Readability and legibility
10th October 2007 by Liam McGee
We’ve been thinking about accessible typography lately. The readability of text, to be precise.
Readability can be considered to be a function of margin space, leading (line height) and line-length. Traditionally, a line-length of approx 3 alphabets (the actual em-width of this will vary according to the font) seems to be ideal in balancing the number of new-line interruptions (for which longer lines are better) with the ease of returning to the beginning of the next line without losing your vertical position (for which shorter lines are better). Leading needs to take into account the difference in height of lower case and upper case letters in the font, known as the x-ration (the ration between the height of the capital X and the lower case x).
The other main typographic properties which affect legibility are font size and contrast.
Font-size
It’s rather hard to categorically state a minimum font size for legibility, as the legibility of a letter is based on the angle subtended by that letter on the reader’s retina — which is a function of the actual size of the letter and the distance of the reader.
Unfortunately as soon as you introduce a computer screen, there is an added complication in that the size of a character 12px high will vary significantly depending on the physical size and resolution of thescreen.
Contrast
There are specific rules about foreground/background contrast within the W3C WAI WCAG guidelines, and it’s one of the most common and significant design problems we see in accessibility testing. The palest allowable grey on a white background, for example, is #585858… or at least this is the case under WCAG1.0. WCAG2.0 is a different story, more about that in a later article.