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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
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the retina.
Visual Abilities:
1.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.
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•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.
VA (minutes)

3438 H
D
Where H is the height of the object and D is the distance
from the eye (see figure4.3).
The scoring system uses the reciprocal of the visual angle.
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•Vernier acuity refers to the ability to differentiate the
lateral displacement of one line from another.
•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 three-dimensional single object. Most difference is
when the object is near the eyes.
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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).
•
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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.
4.
•
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.
•
•
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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.
2100
Contrast
 B1 B
•Measured according
to the following
formula:
B1
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.
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
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features of the 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 4-6 and figure 47). 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.
•14the letters must be thicker as the illumination is reduced
(applicable in both cases).
•
•
•
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.
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III.
IV.
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
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 a
geometric form. 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,
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the character heights should be increased .
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.
9.Sequence of Numerals and Letters:
Mostly depends on the concept of chunking.
(sequences could be remembered easily if they are broken
into groups).
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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 that no more than two dimensions be
used together if rapid interpretation is required.
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•Certain
combinations do not ‘go well’ together (see figure
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
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intended to represent).
•
This depends on the strength of association of a code
symbol with its referent. This association depends on
either of two factors:
A.
B.
•
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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, …..)