GB1235427A - Automatic focusing device for slide projectors - Google Patents

Abstract

1,235,427. Automatic position control. ERNST LEITZ G.m.b.H. Nov.21, 1969, No. 57133/69. Heading G3R. In a system for automatically adjusting a transparency to a correct focusing position in a slide projector, a cyclically scanned beam of light reflected by the slide produces, in a photo-cell, successive pulses corresponding to the surfaces of the rear cover glass and the transparency, of which the latter only controls a bi-stable circuit and thereby, if necessary, causes operation of a motor to correct the position of the slide. In Fig.3, a slide projector comprises a light source 1, heat shield 2, condenser 3, slide 5 with cover glasses 5a, 5b, and projection lens 6. Light from a source 7 is projected through a slit 9 on to the slide and reflections are scanned by a rotating prism 30 and directed on to a photo-cell 11 which produces pulses corresponding to cover glass and transparency reflections in succession. The first pulse exceeding a predetermined value corresponds to that from the transparency and is effective through a limiting amplifier to set a bi-stable circuit 32-38 which is later re-set by a pulse produced by a generator 31 at a predetermined point in the scanning cycle. Opposite polarity outputs are produced during the set and re-set periods of the bi-stable circuit so that, through a servo-amplifier 20, a reversible motor 21 is operated, by a capacitively derived average signal, in the appropriate direction to ensure that the slide is in a position for correct focus. The accuracy of this position is ensured by initially adjusting the directions of the optical axes of source 7 and photo-cell 11 and, if a mixture of glazed and unglazed slides is to be projected, by the provision of a Schmitt trigger circuit which responds to pulses produced by the front cover glass and slightly modifies the average signal applied to control the motor for glazed slides.