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SCREENS 147
(7) Loosely woven, untreated white cloth, for example, while it may be pure white-, is nevertheless a very poor reflecting surface for the double reason that it permits a considerable percentage of the light to pass through, and because much light is absorbed by the fabric itself, into which it enters but does not pass through.
White cloth made of line, hard-twist, closely woven thread is a good reflecting surface, (8) provided its back is properly treated with a pure white coating such as kalsomine, or white lead and white zinc (50-50 mixture) paint.
(9) White lead paint properly mixed and properly applied reflects a high percentage of incident light. White zinc is still better, but there are objections to its being used alone. A proper mixture of the two, half and half, with its slight yellow content killed with blue, is excellent. It provides a widely diffusing and efficient reflecting surface.
(10) Kalsomine also reflects well and has high powers of diffusion. Both paint and kalsomine are relatively low in cost, hence such surfaces may be easily and cheaply renewed when soiled or discolored by age.
(11) Metallic screen surfaces (aluminum paints) reflect well, but usually only through narrow angles, concentrating a large percentage of the light at the center of the auditorium. They have more or less "fadeaway" characteristics at side angles. Moreover, they are subject to chemical changes which reduce reflection power and in time change the tone of the reflected light.
(12) Glass bead screens have high frontal reflection powers and very low powers of diffusion. They may be cleaned better than other surfaces, but (13) their original reflection power cannot be restored, because the white pigment in which the beads are embedded is a part of the screen's reflecting surface and discolors with time.
(14) Glass bead screens are not suited for wide auditoriums or for theatres with balconies or steep projection angles. Bead screens are better suited for use in