Radiant energy – Radiant energy generation and sources – Plural radiation sources
Patent
1990-06-21
1991-07-30
Berman, Jack I.
Radiant energy
Radiant energy generation and sources
Plural radiation sources
2504931, 2504941, 428919, G01J 100
Patent
active
050362119
DESCRIPTION:
BRIEF SUMMARY
This invention relates to means and method for reflecting and emitting electro-magnetic energy and although not directly restricted to infrared energy, the invention is particularly suitable for reflectance and emittance for such energy to a degree which will provide a composite energy emission which corresponds to the surrounding environment so that the device can be used for camouflage purposes and thereby reduce the danger of detection by surveillance systems or the danger of a tracking device of a missile detecting its target against the background.
BACKGROUND OF THE INVENTION
Surveillance systems and missiles frequently make use of infrared detectors capable of comparing temperature differentials between objects and their backgrounds by comparing their emitted infrared energy, in some cases where the differential is as low as 0.1.degree. K. Missile detectors usually rely largely on the existence of a radiation contrast between the target and the background area, the net radiation from each being caused by both reflection and emission from their surfaces. The detectors are often operable in the wavelength ranges of from 3 to 5 and 8 to 14 micrometers. The wavelength band with which this invention is concerned extends throughout the infrared range and can also be applied to the ultraviolet, visible and millimeter wavebands.
It is known that reflectors have been made to reflect for example the energy from the local environment in the case of an object to be camouflaged so that the detector will fail to "recognise a target", but that system has limitations and is only partially effective, due to difficulty in selecting a region of the local environment to be reflected which has the same radiance as the background to be matched.
It is therefore an object of this invention to provide means which will minimise the differential of the combination of radiation and reflectance between a potential target and its background thereby constituting a "camouflage".
BRIEF SUMMARY OF THE INVENTION
In this invention there is provided on an object to be camouflaged an arrangement of surfaces which comprise reflecting surfaces and energy radiating surfaces, and by measuring the background emission and object emission, control of the energy radiating surfaces can effect a match between the two.
The energy which is received from the sky at high elevation angles, particularly at night is equivalent to that from a black body source at a low temperature, often in the range of 240.degree. K. to 250.degree. K. The energy emanating from a sea surface (for example) is equivalent to that of a black body in the range of 270.degree. K. to 300.degree. K. A background at sea therefore is likely to have a wavelength emission approximating that of a black body of temperature between 260.degree. K. and 290.degree. K. depending on aspect which effects the sea surface emissivity and reflectivity.
Energy from a grey-body source having a temperature of T.degree. K. and emissivity of .epsilon. is a function of the temperature and emissivity according to the formula .sigma..epsilon.T.sup.4, where .sigma. is Stefan's constant.
Energy reflected from a surface of emissivity .epsilon. is a function of the black body source of Temperature T.sub.1 which is reflected according to the formula .sigma.(1-.epsilon.) T.sub.1.sup.4. If the source being reflected is a grey-body of emissivity .epsilon..sub.1 this formula becomes .sigma.(1-.epsilon.).epsilon..sub.1 T.sub.1.sup.4.
The wavelength at which the maximum emission occurs is also a function of source temperature. For a comprehensive understanding of radiated and reflected energy, reference may be made to chapter 1 of the publication RADIATION THEORY by W. L. Wolfe and George J. Zissis, published by the Office of Naval Research, Department of the Navy, Washington, D. C., U.S.A.
The energy difference normally detected by an infrared seeker system at maximum range is usually that equivalent to a black body temperature difference in the range of 1.degree. K. to 5.degree. K. Detectors do not discrimin
REFERENCES:
patent: 3967026 (1976-06-01), Dalblom
patent: 4289677 (1981-09-01), Supcoe et al.
patent: 4611524 (1986-09-01), Ferris
patent: 4634293 (1987-01-01), D'Agostino et al.
patent: 4743904 (1988-05-01), Morton et al.
Berman Jack I.
Callan Edward W.
Nguyen Kiet T.
The Commonwealth of Australia
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