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492d HANDBOOK OF PROJECTION FOR
where the angle of observation does not exceed 20°. In order now to obtain the maximum results in screen brightness for any given current consumption it is only necessary to choose that screen, or screens, which shows the highest value in the column marked R.
Next assuming a maximum angle of 30° we -find that only two screens of class A have a ratio of normal to extreme (30°) reflection factor which is less than 4.0. Hence on the basis of our assumptions only two screens of class A are suitable for use in an auditorium where the maximum angle of observation is 30°. Turning to class B however we find that all of the screens have a ratio at 30°, less than 4.0. Again the screen having the highest average reflection factor, R, will give the most brilliant picture for a given energy consumption. (Author's note : This average reflection factor is found by adding the reflector factor for the 0, 5, 10, 15, 20 and 30 degree angles and dividing by 6).
TABLE No.
3
CLASS A
>urfac<
; 20 deg.
30 deg-.
40 deg.
50 deg
No.
Ratio R
Ratio R
Ratio
R
Ratio
R
8
2.70 306
5.83 259
12.9
221
24.3
192
9
2.70 32S
5.97 275
12.8
235
21.8
204
10
2.62 318
5.85 269
11.9
230
23.7
200
12
2.15 241
3.96 208
7.00
180
13.1
158
13
2.07 221
3.75 191
6.37
167
11.4
146
16
2.39 271
4.89 232
9.38
199
17.0
174
18
2.62 256
4.50 220
8.04
190
15.4
166
21
3.51 213
5.28 181
6.06
159
6.19
143
23
2.46 278
5.41 235
9.92
202
17.0
176
29
3.00 337
6.72 282
12.8
241
20.2
209
30
3.31 236
6.00 200
11.2
172
19.4
150
31
2.47 240
4.62 206
8.62
178
15.2
156
32
3.43 295
7.26 248
10.1
214
21.8
186
34
4.20 185
5.85 157 '
7.20
138
7.57
123
35
1.99 242
4.11 208
8.65
179
15.00
156
36
2.20 333
4.98 283
10.90
242
20.80
210
37
1.90 259
4.00 223
8.00
192
14.70
168
Considering now a maximum observational angle of 40° we find that no screens of class A fulfill the requirement of a ratio less than 4.0'. In class B, four of the screens however are suitable and all of the screens of class C fall within the specified limit.