Scales of Apparent Brightness

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Abstract
The historical background of the subject is given, together with a discussion of the types of scales with which attempts have been made to measure subjective brightness. The apparatus used for direct estimation experiments is described, and the method of direct estimation is explained in detail. In this method the observer is presented with a series of test fields in random order of luminance and asked to grade them by directly estimating the magnitude of their apparent brightness. The method was exhaustively explored and the effects were found of different groups of observers, the size of luminance step, the order of presentation of the luminances, field size, foveal and extra foveal viewing, and the colour of the test field. Scaling is discussed, and statistical evidence showing that the directly estimated brightnesses are on a ratio scale is given. Field work confirmed that the apparent brightness data obtained with laboratory apparatus is applicable to practical situations. The effect of the adaptation field is discussed. Binocular matching experiments were used to relate more accurately apparent brightness data to different adaptation levels. Apparent brightness scales for light different adaptation levels are given which can be used for the luminance design of interiors.

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