Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1996-06-10
1998-02-24
McGraw, Vincent P.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
359589, G01J 318, G02B 528
Patent
active
057216142
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optoelectronic camera of the spectral imager or spectrophotometer type which minimizes phantom images, as well as to a set of photosensitive elements and a protective window for such a camera and to a method for producing such a set of photosensitive elements and such a protective window.
Optoelectronic cameras including an optical system, a CCD-type set of photosensitive elements which is arranged at the focus of said optical system and a transparent protective window arranged in front of said set of photosensitive elements are already known.
Such cameras are increasingly used and they have the advantage of delivering a digitized image.
In addition, such CCD cameras of the spectral imager or spectrophotometer type allow spectral analysis of the scene which they observe. In this case, they include a disperser (diffraction grating, prism, etc.), for example introduced into the optical system. Thus, the spectrum of the imaged scene is focused onto the set of photosensitive elements which are, for example, formed by a matrix or a linear array.
However, such optoelectronic cameras have the drawback of generating what is called in the art phantom images, which are added to the image of the scene. This results in a non-negligible error in the main image, which introduces errors into the analysis of this image, more particularly in the presence of objects with high luminance.
These phantom images are generated by the reflection of the polychromatic incident light from the sensitive face of the CCD set, from both of the faces of the protective window and from the last optical face of the optical system, which face points toward said protective window and toward said set of photosensitive elements.
These phantom images are spatially delocalized relative to the observed image and are recentered at other points on the set of photosensitive elements.
In order to try to eliminate these phantom images, consideration has already been given to depositing antireflection coatings on the faces which generate said phantom images, namely the photosensitive face of the set of said elements, both of the faces of the protective window and the last face of said optical system, which face points toward said protective window and said set of photosensitive elements.
Such antireflection coatings consist, for example, in a manner which is known, of products based on tungsten oxide or magnesium fluoride.
If the spectrum analyzed by the set of photosensitive elements is extended (for example between 400 and 1000 nm), it is not easy to produce wide-band anti-reflection coatings. Further, it is very difficult to obtain satisfactory antireflection multilayer coatings on the sensitive face of said set of photosensitive elements, in particular because this sensitive face consists of silicon.
The object of the present invention is to overcome these drawbacks, by profiting from the spatial spreading of the spectrum at the detector.
To this end, according to the invention, the optoelectronic camera including an optical system, a CCD-type set of photosensitive elements which is arranged at the focus of said optical system, a transparent protective window, for example made of glass, arranged in front of the sensitive face of said set of photosensitive elements and a spectral disperser allowing spectral analysis of the scene observed by said camera is note-worthy in that said sensitive face of said set of photosensitive elements, which face points toward said protective window, is coated with a monolayer or multilayer coating of antireflection material whose thickness varies continuously as a function of the wavelength of the light received by said sensitive face, parallel to the dispersion axis of said disperser.
It will be noted that the present invention is particularly advantageous. Indeed, since it is only known to optimize said monolayer at a single wavelength, up to 40% reflection may be reached away from the centering wavelength. In addition, as regards multilayer coatings, it is currently impossible to
REFERENCES:
patent: 3442572 (1969-05-01), Illsley et al.
patent: 4954972 (1990-09-01), Sullivan
patent: 5173368 (1992-12-01), Belmares
Cerutti-Maori Guy
Chessel Jean-Philippe
Aerospatiale - Societe Nationale Industrielle
McGraw Vincent P.
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