Abstract: An overhead projector comprises a hollow base comprising a writing plate and a reflective Fresnel lens below the writing plate and an air space below the Fresnel lens. A hollow support arm projecting up from one side of the base carries a projection head having an objective lens and deflection mirror. A lamp mounted in the transition between the support arm and the projection head directs light on the writing plate. The hollow support arm is in communication with the interior of the base to provide a cooling air flow path between openings in the base at the side opposite the support arm and openings in the support arm adjacent the projection head, behind which latter openings a blower is mounted. Ribs in the hollow base define air flow distributors for effectively cooling the Fresnel lens and electronic circuit components mounted in the hollow base.

Abstract: An overhead projector comprises a housing having an upwardly facing writing plate and a Fresnel lens below the writing plate. A hollow support arm extending up from one side of the housing supports a projection head comprising a lamp directing light down onto the writing plate, a variable objective lens receiving an image from the writing plate and a deflection mirror for directing the image onto a projection screen. A power supply in the support arm supplies power for the projector including the lamp and a blower in the support arm for cooling the power supply and lamp. A microprocessor in the support arm has inputs receiving signals from sensors for sensing the distance from the projector to the screen, ambient brightness, brightness contrast in the projected image on the screen and the temperature of the power supply and the microprocessor. The microprocessor has outputs for regulating the focus of the objective lens, the speed of the blower, the brightness of the lamp and the voltage of the power supply.

Abstract: An overhead projector comprises a flat base accommodating a reflective Fresnel lens below a writing surface. A hollow arm projecting up from one side of the base supports a projection head comprising a light source, an objective lens assembly spaced horizontally from the light source, a first deflection mirror for directing rays from the light source down onto the writing surface and reflective Fresnel lens and a second deflective lens for directing rays from the writing surface and reflective Fresnel lens out through the objective lens assembly. The objective lens assembly is tiltable to direct light rays on an upwardly inclined path and the second deflection mirror is also tiltable and is coupled with the objective lens assembly so as to be tilted half as much as the objective lens assembly.

Abstract: In an overhead projector comprising a writing plate, and under this arranged Fresnel lens, a light source and an objective carried by a carrying arm, the problem to be solved is to overcome the disadvantages of the state of the art and to provide a possibility for a lecturer to make information retrieved from an electronic information source, for example, a computer or an electronic game, accessible to a large viewer and listener circle. The invention solves this problem thereby, that between the Fresnel lens and the writing plate there are arranged a layer of liquid crystals which are aligned in an electric field and transparent electrodes on both sides thereof, whereby two of the plates or films traversed by the light beam have light polarizing properties.

Abstract: An overhead projector comprises a base, a hollow post extending up from the base and a projection head on an upper portion of the post. The projection head comprises a light source, an objective lens and a deflection mirror. A power supply unit housed in the post supplies power to the light source and to a ventilating fan inside the post between the power supply unit and the projection head. Openings in the post provide for circulation of cooling air by the fan over both the light source and the power supply unit. The power supply unit includes interchangeable input stages which can be plugged into a socket in the post to adapt the power supply unit to different voltages and currents. A slide or microfilm projector mounted adjacent the projection head projects on the base an image which is then re-projected onto a wall or screen by the projection head. For use at conferences, a conference table top is provided with a plurality of sockets into which the posts of overhead projectors can be plugged.

Abstract: Apparatus for wireless transmission of a teaching program in a classroom comprises a transmitting unit comprising a casing which houses one or more tape recorders with operating elements, a microphone and a plurality of infrared transmitters. A removable cover which can be hung on the wall has additional infrared transmitters connected by a cable to the transmitting unit. Each student has a head set with an infrared receiver, earphones and a microphone. The transmitting unit is remotely controlled by a portable ultrasonic transmitter. When not in use, the headsets are placed in a cabinet where the batteries of the headset are charged by means of induction coils on the head sets being magnetically coupled with induction coils in the cabinet to which high frequency current is supplied.