A typical CRT terminal workstation is characterized by 3 different kinds of visual tasks:
Good visual conditions are only satisfied when all these three visual tasks can be performed with optimum ease and comfort. One of the difficulties in obtaining such optimum conditions is that the text on the screen is generally presented as bright letters on a dark background (positive contrast), while the printed or typed text on the document is made of black or dark letters on white paper (negative contrast).
It is widely believed that thermal confort depends on differences of race, age, and sex. However, systematic testing in an environmental test chamber on elderly and youth subjects, and people from countries with widely differing climates, has shown that the Comfort Equation is independent of these factors. All together, more than 1300 subjects have been tested. Possible national differences, or differences between the sexes in preferred ambient temperature, seem to be exclusively a function of differences in clothing fashions. Such differences are taken into account in the Thermal Comfort Meter when a cIo value is selected for the clothing parameter
The operator has to perform almost simultaneously two visual tasks corresponding to two completely different adaptation levels of his/her eyes. But this is only the first part of the problem. The second part of the difficulty, as shown below, is that the requirement to obtain optimum conditions in each case are not compatible. Owing to the special nature of a workstation with a CRT terminal, there are many areas where the light sources can produce veiling reflections and glare effects.
Good readability of the text on the source document requires a higher light level, since good illumination corresponds directly with visual acuity. However, the use of a higher ambient light level may reduce the readability of the text displayed on the video screen.
Many parameters influence the readability of the screen display, as for instance the dimension of the characters and the sharpness of their contours, and the colours of the characters and the background. But undoubtedly the most important being the contrast between the letters on the screen and the background.