Abstract

In the first part of this article, results of an experimental investigation of the action of a light-source in the field of view in changing the adaptation of the fovea of the eye, and in increasing the minimum perceptible brightness difference are presented. In the second part are presented results of a study to determine whether a given light-source in the field of view is an advantage or disadvantage; and also a method is presented for expressing visibility by an exact ratio.

© 1927 Optical Society of America

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Figures (11)

Fig. 1
Fig. 1

Arrangement of apparatus and test-object used.

Fig. 2
Fig. 2

Curve A shows variation of minimum perceptible brightness difference (ΔF)τ with brightness F of adapting field. Curve B shows corresponding contrast sensitivity S or ratio F/(ΔF)τ.

Fig. 3
Fig. 3

Shows variation of minimum perceptible brightness difference (ΔF)τ with illumination E at eye from a dazzle-source located 5 degrees above the line of vision. Curves A, B and C are for screen or background brightnesses F of 1, 0.1 and 0.001 ml respectively.

Fig. 4
Fig. 4

Curves A, B, C and D show the variation of minimum perceptible brightness difference (ΔF)τ with illumination E at eye from a dazzle-source located respectively 2.5, 5, 10 and 25 degrees above the line of vision when the background brightness F was 0.01 ml.

Fig. 5
Fig. 5

Shows the millilamberts increase of adaptive brightness equivalent to one meter-candle of unidirectional illumination E at eye from a dazzle-source located at various angles D to the line of vision.

Fig. 6
Fig. 6

Showing how the minimum perceptible brightness difference (ΔF)τ varies with the background brightness F for unidirectional illuminations E at the eye of 0, 1/4, 1/2, 1, 2, 4 and 8 meter-candles from a glare-source 5 degrees out of the line of vision.

Fig. 7
Fig. 7

Sketch A shows conditions assumed. Curve B shows how the safety-factor of visual-contrasts varies as angle D of glare-source varies when the reflection-factor of object is 0.5. Curve C shows how the reflection-factor of the object would vary if the safety-factor were to be maintained at a constant value of three.

Fig. 8
Fig. 8

Sketch A shows conditions assumed. Curves B, C and D show how the safety-factor of visual-contrasts varies with the background brightness F for values of the illumination E at the eye from the glare-source of 0.1, 1 and 10 mc respectively.

Fig. 9
Fig. 9

Sketch G shows conditions assumed. For equal illuminations at the eye and upon the screen from the light-source L1, the curves show how the safety-factor of visual-contrasts varies with the illumination E at the eye and screen from the light-source L1. Curves A, B, C, D, E and F are for initial brightnesses of the screen of 0.0, 0.001, 0.01, 0.1, 1 and 10 ml respectively.

Fig. 10
Fig. 10

Sketch D shows the light-source L1 to be 5 degrees out of the line of vision and to give illuminations of E at the observer’s eye and rE upon the screen. The illumination from light-source L2 upon the screen produces an initial brightness F0 of 0.1 ml. Curves A, B and C show how the safety-factor of visual-contrasts varies with the ratio r for illuminations E of 0.1, 1, and 10 mc, respectively.

Fig. 11
Fig. 11

Shows the initial brightness F0 of the background in order that the safety-factor of visual-contrasts may be ten when the conditions are as shown in Sketch D, Fig. 10. Curves A, B, C, D and E are for ratios r of the illumination upon the screen to that at the eye of 0, 1, 2, 2.5 and 2.7, respectively.

Tables (2)

Tables Icon

Table 1 Showing the minimum perceptible brightness difference (ΔF)τ in millilamberts between test-objects and background for various brightnesses F in millilamberts of the background or adaptive field. The test-object consisted of a bright annular ring with a mean diameter that subtended a visual angle of about 55 minutes and a radial width of about 37 minutes as shown in Fig. 1.

Tables Icon

Table 2 Table showing the minimum perceptible difference in brightness (ΔF)τ between test-object and background when the background had a brightness of F millilamberts and a unidirectional illumination of E meter-candles shone into the observer’s eyes from a dazzle-source located D degrees above his line of vision.

Equations (3)

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M = 2.9 / D 2
equivalent total brightness = F + 2.9 E / D 2
Δ F ( Δ F ) τ = Actual brightness difference Minimum perceptible brightness difference = a s a f e t y - f a c t o r o f v i s u a l - c o n t r a s t s