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COLOR LEGIBILITY Color graphics are the foundation of the post-industrial information economy. Yet there is little data on how easy (or difficult) it is to see letters and other graphics when these are presented in color. Several experiments have shown that hue (eg. red, orange, yellow, etc.) makes no difference. Therefore, most authoritative texts state that legibility depends only on lightness contrast. Yet many artists and designers know this to be false. Only recently was the flaw in those experiments recognized. Legibility had been measured in terms of how fast people could read or recognize the graphics. The rational was that graphics which required more time to be recognized were less legible. While that seemed to make sense, it overlooked the fact that color information is carried from the eye into the brain by small, slow conducting nerve fibers, while lightness contrast is carried by fast conducting fibers. Therefore, rapid recognition is based only on lightness contrast and not the effects of hue. Some recent experiments got around this problem by measuring legibility in terms of how many nerve fibers are needed to carry enough information for recognition. The fewer pathways needed, the more legible the graphics. These measurements revealed several color combinations that were more legible than black and white. This proved that lightness contrast is not the only factor, since nothing has a higher lightness contrast than black/white. Measured in terms of the required number of nerve fibers, the ten most legible printed combinations of letter / background colors are: black/pink, black/yellow, black/orange, dark-green/white, black/white, white/purple, blue/yellow, orange/black - in descending order. The ten worst are: purple/green, light-blue/brown, pink/lime, orange/lime, lime/pink, lime/orange, brown/purple, purple/brown, lime/light-blue, light-blue/brown - in descending order. For a complete listing of relative legibility of 100 color combinations, see: The International Encyclopedia of Ergonomics, 2nd Edition, 2006, CRC Publishers, pages 1440-1452 - Legibility of Colored Print. Three caveats pertain to this list. 1) It is based only on printed colors which arise from light reflecting pigments. There are good reasons to believe that colors produced by light-emitting surfaces such as computer displays differ somewhat in legibility. 2) Color legibility also depends on the stroke width of the lines that form letters or other graphics. This is so even for black/white print which for the same stroke width is more legible than white/black. For example, reducing the stroke width of white/black letters improves their legibility (MS Sanders & EJ McCormick (1987) Human Factors in Engineering and Design, McGraw-Hill.) 3. Reduction in sharpness of the graphics image may make legibility more dependent on hue and less on lightness contrast. This would be relevant for seniors and persons with certain visual impairments. Measurements of these effects are underway. |
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