Glossary A–ZLetter C
Contrast (legibility)
Contrast is the difference in brightness between text and background and can be measured as a ratio.
The higher the ratio, the easier the text is to read, especially from a distance and for people with impaired vision.
The WCAG web guidelines call for at least 4.5 to 1 for normal text and 3 to 1 for large text. They apply to web content but also serve as a yardstick for screens in a room. In practice this means dark text on a light background or light text on a dark background, no colour pairs of similar brightness such as red on green, and no thin typefaces.
It should be checked at every point, especially for text on photos and gradients, where the background changes behind individual letters. A semi-transparent panel behind the text helps there. The measured value is only part of the story: reflections, sunlight and a screen brightness that is too low reduce contrast on site. Operators should therefore look at content in the actual location.