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446 DARKENING THE PROJECTION ROOM [Cn. XII
Much of the projection at the present time requires daylight rather than twilight vision from the brilliancy of the screen images, but one should keep in mind that good screen images may be obtained by two methods (i) brilliant illumination and daylight vision; or (2) moderate illumination and twilight vision.
§ 610. Method of darkening a room. — As many rooms used for projection are well supplied with 'windows there must be some method of excluding daylight or other outside light. The two means usually employed are wood or metal shutters and opaque cloth curtains.
Shutters may be on hinges and swing sidewise, or they may be hung, and by means of pulleys raised and lowered. In many laboratories where the shutters are opened and closed several times during a lecture, there is a water or electric motor to move the shutters.
If curtains are used they should be of dark colored opaque cloth on a spring roller, so that they can be opened or closed as much or as little as desired. These are usually opened and closed by hand (fig. 238).
§ 611. Excluding light at the window margins. — As curtains are usually hung, there is a space admitting light at the top, bottom, and sides of the window. This can be avoided by having the edges of the curtain in a groove at the sides and bottom of the window frame, and having the curtain roller above the opening of the window frame (fig. 238). If one has the designing of the building, proper grooves can be planned for and put in when the window frames are made. If this has not been planned for in designing the building, then the light-excluding devices can be added afterwards. That is, a light-excluding shield can be put all around the window frame (fig. 239). This will, of course, cut down somewhat the opening of the window frame.
POSITION OF THE PROJECTION APPARATUS IN THE ROOM § 612. The best position for the projection apparatus in a lecture room or exhibition room is at the back of the room, where it is entirely free from the audience. This also gives the operator greater freedom (fig. 240).