Imaging system comprising a Fresnel lens
AssigneesOffice National d'Etudes et de Recherches Aerospatiales (O.N.E.R.A.), Ulis
InventorsGuillaume Druart, Florence De La Barriere, Arnaud Crastes
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Abstract
The imaging system for imaging field rays comprises a detection surface, a device for focusing the field rays with said detection surface, and a diaphragm. Said device comprises a Fresnel lens comprising a first dioptre, the non-plane surface of which, called the active zone, makes it possible to focus the field rays towards said detection surface, and said diaphragm allows controlled distribution of the field rays over said active zone.
Claims16
A system for imaging field rays originating from any point in an observed scene, comprising:
- a detector;
- a diaphragm placed upstream of the detector and located at a distance d0 from said any point in the observed scene;
- and
- a device for focusing the field rays onto the detector, the focusing device being designed for a working wavelength λ0 comprised in the spectral band of the observed scene, the active dioptre of the focusing device being placed between the diaphragm and the detector:
- at a distance d1 from the diaphragm minimizing astigmatic aberration at the detector;
- and
- at a distance dd from the detector, the distance dd depending on the distance d0 and being the focal distance of said active dioptre,
- the imaging system wherein:
- the focusing device comprises a plate having parallel sides, said plate comprising at least one Fresnel dioptre that is the active dioptre of the focusing device and that supplies most of the optical power of said focusing device, this optical power being generated by phase-continuity zones of said active dioptre, said phase-continuity zones essentially functioning in refractive mode, said optical power being adapted to focus the field rays onto the detector;
- said active dioptre is designed to the order p for the working wavelength, the order p being greater than or equal to three, in order to obtain many diffraction orders in the spectral band of the observed scene;
- and
- the aperture Ld of the diaphragm placed upstream of the active dioptre is smaller than Ldmax=L4·d0/(d0+d1), where L4 is the smallest distance between four consecutive discontinuities of the Fresnel lens, in order on the one hand, to limit to 3 the maximum number of phase-discontinuity zones illuminated by said field rays for a given angle of incidence, and on the other hand, to make allowance for a local correction of aberrations, which correction is adapted to the angle of incidence of said field rays;
- said active dioptre is designed to make said local correction of aberrations.
A method for imaging, on a detector, using a focusing device comprising a plate having parallel sides and comprising at least one Fresnel dioptre which is the active dioptre of the focusing device of working wavelength λ0 comprised in the spectral band of an observed scene, the focusing device being placed downstream of a diaphragm, field rays originating from any point in an observed scene located a distance d0 from the diaphragm, comprising the following steps:
- calculating the distance d1 between the diaphragm and the active dioptre of the focusing device allowing astigmatic aberration at the detector to be minimized,
- choosing an optical power of the active dioptre of the Fresnel lens, said active dioptre supplies most of the optical power of said focusing device, this optical power being generated by phase-continuity zones of said active dioptre, said phase-continuity zones essentially functioning in refractive mode, said optical power being adapted to focus the field rays onto the detector;
- focusing the field rays onto the detector, by means of said active dioptre designed to order p for the working wavelength, said order p being greater or equal to three, in order to obtain many diffraction orders in the spectral band of the observed scene;
- calculating the smallest distance L4 between four consecutive discontinuities in the Fresnel lens, and the distance dd between the focusing device and the detector, the distance dd depending on the distance d0 and being the focal distance of said active dioptre;
- and
- decreasing, if necessary, the aperture Ld of the diaphragm placed upstream of the active dioptre, until it is smaller than Ldmax=L4·d0/(d0+d1), in order on the one hand, to limit to 3 the maximum number of phase-discontinuity zones illuminated by said field rays for a given angle of incidence, and on the other hand, to make allowance for a local correction of aberrations, which correction is adapted to the angle of incidence of said field rays;
- said active dioptre is designed to make said local correction of aberrations.
Dependencies are inferred from the claim text.