Legibility can be defined as the ease with which a reader is able to decode symbols. In addition to written language, the term is also used to refer to behavior or architecture. In everyday common language, this term is synonymous with readability even though the fundamental meaning is different. Readability can be defined as the ease that a reader has when following and understanding words, sentences, and paragraphs. However, legibility refers to the visual clarity of each individual symbol. Readability is concerned more with the arrangement of words or paragraphs or even the choice of words. Hence, it can be said that it is a component of readability.
The legibility of the text is most commonly examined by a controlled deterioration of viewing conditions as well as the determination of threshold detection. This does not mean that all written text benefits from optimizing for legibility. Texts that are created specifically to be eye-catching or the appearance of which is supposed to hold a certain connotation could deviate from easy legibility for deliberate means. Corresponding typefaces are referred to as display typefaces.
The legibility of a piece of visual text depends on:
- The environmental conditions or interferences like lighting or vibrations, which affect the human visual system
- The distance from which the observer when viewing the text
- The design of the font
- The brightness and contrast between symbols and their background, which might be limited by the technology of the presentation medium
- The abilities and fitness of the readers
Even though a difference in viewing distance equally does affect the angular size of symbols and their visual resolution, the former has a much greater impact on legibility. With the advancement of screens, now screen characteristics are also a factor affecting legibility.