GB2100548A - Thermal imagers - Google Patents

Abstract

Edge portions (8) of two thermally emissive members 7 protrude into the field-of-view of a thermal imager near the image plane thereof to provide image temperature references, the temperatures of the two members being controlled by thermoelectric cooling elements (10) for example. <IMAGE>

Description

SPECIFICATION Thermal imagers Athermal imager is a device for scanning a viewed scene to produce a video signal representative of the infra-red radiation image ofthe scene and may comprise an optical system for receiving the radiation, an infra-red sensitive detector device and an opto-mechanical scanning assembly (comprising moving mirrors for example) or electronic scanning means for, in effect, causing the device to scan the scene linewise.

For processing the video signal, particularly for automatic calibration thereof within picture signal generating apparatus which produces a signal for a display on a t.v. monitor, it is desirable to introduce at least two reference signal levels, for example, where the detector device has several elements or photosites, to allow proper equalisation of gain and d.c. level for each of those elements and/or any associated signal amplifier.

According to the invention there is provided, in a thermal imager, means for providing a video reference level, which means comprises two thermally emissive members positioned for appearing in focus in the field-of-view of the imager, and temperature control means for stabilising the temperatures of the two members at respective different levels.

The two members may comprise respective highconductivity plates coupled to respective temperature control and sensing devices and positioned so that an edge portion of each plate just extends into the field-of-view of the imager near the focal plane of an objective optical element, e.g. a lens, a forming part of a telescopic viewing system of the imager. By way of example, the plates may be mounted on an apertured mask positioned at said image plane and operable for shielding the imagerfrom unwanted radiation.

For a better understanding of the invention, reference will now be made by way of example, to the accompanying drawing, in which: Figure lisa diagrammatic view of a part of a thermal imager, and Figure 2 is a sectional diagrammatic view of a mask and temperature reference assembly used in the imager of Figure 1, and Figure 3 is a diagrammatic view of a modification of Figure 2.

The thermal imager comprises a telescopic lens system including an objective lens 1 and a focussing lens 2 for receiving radiation from a viewed scene and directing it to an infra-red detector device (not shown) via a scanning assembly including a rotatable octagonal mirror 3, only part of the octagonal mirror being shown. The mirror 3 is rotated by a drive motor (not shown) and the scanning assembly may comprise further reflective elements for receiving radiation from mirror 3 and directing it onto the detector device. The detector device could comprise an array of several photosites so that no frame scanning means is needed in the scanning assembly all the lines of the scanned raster being executed simultaneously by the effect of the rotating mirror 3.

Alternatively, a suitable frame scanning means (not shown), for example an oscillating or nodding mirror, can be provided either two provide the frame scanning effect in a case where the detector device comprises only one photosite orto enlarge the field-of-view or increase the resolution where the device has an array of photosites. Between the lenses 1 and 2, at the focal plane of lens 1 ,there is provided an apertured mask 4 which blocks the entry of any unwanted radiation and defines the field-ofview of the imager. At two opposite sides of the mask aperture, are mounted respective temperature reference devices 5 and 6.Each device 5 and 6, comprises a plate 7 made of high thermalconductivity material and arranged so that one edge 8 thereof just extends into the field-of-view defined by the mask aperture so as to present a focussed, rectangular image portion along a respective side of the viewed scene. At least these edges are of high-emissivity, i.e. they are thermally "black".Fixed to or within each plate 7, near the protruding edge 8, is a temperature sensing element 9, for example a thermistor, and each plate is coupled to a temperature control component 10, for example a Pletiereffect component which suitably electrically driven presents a cooled surface on one side and a warmed surface at the other side, the components being arranged in the illustrated embodiment so that the plate 7 of reference device 5 is warmed by the associated component 10 while the plate 7 of device 6 is cooled by its component 10.By means of the components 10 and the temperature sensing elements 9, the two plates 7 are maintained at respective regulated temperatures which are within the dynamic operating range of the thermal imager and preferably but not necessarily within the dynamic range of whatever display device e.g. the aforementioned t.v. monitor, is used to display the resultant image. The sides of the mask aperture at which the devices 5 and 6 are positioned depends upon whether it is required to provide the reference levels at the ends of each of some or all of the raster lines or at the beginning and end of each frame, i.e. the edges 8 are arranged perpendicular or parallel to the line direction as desired. If both end of line and end of field references are required or some intermediate combination, the positioning and number of devices 5 and 6 can be chosen to suit.

It will be appreciated that, in the illustrated embodiment, the edges 8 of plates 7 preferably protrude into the field-of-view at relatively undistorted parts thereof and present image portions which are in focus.

In the embodiment of Figure 3, two plates 7a and 7b are coupled to respective ones of the surfaces of a Peltier-component 10a and the plate 7a is also coupled to a Peltier effect component 10b. Highemmissivitiy edge portions 8 of the plates 7a and 7b protrude into the imager field-of-view so as to lie substantially adjacent one another at a position which could be at the top, bottom or either side of the image plane. Near its protruding edge 8, each plate has a temperature sensing element 9 and, under the control of these elements, the components 1 Oa regulates the difference in temperature between the two plates while component lOb steers the temperatures of the two to above or below ambient as desired.

As will be realised, various modifications could be made to the apparatus shown and described herein.

For example, it is not essential that the optical radiation receiving system used should be telescopic or that it should comprise transmissive lens elements as opposed to say reflective or mirror lenses.

The element 3 need not be octagonal, i.e. it could have other than eight sides or some form of scanning system other than one comprising a rotating polygonai mirror could be used, for example some other kind of opto-mechanical system or even an electronic sysem may be suitable.

Claims (5)

1. Athermal imager including video reference level providing means which includes two thermally emissive members positioned for appearing in focus in the field-of-view of the imager, and temperature control means for stabilising and/or controlling the temperature of the two members at respective different values.

2. A thermal imager according to claim 1, wherein said members comprise respective highconductivity plates coupled to temperature control and sensing means and positioned so that an edge portion of each plate just extends into the field-ofview of the imager at least near an image plane thereof.

3. A thermal imager according to claim 2, wherein said plates are mounted upon an apertured mask positioned at said image plane.

4. A thermal imager according to claim 1, wherein said temperature control means includes one or more thermoelectric cooling elements.

5. A thermal imagersubstantially as hereinbefore described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.