Handicap is when the individual actually wishes to perform that task. Maximum legibility is achieved if print is well spaced, sans serif, bold and well contrasted with the background. The process of reading is not a smooth continuous flow across the print. Most ink-print uses a light coloured background with dark print. This arrangement is so familiar that it is not usually appreciated that the eye sees the background rather than the figure.
Public displays present special and different problems.
Unlike print which is hand held and can be moved to the appropriate position, terminals are usually at a fixed height. Other problems, such as glare on screen surfaces are outside the scope of this study but are complicating factors. Because cataract is treatable it is often not quoted as a cause of permanent visual loss. Macular degeneration which extinguishes some or all of the macula function is the leading cause of permanent visual impairment. Diabetic eye disease and glaucoma are also important causes.