The proper location of the projection room is also a serious question in designing a motion picture theatre, a factor of greater importance than is generally imagined. Yielding to a tendency to imitate rather than originate, architects are generally inclined to copy blindly the bad examples set by incompetent designers. Whatever the distance of throw, the angle of projection should be as nearly level as possible, and ought never to exceed fifteen degrees. The flatter the throw, the better the projection. In Chicago and other Western cities projection rooms formerly placed high are now being removed to the main floor.
The center beam of light should strike the center of the screen at a right angle. Any departure from this condition must produce a distortion that is made manifest in what is called a "keystone effect." That is, when the beam of light comes from an altitude, the picture will resemble in appearance an inverted keystone, larger at the base than at the top. The higher the altitude the greater the degree of distortion. When the angle of light is above fifteen degrees the distortion becomes evident to the eye, and the screen must then be tilted backward to offset this defect. This again affects the view of the spectators in another way, for the screen will not appear to be plumb. Unless the tilting be artfully concealed, it is sure to be detected by the observant.
Level conditions for a projection room are rarely feasible in front projection, unless the booth be placed on the main floor. This is not generally done because of its impracticability and the disfigurement that results from placing a projection booth in the main foyer. Rear projection is far preferable where the conditions make it possible. It admits of placing the projection booth, with all its heat, noise and danger, completely away from the audience, and also saves the extra electric current required for projection through a lighted auditorium.
Where the distance from the machine to the screen is about fifty feet, so that a four-inch E.F. objective lens, or even a larger one, may be used, rear projection is ideal. The projection rays may be confined in a horn shaped, black lined funnel, extending from the projection machine to the screen with the small end of the funnel attached to the projection machine. With rear projection, the film must be placed in the machine with the emulsion side toward it instead of toward the light, which is the practice in front projection.
Where the balcony is deep and its rear too high the projection room may be placed on the mezzanine that forms the lower-balcony, with independent means of entrance and exit. The lens and observation ports are then made to face an extra wide tunnel piercing the balcony. Otherwise it should be placed on the rear level of the balcony, with the lens openings sufficiently high to permit the free passage of the light rays emanating from these ports above the heads of passers-by. Unless the throw is unusually long the projection room should never be placed like a conning turret, high above the rear level of the balcony, because of certain evident distortion.
A projection room must not be less than forty feet distant from the screen, and, to accommodate two machines, should not be less than seven feet high and nine feet square. If three machines are to be employed, the width should be increased to twelve feet. The floor of the projection room must be fire-proof and absolutely rigid, to prevent vibration. Self-closing exit doors, preferably of the gravity sliding type, should be placed on each side of the room. If an ordinary hinged door be used it should swing outward, as usually required by law for all exits.
The lens and observation ports should be equipped with automatic closing shutters. The law usually requires that these openings be operated by fusible links to guard against fire. The writer has always entertained a prejudice against fusible links, on the ground that they may, or may not, burn at the critical moment. For this reason he favors an ingenious device employed in Germany, that insures the constant attendance of an operator at his post. It is a special attachment linked to a depressible metal platform fitted in the floor, and upon which the operator stands to manipulate his machine. His weight depressing this plate acts through the linked connections as a pedal to hold the shutters open. The instant he abandons his post the relief of weight from the platform pulls the link and automatically closes the shutters. In case of sudden fire fright would impel the operator instinctively to jump off the platform and thereby automatically close the shutters before the audience become aware of any danger.
Where two machines are used, these platforms could be so connected that a person standing on one would depress the other and hold open both sets of ports. If the operator desired to dissolve one picture into another, his assistant, always present by legal requirement, might temporarily stand on one platform while the desired change was made. A Boston ordinance, recognizing the necessity of operators remaining constantly at their post, demands that the light control valve be always held open by an extension spring, so that a steady hand pressure is required to keep it closed and thereby to give light.
Observation ports are usually designed too small, and at improper heights. To be at all use-ful, an observation port should be about twelve inches wide and six inches high, so that the operator, standing at his machine, two or three feet back from the opening, may see the full screen without craning his neck. The height of the observation port should conform to the height of the operator and the opening should be cut twenty-four inches high. This open space should be fitted with movable metal plates, working in grooves of similar material, one above the other, each six by twelve inches in size. In number, these metal plates should be one less than enough to cover the entire opening, and a clear glass plate should be employed to cover the space left vacant by the missing metal plate. Many operators, in addition to covering the observation ports, also cover the lens port with a clear glass plate. This is, however, an unnecessary practice, and it only provides another dirt collecting glass through which the picture must be projected.