Lighting

	Overview   |   Measurement of light   |   Luminance balance   |   Illuminance   |   Workstation light level   |   Transient Adaptation Factor (TAF)   |   Disability glare   |   Other considerations

The Transient Adaptation Factor was devised to predict the momentary reduction in contrast sensitivity that occurs when the point of fixation is shifted to surfaces of deferent luminances. The TAF would, for example, estimate the visibility of one source document 300 milliseconds after having viewed another, otherwise similar, source document illuminated by a significantly different amount of light.

Figure 56 shows the TAF function. The luminance ratio between the two surfaces may be located on the abscissa. The ordinate of the curve at a given point gives the multiplier that predicts the temporary reduction in contrast sensitivity. For example, if there is a 10:1 difference between the luminances of two otherwise similar source documents, there will be a momentary reduction of about 25% of effective contrast whenever fixation
is shifted from one source document to another.

The Transient Adaptation Factor. The relationship between the temporary loss in contrast sensitivity and the ratio of luminances of successive visual fields.

The TAF values characterize task visibility only for a brief period following eye movements between surfaces of different luminances. They do not
apply to the considerably greater periods during which the task is steadily
fixated. The TAF is not particularly important when dealing with visual tasks
that are well above threshold. The visual tasks usually associated with VDT
use are well above threshold and not particularly sensitive to the TAF function.

Although it is sometimes recommended, there seems to be no physiological reason for intentionally creating an imbalance in the light falling on a work surface. Surfaces away from the prime working area do not need as much light, and energy may be saved by illuminating them at a lower level. The Illuminating Engineering Society Handbook (Kaufman and Christensen, 1987), however, recommends that surfaces excessively brighter or darker than the point of interest be no closer than 5 degrees to the point of interest so as not to affect the visibility of the task. That recommendation is reasonable in the case of the very bright surfaces in the field of view; the dark surfaces in the field of view will not have an adverse effect.

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