| 摘要 |
<p>1,258,519. Overhead projector head; diaphragms. MINNESOTA MINING & MFG. CO. March 3, 1969 (March 4, 1968], No.11186/69. Headings G2A and G2J. An overhead projector head of the kind having two meniscus lenses 12, 14 at right-angles and a plane mirror 13 at the hypotenuse has in addition an apertured screen 15 at right-angles to the mirror 13 to reduce spurious reflections within the head. The aperture is semi-elliptical in shape and is positioned so that-substantially the entire beam passing through lens 12 falls on to the mirror 13. The screen 15 may also be used in projection heads without the second meniscus lens 14.</p> |
