1982-02-25
1985-08-27
Corbin, John K.
350122, 350125, 350412, 350448, G02B 304, G02B 902, G03B 2156
Patent
active
045374740
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an optical system for a projection apparatus, which is preferably intended for projecting an image on a display screen with generally negative curvature.
BACKGROUND ART
In the construction of projection objectives with relatively short Gauss focal length, e.g. for portable overhead projectors, the conventional method has been to make the objective with a small aperture, in order to obtain with few lenses, a generally acceptable image quality. An optical system is thus afforded which can be produced at relatively low costs and which has relatively small dimensions and low weight, so that the condition of portability is satisfied, but with the disadvantage that such an optical system at best only allows acceptable projection of very highly luminous objects such as transparencies or the like.
An attempt to make an objective allowing acceptable image quality and light strength, even when projecting objects of relatively small luminosity, is disclosed in the Swedish Pat. No. 7407134-1. Such an objective includes a modified triplet supplemented by a large positive lens a long way in front of the triplet. This kind of objective has indeed a large aperture, and affords acceptable image quality, but as indicated, it requires four large lens elements and the Gauss focal length needs to be about 600 mm, in order that the projector operator can manipulate the image of the object. Due to the necessary size and weight of the objective, as well as the large number of lenses necessary for it, this previously known objective is neither economically acceptable nor compatible with the portability requirement.
The conviction of those skilled in the art has been that a proper projection objective must include a plurality of lens elements, even if some lens surface has been given an aspheric correction. The technique of aspherically correcting a lens surface for reducing certain image reproduction errors or aberrations is known, but is commercially limited, since the cost of producing such lenses is relatively high. It has therefore appeared as being more economically favourable in the construction of a projector objective to increase the number of lens elements therein, if it is desired to improve the resulting image quality, instead of aspherically correcting any of the lens surfaces in the objective since, as mentioned, such persons were convinced that the objective necessarily must include a plurality of lens elements in order to avoid the most serious aberrations.
PURPOSE OF INVENTION
One purpose of the invention is to propose an objective structure for projectors, said structure having a short Gauss focal length, great numerical aperture, high image quality, low weight and a small number of lens elements, so that an overhead projector, for example, equipped with such an optical system will actually be light in weight, meet the portability condition, and with acceptable visual quality be capable of depicting objects which are less luminous, such as epidiascope pictures, i.e. non-transparent pictures illuminated by an ordinary light source from above.
Extremely surprisingly, it has now been found possible to achieve said purpose by means of an objective of the kind apparent from the appended patent claims. Especially surprisingly is the fact that the objective only needs to contain a single lens element of generally concavo-convex or plano-convex type, the convex surface of which is directed towards the plane of the object. Both the surfaces of the lens will be aspherically corrected, preferably so that in annular areas concentric with the lens axis there will be a positive deviation in the vicinity of said axis, negative deviation in an intermediate annular area and positive deviation in an outmost annular area, the deviation being related to the sphere best fitting the respective lens surface. The correction has continuous first and second order derivatives which are =0 in the axial point. The aspherical correction of both lens surfaces can be simultaneously calculated by me
REFERENCES:
patent: 3778133 (1973-12-01), Tatian
patent: 4181409 (1980-01-01), Whitney et al.
patent: 4289387 (1981-09-01), Jalie
Corbin John K.
Optik Innovation AB OIAB
Sugarman Scott J.
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