Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-02-21
2003-10-14
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S676000, C359S680000, C359S682000, C359S683000
Reexamination Certificate
active
06633436
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system having a long back focus, a compact structure, and good optical characteristics and, for example, to an optical system suitable for a projection lens of a high-definition mobile liquid crystal projector.
2. Related Background Art
In recent years, the liquid crystal projectors have commonly been utilized as devices capable of presenting a projection of an image of a personal computer or the like on a large screen in conferences, presentations, and so on. Among others, it is preferable to use a three-panel-type liquid crystal projector using three liquid crystal displays (LCDs) for the respective colors of R (red), G (green), and B (blue) and arranged to project a synthetic image of image information displayed on the respective LCDs, onto the screen, in order to present a high-definition image with high contrast. Projection lenses of such three-panel-type liquid crystal projectors are desirably those satisfying the following requirements:
1) The back focus of a certain length is ensured in order to provide a space for placement of such elements as a dichroic prism and polarizers for combining color beams from the liquid crystal displays for the respective colors of R, G, and B, between the liquid crystal displays and the projection lens;
2) The projection lens is an optical system telecentric on the liquid crystal display side whose pupil viewed from the liquid crystal display (reduction) side is at infinity, in order to minimize influence of angle dependence of color combining films and in order to ensure good pupil matching with an illumination system;
3) Corresponding pixels of the respective color liquid crystal displays have to be well superimposed throughout the entire screen in order to prevent degradation of resolution sense and quality due to doubling of characters etc. of the personal computer when the synthetic image of the image information of the three color liquid crystal displays is projected onto the screen.
For that purpose, chromatic deviation (chromatic aberration of magnification) occurring in the projection lens has to be corrected well in the visible band (to realize an apochromat system);
4) The optical system is corrected so that the maximum distortion is not more than about 2% in order to prevent the projected image from being distorted in the contour part and degrading (particularly, if there remains sudden distortion change or the like in the periphery and intermediate part or the like the quality of image will degrade and thus it is not preferable); and so on.
The prior arts intended for meeting the above requirements include, for example, the projection lenses for liquid crystal projectors as described in Japanese Patent Applications Laid-Open No. H10-186235 and No. H10-268193. The projection lenses disclosed in these applications are comprised of five lens units having their respective optical powers which are negative, positive, positive, negative, and positive in the order named from the enlargement conjugate side, and magnification variation (zooming) from the wideangle extreme to the telephoto extreme is implemented by fixing the first and fifth lens units and moving all the second to fourth lens units in the lens system toward the enlargement conjugate side. Since the first and fifth lens units closest to the enlargement conjugate side and to the reduction conjugate side are fixed, the total length is maintained constant during zooming. This optical system is also a telecentric zoom lens having the back focus of a fixed length and realizing low distortion and low chromatic aberration.
There are, however, strong market needs for achievement of much smaller mobile liquid crystal projectors at present and toward the future.
For this theme of “downsizing of liquid crystal projector,” the size of the liquid crystal displays providing the original images first has to be decreased. However, for ensuring the resolution of the same level as before, the aperture ratio of the liquid crystal displays is reduced and the ratio of sizes of illuminated areas and emission light sources (=size of illuminated areas/size of light sources) becomes smaller, so that illumination efficiency is usually lowered. This will results in losing commercial values in respect of brightness even though the apparatus can be downsized.
Against the above problem, in order to ensure screen illuminance enough for use even under fluorescent tubes and the daylight, it was difficult to attain sufficient brightness with marketability by the full-aperture F-value at the wideangle extreme of about 2.5 (the F-values hereinafter representing full-aperture values) in spite of the projection lens for compact projector. It is thus necessary to attain brightness enough to maintain marketability, by setting the F-value to approximately 2.0.
However, the decrease of the F-value to about 2.0 or to below 2.0 will raise the possibility of forcing constraints from the F-value on design, decreasing freedom of correction of aberration, making correction of aberration, particularly, spherical aberration very hard, and thus degrading the optical characteristics. It is possible to well correct the spherical aberration by use of many lenses, but it will result in increasing the scale of the optical system and thus lowering the commercial values as a mobile liquid crystal projector.
The cross dichroic prism mainly used in the liquid crystal projectors and disclosed in Japanese Patent Application Laid-Open No. H11-95098 is very compact, but it is very difficult to fabricate the prism and join its prism elements. In order to decrease the production cost of the color combining prism, a method in which the dichroic mirror layers do not intersect with each other inside the prism, is also investigated as a new color combining method.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical system having good optical characteristics while realizing a large aperture ratio. A further object of the present invention is to provide an optical system that can be constructed in a total length of very small value in combination with a cross dichroic prism and that can be constructed without increasing the scale of the entire system and realized in the scale approximately equal to that of the conventional systems using the cross dichroic prism, even in combination with a prism of a color combining method with a longer prism path length, other than the cross dichroic prism.
An optical system according to the first aspect of the present invention is an optical system comprising a first optical component, a second optical component, a third optical component, a fourth optical component, a fifth optical component, and a sixth optical component in the order named from the enlargement conjugate side,
wherein at least four optical components out of said plurality of optical components move during zooming and said first optical component closest to the enlargement conjugate side has a negative optical power.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein during zooming the optical component closest to the enlargement conjugate side and the optical component closest to the reduction conjugate side both are fixed relative to a reduction-side conjugate point.
An optical system according to another aspect of the invention is the optical system according to the first aspect, which comprises at least one optical element having an aspherical surface.
An optical system according to another aspect of the invention is the optical system according to the first aspect, which comprises at least one optical element comprised of a material with an Abbe's number of not less than 80.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein during zooming at least two movable optical components integrally move.
An optical system according to another aspect of the inv
Sugawara Saburo
Wada Ken
Canon Kabushiki Kaisha
Morgan & Finnegan L.L.P.
Raizen Deborah
Sugarman Scott J.
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