Projector apparatus including a light source, a projection head and a projection stage aperture for an object, the image of which is to be produced. The aperture is situated in the ray path between the light source and the projection head. An ellipsoidal mirror is situated in the light path between the light source and the aperture. One focal point of the ellipsoidal mirror substantially coincides with the light source and the other focal point of the mirror substantially coincides with the projection head. The projection head includes a deflection means. The projection head is situated substantially in a plane constituting a normal plane to the aperture plane and containing the front edge line of the aperture, as observed in the generally horizontal projection direction.

An optical lens having smooth outer surfaces includes at least two layers of different materials and different indices of refraction. Each layer has Fresnel type undulations forming optically active surfaces which interfit or mate with complementary undulations on an adjacent layer in a surface-to-surface and void-free condition. The layers are either fused together or held together by a transparent cement.

A compact light condensing illumination system for the concentration of light at an area provides a light source capable of having a relatively shallow reflector with a relatively small aperture directing collimated light toward a planar reflector of rectangular configuration having a mirrored surface configured with relatively short grooves extending at an angle to the direction of the collimated light to uniformly reflect the light toward a planar grooved mirror disposed substantially at a right angle to the first mirrored surface and of relatively large surface to reflect the light in substantially parallel rays to a Fresnel lens of relatively low power for condensing of the light rays to concentrate the light at a preselected distance from the Fresnel lens.

The invention concerns a transparency overhead projector having an illumination unit of which the height has been extremely reduced. The main component of the illumination unit is a specular surface comprising mutually parallel, mirrorized prismatic shoulders either on its top or on its lower side, which reflect the light from at least one lateral source toward the Fresnel lens, the platen and the projecting lens. The specular surface may be plane, however it may also be curved in circular, hyperbolic etc. manner in the longitudinal direction of the prismatic shoulders. This curvature may be constant or it may vary. Further, the specular surface also may be twisted, that is, it may also be curved in a direction orthogonal to the prismatic shoulders, and this curvature also may be constant or variable. However the prism angles of the individual prismatic shoulders are always individually determined so that the light shall appear to come from a source which seems to be mounted centrally along the optical axis underneath the Fresnel lens.

A low-profile, transmissive overhead projector (OHP) employs a novel condensing lens system having a catadioptric Fresnel lens and a dioptric Fresnel lens. Each of the lenses exhibits chromatic aberration, but the dispersion of one lens is selected to cancel out the dispersion of the other, e.g., if the catadioptric lens has negative dispersion then the dioptric lens is selected to have positive dispersion. The mutual cancellation of the dispersion of each lens practically eliminates longitudinal chromatic aberration through the lens system. The lens system provides a fast condensing system which allows collimation, dispersion or concentration of light striking the lens at a high angle of incidence. This characteristic is particularly suited for a low-profile OHP as it allows the light source to be placed directly under, but very close to, the stage area.