When you are dealing with the United States federal government, accessibility isn’t a nice to have, it is the law. As I was researching the law, I came across a related item, the Bond (as in 007) rule, which was proposed by Sidney Smith in the journal Human Factors in 1977.
The federal government has an entire website section508.gov describing the 1998 law that requires federal agencies to make their applications and websites accessible. I encourage you to go to the site and learn more. Of particular interest to application developers is the Technology Tools page. It contains a lot of great links. You’ll find references to many things things including some I’ve blogged about before – color blindness and epilepsy.
Smith discovered that to be readable by the masses, a character needs to be at least .007 radians. That’s why some people refer to this as the Bond Rule. Using radians to specify character height provides a measurement independent of the viewing distance. To know the exact character height needed to be visible at a specific distance, multiply the viewing distance by 0.007 to find the character height in the same units as the viewing distance. If you have 20/40 vision and are reading text that is 100 cm away, the character size should be at least 0.7 cm (7 mm). 20/40 is used instead of 20/20 because 20/40 is more representative of the real world than the idealized 20/20.