International projectionist (Jan 1961-Dec 1962)

reflector, namely, silvered-glass, metal, and interference-type ("cold") mirrors The conclusions of testing engineers and the actual experience of practicing projectionoists attest overwhelmingly to the superiority of glass mirrors, both silver-coated and "cold". Glass Reflectors Preferred The earlier metal mirrors were made of steel and plated with rhodium, a platinum-like metal. A much-publicized advantage of these mirrors was freedom from breakage and resistance to pitting by copper splashes frcm copper-coated carbons. Unfortunately, the substantially lower light-reflecting power of metal mirrors was often overlooked. The metal surface is generally not as specular as is a polished glass reflector since the metal surface cannot be polished to the same degree as glass. This fact, alone, accounts for much of the lower efficiency of metal mirrors. The rhodium reflective coating of the earlier metal mirrors has, on the average, only 83% of the reflectivity of silvered-coated glass. Also, the reflectance of rhodium is spectrally nonuniform, imparting to he reflected arc illumination a slightly amber tint. Most serious of all, the heat -producing infra red radiation which is partially absorbed by the glass of a silvered mirror is almost totally reflected by metal mirrors of all types. Accordingly, the use of metal mirrors involves the risk of excessive film buckling, the cracking of heat filters, and possible irreparable damage to the projection lenses. A more modern type of metal mirror has a somewhat higher reflectance than rhodium-plated steel. This is the aluminum mirror having a specially anodized reflecting surface. The reflectivity of this surface is only a few per cent lower than that of silver; but the absence of optical precision results in a serious loss of light. Glass mirrors yield to the screen up to 25% more light than metal mirrors of the same sizes. Mirror Optics Based on Ellipse Owing to the fact that arc-lamp reflectors are required to form an accurate optical image of the radiant crater of the positive carbon on or near the film aperture of the projector, the adjustment of the mirror in relation to the optical components of the arc lamp and projector merits the most careful attention. Imagine an arc-lamp reflector cut in half through its center. The gently curved cross-section is a segment of an oval-shaped geometric figure known as an ellipse. For this reason, standard FIG. 4 — The polished mirror is handfinished, chemically cleaned, silvered, and then backed by copper-plating and either an aluminum or water base high temperature paint and then baked. Even though inspection follows each step of manufacture; the finished silvered mirror is carefully tested for optical accuracy before being packaged for shipment. The results of the optical tests are recorded and filed for each Strong mirror according to the serial number etched thereon. arc reflectors are called "elliptical" mirrors. Now, a straight line drawn through the long dimension of the imaginary ellipse intersects the mirror at its center. This line is called the major axis of the ellipse. And when the mirror is correctly installed in the lamphouse, and the lamp properly adjusted, the major axis of the mirror coincides exactly with the optical axis of the entire projector, passing through the centers of the mirror, positive crater, film aperture, and projection lens. The all-important procedure of "lining up" an arc lamp consists in making the major axis of the imaginary mirror-ellipse coincide in this way with the lamp and projector optical axis, which is a straight line passing through the centers of all the aforementioned components. This can be accomplished by the use of a tightly stretched string and a dummy lens, by means of special alignment tools supplied by the lamp manufacturer or his dealer. There are two definite points on the mapor axis of an ellipse called foci, the center of the ellipse lying halfway between them. When an arc lamp is correctly adjusted, the bright gas-ball in the crater of the high-intensity positive carbon will occupy the focus nearest the reflector (3% to 67/16 inches from the center of the reflector, depending upon the make and model of lamp), while the "spot", or magnified image of the luminous crater, will be found at the other focus on or near the film aperture of the projector. This set-up is illustrated in the accompanying diagram (Fig. 5). Working Distance and Arc Focus Screen illumination is at its best only when the arc-lamp reflector is new and bright and when the optical line-up has been accurately established and the positive crater placed in the geometric focus of the reflector. To meet the last-named requirement, it is necessary to place the mirror at the prescribed "working distance" from the 35-mm projector aperture (24 to 41 inches, depending upon make and model of lamp). This distance is measured from the center of the mirror (or the edge of the center hole) to the film aperture, and is usually shortened to place the actual crater image closer to the projection lens when 70-mm prints are shown with special 35/70-mm all-purpose lamps. This expedient places a wider beam of light at the 70-mm aperture, insuring adequate coverage with acceptably uniform illumination. If the tip of the positive carbon be positioned a fraction of an inch too close to the mirror, the yellowish "shell light" will be focused upon the aper 8 International Projectionist March 1961