Readability is concerned with comprehension: the ease in which text can be read and understood. It refers to entire words, sentences, and paragraphs.
Legibility is concerned with perception: the degree to which individual characters or letters (glyphs) in text are understandable or recognizable as distinct from each other based on appearance.
Various factors to measure readability have been used, such as “speed of perception,” “perceptibility at a distance,” “perceptibility in peripheral vision,” “visibility,” “the reflex blink technique,” “rate of work” (e.g., speed of reading), “eye movements,” and “fatigue in reading.”
Readability is influenced by:
- typeface and legibility
- point size and contrast
- column spacing: generous vertical space separates lines of text, making it easier for the eye to distinguish one line from the next, or previous line.
- column width and line length
- alignment (design of column edge),justification, and hyphenation
- vertical spacing: linespacing and primary and secondary leading: is it too tight or too loose?
- horizontal spacing: kerning, tracking or letter spacing and word spacing
- colour contrast between type and background. Contrast, without dazzling brightness, has also been found to be important, with black on yellow/cream being most effective.
- hierarchy contrast and predictability between different elements on the page
The saccadic rhythm of eye movement is important for readability—in particular, the ability to take in (i.e., recognise the meaning of groups of) around three words at once and the physiognomy of the eye, which means the eye tires if the line required more than 3 or 4 of these saccadic jumps. More than this is found to introduce strain and errors in reading (e.g. Doubling). Proper fonts for people with reading difficulties such as dyslexia, have continued to be subjects of debate.
Legibility is usually measured through speed of reading, with comprehension scores used to check for effectiveness (that is, not a rushed or careless read). The typeface chosen should be read without effort. This is a key issue in typeface design to ensure that each individual character or glyph is unambiguous and distinguishable from all other characters in the font. Legibility includes factors such as:
- character shapes
- stroke contrast
- size of its counters
- serifs or lack thereof
Some commonly agreed findings of legibility research include:
- Text set in lower case is more legible than text set all in upper case (capitals), presumably because lower case letter structures and word shapes are more distinctive.
- In general typefaces that are true to the basic letterforms are more legible than typefaces that have been condensed, expanded, embellished, or abstracted.
- Regular upright type (roman type) is found to be more legible than italic type.
- The upper portions of letters play a stronger part than the lower portions in the recognition process.
- Extenders (ascenders, descenders and other projecting parts) increase salience (prominence).
- Positive images (e.g. black on white) are easier to read than negative or reversed (e.g. white on black). However even this commonly accepted practice has some exceptions, for example in some cases of disability.