Invention Grant
US5870228A Projection lenses having larger back focal length to focal length ratios 失效
投影镜头具有较大的后焦距与焦距比

Projection lenses having larger back focal length to focal length ratios
Abstract:
A projection lens for use with LCD or DMD panels is provided. The lens has three lens units, the first unit having a negative power and at least one plastic lens element having two aspheric surfaces, a second lens unit having a negative power or a weak positive power and at least one color correcting doublet, and a third lens unit having a positive power and an aspheric surface either on a glass element or on a weak plastic element. The projection lens has a back focal length to focal length ratio of at least 3.0, the ratio being achieved by arranging the first, second, and third lens units so that:D.sub.12 /f.sub.0 >1.0,D.sub.23 /f.sub.0 >0.7, and1.5
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