ABSTRACT
Text that conveys critical information should be legible to ensure users can read the text easily and accurately. In most cases, character height subtending a visual angle of 20 to 22 minutes assures legibility. An angle of 16 to 18 minutes might be acceptable, depending on several factors, including the text-to-background contrast ratio. The US Federal Aviation Administration suggests using character heights of 1/200th the viewing distance to ensure legibility. To be legible, text must have sufficient contrast against its background. One might think that Times New Roman and Helvetica fonts, although different, are equally legible. Due to their simpler geometry, sans serif fonts are considered superior for rapid visual acquisition of shorter text strings, whereas serif fonts are considered better at guiding the eye across long text strings. There are many other factors that affect legibility, such as stroke thickness, height / width ratio, letter spacing, and line spacing.
