Reflection and contrast
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Character size
The requirements for display terminal character size are about the same as for
various forms of written material. Although the resolution of a CRT display
is usually less than that of printed material, contrasts may be higher. Character
sizes as small as 10 minutes of arc are usable providing there is sufficient
contrast. Character sized that small, however, should not be used unless the
trade-off between character size and some other parameter results in an overall
benefit to the user.
Tasks that require rapid and accurate legibility of individual characters
and where ease of reading is important are common display terminal tasks. The
legibility of single characters is not significantly improved for characters
larger than about 16 to 18 minutes of arc. Larger character sizes may certainly
be used, but they may require a larger screen, which may hamper some tasks
that require visual searching. This is one of the trade-offs that should be
considered. Readability, though, is different from legibility.
ISO 9241, part 3, makes the following distinction in requirements between
legibility of characters and readability of text:
"Character heights of from 20 to 22 minutes of arc
are preferred for the most tasks. The minimum character
height shall be 16 minutes of arc.
For applications where readability is incidental to the task, smaller characters
may be used (e.g., for footnotes, superscripts, subscripts)."
A person reads, not by looking at individual characters, but by perceiving
the shape of a word or group of words. If the characters are too small, there
may be some difficulty in perceiving the distinctive characteristics of a word.
If the characters are too large, however, the person may be required to make
more fixations per unit of material and thus disrupt the reading process. Assuming
reasonable definition and contrast and tasks where the readability of continuous
text is important, character sizes should not be smaller than 14 or larger
than 22 minutes of arc in height.
Range of viewing distances for various character sizes.
Figure 47 shows a range of viewing distance for various character sizes for
the readability condition where the characters subtend visual angles between
14 and 22 minutes of arc. This corresponds to 8- to 12-point type when viewed
at typical reading distances. Ten- and 12-point type are generally preferred
and are the most frequently used sizes.
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