Glare and the Difficulty
of Adaption

Work at a CRT display terminal requires frequent changes between the three visual tasks: reading of the text on the screen and on the document, and the recognition of symbols on the keyboard.

Too much difference between the luminance values of these three visual tasks can and usually does disturb the adaptation process of the eyes.

The eyes of the operator watching a dark screen are adapted to a low level of light and the general level of illumination of his field of vision.

While adaptation from dark to light conditions requires only a few seconds, the adaptation from light to dark condition can take up to half a minute.

That means that an operator watching successively a dark screen and a light source document and changing frequently between these two visual tasks will almost never reach the complete adaptation required for the screen.

Besides the difficulty of adaptation, veiling reflections from various light sources on screen and document as well as keyboard, can cause glare effects of annoying proportions.