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, bot- 

 tom, 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 lec- 

 ture 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). 



