velocity, thereby to compensate for the non-linearity of the optomechanical scanner. Apparatus according to claim 1, wherein said means for varying (24) comprises a feedback loop coupled to the scanner (10,11). Apparatus according to claim 1, wherein said means for varying (24) comprises a buffer memory (33). Apparatus according to any one of the preceding claims and arranged to scan in either of two opposite directions and including means (31,42) for selectably switching the polarity of the bias voltages across said contacts. Apparatus according to claim 4 and comprising means for selectably obtaining an output signal (31,16) at a selected end of said SPRITE detector in accordance with the desired direction of scanning.

Inventors: GAL ELI, GAL, ELI
Applicants: BENDIX AVELEX INC, BENDIX AVELEX INC.
Application year: 1992
Main classification: H04N3
IPC codes: H04N3