Optics: image projectors – Particular condenser
Patent
1996-08-01
1998-09-08
Dowling, William
Optics: image projectors
Particular condenser
353 38, 353121, G03B 2114, G03B 21132
Patent
active
058035680
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates generally to Fresnel lenses, and particularly to Fresnel lenses for overhead projectors.
BACKGROUND OF THE INVENTION
Conventional Fresnel lenses for use as condensers, concentrators and field lenses are usually of the form of a single element with prismatic, circular grooves on one side and a planar surface on the other. When high transmission is required at a smaller F
umber, for example in overhead projectors, two lenses of this type are sealed together, grooves adjacent, at the periphery. Since these lenses usually lie close to the imaging plane or stage of the projector, Moire patterns can appear on the projected image when the centers of the circular groove structures are not coincident to a certain tolerance.
A single element Fresnel lens with identical groove structures on both sides is described in Japanese Patent 57-109618. In that patent, two identical dies are accurately aligned in a fixture using guide posts. The lens is formed by pouring a thermoplastic resin between the dies and subsequently applying heat and pressure. Such a single element lens is economically advantageous over the conventional two lens arrangement because only one sheet of lens material is necessary. Unfortunately, the requirement that the center of the grooved surfaces be aligned to a close tolerance increases the expense of a workable single element lens, and therefore somewhat offsets its advantages.
U.S. Pat. No. 4,900,129 improves on the aforementioned Japanese patent by increasing the groove frequency of one surface of the single element Fresnel lens with respect to the other. This reduces the contrast of the Moire pattern to the point that it is invisible. In this design, the angle of incidence of a light ray on the first surface is equal to its angle of refraction on the second surface.
Although these two documents address the important issues of cost and Moire, they fail to address two others: transmission efficiency and filament image size. Transmission efficiency is the amount of light which exits the Fresnel lens in the preferred direction as a fraction of the incident light. It is important because higher efficiency leads to a brighter image on the screen or less energy used for a given illumination level. Filament image size is the size of the image of the illumination source produced by the Fresnel lens. It is important because the size of the filament image determines the size, and therefore cost, of the projection lens. Minimizing the filament image size can decrease the cost of a projector.
SUMMARY OF THE INVENTION
The present invention produces a single-element, dual-grooved Fresnel lens having nonidentical groove structures on each side of the lens. The groove frequencies are carefully controlled to reduce or eliminate the need for accurate alignment of the centers of the groove patterns on each side of the lens. Also, in one embodiment of the invention, the groove angles are designed to optimize light transmission through the lens by minimizing the combined losses due to riser blockage and surface reflection. In another embodiment, the groove angles are designed to reduce the size of the filament image. The techniques employed to increase the transmission efficiency of light through the lens and achieve a reduction of the size of the filament image may be combined to increase the overall efficiency of the optical system used in an overhead projector.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described with respect to the accompanying drawings, wherein like numbers refer to like parts in the several views, and wherein:
FIG. 1 is an enlarged, partial cross-sectional view of a single-element, dual-grooved Fresnel lens of the type designed according to the present invention;
FIG. 2 is a view similar to that of FIG. 1 illustrating the parameters for calculating riser blockage of a Fresnel lens;
FIG. 3 is a schematic view of a portion of the optical elements of an overhead projector illustrating the formation of a filament i
REFERENCES:
patent: 4741613 (1988-05-01), Vanderwerf
patent: 4900129 (1990-02-01), Vanderwerf
patent: 5296882 (1994-03-01), Nelson et al.
patent: 5317349 (1994-05-01), Vanderwerf
patent: 5504544 (1996-04-01), Dreyer et al.
Dowling William
Ho Nestor F.
Minnesota Mining and Manufacturing Company
Musgrove Jack V.
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