Studies of von Bezold type of “spread” or assimilation, wherein white lines lighten and black lines darken contiguous areas, have shown that the effect depends upon both absolute and relative widths of the lines and intervening areas and their respective reflectances. Moreover, there is a continuum from assimilation (reversal of classical contrast) to contrast with an interval in which neither assimilation nor contrast occurs. With grays of about 36% reflectance, assimilation is found with lines up to 8–10 mm after which classical contrast is found. With grays of 80% and 14% reflectance, assimilation occurs when the white and black lines and the line separations are as wide as 29 mm. Another type of anomalous contrast, the Gelb spotlight effect, proves to be a case of ‘trigger’ contrast, so-called here because of the very large contrast effect of the very small patch of white. Here the area and position of the white surface exert measurable, consistent effects. A theory of retinal and/or central facilitation and inhibition seems to account for the data.
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