Identification of Text and Symbols on a Liquid Crystal Display Part II: Contrast and Luminance Settings to Optimise Legibility

This study aimed to identify the luminance and contrast levels necessary to minimise the threshold identification size of bright letters displayed over a dark background on an LCD under low (1.5 lux) and high (260 lux) lighting conditions. White, pure red, pure green and pure blue upper-case Bailey Lovie letters with a contrast between 3:1 and 300:1 were displayed on an LCD to 20 participants. The threshold identification size was equivalent for all colours except for blue, which had larger threshold identification size than white, red and green at all character luminance levels tested. The threshold identification size initially decreased as character luminance increased, but asymptoted to a minimum size after which further increases in character luminance yielded no additional significant reduction in the threshold identification size. The threshold identification size was relatively insensitive to contrast provided the contrast level was above 6:1. Self-report preferred character luminance levels were generally higher than the character luminance levels necessary to achieve minimum threshold identification size. It is recommended that character luminance should be approximately 20 cd/m2 under dark lighting conditions and approximately 60 cd/m2 under bright lighting conditions. These values translate to contrast levels of 20:1 and 60:1 under the suggested screen background luminance of 1.0 cd/m2.