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MANAGERS AND PROJECTIONISTS
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practical use, it was decided to define the range covered by these classes as follows :
Class A includes those screens which are adapted for use in theatres where the maximum angle of observation does not exceed 30 degrees ; Class B includes the screens adapted for use where the maximum angle of observation does not exceed 50 degrees, while the Class C screens should be used in all cases where the angle of observation is greater than 50 degrees. The point should be again emphasized that . these classifications are not rigid, but of an approximate ; character. '
TABLE 15 ;
CLASS A
Surface No.
20°
30°
40°
50°
Ratio
R
Ratio
R
Ratio
R
Ratio
R
8
3.91
305
7.62
235
11.8
194
IS. 8
165
9
2.61
339
6.30
270
14.4
217
22.4
183
34
2.70
209
5.94
167
10.3
137
16.8
116
31
3.22
159
5.48
127
6.10
108
6.17
96
22
2.38
265
4.96
216
9.92
167
16.3
151
15
2.21
218
4.34
178
8.70
147
13.4
134
21
2.06
253
4.30
207
8.47
172
13.2
145
10
2.23
205
4.14
169
7.82
140
12.3
119
24
2.08
253
4.02
209
8.35
174
13.6
147
23
2.08
245
4.00
201
8.10
167
13.7
141
11
1.80
242
3.20
202
5.75
172
11.3
147
16
1.69
2^8
3.20
181
6.03
152
9.75
130
36
2.00
112
2.96
96.4
4.34
83.3
5.59
73.2
30
1.70
151
2.82
128
4.35
110
6.36
87
17
1.63
192
2.77
165
4.85
141
7.77
121
18
1.64
172
2.58
150
4.25
128
6.07
112
Note: Columns marked "R" indicate mean (average) reflecting power within the entire angle.
After careful consideration of the subject, it was decided that the value of the ratio of the reflecting power measured at normal observation to that measured at the maximum angle of observation in a particular installation would serve as the most logical criterion by which to select the most suitable screen for any particular case.
This value will then represent the ratio of the brightness of the screen as observed by a person in the center of the auditorium to that of the brightness as observed by a person occupying a seat at the side and near the front. By keeping this ratio below a certain limiting value, satisfactory brightness will be obtained for all observers. This factor alone, however, is not sufficient for the classifying of screens ac