Glass used in projectors.

Yakov Jacobson / November 1, 2018 / Industry News

Light Projection

One of the reasons for commonly found problems in optical digital display, are improper cinema projection port installations. This when accompanied by the use of inferior glass, produces less-than-favorable results where projection and viewing are concerned.

Optical glass that has undergone grinding and polishing on each side is widely accepted as ideal for installing in a cinema projection port. This has for a long time been recognized as the best way to ensure maximum light transmission. Optical glass of this type comes highly recommended for clear projection, but it is also very expensive. Following are some of the things to be taken care of when installing such glass.

Placement

The glass in the cinema projection port should be mounted exactly perpendicular to the line of projection. If the angle of projection is zero degrees, then the glass should be vertically situated. Also, it should be kept in such a way that removal is easy. And if the projection angle is -10 degrees, then the glass should be tilted towards the screen at 10 degrees, where it is again perpendicular to the line of projection. This allows light to find the shortest path through the glass.

Optical Glass Vs Plate Glass

Movie Projection

Optical glass is found to be a lot more effective than plate glass, and allows only a smaller percentage of the projected light to be lost in transmission. Ground, polished, and coated both sides, the former can cut down external reflection losses to 1%, an absorption to 2%. For ordinary plate glass, this stands at 8% and 20% respectively. This extra loss is significant in many theaters, especially at drive-in theaters where the effect is compounded by atmospheric dispersion.

Glass to be used in a cinema projection port should be tested before it is permanently installed. Keeping the white light on the screen, the glass should be moved into the path of the light beam. Good optical glass will not produce much difference in this instance. With plate glass however, the light reaching the screen will be reduced, and will also take on a slightly more greenish appearance. Either of these is enough to bring down the quality of picture viewing.

The glass should also be checked to see if it distorts the focus of the picture. For this, a known good target should be projected on, and the glass should be brought into the path of the beam to check for significant change in focus.