Transcript Section II The Human Mind

Section II The Human Mind
CHAPTER 5 HOW WE SEE
CHAPTER 6 HOW WE HEAR
CHAPTER 7 HOW WE SEE OBJECTS AND ENERGY
CHAPTER8 HOW WE EXPERIENCE INDOOR AND OUTDOOR
CLIMATES
How We See
 Eyes sense energy from the outside world in the form
of light rays
 Light rays are converted to nerve impulses which the
brain integrates into a visual picture
 Perceived picture is a subjective modification of what
the eye reports – what?


Depends on age, experience, attitude, preconceived ideas
People differ in their ability to see colors and focus
 General similarity in human visual sense function
allows the development of ergonomic
recommendations
Our Eyes
 Continually adjust:
 to the amount of light they let in
 To focus on near and far objects
 To produce continuous images
 Anatomy
 Spherical and 2.5 cm in diameter
 Surrounded by the sclera
 Light enters the cornea

Helps protect the eye and focus light on the retina
Our Eyes
 Pupil in the iris
 Light then enters the pupil
 Iris is the colored area
 Pupillary dilator and
sphincter muscles control
the amount of light let in
Our Eyes
 The lens focuses
 Accommodation - increase
in curvature of the lens to
achieve the greater
refraction needed for near
vision
 Normal adult can focus on
an object 10 cm away
 Lens becomes less flexible
with aging, less able to
accommodate
 Light rays are refracted as
they pass through the
vitreous humor to the retina
Our Eyes
 Rods for white/grey/black perception
 Retina carries about 130 million light sensors
 Densely collected at the center, the fovea
 2 kinds: rods and cones
 120 million are rods
 Contain only 1 pigment
 Respond to even very low light intensity
 Electrical impulses travel along the optic nerve to the brain for
black and white perception
 Provide us with the most and most important visual
information
Our Eyes
 Cones signal color
10 million cones
 Majority are in the fovea
 Each cone contains one pigment it is most sensitive to: blue,
green or red
 If light is intense enough, cones will
respond, chemical reaction is
triggered, electrical impulse
is sent to the brain via the optic
nerve
 Can distinguish
150 color hues

Our Eyes
 Optic nerve
 Exits eye at the rear,
medially offset about
15degrees
 Area where blood vessels
enter and there are no light
receptors
 Optic disk is an area which
cannot form an image and is
known as the blind spot
Our Eyes
 Visual Control System (Figure 5.2)
1.
Adjustment of the lens
2.
Focused image on the retina
3.
Image information is transmitted along the optic nerve to the
brain
4.
Nervous control mechanisms provide feedback to readjust
the position of the eye, the size of the pupil and adjust the
curvature of the lens continually
5.
Visual image reaches the conscious area within the brain
6.
Initiates command signals that travel through the spinal cord
to prompt appropriate actions by the body
Seeing the Environment
 The Visual Field
 Area in front of the eye in which we are able to see objects
 Able to discern visual objects with high acuity only if they
appear within this area
 Ex. reading
Seeing the Environment
 Fixated eyes
 Visual field ranges:
Lateral- 90 degrees
 Medial – 65 degrees
 Upward – 45 degrees total, 30 degrees color
 Downward – 70 degrees total, 40 degrees color

Seeing the Environment
 Moving the eyes
 Several muscles attach to the outside of the eye
 Rotational movements
Pitch – 50 degrees up and down
 Yaw – 50 degrees left and right
 Roll
 Some forward and backward movements occur

Seeing the Environment
 Moving the eyes
 Peripheral vision is initiated by eye rotation and then replaced by
head movement
 Adjustments in body posture allow eyes to operate close to their
resting position
 Eye tracking
Visual targets moving left to right at less than 30 degrees/second or
less than 2 hertz
 Greater rates of movement require eye movements is saccades: lags
behind and catches up

Seeing the Environment
 Avoid eye fatigue
 Excessive demands on extrinsic and intrinsic eye muscles
 Elderly lenses cannot accommodate well and stiff musculature
requires more strain on eye muscles
 Ex. Computer users
Fatigue due to:
1.
Poor placement of monitors/visual targets
2.
Unsuitable light conditions

Proper arrangement of visual targets:
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In front

At a distance in which eyes can easily accommodate

Low so we can slightly rotate eyes while keeping the trunk in
a comfortable position

Seeing the Environment
 Line of sight
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When viewing an object in front of us, we adjust 2 angles: the
eyes within the head and head against the trunk
Line of site (LOS): from the center of the retina through the
midpoints of the lens and pupil and then to the target
 Looking down on the job
Ear-eye (EE) line is used to establish a target reference
(Figure 5.3)
 Angle between LOS and EE is the pitch angle of the eye, LOSEE
 Best around 45 degrees for reading
 Gets smaller as the object moves farther away
 Position angle P describes how we hold our head compared to the
horizon, P= 15 degrees

Seeing the Environment
 Size of the visual target (Figure 5.4)
 Expressed as a subtended visual angle, the angle formed at the
pupil
 Magnitude = α
 Size of the object = L
 Distance from the eye = D
 α (in degrees) = 2 x arctan (0.5 x L x Dˉ¹)
 1 degree = 60 minutes = 60 x 60 seconds of arc
 The minimum a human eye can perceive is a visual angle of 1
minute of arc
 Table 5.1 lists the visual angles of familiar objects
 Design angles should subtend 15 minutes of arc increasing to
21 minutes at low light levels
Seeing the Environment
 Diopter
 The reciprocal of the distance of the object in meters, 1/D
 The optical refraction needed for best focus
 Table 5.2 shows typical target distances
 Focusing
 Accommodation is the ability of the eye to bring objects of
varying distances into focus
 We can only see those objects whose image is clearly focused
on the retina
Seeing the Environment
 Incessant changes
 Lens continuously adjusts its curvature to maintain sharp
images on the retina as our gaze moves


Ex. Reading – lens oscillates at a rate of 4 times/ second
Iris continuously changes pupil size
Dilator and sphincter muscles are constantly adjusting the pupil
size based on the light conditions
 Daylight pupil size: 3-5 cm; evening pupil size: 8 cm
 Emotions and pupil sizes
 Dilates during strong emotions, i.e. alarm, joy, pain, intense
mental concentration
 Narrows with fatigue and sleepiness

Seeing the Environment
 Overcoming ocular problems
 Healthy, young eyes can accommodate from infinity to about
10 cm, diopter range form 0 -10
 At age 40, minimal diopter range increases to 5 (20 cm)
 At age 60, minimal diopter range increases to 1 (1 m)
 Accommodation ability decreases with increasing age as the
lens loses water content and stiffens
 Myopia: convergence in front of the retina, nearsighted


Improves with age
Hyperopia: convergence behind the retina, farsighted

Worsens with age
Seeing the Environment
 More light!
 The aging eye
Pupils shrink with age
 Vitreous humor yellows
 Floaters – small clumps of gel suspended in the vitreous humor
 Visible only when in the line of sight and usually harmless
 Cataracts – patterns of cloudiness inside the lens
 Lens may be removed and replaced with an artificial implant
 Glaucoma – disease of the optic nerve due to increase pressure
inside the eye
 Leading cause of blindness in the U.S.

Seeing the Environment
 Vision deficiencies unrelated to aging
 Astigmatism
Cornea is not uniformly curved
 Object is not sharply focused on the retina
 Light rays are either more strongly refracted at the center or the
periphery of the field of view


Chromatic aberration
Eye is hyperopic for long waves (red) and myopic for short waves
(blue)
 Artificial lens solves
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Night blindness
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Less than normal vision in dim light
Seeing the Environment
 Color vision deficiencies
 8% of the male and 0.5% of the female
population
 Reduced color discrimination
 Some are missing an element of the cone
system and cannot distinguish basic colors
 Others with color weakness cannot distinguish
as many gradations of colors
Dim and Bright Viewing Conditions
 The eye adapts to increases and decreases in
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illumination
Visible wavelength spectrum between 380 – 720 nm
(violet to red)
Minimal intensity to trigger light perception = 10
photons
Minimal illuminance is 0.1 lux, activating only rods
>0.1 lux both cones and rods respond
Dim and Bright Viewing Conditions
 Unique visual effects of the darkness
 Autokinetic phenomenon
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While staring at a single light on a dark background, the light
seems to move
Night vision deteriorates with decreasing oxygen
Vision is reduced at higher altitudes
 Reduced by 40% in smokers whose blood has lost oxygen carrying
capabilities
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Night myopia

When there is a lack of visual contrast, the lens relaxes and focuses
at about 1-2 m away, making it hard to see distant objects
Dim and Bright Viewing Conditions
 Adaptation to Light and Dark
 Illumination – the light falling on a surface (lux)
 Luminance –reflected light (candela/m²)
 Eye can change its sensitivity through a large range of
illumination and luminance
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
Due to pupil adjustments, spatial summation of stimuli and
stimulation of rods and cones
Full adaptation from light to dark takes 30 minutes
Initially cones are more sensitive, eventually becomes rods
 After adaptation, sensitivity at the fovea is 1/1000th of the
periphery – why?

Dim and Bright Viewing Conditions
 Adaptation to light and dark
 Adaptation to darkness depends on cone thresholds
 Dark blindness – nonfunctional rods
 Color blindness - cone deficiencies
 Adaptation to light is very quick
 During this period, the fovea perceives wavelengths in the
yellow region most easily
 Best to illuminate vehicles with yellowish light at night
 Allows driver to maintain dark adaptation of the rods to
observe events on the road and illuminate the path
 We adjust faster to red and yellow lights in the dark than blue
Dim and Bright Viewing Conditions
 Seeing requires light
 Some objects generate light, others reflect it
 Luminance is the most important factor in vision with
reflected light
 The eye adapts to lighting and therefore does not give reliable
information on the absolute lighting level
 Visual Acuity
 Ability to detect small details and discriminate small objects
 Depends on:
Shape of the object
 Wavelength, illumination, luminance, contrast and duration of the
light stimulus


Measured at viewing distances of 6m (20 ft) and 0.4 m (1.3 ft)
Dim and Bright Viewing Conditions
 Acuity testing (Figure 5.7)
 High-contrast patterns are presented to the observer at a fixed
distance
 Snellen letters and Landolt rings are the most common
 Measures depend on the ability to see the edge differences
between the black and white stimuli at high illuminance levels
Dim and Bright Viewing Conditions
 Color perception
 Sunlight contains all visible wavelengths of the spectrum
 Objects onto which the sun shines absorb some of the
radiation
 Light that we see is what the objects reflect
 Trichromatic vision
 The human eye perceives colors stemming from the
combination of the three primary colors: red, green and blue
 Color is an experience
 Brain classifies different signals from different wavelengths as
different colors
 Psychological experience
Dim and Bright Viewing Conditions
 Aesthetics and psychology of colors
 Perception, interpretation and reaction to colors are highly
individual and variable
 Emotional reactions to color
Red, orange, yellow – very warm and stimulating
 Violet, blue, green – cool, relaxing and clean

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Varies culturally and regionally
Dim and Bright Viewing Conditions
 Designing illumination
 Proper vision requires appropriate illuminance
 Luminance of an object is determined by its incident
illuminance
 Illumination direction and quantity should be carefully
selected
 Special requirements should be noted (elderly require more
light)
 Proper color usage can be helpful but requires sufficient light
Summary
 Eyes provide a large portion of information we need
in daily life
 To see objects we need sufficient light
 A well lit visual target needs to be at a proper
distance to be able to distinguish particulars
 Eyesight deteriorates as we age and it becomes more
important to have proper lighting