Abstract
With spread of office automation system, Visual Display Terminal (VDT) became offen and usually used in office works. CRT (Cathode Ray Tube) displays are most usual unit of VDT. For most common positive contrast type display, wherethe character luminance is higher than the background luminance, lighting conditions on the display screen bring essential influence on visibility of the characters on the screen.
Especially in these conditions, reflection of external light sources on the display screen has very important influence on the visibility of the characters.
Experimental study using subjective appraisal was carried out to find out permissible range of reflected luminance related to the charactor luminance and the backgroud luminance.
The results are as follows:
(1) Preferable luminance of the charactor Lc' related to the background luminance Lb', which is proportional to the normal or vertical illuminance on the screen, is expressed with the empirical formula
Lc'=16×Lb'0.71
(2) Under above condition, permissible value of the refrlected luminance Lr' is expressed with the empirical formula
Lr'≅1.9Lb'0.19
or with the contrast form (luminance ratio)
C=Lr'/Lb'≅1.9×Lb'-0.09
(3) Furthermore, for the practical range of the Lb', this contrast (Lr'/Lb') nearly constant and about 1. 2, for the display used in the experiment. To avoid reflected luminance, Lr' should be kept under 1. 2×Lb'.
Where every Lc', Lb' and Lr' is total luminance overlapped with reflected component of the diffuse reflection on the screen.
