Automatically focussing slide projector - has 2 sets of photocells monitoring light passing slide at sharp angle for slide or lens servo drive adjustment

摘要

The automatic slide projector has two sets of photocells which monitor light passing through the slide at a sharp angle. They alter the slide position, or operate lens servo drive to obtain the sharpest focus. The user manually sets the focus for the first slide in a series and the system automatically corrects the focus for subsequent slides. A microswitch in the slide frame operates an inhibit mode if no slide is in the projection position to prevent the automatic focus control from hunting, and moving too far out of setting. The system compensates for different types of slides in the same series as well as heating effects during long projection.