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Vision
•The human eye is like a camera.
•It has an adjustable lens through which light
rays are transmitted and focused.
•The light falls on a sensitive area called the
retina.
•Light rays that are reflected from an object
enter the transparent cornea and pass
through a clear fluid (aqueous humor) that
fills the space between the cornea and the
pupil (a circular variable aperture) and lens
behind the cornea.
•The muscles of the iris can change the size
of the pupil making it larger in the dark and
smaller in bright conditions.
•Light rays transmitted through the pupil to
the lens are refracted by the adjustable lens
and then transverse the vitreous humor (a
clear jellylike filling the eyeball back of the
lens).
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• In normal or corrected vision persons, the
light rays are exactly focused on the
retina.
• The retina consists of about 6 to 7 million
cones concentrated near the center and
about 130 million rods distributed in the
outer areas of the retina around the sides
of the eyeball.
• The cones receive daytime vision and the
rods are important in dim light and at
night.
• Greatest sensitivity is in the fovea (the
dead center of the retina).
• For clear vision, the eye must be directed
so that the image of the object is focused
on the fovea.
• the image on the retina is inverted.
• The cones and rods are connected to the
optic nerve which transmits neural
impulses to the brain which integrates
them to give the visual impression of the
object. This process also corrects the
inverted image on the retina.
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Visual Abilities:
1. Accommodation
1.
•
•
•
•
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Visual Acuity:
The ability of the eye to differentiate
between detailed features of what we
see such as identifying a person across
the street.
It depends on the accommodation of
the eyes (adjusting the lens for proper
focusing of the light rays in the retina.
In normal accommodation, the lens
flattens to see far objects and bulges to
see near objects.
If accommodation of the eyes is
inadequate, nearsightedness (the lens
remains in a bulged condition
preventing proper focusing to see far
objects) or farsightedness (the lens
remains in a flat condition preventing
proper focusing to see near objects) will
occur. In both cases, vision needs to be
corrected through the use of proper
lenses.
•Visual acuity is measured using minimum
separable acuity which refers to the
smallest feature or space between the part
of the target that the eye can detect.
•The reciprocal of the visual angle (VA) in
the case of the smallest object a person can
see can be used as a measure for visual
acuity.
3438H
VA (minutes) 
D
Where H is the height of the object and D is
the distance from the eye.
The scoring system uses the reciprocal of
the visual angle.
Vernier acuity refers to the ability to
differentiate the lateral displacement of one
line
from
another
while
minimum
perceptible acuity refers to the ability to
detect a spot from its background.
Stereoscopic acuity refers to the ability to
differentiate the different images received by
the retinas of the two eyes of a threedimensional single object. Most difference is
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when
the object is near the eyes.
2.Convergence:
•The two eyes must converge on the object
to be seen so that the images of the object
on the two retinas are in corresponding
positions to get the impression of a single
object (the images are fused).
•Convergence is controlled by the muscles
surrounding the eyeball. Some individuals
tend to converge to much and others tend
not to converge enough. These two
conditions are called phorias and cause
double images which are visually
uncomfortable and may cause muscular
stresses and strains.
3.Color Discrimination:
•The cones of the retina are the basis for
color discrimination.
• Some people find it difficult to discriminate
between red and green, blue and yellow.
Few people are color blinded.
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4.
•
•
•
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Dark Adaptation:
The eye pupil increases in size as we
enter into dark rooms enabling more
light to enter the eyes, and contracts in
bright light conditions to limit the
amount of light entering the eyes.
The cones lose much of their sensitivity
as we enter dark rooms where our
vision will then depends largely on the
rods. In such case, color discrimination
is limited.
Complete dark adaptation usually
needs 30 min or more while reverse
adaptation (from dark to light) takes
place in 30 seconds to two minutes.
Conditions Affecting Visual
Discrimination
Visual discrimination depends on the visual
abilities and especially on the visual acuity.
Some factors external to the individual affect
visual discrimination. Some of these factors
are listed below:
1.Luminance Contrast:
•It is some times called brightness contrast or
simply contrast. It refers to the difference in
luminance of the features of the object being
viewed. The most important is the difference
in luminance between the object and its
background.
•Measured according to the following
formula:
B1 B2
Contrast 
B1
100
Where B1 and B2 are the percent of
reflectance of brighter and darker of
contrasting areas respectively.
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•When the contrast is low, the target must
be larger for it to be equally discriminated to
a target with a greater contrast.
Figure 4.5 page 83
2.Amount of Illumination:
High illumination is needed for delicate and
difficult tasks.
3.Luminance Ratio:
It is the ratio between the luminance of any
two areas in the visual field. It is
recommended to have a luminance ratio of
3:1 , 10:1, and 1:10 for task: adjacent
surrounding, task: remote darker areas, and
task: remote lighter areas respectively.
4.Glare:
it is produced by brightness within the field
of vision that is greater than the luminance
to which the eyes are adapted and may
cause annoyance.
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5.Movement:
Visual acuity decrease as the viewed object
is moved.
The visual acuity when viewing a moving
object is called dynamic visual acuity and
will decrease significantly as the rate of
movement is increased.
6.Combination of Variables:
If any of the above variables are combined,
an interaction effect on visual performance
is expected.
7.Age:
Visual acuity tends to deteriorate through
age. This fact must be considered in the
design process.
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PERCEPTION
•
The ability to see the relevant features
of visual displays is not enough to
make appropriate decisions based on
the information in question. The
meaning of the displayed information
must also be understood.
•
Perception is related to the
interpretation of the sensed information.
•
The interpretation process is
sometimes straightforward. But in the
use of most displays, it depends on
previous learning (due to experience or
training).
•
Visual displays design must meet two
objectives:
1. The display must be seen clearly.
2. The design should help the viewer to
correctly perceive the meaning of the
display.
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ALPHNUMERIC DISPLAYS
The most important characteristics of
alphanumeric displays are:
1. VISIBILITY: The quality of the
character to make it separately visible
from its surroundings (some times
called detectability).
2. LEGEBILITY: The attribute that makes
a character to be identifiable from the
others. This feature depends on the
stroke width, form of characters,
contrast, and illumination.
3. READABILITY: It makes possible the
recognition of the information content of
material when represented by
alphanumeric characters in meaningful
groupings such as words and
sentences. It depends on the spacing
between lines and letters, the letter
combinations, and margins more than
on the specific features of the
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characters.
Typography
•It refers to the various features of
alphanumeric characters.
•Most variations in typography fulfil the three
previously mentioned human factors criteria
(visibility,
legibility
and
readability).
However, there are at least four types of
circumstances in which it may be important
to use preferred forms of typography:
1.When
viewing
conditions
are
unfavourable (such as limited viewing
time or poor illumination).
2.When the information is critical or
important (as when emergency labels
are to be read).
3.When viewing occurs at a distance.
4.When dealing with people with poor
vision.
when faced with one or more of these
conditions, the following 9 typography
features must be considered:
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1.Stroke Width:
Expressed as the ratio of the thickness of
the stroke to the height of the letter or
number (see figure 4-7). The effect of the
stroke width depends on the nature of the
background (black on white or white on
black) and illumination.
Irradiation causes white features on a
black background to appear to ‘spread’ into
adjacent dark areas, but the reverse is not
true. So black-on-white letters should be
thicker (because they have low radiation)
than white-on-black letters.
Based on the above:
•With good illumination, 1:6 to 1:8 ratios for
black-on-white and 1:8 to 1:10 ratios for
white-on-black are reasonable ratios.
• the letters must be thicker as the
illumination is reduced (applicable in both
cases).
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•
•
•
With low illumination levels or low
background contrast, printed letters
should be boldface type with a low
stroke width-height ratio (such as 1:5).
For highly luminous letters, ratios
should be from 1:12 to 1:20.
For black letters on a very highly
luminous background, very thick
strokes are needed.
2. Width-height Ratio:
It is the relationship between the width and
height of the character. It could be
expressed as a ratio such as 3:5 or as
a percentage such as 60 percent. Most
letters have a ratio of 3:5 which is used
commonly for most of the letters. But
the legibility of certain letters could be
enhanced if their width were adjusted to
their basic geometric forms such as O,
A and V.
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Although a 3:5 width-height ratio is
satisfactory for most purposes, wider
letters are appropriate for certain
circumstances, such as when the
characters are to be transilluminated
or are to be used for engraved
legends (see figure 4.8)
3.
Styles of Type:
There are more than 30,000 type styles (or
typeface, or fonts of type) used in the
printing trade. These styles fall into
four major classes:
I.
II.
III.
IV.
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Roman: The most common class.
The letters have sherifs (little
flourishes).
Sans serif: modern in appearance
and used for headings, labels, etc.
Script: stimulate modern hand
writing.
Black letter: resembles the
German manuscript handwriting
used in the fifteenth century.
The most used styles of type for
conventional text are roman. Script and
black letter styles should not be used when
visibility, legibility, and readability are critical.
(see figures 4.8 and table 4.1).
4.Illuminated Alphanumeric Characters:
The technological revolution has yielded
new methods of presenting alphanumeric
material than the traditional hard copy
(printed text on paper). Two of these new
methods are of important in this area:
segmented characters (especially
numerals) and dot matrix characters.
•Segmented numerals are formed from
selected combinations of some of the seven
segments of the geometric form shown in
figure 4.9. the numerals are presented by
illumination of the specific line segments in
question.
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•Dot matrix characters are generated with
CRTs (TVs VDTs) or dot matrix printers. The
image is a combination of thousands of
elements of a matrix or grid consisting of
many horizontal raster scan lines, each of
which is made up of many separate
elements called pixels. (see figure 4.10).
5.Size of Characters at Reading Distance:
•The size of type is measured in points. One
point equals 1/72 in (0.35 mm). But this is
the height of the slug on which the type is
set. This height is approximated as
equivalent to 1/100 in (0.25 mm). The type
size in most printed material ranges from 7
to 14 pt with the most common about 9 to
11 pt.
•When the reading is critical or is performed
under poor illumination, or when the
characters are subject to change, the
character heights should be increased (see
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table
4-2 and figure 4-12).
6.Size of Characters at a Distance:
The legibility and readability for characters
are equal for various distances if the
characters are increased in size for distance
viewing so the visual angle remain the
same.
The variations of the levels of visual acuity
for people must be considered when
choosing the size of the characters.
7.Size of Characters on CRTs:
It is recommended to point sizes that range
from 2.3 to 2.5 mm at a conventional
reading distance of about 71 cm. Legibility
and readability could be enhanced with
larger sizes (3.56 to 5.08 mm) at the same
reading distance.
VDTs are usually used at shorter distances
(about 46 cm). At such distances, the sizes
can be reduced considerably without
sacrificing the legibility and readability of
the characters.
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8.Density of Characters:
It refers to how compact the characters are
(how ‘tightly packed’ they are). It refers to
the spacing between lines.
Densely packed letters are preferred for
reading tasks because reading densely
packed characters require less ocular work
than reading less densely packed
characters. (see figure 4-9).
9.Sequence of Numerals and Letters:
Mostly depends on the concept of chunking.
(sequences could be remembered easily if
they are broken into groups). (see figure 415)
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Visual Coding Dimensions:
The items to be coded are called referents.
The types of visual stimuli used are called
coding dimensions (such as colours, shapes
and sizes. Codes could have single
dimension or more than one dimension
(multidimensional).
Single Coding Dimension:
•If various coding dimensions (coding
systems) can be used, an experiment could
help in specifying the best coding system
(see figures 4.18 and 4.19).
•Table 4.5 presents a summary of certain
visual coding methods.
•Multidimensional codes:
•It is recommended than no more than two
dimensions be used together if rapid
interpretation is required.
•Certain combinations do not ‘go well’
together
(see figure 4.22).
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Colour coding:
•Colour is a widely used visual code.
•What is the number of distinct colours that
a normal colour vision can differentiate on
an absolute basis?
Jones (1962) found that the normal
observer could identify 9 surface colours.
With training, people are able to identify
around 24 colours.
But when dealing with untrained people, it is
wise to use a smaller number of colours.
Colour coding is very useful in searching
tasks which involve scanning an array of
many different stimuli
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VISUAL CODES AND SYMBOLS
Comparison of Symbolic & Verbal Signs:
•
When to use symbolic or verbal signs?
•
Symbolic signs are preferred if they
reliably represent visually what they are
intended to represent. This is because
symbols do not require recording
(words and short statements require
recording).
•
Ells and Dewar (1979) conducted a
study that supports the advantages of
symbols (see figure 4.15).
Objectives of Symbolic Coding System:
•
The objective is to use the symbols that
best represent their referents (the
concept or things the symbols are
intended to represent).
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•
This depends on the strength of
association of a code symbol with its
referent. This association depends on
either of two factors:
A.
B.
•
Any already established association
(sometimes called recognizability).
The ease of learning such an assocaition.
Some guidelines for using coding
systems were discussed earlier
(detectability, discriminability, …..)
Criteria for Selecting Coding symbols:
•
If there is any question about the
suitability of coding symbols, they
should be tested by the use of some
experimental procedures.
•
Various criteria have been used in
studies with symbolic codes. Some of
these criteria are listed:

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Recognition: Subjects are presented
with experimental symbols and asked to
write down or say what each represents
(see figure 4.16 as an example).


Matching: several symbols are presented
to subjects along with a list of all the
referents represented. The subjects are
then asked to match each symbol with its
referent. This process yields a confusion
matrix that indicates the number of times
each symbol is confused with every other
one (see figure 4.25 and table 4.4 as an
example).
Preferences and Opinions: Subjects are
asked to express their preferences or
opinions about symbol designs (see
figure 4.27 as an example).
In general, Filled figures are clearly
superior to outline figures, circular figures
are less reliably identified than those with
square or rectangular backgrounds, and
simplified figures (reduced number of
symbol elements) are better than complex
figures.
Perceptual Principles of Symbolic Design:
The basic principles are: Figure/Ground,
Figure Boundaries, Closure, Simplicity, and
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Unity. See figure 4.18 as an example.
Principles of Designing Pictorial Displays:
•
Pictorial displays represent a visual
representation of something, such as
land areas, traffic routes and maps.
•
The design should be simple.
Simplification can be done by:
1.
2.
Removing any external details.
Using schematic representations. (see
figure 4.30 as an example).
Graphic Representations:
A graph should convey a visual representation
of data that is consistent with the numeric
representation.
There are two basic principles in designing
graphical representations:
•
The representation of numbers physically
measured on the surface of the graphic
itself should be directly proportional to the
quantities represented.
•
Clear, detailed, and thorough libelling
should be used.
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