An intensity modulated cathode-ray screen of the PPI type used in radar depends on differential brightness discrimination for detection of weak signals. The brightness quantities have been largely unknown, because of photometric difficulties, but it has commonly been assumed that dark-adaptation is required for efficient visual detection. Therefore, an experimenal test was undertaken of the effects of light- and dark-adaptation on the contrast threshold and on time required for detection of supra-threshold signals appearing on a typical screen (P7 phosphor). Adapting brightnesses ranged from zero to 2000 millilamberts (ml.). By special photometry, the four screen intensities used were estimated as ranging from 0.0001 ml. to 0.22 ml. Length of exposure to the brightest adapting light and the degree of contrast between signal and background were also varied. The lowest contrast thresholds and the shortest detection times were found to occur when the test and adapting illuminations were equal or nearly so. When the two illuminations were unequal, the loss in sensitivity was roughly in proportion to their difference. The optimal field brightness of the phosphor screen used was on the order of 0.1 ml. With this field the level of adaptation of the eye can be a relatively insignificant factor in radar detection, even of weak signals. Increasing values of signal contrast allow progressive reduction in detection delay after exposure to the highest brightness.
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Brightness in millilamberts of each of four test fields and the corresponding CRT bias necessary to produce it. Probable brightness values are estimated for 0 and 2 volts from VRI by extrapolation of measurements made 1/6th second after excitation; the value for 4 and 6 volts from VRI are actual measurements.
Volts from VRI (22 v)
Brightness of field of pip
0
0.0001
2
0.0005
4
0.022
6
0.22
Table II
Brightness in millilamberts of pip and background, and the ratio between them.
Test field brightness
Brightness of pip
Contrast ratio
0.0001
0.0061
60.00
0.0005
0.0022
3.40
0.022
0.0286
0.30
0.22
0.2600
0.18
Table III
Time for detection (in seconds) at different lengths of exposure to a constant intensity for four test field brightnesses.
Test field brightness (millilamberts)
Length of exposure to 2000 ml. (minutes)
Statistic
1
2
3
4
5
0.0001
Mean
15.3
19.2
27.8
35.4
46.2
48.5
43.1
43.7
Range
14.4
15
15
15
27
27
27
21
19.2
26.4
45
54
66.6
63
61.2
63
N
8
8
8
8
8
8
4
4
0.0005
Mean
9.5
12.5
15.1
27.4
23.8
25.4
27.6
32.7
Range
7.8
8.4
8.4
15
8.4
8.4
8.4
15
15
15.6
24
37.8
49.8
47.4
55.8
63
N
10
10
10
6
10
10
10
6
0.022
Mean
6.9
7.0
6.6
6.8
7.2
8.0
8.0
7.4
Range
6
6.6
5.4
6
6
5.4
5.4
4.8
7.8
7.8
7.8
7.8
7.8
11.4
11.4
9.6
N
4
4
4
4
4
4
4
4
0.22
Mean
15.9
19.3
34.1
36.6
37.7
55.3
56.5
62.6
Range
7.2
8.4
19.8
19.8
14.4
21.0
15.0
37.8
25.2
33.6
63.0
66.0
67.8
87.0
90.0
85.8
N
10
10
10
10
10
10
10
10
Table IV
Visibility threshold for four degrees of light adaptation at each of four screen brightnesses. Visibility thresholds recorded as decibels attenuation of a standard signal. The mean threshold for each condition is expressed both in db and, in parentheses, as a brightness contrast. Contrast ratios were computed from brightness measures and their extrapolations, as explained in the text.
Screen bright ness-
Dark-adapted eye
Adapted to screen
Adapted to 1000 ml.
Adapted to 2000 ml.
recorded values
mean
recorded values
mean
recorded values
mean
recorded values
mean
0.0001
23
23
11
10
24
23.0
23
22.67
10
10.0
9
9.0
22
(3.10)
22
(3.20)
9
(385.0)
8
(860.0)
0.0005
31
32
16
11
31
31.0
32
31.67
16
15.67
10
10.0
31
(0.72)
31
(0.66)
15
(12.0)
9
(60.0)
0.022
38
41
26
20
39
38.67
41
41.0
26
25.67
20
19.67
39
(0.12)
41
(0.09)
25
(0.60)
19
(1.40)
0.22
37
39
27
21
36
36.67
39
39.33
26
26.67
22
22.0
37
(0.08)
40
(0.06)
26
(0.24)
23
(0.45)
Table VI
Time for detection of pip for various degrees of light adaptation at each of four screen brightnesses. (Target contrast: 3 db above threshold.)
Note: Figures in parentheses were not used for various reasons such as equipment variability, lack of certainty by observer, etc.
Table VIII
Brightness equivalents of pips produced by various signal voltages. Signal intensity given in decibels above threshold, which is about 41 db of attenuation of an 8 volt signal. ΔI is the difference between pip and background in millilamberts (calculated) and ΔI/I is the contrast ratio.
Signal (db)
ΔI
I+ΔI
ΔI/I
3
0.0028
0.0248
0.13
6
0.0040
0.0260
0.18
10
0.0066
0.0286
0.30
15
0.0128
0.0348
0.58
20
0.0264
0.0484
1.20
25
0.0550
0.0770
2.50
Table IX
Time for detection of pip for various degrees of contrast between target and background. (Constant scope brightness: 0.022 ml.)
Contrast ratio
Dark-adapted eye
0.20 ml.
2.0 ml.
6.0 ml.
200 ml.
1000 ml.
2000 ml.
rec. val.
mean
rec. val.
mean
rec. val.
mean
rec. val.
mean
rec. val.
mean
rec. val.
mean
rec. val.
mean
Time for detection (seconds)
15
5
5
7
79
140
199
0.13
14
5
5
6
90
81
141
141.67
194.5
13
5
5
5
74
144
190
5
5
5
5
13
90
135
0.18
5
5
5
5
12.5
89
131.5
5
5
5
5
12
88
128
5
5
33
101
0.30
5
5
54
49.33
92
5
5
61
83
4
5
58
0.58
4.5
5
54
5
5
50
5
20
1.20
5
19.5
5
19
2.50
5
5
5
5
Tables (7)
Table I
Brightness in millilamberts of each of four test fields and the corresponding CRT bias necessary to produce it. Probable brightness values are estimated for 0 and 2 volts from VRI by extrapolation of measurements made 1/6th second after excitation; the value for 4 and 6 volts from VRI are actual measurements.
Volts from VRI (22 v)
Brightness of field of pip
0
0.0001
2
0.0005
4
0.022
6
0.22
Table II
Brightness in millilamberts of pip and background, and the ratio between them.
Test field brightness
Brightness of pip
Contrast ratio
0.0001
0.0061
60.00
0.0005
0.0022
3.40
0.022
0.0286
0.30
0.22
0.2600
0.18
Table III
Time for detection (in seconds) at different lengths of exposure to a constant intensity for four test field brightnesses.
Test field brightness (millilamberts)
Length of exposure to 2000 ml. (minutes)
Statistic
1
2
3
4
5
0.0001
Mean
15.3
19.2
27.8
35.4
46.2
48.5
43.1
43.7
Range
14.4
15
15
15
27
27
27
21
19.2
26.4
45
54
66.6
63
61.2
63
N
8
8
8
8
8
8
4
4
0.0005
Mean
9.5
12.5
15.1
27.4
23.8
25.4
27.6
32.7
Range
7.8
8.4
8.4
15
8.4
8.4
8.4
15
15
15.6
24
37.8
49.8
47.4
55.8
63
N
10
10
10
6
10
10
10
6
0.022
Mean
6.9
7.0
6.6
6.8
7.2
8.0
8.0
7.4
Range
6
6.6
5.4
6
6
5.4
5.4
4.8
7.8
7.8
7.8
7.8
7.8
11.4
11.4
9.6
N
4
4
4
4
4
4
4
4
0.22
Mean
15.9
19.3
34.1
36.6
37.7
55.3
56.5
62.6
Range
7.2
8.4
19.8
19.8
14.4
21.0
15.0
37.8
25.2
33.6
63.0
66.0
67.8
87.0
90.0
85.8
N
10
10
10
10
10
10
10
10
Table IV
Visibility threshold for four degrees of light adaptation at each of four screen brightnesses. Visibility thresholds recorded as decibels attenuation of a standard signal. The mean threshold for each condition is expressed both in db and, in parentheses, as a brightness contrast. Contrast ratios were computed from brightness measures and their extrapolations, as explained in the text.
Screen bright ness-
Dark-adapted eye
Adapted to screen
Adapted to 1000 ml.
Adapted to 2000 ml.
recorded values
mean
recorded values
mean
recorded values
mean
recorded values
mean
0.0001
23
23
11
10
24
23.0
23
22.67
10
10.0
9
9.0
22
(3.10)
22
(3.20)
9
(385.0)
8
(860.0)
0.0005
31
32
16
11
31
31.0
32
31.67
16
15.67
10
10.0
31
(0.72)
31
(0.66)
15
(12.0)
9
(60.0)
0.022
38
41
26
20
39
38.67
41
41.0
26
25.67
20
19.67
39
(0.12)
41
(0.09)
25
(0.60)
19
(1.40)
0.22
37
39
27
21
36
36.67
39
39.33
26
26.67
22
22.0
37
(0.08)
40
(0.06)
26
(0.24)
23
(0.45)
Table VI
Time for detection of pip for various degrees of light adaptation at each of four screen brightnesses. (Target contrast: 3 db above threshold.)
Note: Figures in parentheses were not used for various reasons such as equipment variability, lack of certainty by observer, etc.
Table VIII
Brightness equivalents of pips produced by various signal voltages. Signal intensity given in decibels above threshold, which is about 41 db of attenuation of an 8 volt signal. ΔI is the difference between pip and background in millilamberts (calculated) and ΔI/I is the contrast ratio.
Signal (db)
ΔI
I+ΔI
ΔI/I
3
0.0028
0.0248
0.13
6
0.0040
0.0260
0.18
10
0.0066
0.0286
0.30
15
0.0128
0.0348
0.58
20
0.0264
0.0484
1.20
25
0.0550
0.0770
2.50
Table IX
Time for detection of pip for various degrees of contrast between target and background. (Constant scope brightness: 0.022 ml.)