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in mind that the sensation produced when light falls upon the retina depends upon several factors, such as the intensity of. the light, the length of time during which the stimulation continues, wave-length of the radiation, the size and shape of the retinal area stimulated, and the physiological conditions of the retina due to previous action of light upon it. It is sufficient for the present purpose to consider only the reactions of the retina to the intensity factors of the stimulus and neglect those which are functions of its quality.
Of first importance among the factors requiring consideration is the sensibility of the retina to brightness. There are three types of brightness sensibility: (1) threshold sensibility, which is measured by the least brightness perceptible; (2) contrast sensibility, which is measured by the least brightness difference perceptible; and (3) glare sensibility, which is measured by the brightness just sufficient to produce discomfort when observed.
Now the sensibility of the eye to brightness, contrast, and glare depends upon the condition of the retina at the particular time the determination is made, and that condition is in turn dependent upon the previous stimulation. Hence, it is necessary to specify the condition of the retina at the time the measurement of sensibility is made. This is done by specifying the brightness to which the eye is adapted, and is termed the "adaptation level" of the retina. For instance, when an observer looks for some time (10 to 30 minutes) at a uniformly illuminated surface so large as practically to fill the field of vision, a condition of equilibrium in the retinal process is reached, and the observer's eye is said to be adapted to the brightness of the field, and his "adaptation level" is specified by stating the brightness of the illuminated surface, which is sometimes termed the "sensitizing field." It is found that the sensibility of the retina varies over very wide limits depending upon the adaptation level ; in fact due to this variable sensibility it is able to operate over a range of brightness from 1 to 100,000,000 (approximately). A complete expression of sensibility therefore requires measurement over the entire adaptation range, and the results are most conveniently expressed in graphic form as curves plotted with values of the various types of sensibility (threshold, contrast, and glare) as ordinates and the adaptation level as abscissae. These three sensibility curves are given in Fig. 1.*
Since the variation in value of adaptation level (field brightness) is enormous, it is necessary in plotting the curves to use the logarithms of these values. The ordinates are expressed in units appropriate to the various types of sensibility. For any specified adaptation level, it is now possible to read from the curves the brightness which is just uncomfortable (producing glare), the least perceptible difference in brightness (contrast), and the least perceptible brightness (threshold). Applying this to the case of an
* Nutting, P. G., The Fundamental Principles of Good Lighting, J. Frank.
Inst., 183: 1917, p. 287. Blanchard, Julian, The Brightness Sensibility of the Retina, Phys. Rev..
XI, No. 2, 1918, p. 81.
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