Human Senses : Vision week 11 Dr. Belal Gharaibeh 1 Body senses - - PowerPoint PPT Presentation

Human Senses : Vision week 11

• Dr. Belal Gharaibeh

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Body senses

• Seeing
• Hearing
• Smelling
• Tasting
• Touching

– Posture of body limbs (Kinesthetic) – Motion (Vestibular )

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Kinesthetic

• Perception of stimuli relating to body parts’

position, posture, equilibrium, or internal condition.

– The sense of position and movement of the limbs and the sense of muscular tension. – The awareness of the orientation of the limb in space – awareness of the direction, extent, and rate of movement of the limbs – Kinesthetic senses depends in part upon information derived from sensory receptors in the joints, tendons, and muscles.

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Vestibular

• Awareness of body balance and movement

(the sensations of body rotation and translation) are monitored by the vestibular system.

• It arise in the inner ear;
• the sense organs are the auditory hair cells

that send out signals over the auditory nerve.

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Vision

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Elements of the visual system

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Retina

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Rods

–120 million rods –Outside the Fovea area –located throughout periphery –Peripheral vision uses Rods –more sensitive, THEY RESPOND TO LOWER light INSTENSITIES –more sensitive to light levels than cones –Produce colorless vision

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cones

– 7 million cones –provide visual acuity (sharpest vision- Fine details) –color vision (color sensitivity) –Best with higher illumination –Most vision uses Cones –sole receptors within fovea - less common as move further away from fovea

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The visual Field

• Is the area measured in degrees within which

the form and color of objects can be seen by both fixated eyes.

• To the sides the eyes can see a bit over 90

degrees, but only within the inner 60 degrees can color be perceived.

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The Visual Field

• Upward the visual field extends through about 55

degrees, occluded by the orbital ridge of the eyebrows.

• Color can be seen up to 30 degrees upward.
• Downward vision is limited by the cheek at about

70 degrees.

• Color can be seen up to 30 degrees downward.
• The head moves nearly everything in the

environment can be seen

• Operators and people with stiff necks have limited

head mobility and should locate visual targets close to their chosen line of sight.

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Visual Problems

• The accommodation capability of the eye

decreases with age, because the lens become stiffer by losing water content.

• In many people the pupil shrink with age, less

light strikes the retina. So older people need to have increased illumination on visual objects.

• Myopia (near slightness)- can’t see far objects
• Hyperopia (Far sightness)- can’t see near
• bjects

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Visual Problems

• Night blindness: having less than normal

vision in dim light.

• Color weakness: if a person can see all colors

but tends to confuse them, particularly in low illumination.

• Color blind: they confuse red, green and grey.

Only few people see no color at all or only

• ne color.

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red-green color blindness

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a red/green color deficit

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Accommodation

• Accommodation is the action of focusing on

targets at various distances.

• The young eye can accommodate from

infinity to very close distances, meaning a diopter range from 0 to about 10.

• The lens retracts for close objects and relaxes

for distances more than 6 meters. Focusing the image on the retina.

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Diopter

• If the target distance D is measured in meters

the reciprocal, 1/D is measured in Diopters.

Target Distance D(m) Focal Point (Diopter) Infinity 4 .25 2 .5 1 1 .67 1.5 .2 5

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Accommodation

• The closest distance focused is called near

point.

• The farthest point that can be focused

without conscious accommodation is far point.

• The difference between far and near point is

called the amplitude of accommodation.

• Near point for young people is 10 cm, at age

40 is 20 cm and at age 60 is about 100 cm.

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Contrast

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Examples of contrast levels

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Contrast sensitivity

• Contrast increases with the contrast between

the symbol and its immediate background

• Displays should have dark characters on a

light background

• Visual acuity is greater for dark targets on a

bright background than for the reverse.

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Visual Acuity

Human factors Human factors

Human Factors Visual Acuity

Visual Acuity Visual Acuity 30

Visual Acuity

• Ensure appropriate size of text/graphics in

relation to:

– viewing distance – viewing environments

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Visual Adaptation

• The change in eye sensitivity through a large

range of illumination.

• Adaptation from dark to light is very fast, fully

achieved within few minutes.

• Full adaptation from light to dark takes about

30 minutes.

• It’s achieved through several measures:

– Adjustment of pupil – including the stimulation of rod and cones.

• Fovea (cones)
• Periphery retina (rods)

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Some facts about our Vision

• The eye is particularly sensitive to the colors

green, red and orange.

• Best vision is achieved when:

– Image is focused on the fovea (cones). – Lighting is adequate. – Target is in the direction of viewing. – Target is fixed (not moving). – Contrast is high.

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Examples of design implications

• For low levels of illumination, do not use color

coding

• Design for monochrome first - add color

afterwards (particularly for coding)

• Use appropriate color combinations

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Visual Fatigue

• Visual fatigue, e.g. people working at a screen
• f a computer display, often complain of eye

discomfort, visual fatigue or eyestrain.

• This is subjective visual symptoms or distress

resulting from the use of one’s eyes

• This is related to the effort of focusing at a

distance that is different from the personal minimal resting distance.

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Visual Fatigue

• Some Causes:
• Strenuous fine work
• Lack of eye movements
• Reading poorly printed texts or low quality

computer images

• poor monitor placement
• Inadequate lighting
• Exposure to flickering light
• Elderly people are more prone to visual fatigue.

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Line of Sight (LOS)

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Visual target size

• If the visual target is not a point but can be

expressed as the length of a line perpendicular to the line of sight, the target is usually expressed as the subtended visual angle- the angle formed at the pupil.

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Visual Angle

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Visual target size

• α (in degrees)= arctan *L/D

=2 arctan (0.5L/D)

• For visual angles not larger than

10 degrees

– α (in degrees)= 57.3 L/D – α (in minutes of arc)= 60* 57.3 L/D = 3,438 L/D

• The distance between pupil and

the lens is neglected (7mm)

• For design purposes: the angle is

15 minutes of arc.

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Visual field

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Visual field

c

b

a

a = zone of sharp vision; angle of view of 1 deg. b = middle field; unsharp vision; angle of view from1 deg. to 40 deg. c = outer field, only movements perceptible; angle of view from 41

• deg. to approximately 70

deg.

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Convergence

• If one aims both eyes at the same point an angle

exists between the two lines of sight connecting each eye with the target.

• For large distances the angle is small and

negligible

• For small distances the angle is larger
• When the observer is greatly fatigued, “phoria”

exists if the images of one target are not focused

• n the same spots on the retinas of both eyes,

resulting in double images.

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Aspects in designing work systems

• 1. the accommodation distance are different from

individual to individual, but are constant for a given person.

– We should encourage each person to select the “ personal distance” from a target that must be visually fixated, such a computer screen.

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Aspects in designing work systems

• 2. Tilting either the head or eyes brings about a

similar effect on the natural vergence distance (people find it more comfortable to lean back in a chair while tilting the head forward and down to look at a close object.

• 3. Targets at or near “reading distance” e.g.

computer displays should be distinctly below eye level, particularly if the viewer is elderly.

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Illumination Concepts in Engineering and Design

• Proper vision required sufficient quantity and

quality of illumination

• Special requirement of illumination for the elderly.
• Use of color if selected properly can be helpful, but

with sufficient light.

• Luminance of an object is the amount of energy

reflected or emitted from it which meets the eye.

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Illumination Concepts in Engineering & Design

• The ability to see an object depends largely on

the luminance contrast between the object and its background, including shadows.

• Avoid unwanted or excessive glare. There are

two types of glare

– Direct glare ( from the light source) – Reflected glare (reflected on the surface)

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Ergonomic Principles (from fitting the task to the man)

• All important surfaces within the visual field

should be of the same order of brightness

• The general level of illumination should not

fluctuate rapidly because pupil and retina reaction times are relatively slow.

• The disturbance is greater in a dim room than a

bright one (retina is most sensitive in dim conditions)

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Ergonomic Principles (from fitting the task to the man)

Type of work Examples Recommended illumination (Lx)

General Moderately precise

Storeroom, packing, dispatch, simple assembly, work on carpenter’s bench Turning, milling, locksmith work 80-170 200-250 250-300

Fine work

Reading, writing, book keeping, lab technician, assembly of fine equipment, winding fine wire, woodworking by machine 500-700

Very fine to precision work

Technical drawing, color proofing, adjusting and testing electrical equipment, assembly of delicate electronics, watch making 1000-2000

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Ergonomic Principles (from fitting the task to the man)

• Accident rate:

– Expert at the American National Safety Council- bad lighting was the cause of 5% of all industrial accidents, together with optical fatigue it engendered, contributed to as much as 20% of them.

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The end

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Vergence

• A vergence is the simultaneous movement of both

eyes in opposite directions to obtain or maintain single binocular vision

– The two eyes converge to point to the same object where the projection of the image is in the centre of the retina in both eyes

• Dark vergence dark focus is a resting position of

vergence , measured in a dark visual field.

– Dark vergence differs reliably among subjects: the average subject converges at a viewing distance of:

• Looking ahead: about 1 m,
• Looking down, 80 cm
• Looking up 140 cm

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Factors that Affect Target Detection and Recognition

• Target size and shape
• Color:

– Luminance: relevant amount of light reflected give

• ur sensation of brightness

– Saturation:

• Saturation = the amount of light intensity
• Where in the visual field
• Visual Defects
• Motion, Clarity,
• Contrast
• Time to view

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Illumination Concepts in Engineering & Design

• Direct glare can be avoided by the following practices.

– Placing high intensity light source outside the cone of 60 degrees around the line of sight. – Using several low intensity light sources placed away from the line of sight, instead of one intense source. – Using indirect lighting, – Using Shields or hoods over reflecting surfaces, or a visor

• ver a person’s eyes.

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• Indirect glare can be reduced by:

– Diffuse, indirect lighting

• Dull, matte, or other non-polished surfaces.
• Properly distribute light over the work area.

– Use indirect lighting – Use of “glare protection” screen

Illumination Concepts in Engineering & Design

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