Auto-focus slide projector

Claim 1

APPARATUS FOR THE AUTOMATIC FOCUSING OF A SLIDE PROJECTOR TO COMPENSATE FOR "POPPING" OF THE FILM DURING PROJECTION WHEREIN THE FILM IS ILLUMINATED FROM A LIGHT SOURCE AND IS PROJECTED ALONG A PROJECTION AXIS BY A PROJECTION LENS WITH ITS FOCAL PLANE LOCATED AT THE FILM PLANE COMPRISING: LIGHT MEANS ADAPTED FOR MOVEMENT WITH THE PROJECTION LENS FOR DIRECTING A CONSTANT LIGHT BEAM ONTO THE FILM ALONG AN AXIS AT AN ANGLE TO THE NORMAL FILM PLANE TO BE CONSTANTLY REFLECTED FROM THE FILM ALONG AN AXIS WHOSE POSITION CHANGES WITH THE CHANGES OF FILM POSITION; LIGHT DETECTING MEANS INCLUDING ONE LIGHT ACTIVATED SILICON CONTROLLED RECTIFIER ALSO ADAPTED FOR MOVEMENT WITH THE PROJECTION LENS FOR INTERSECTING SAID REFLECTED LIGHT AND FOR GENERATING AN OUTPUT WHICH IS REPRESENTATIVE OF THE LIGHT INTERSECTED; UNIDIRECTIONAL DC MOTOR MEANS CONNECTED IN PARALLEL WITH SAID LIGHT ACTIVATED SILICON CONTROLLED RECTIFIER, OPERABLY CONNECTED TO THE PROJECTION LENS, AND ADAPTED TO MOVE SAID PROJECTION LENS IN A LONGITUDINAL DIRECTION PARALLEL TO THE PROJECTION AXIS OF THE PROJECTOR THEREBY SHIFTING THE FOCAL PLANE OF THE PROJECTION LENS TO BRING IT INTO COINCIDENCE WITH THE FILM PLANE; AND CONTROL MEANS ELECTRICALLY CONNECTED TO THE OUTPUT OF SAID LIGHT DETECTING MEANS FOR CONTROLLING SAID UNIDIRECTIONAL DC MOTOR MEANS.