Glare and the Difficulty
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
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
changing frequently between these two visual tasks will almost never reach the complete
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.