Infrared optical system

Details Infrared optical system Infrared optical system

1998-03-27

2000-07-18

Hannaher, Constantine

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

2504931, 250504R, 257147, 257192, G01J 506

Patent

active

060910690

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an infrared optical system, and more particularly to noise reduction in such a system incorporating a background limited detector.
An infrared detector is said to be "background limited" when the detector noise is predominantly due to fluctuations in the rate at which photons reach it. Such a detector in an infrared optical system will receive photons both from an imaged scene and from sources of stray radiation which do not contain scene information. If stray radiation can be reduced or eliminated, detector noise is reduced without affecting photons from the scene and the system signal to noise ratio is therefore improved.
To reduce stray radiation in infrared optical systems employing a cooled array of detector elements, it is known to employ a shield which is itself cooled by the detector cooling apparatus. The shield is cooled to reduce its thermal emission reaching the detector array. However, in practice it is difficult to provide an efficient cold shield. The problem occurs because of the finite size of the detector array. In order to avoid vignetting of the elements at the edge of the array, the size of the cold shield aperture must be increased, making it less efficient. The problem is reduced by using a larger cold shield more remote from the detector array, but this gives rise to cooling difficulties. A large cold shield increases cooling apparatus requirements and cool down time, and a small cold shield is inefficient in excluding stray radiation. This problem is particularly severe in infrared optical systems having a high f-number and/or in those incorporating a large detector such as a long linear array or a two-dimensional array of detector elements.
One approach to reducing stray light is to position an uncooled concave mirror around a lens in the infrared optical system responsible for imaging a scene onto the detector. The mirror has a central hole to accommodate the lens. The mirror radius of curvature is equal to the mirror-detector separation so that the detector coincides with its image. The mirror necessarily has low emissivity and so generates relatively few photons. However, the mirror can reflect stray radiation to the detector, there remains residual emission from the mirror which causes difficulty and the mirror gives alignment problems.
It is an object of the invention to provide an alternative form of infrared optical system.
The present invention provides an infrared optical system including detecting means and an optical stop arranged to exclude stray radiation from reaching the detecting means, characterised in that the optical stop is arranged to exhibit negative luminescence to reduce radiation incident on the detecting means.
The invention provides the advantage that the detecting means is shielded from stray radiation by the optical stop without the penalty of receiving as much radiation as that from a stop without negative luminescence characteristics but of equivalent nature otherwise.
The optical stop preferably incorporates a hole which is disposed coaxially with an optical axis of the system.
The system may have a single objective lens for directing infrared radiation to the detecting means and a cold shield for the detecting means, the optical stop being located between the lens and cold shield.
The system may alternatively have a plurality of focusing elements arranged in combination to image a scene onto the detecting means, the optical stop being positioned interjacent the focusing elements and detecting means of the system. The optical stop may be positioned between the detecting means and that focusing element which is nearest to it or at an intermediate focal plane within the plurality of focusing elements. The system may include means for scanning a scene over the detecting means.
The system may incorporate scanning means arranged to scan a scene over the detecting means the optical stop being disposed between the detecting means and an optical element which is nearest to the detecting means.
The optical stop may at least partly c

REFERENCES:
patent: 5444250 (1995-08-01), Dietmar
patent: 5834778 (1998-11-01), Veyrier et al.
Berdahl et al, "Negative Luminescence of Semiconductors" Infrared Physics vol. 29, No. 2-4, 1989, pp. 667-672.

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