GB1263129A - Projector with automatic focussing arrangement - Google Patents

Abstract

1,263,129. Photo-electric projector focus control. RANK ORGANISATION Ltd. May 20, 1970 [Sept. 5, 1969], No.44077/69. Heading GlA. In a projector in which a transparency 1 or the objective 2 is automatically positioned for correct focus a pattern of light of cyclic waveform other than square wave is moved across the plane, of a transparency in a direction at right angles to the plane containing the incident beam and the specularly reflected beam and the signal developed by a photosensitive device viewing a small area of the moving pattern is used to control the correcting servo. A moving saw tooth pattern Fig.2 (not shown) is produced by a rotating toothed disc 4 in the beam from a source 5 and the level of the waveform at which the sensitive field of the detector is traversed, is determined by the position of the plane of the transparency producing pulses of more positive or more negative duration, as demonstrated in Figs.4-6 (not shown) according to whether the transparency is in front of or behind the focal plane. The objective, source, and sensor may be on a common assembly and may be driven by a cam 16. Suitable transistor amplifying and detecting circuitry using an integrator to produce a positive going pulse-negative going pulse signal is described and a focusing suppression switch is included for actuation during transparency change.