Overhead projector with achromatic fresnel lens

An overhead projector comprising:

• a base having a stage area;
• a projector head mounted to said base, proximate said stage area;
• a light source in said base;
• means for directing light from said light source towards said stage area;
• and
• an achromatic singlet lens located at said stage area, said singlet lens including:
• a generally planar substrate having first and second surfaces,
• said first surface having a plurality of prismatic ridges and grooves forming a catadioptric surface, said ridges and grooves of said catadioptric surface being generally circular, defining an effective center, and said catadioptric surface exhibiting dispersion,
• said second surface having a plurality of prismatic ridges and grooves forming a dioptric surface, said ridges and grooves of said dioptric lens being generally circular, defining an effective center, and said ridges and grooves of said dioptric surface being oriented to essentially cancel said dispersion of said catadioptric surface, and
• said effective center of said dioptric lens coinciding with said effective center of said catadioptric lens.

An overhead projector comprising:

• a base having a stage area;
• a projector head mounted to said base, proximate said stage area;
• a light source in said base;
• a condenser lens located adjacent said light source, between said light source and said stage area;
• a back reflector located adjacent said light source, opposite said condenser lens;
• and
• a condensing lens system located at said stage area, said condensing lens system including:
• a convergent, catadioptric Fresnel lens having an upper plano surface and a lower surface, said lower surface having a plurality of generally circular, prismatic ridges and grooves defining an effective center, said catadioptric lens having negative dispersion, and
• a convergent, dioptric Fresnel lens generally parallel with said catadioptric lens, having an upper plano surface and a lower surface, said lower surface having a plurality of prismatic ridges and grooves defining an effective center and being located adjacent said upper plano surface of said catadioptric lens whereby said effective center of said dioptric lens overlies said effective center of said catadioptric lens, said dioptric lens having positive dispersion,
• said catadioptric and dioptric lenses being constructed of a polymeric thermoplastic having an index of refraction of at least about 1.4, and conforming to the conditions:
• F.sub.1 <(.phi..sub.1 -.phi..sub.2)
• and
• .phi..sub.2 <.phi..sub.3,
• where F.sub.1 is a refracting groove angle of said catadioptric lens, .phi..sub.1 is an entrance angle of a light ray entering said lower surface of said catadioptric lens, .phi..sub.2 is an internal angle of a light ray between said upper plano surface of said catadioptric lens and said lower surface of said dioptric lens, and .phi..sub.3 is an exit angle of a light ray exiting said upper plano surface of said dioptric lens.