Diminishing Returns Lawprinciple Of See Josts Laws

DIMMING EFFECT AND DIMMING CONTRAST EFFECT. The American psychologist Leonard Thompson Troland (18891932) - who studied visual perception and retinal illuminance (the illuminance unit called the Troland is named after him, and is defined as the intensity of light falling on the retina, calculated as the product of luminance and pupil area), and who is the co-inventor of colored movies - described the dimming effect and the principle of the dimming contrast effect in 1917 in his investigations of afterimages produced under carefully controlled conditions. Troland found that if an afterimage, once formed and projected, is permitted to fade and, if then, the afterimage of the projection field itself be observed against a completely dark ground, a demarcation of the field into halves may be noted. This effect is always obtained with short pre-exposures and high intensities, but is absent for pre-exposures that approach in duration the "equilibrium time," or maximal duration of the image. If the intensity of the projection field is decreased, but not made zero, a similar rejuvenation of the afterimage contrast occurs. The pre-exposed half appears darker and complementary in hue as compared with the other half of the field. In its general form, this principle is that dimming the reaction light enhances the processes that are "antagonistic" to the original stimulation. However, to account for the increase of the afterimage, the principle requires a special mathematical form because if dimming influenced both halves of the field equally, the contrast would not be augmented, no matter how great the absolute alteration of the compared areas. Consequently, under these conditions, the principle of the dimming contrast effect refers to a very slight difference in the level of fatigue of two visual areas that furnishes the basis for a large difference in the magnitude of the dimming effect producible on the two respective areas. Such a contrast, which has been re-established by dimming, fades rapidly on the dimmed field, but - if after it has disappeared - the field is brightened to its original intensity and then dimmed again, the contrast returns. This process may be repeated successfully over a period sometimes twelvefold the life of the image on the undimmed field. The factor of the "relative durability" of the dimming contrast effect decreases rapidly to an asymptote with increase in the original pre-exposure time (i.e., the effect is less marked in conjunction with a generally low level of visual sensitivity than it is with higher levels). Troland stated that certain aspects of the dimming (and "reversal") phenomena are explainable by the earlier work of the German physiologist Ewald Hering (1834-1918), but Troland attempted to modify Hering's views (cf., Troland, 1915) and concluded that the dimming effects depend either upon the laws governing certain of the constants/parameters of the simple equations for retinal excitation, or else they rest on conditions present in the visual system posterior to the retina. See also ADAPTATION, PRINCIPLES/LAWS OF; VISION/SIGHT, THEORIES OF. REFERENCES

Hering, E. (1878/1964). Outlines of a theory of the light sense. Cambridge, MA: Harvard University Press. Troland, L. T. (1915). Adaptation of the chemical theory of sensory response. American Journal of Psychology, 25, 500-527. Troland, L. T. (1917). The influence of changes of illumination upon afterimages. American Journal of Psychology, 28, 497-503.


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