Readability of Display Screen,
Keyboard, and Document
When the ambient illumination is increased it will be superimposed on the screen. This means that the
contrast between background and letters displayed will almost always be lower than the normal contrast.
On a screen, in fact, the additional or excess luminance called veiling luminance or veiling reflection, may
be due to two different effects since the reflection factor of the screeen is usually a combination of diffuse and
Increasing the ambient light level around the terminal may raise the mean luminance of the background of
the screen (diffuse reflection) but at the same time lighted objects in the field of reflection of the screen also
become brighter and are therefore much more easily reflected and visible on the screen (specular reflection).
Such images, which are seen on the screen at different viewing distances, can disturb the accomodation of
the eyes, which try to concentrate on the text displayed at the screen surface. This drawback can at times be
partly eliminated with some of the modern terminal screens where the background brightness can be set and
controlled independent from the character brightness. Proper positioning of the terminal will also greatly help
in avoiding reflections.
Even if the operator can touch-type, he looks frequently at the keyboard -- whenever a function key needs
to be activated. This visual task therefore should not be ignored. Reflections on keys may irritate the
operator, who begins to make more typing mistakes than usual.
Unfortunately, veiling reflections on keys are more difficult to avoid, because the initially matte surface of
the keys becomes glossy by wear and use, and because the keys which are almost always concave, reflect
light sources from a relatively wide solid angle.
The reflections can be less disturbing if the keys themselves are of a light colour and the embossed
symbols dark. However, since the keys are observed frequently and for very short periods of time, a contrast
of the same sign on the screen and on the keyboard (i.e. light symbols or characters on a dark background
may be of advantage in some cases.
Situations where work at a CRT terminal does not require the use of any sort of source document are
very few. This source document can be printed, typed or handwritten, or a computer listing which is related
to the information the operator wants to read or write on the screen. Readability of the document close to
the terminal (where of course it should be placed whenever possible) depends on both the contrast of the
text or information as seen by the operator and the level of adaptation of the operator's eyes.
The contrast of a task depends not only on the reflective properties of the paper, but also on the way it is
illuminated. For visual tasks with perfectly matte surfaces the light is diffusely reflected and therefore causes
no veiling reflections. But unfortunately such highly desirable materials are seldom found in practice. In most
cases the source document reflects what incidental and ambient light sources happen to be in evidence. This
can at times, particularly with glossy paper, make the document completely unreadable.