GB1177567A - Improvements in and relating to Arrangements for Automatic Focusing of Projectors. - Google Patents

Abstract

1,177,567. Slide projectors; focusing. ERNST LEITZ G.m.b.H. Sept.7, 1967 [Sept. 10, 1966], No.40925/67. Heading G2J. [Also in Division G1] Automatic focusing mechanism for a slide projector using differently mounted slides comprises a motor 16 driving a slide carrier 11 through rack 10 and pinion 14, the motor being controlled by a photo-electric device 31 responsive to a light beam from source 26 reflected from the surface of a transparency or its cover glass when said surface is at the focal plane of projection lens 6, there being a further photo-electric device 33 sensing the presence or absence of a cover glass and over-riding the effect of reflections from the outer face of a cover glass when present. Slide carrier 11 and rack 10 are mounted on two leaf springs 8a, 8b, and are spring-biased towards stop 13. The nose of slide changer 20 contacts two switches which actuate a magnetic clutch connecting motor 16 to pinions 14, and which actuate two bi-stable flip-flops 34, 35 to start motor 16. The reflected beam sweeps across detector unit 25a until, when a glass-less slide is in the focused position, light passes slit 30 to reach cell 31 whereby a trigger 36 switches off the motor through flip-flop 34. The transmitted beam is undeflected and does not reach cell 33. In the case of a glass-mounted slide, the transmitted beam through the slide is displaced by refraction and is passed by mask 33a for detection by cell 33. A first pulse from cell 33 therefore activates gate 39 through trigger 37 to pass the delayed pulse from trigger 36 and multivibrator 38 to close flip-flop 35 and open flip-flop 34. Motor 16 therefore continues to run until the beam reflected from the rear face of the cover glass closes flip-flop 34 as above described. The transparency is substantially co-planar with said rear face, and is therefore focused. The rack-and-pinion drive for carrier 11 may be replaced by a motor driven cam system. In this case the control is entirely by reflected light, a double photo-cell being used.