EP0138398A2
Expired / Lapsed
EP
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Thermal imager
Assignee: UK Secretary of State for Defence
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Filing: 1984-09-14
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Granted: —
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Expires: 2004-09-14
Abstract
Abstract A thermal imager includes an infrared sensitive light valve (1) and a light source (3) arranged to illuminate the full responsive area of the valve (1). The imager also includes an optical stage (5) having focussing optics for forming an infrared image upon the surface of the valve (1) and a chopper (23) for modulating infrared radiation from a scene. Modulated light from the valve (1) is read out in parallel by a detector array (7), and frame signals corresponding to alternate positions of the chopper (23) are subtracted to provide iniformity correction. The light valve (1) may comprise an infrared sensitive optically active liquid crystal cell (31) and an analyser (33) adjusted to near extinction. An optical processor (13) comprising a lens (55) and an apodized stop filter (57) lies in the light path between the valve (1) and the detector array (7). The light source (3) may comprise an array of light emitting diode elements (25, 25') and the filter a number of corresponding off-axis stop regions (59').
Inventors
- 👤 Charles Thomas Elliott
- 👤 Richard George Humphreys
- 👤 Rex Watton
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-10-24 05:33
CPC Classifications
H; H04; H04N; H04N23/00; H04N23/20; H04N23/23
Claims Analysis
Total Claims: 17
Claim 1
Independent Claim
130 words
A thermal imager including:- (a) a light valve (1) responsive to absorption of infrared radiation by variation in reflectivity or transmittivity at shorter wavelengths, (b) imaging means (21) for imaging a thermal scene on to the light valve (1), ( c ) a light source (3) arranged for illumination of the light valve's responsive region (41), and (d) means (7) for detecting light from the source (3) after passage via the light valve (1), characterised in that the imager also includes modulating means (23) arranged for intensity modulation of radiation from the scene falling on the light valve (1), and electronic processing means (11) arranged to provide an output signal indicating changes in detected light intensity produced by modulation of radiation from the scene received by the light valve (1).
Claim 2
Dependent Claim
130 words
A thermal imager including:- (a) a light valve (1) responsive to absorption of infrared radiation by variation in reflectivity or transmittivity at shorter wavelengths, (b) imaging means (21) for imaging a thermal scene on to the light valve (1), ( c ) a light source (3) arranged for illumination of the light valve's responsive region (41), and (d) means (7) for detecting light from the source (3) after passage via the light valve (1), characterised in that the imager also includes modulating means (23) arranged for intensity modulation of radiation from the scene falling on the light valve (1), and electronic processing means (11) arranged to provide an output signal indicating changes in detected light intensity produced by modulation of radiation from the scene received by the light valve (1).
Claim 3
Dependent Claim
54 words
A thermal imager according to Claim 1 characterised in that it is arranged for parallel readout, and wherein the detecting means (7) includes an array of detectors each arranged to monitor a respective light-valve sub-region, and the light source (1) is arranged to illuminate simultaneously all of the light valve's infrared responsive region (41).
Claim 4
Dependent Claim
31 words
A thermal imager according to Claim 2 characterised in that it includes a heat sink (47) uniformly coupled to the light valve responsive region (41) via a high thermal impedance (49).
Claim 5
Dependent Claim
54 words
A thermal imager according to Claim 3 characterised in that the heat sink (47) is a high thermal conductivity sheet of for example BaF 2 , and the high thermal impedance is an evacuated or gas-filled space (49) sandwiched between the heat sink (47) and the responsive region (41) of the light valve (1).
Claim 6
Dependent Claim
30 words
A thermal imager according to Claim 4 characterised in that the light valve (1) comprises an infrared sensitive, optically active, liquid crystal cell (31) and an optical polarization analyser (33).
Claim 7
Dependent Claim
15 words
A thermal imager according to Claim 5 characterised in that the analyser (33) is rotatable.
Claim 8
Dependent Claim
24 words
A thermal imager according to Claim 6 characterised in that the analyser rotation is servo-mechanism controlled to follow changes in liquid crystal cell temperature.
Claim 9
Dependent Claim
18 words
A thermal imager according to any preceding claim characterised in that the modulating means is a chopper (23).
Claim 10
Dependent Claim
41 words
A thermal imager according to any one of Claim 2 to 8 characterised in that it includes one or more optical signal processing components (55,57) arranged in a light path between the liquid crystal cell (31) and the detector array (7).
Claim 11
Dependent Claim
29 words
A thermal imager according to Claim 9 characterised in that the optical signal processing components include a lens (55) with a stop filter (57) in its Fourier transform plane.
Claim 12
Dependent Claim
29 words
A thermal imager according to Claim 10 characterised in that the stop filter (57) has a central region arranged to transmit a proportion of a zero spatial frequency component.
Claim 13
Dependent Claim
17 words
A thermal imager according to Claim 11 characterised in that the stop filter optical density is graded.
Claim 14
Dependent Claim
36 words
A thermal imager according to Claim 10, 11 or 12 characterised in that the light source comprises an array of source elements (25,25') and the filter (57) includes an array of corresponding stop regions (59, 59').
Claim 15
Dependent Claim
45 words
A thermal imager according to any preceding claim characterised in that it is arranged such that infrared radiation from the scene and light from the source (3) traverse different paths to a dichroic device (53), and thereafter the same path to the light valve (i).
Claim 16
Dependent Claim
38 words
A thermal imager according to Claim 10 characterised in that it includes a dichroic mirror (53) arranged to reflect light from the source (3) and to transmit infrared radiation from the scene on to the light valve (1),
Claim 17
Dependent Claim
40 words
A thermal imager according to any preceding claim characterised in that the light source (3) is arranged to provide pulsed light synchronised to operation of the modulating means (23) such that illumination is provided towards the end of modulation half-cycles.
References Cited (Backward Citations)
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