Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Patent
1989-02-01
1990-08-14
Britton, Howard W.
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
358110, H04N 530
Patent
active
049491741
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to thermal imaging devices, and in particular to optically-read thermal imaging devices.
2. Description of Related Art
Conventional thermal imaging devices include an infra-red sensitive detector and one or more infra-red optical components which focus the radiation from a thermal scene on to the surface of the detector. Devices of this type are described in patent application Ser. No. GB 2150387 and in our own patent application Ser. No. GB 2180361. The material of the detector is chosen to have a highly temperature-dependent optical property, such as birefringence or optical rotation. This detector is also illuminated by a visible or near-visible polarized light source the output of which is reflected off a dichroic mirror on to the detector. The visible or near-visible light experiences a modulation of polarisation, on passing through the detector, which is converted to an intensity modulation by a quarter-wave plate and an analyser. This intensity modulation conferred on the light passing through the detector thus corresponds to temperature variations induced in the detector by the infra-red radiation. In conventional devices, the modulated light is then focused by lenses through a Fourier plane filter on to a television camera chip the output of which is read into a frame store.
In order to vary the field of view of the device, an infra-red zoom lens must be used to focus the radiation from the thermal scene on to the liquid crystal. Infra-red lenses are expensive, and infra-red zoom lenses particularly so; so for any individual system, the range of lenses is usually limited.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thermal imaging device in which the field of view may be varied, whilst avoiding the use of infra-red zoom lenses.
According to the present invention, a thermal imaging device comprises: a non-pixellated detector including a component the optical properties of which vary with temperature; means for forming a focused image of a thermal scene on said non-pixellated detector; a visible light source arranged to illuminate the non-pixellated detector such that visible light from the source is modulated by the image formed on the non-pixellated detector; a further detector effective to detect said modulated visible light, to give an output representative of the image of the thermal scene; and a visible light zoom lens interposed in the optical path between the non-pixellated detector and the further detector such that variation of the focal length of the lens varies the field of view of the thermal imaging device.
The component within the non-pixellated detector is suitably a liquid crystal; the material of the liquid crystal may exhibit temperature-dependent optical properties, such as birefringence or optical rotation.
Preferably the further detector which is effective to detect visible light is a pixellated detector such a a charge-coupled device of the kind used as a television camera chip. Alternatively a television vidicon may be used.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
FIG. 1 is a schematic diagram of a thermal imaging device, and
FIG. 2 is a ray diagram.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In both figures, the rays shown incident on a liquid crystal detector 3 are infra-red rays, whilst those emerging from the detector 3 are rays of visible light.
Referring first to FIG. 1, the device comprises means for forming a focused image of a thermal scene on to a detector 3, a visible light source 17 arranged to illuminate the detector 3, and a visible light zoom lens 5 interposed between the detector 3 and a television camera chip 7.
The means for forming a focused image of the thermal scene includes a chopper 9 which chops the infra-red radiation incident on an infra-red lens 1. This lens directs the radiation on to the full area
REFERENCES:
patent: 3283159 (1966-11-01), Robbins
patent: 4670654 (1987-06-01), Ross
patent: 4687933 (1987-08-01), Loy
Britton Howard W.
Kostak Victor R.
The General Electric Company p.l.c.
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