Patent GB-1285870-A (GB)
Title: Focusing system for a projector
IPC: G02B7/32
Filed: 1970-07-21
Abstract: 1285870 Automatic focusing GAF CORP 21 July 1970 [22 July 1969 18 May 1970] 35309/70 Heading G3R [Also in Divisions G1 and H3] Automatic focusing apparatus for a projector comprises a source of radiation, detection means responsive to displacement of a beam or beams from the source, and means directing a beam or beams for reflection from the transparency and from the screen, and thereafter to the detection means, which controls an arrangement for moving the objective lens, transparency or screen thereby altering the screen to lens: lens to transparency ratio such as to focus the image of the transparency on the screen, after which the control arrangement is effectively de-activated. In one arrangement, Fig.1, a rotating pair of blades carrying mirrors 33 periodically diverts radiation from projector lamp 10 along path 35 to mirrors 40, 42, and double sided mirror 45. The beam then follows the paths 46, 47, 48, 49 being reflected from the transparency 16 and the screen 22 and travelling via the outer edge of the objective lens 20. At mirror 45 it is reflected via mirror 52 to a pair of photo-cells 54, 55. If there is inequality of illumination of the cells due e.g. to the initial setting up of the screen and projector, buckling of the transparency &c. e.g. as illustrated in Figs.5 to 8 (not shown), an output to motor 28 causes movement of the objective lens mount 24 to focus the system. Flooding of the photo-cells with light reflected from the screen is prevented by a slotted disc (60) Fig. 3 (not shown), rotating in synchronism with blades (33) Fig.2 (not shown). Alternatively, since the light on path 35 is high in infrared owing to blades 33 being before infrared filter 14, disc 60 may be replaced by infrared filters or the photo-cells may be particularly sensitive to this radiation. Other alternatives are the use of U.V. light or coding arrangements. In a modification of the Fig. 1 arrangement, light for the focusing control system may be provided by a small bulb in the location of mirror 42. Alternatively fixed mirrors only may be used to obtain light from the projector lamp. In another alternative, a modified objective lens arrangement is provided, Fig.4 (not shown). This has a central projection portion and outer lenses of the same or different focal length from the inner portion, for the focusing control system. Modifications in the orientation of mirror 45 are also described, Figs.9 to 11 (not shown). In another arrangement, Fig.12, a separate light bulb is associated with fixed plane and spherical mirrors to form the optical path of the focusing control system. An infrared filter may be used to prevent formation of a visible spot on the screen. Light from the bulb may be chopped and the photo-cell circuit be provided with a suitable high-pass filter. In this arrangement silicon photo-diodes are specified. In the arrangement of Fig.13 (not shown), the projector cooling fan is used to chop light from the lamp, the beam being directed past a knife edge, and after reflection from the transparency and screen, to a photo-transistor module. This may be a planar silicon Darlington amplifier device and is in an A. C. circuit with a field effect transistor and a silicon controlled rectifier, Fig.14 (not shown). When the photo-transistor has a minimal amount of light thereon (out-of-focus condition), SCR 100 is off and motor 29 rotates to tend to focus the projector. As the projector focuses, the detector is illuminated, the SCR conducts and the motor reversed. The effect is a continuous "hunting" operation around the in-  focus condition; which is unnoticed by the observer. The arrangement of Fig.16 is designed to avoid reduction of signal strength affecting accurate focusing control. Light from lamp 120 is directed via mirrors 122, 128 and the transparency to a first photo-transistor module 136 which controls orientation of mirror 128 to maintain beams 127 and 142 colinear. Light via mirrors 140, 128, screen 22"' and mirror 122 reaches detector 148 and the output of this is used to effect focus control. The automatic focusing system may be used in slides or movie projectors. With the former, focusing would not involve movement of the slide and with the latter it would not involve movement of the screen but could movement of the film gate.