Reflection and contrast
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Character size measurement
The convention for measuring character size has been to state the angular or
physical height of non-accented upper case letter. The classical measurement
is from the top edge to the bottom edge of the character. Other conventions,
however, measure character height in terms of dot-center to dot-center or
stroke-center to stroke-center. The center-to-center measurements avoid the
problem of having to define the edge. Since both stroke-width and between-character
spacing are also considerations, some definition of edge location should
be agreed upon.
There are optical reasons for selecting the 50% level as the "edge",
that is, the point halfway in the luminance change between the light and
dark areas that define the edge. If a person views a perfect square wave
edge, or a target of very high resolution, the cross section of the illuminance
distribution on the retina will be sinasoidal, with the 50% level indicating
the location of the edge being viewed (Gubish, 1967). Even if the resolution
of the edge being viewed is reduced, or if the retinal image is defocused,
the location of the 50% level should not change. The 50% point is also insensitive
to aperture size, within certain limits. of the photometer being used to
measure the edge.
ISO 9241, part 3, deals with character size in a discussion of character
height-to-width ratio:
"A width to height ratio of from 0.7:1 to 0.9:1 is recommended
for optimum legibility and readability. For other considerations, (e.g.,
line length, proportional spacing), however, the ratio shall be within
the range 0.5:1 to l:1."
ISO also specifies character formats:
"A 5 x 7 (width to height) character matrix shall be the minimum
used for numeric and upper-case only presentations.
"A 7 x 9 (width to height) character matrix shall be the minimum for
tasks that require continuous reading for context, or where individual alphabetic
character legibility is important for the task, such as proofreading.
"The character matrix shall be increased upward by at least two
dot (pixel) positions if diacritics are used. If lower case is used, the
character matrix shall be increased downward by at least two dot (pixel)
positions, to accommodate the descenders of the lower case letters.
"For higher density character matrices, the number of pixels used
for diacritics should follow conventional designs for printed text.
"A 4 x 5 (width to height) character matrix shall be the minimum
used for subscripts and superscripts, and for numerators and denominators
of fractions that are displayed in a single character position. The 4 x
5 matrix may also be used for alphanumeric information not related to the
operator's tasks, such as copyright information.
"For non-dot matrix techniques, equivalent character shapes should
be achieved."
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