Abstract: The invention disclosed here is a method of encoding serial numbers indicating "confidentiality" or "proprietary information" on printed documents. A group of marks in the form of small-sized dots or equivalent markings are repetitively printed across the face of the document. The placement and location of the markings defines a readable binary serial number. Intermixing the markings with the printed matter on the document makes it difficult or impossible to remove the serial number created by the markings, and they will remain on the document after repeated photocopies of it are made.

Abstract: A lamp system for a light projector for continuous light projection upon burn-out of a main lamp. A main lamp with a highly directional light pattern is mounted to the frame for light pattern alignment with the optical axis of the projection lens. A spare lamp is mounted at a location remote from the optical axis. Electronic circuitry detects failure of the main lamp and energizes the spare lamp. A mirror pivots about a point remote from the optical axis for positioning the mirror between the first position which does not effect nor obstruct light transmission between the main lamp and the projection lens and between a second position where the mirror reflects light from the automatically energized spare lamp along the optical axis of the projection lens upon failure of the main lamp.

Abstract: A control circuit for a light projection system for energizing a spare lamp when the circuit detects failure of a main lamp. A pair of input lines apply an alternating-current voltage source to the circuit, to the main lamp and to a spare lamp which is substantially in parallel connection with the main lamp. A switch is in series connection with the main and spare lamp for selectively energizing one of the lamps. Current sensing means, generally a resistor, in series connection with the switch and the lamps, senses the current through the main lamp thereby dropping a small portion of the voltage source. A pair of opposite polarity voltage references are established by rectifying the voltage dropped across the current sensing resistor and by rectifying the voltage across the input lines of the voltage source.