Chapter 20: Illumination and...
Transcript of Chapter 20: Illumination and...
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Chapter 20: Illumination and Vision
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After successful studying this chapter, You should be able to
• Explain how we see objects?
• Discus the anatomical structure of the eye,
• Describe the differences between Rods and Cones,
• Explain light and dark adaptation
• Discuss typical eye problems,
•Discuss color weakness,
• Define what is light and what are colors?
• Calculate the needed quantity of Illumination for specific tasks,
Learning Outcomes
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Normal Vision
Red-Green Colorblindness
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Color Sensitivity of Human Eye
400 460 530 650600 700500
Wavelength (nm)
Rela
tive r
esp
on
se
Blue Cyan Green Red
490
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Optically, Eye is Similar to a Camera
• Contains a lens system that inverts the image
• Has an opening (the pupil) for controlling light exposure
• Film (the retina) that records the image
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VISION
• More than half the sensory receptors in the human body
are located in the eyes.
• A large part of the cerebral cortex is devoted to
processing visual information.
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When you look at any object
•Light waves from that object enter the eye first through
the cornea, which is the clear dome at the front of the eye
• Light waves progress through the pupil, the circular
opening in the center of the colored iris
• Immediately behind the iris (and pupil) is the crystalline
lens, and light passes through that also
How we see objects?
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Major Processes of Image Formation
Refraction of light
• By cornea and lens
•Light rays must fall upon the retina
Accommodation of the lens
• Changing shape of lens so that light is focused
Construction of the pupil
• Less light enters the eye
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Normal Vision
• Visual acuity = 1 ÷ Visual angle of minimum object detectable
• The ability to detect an object with 1 min of arc at 6 m (20/20 vision).
• Measurements:
– Gap detection
– Spot detection
– Lateral displacement of two lines
– Dynamic
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Sensitivity vs Acuity
• Sensitivity is a measure of the dimmest light the
eye can detect.
• Acuity is a measure of the smallest object the eye
can see.
• These two capabilities are in competition.
– In the fovea, cones are closely packed. Acuity is at its
highest, sensitivity is at its lowest (30 cycles per
degree).
– Outside the fovea, acuity decreases rapidly.
Sensitivity increases correspondingly.
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Anatomy and physiology of the human eye
Retina
Optic
NerveLens
Pupil
Iris
Cornea
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Retina
• Thin membrane lining rear of eye
• Contains light sensitive cells
• Rods & cones
– Rods are sensitive to light
• 120 million rods
– Cones are sensitive to colors
• 6 million cones
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Cones and Rods
• Cones
– Provide color, daylight vision.
– Are more sensitive to some wavelengths of light than
others.
• Rods
– Provide black and white, night vision.
– Are more sensitive to light than are cones.
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Layers of Retina• Pigmented epithelium
– nonvisual portion
– absorbs stray light &
helps keep image
clear
• 3 layers of neurons
(outgrowth of brain)
– photoreceptor layer
– bipolar neuron layer
– ganglion neuron
layer
• 2 other cell types
(modify the signal)
– horizontal cells
– amacrine cells
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External Anatomy of Eye
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Eyelashes & Eyebrows
• Eyelashes & eyebrows help protect from foreign objects,
perspiration & sunlight
• Sebaceous glands are found at base of eyelashes (sty)
• Palpebral fissure is gap between the eyelids
Eyeball = 1
inch diameter
5/6 of Eyeball
inside orbit &
protected
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Photoreceptors
• shapes of their outer segments differ
• Rods
– specialized for black-and-white vision in dim light
– allow us to discriminate between different shades of dark
and light
– permit us to see shapes and movement.
• Cones
– specialized for color vision and sharpness of vision (high
visual acuity) in bright light
– most densely concentrated in the central fovea, a small
depression in the center of the macula lutea.
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Photoreceptors (cont.)
• The macula lutea is in the exact center of the posterior
portion of the retina, corresponding to the visual axis of the
eye.
– The fovea is the area of sharpest vision because of the high
concentration of cones.
– Rods are absent from the fovea and macula and increase in
density toward the periphery of the retina.
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Application: Color Blindness & Night Blindness
• Color blindness
– inability to distinguish between certain colors
– absence of certain cone photopigments
– red-green color blind person can not tell red from green
• Night blindness (nyctalopia)
– difficulty seeing in low light
– inability to make normal amount of rhodopsin
– possibly due to deficiency of vitamin A
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Color Blindness
• Most forms of colorblindness (inability to distinguish certain colors) result from an inherited absence of or deficiency in one of the three cone photopigments and are more common in males. A deficiency in rhodopsin may cause night blindness (nyctalopia)
• Inability to perceive colors
– Total color blindness is rare
• Color Weakness: Inability to distinguish some colors
– Red-green is most common; much more common among men than women
• Ishihara Test: Test for color blindness and color weakness
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Light and Dark Adaptation
• Light adaptation
– adjustments when emerge from the dark into the light
• Dark adaptation
– adjustments when enter the dark from a bright situation
– light sensitivity increases as photo pigments regenerate
• during first 8 minutes of dark adaptation, only cone pigments are
regenerated, so threshold burst of light is seen as color
• after sufficient time, sensitivity will increase so that a flash of a
single photon of light will be seen as gray-white
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Accommodation & the Lens
• Accommodation is an increase in the curvature of the lens,
initiated by ciliary muscle contraction, which allows the lens to
focus on near objects
• Convex lens refract light rays towards each other
– Lens of eye is convex on both surfaces
• Viewing a distant object
– lens is nearly flat by pulling of suspensory ligaments
• View a close object
– ciliary muscle is contracted & decreases the pull of the
suspensory ligaments on the lens
– elastic lens thickens as the tension is removed from it
– increase in curvature of lens is called accommodation
– The near point of vision is the minimum distance from the eye
that an object can be clearly focused with maximum effort.
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Vascular Tunic -- Muscles of the Iris
• Constrictor pupillae (circular) are innervated by parasympathetic fibers while Dilator pupillae (radial) are innervated by sympathetic fibers.
• Response varies with different levels of light
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Mixing Colored Light
When red, blue,
and green light
are projected
onto a screen,
the overlapping
areas appear
different colors.
Where all the
three overlap,
white is
produced.
RED + GREEN =YELLOW
RED+BLUE =MAGENTA
GREEN + BLUE = CYAN
Additive primary colors are red, blue,
and green because these colors
produce the highest number of different colors.
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Complementary Colors
• (trichromatic, tricolor)
• When two colors are added together to produce white, they are called complementary colors.
• YELLOW + BLUE =WHITE ( Yellow a combination of Green + Red )
• MAGENTA + GREEN = WHITE ( Magenta a combination of red + blue)
• CYAN + RED = WHITE ( Cyan a combination of green+ blue)
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cont…
• For example : if white light falls on a pigment that absorbs red light, the light reflected appears cyan.
• Not all light incident upon an object is reflected. The ones that are absorbed are subtracted from the incident light.
• Whenever you subtract a color from white light, you end up with the complementary color.
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Color Perception Deficiencies
• Result from defective pigments in cones.
• Influenced by X chromosome, so female have advantage.
• 8% of males and 0.4% of females have color perception deficiency.
• Deficiency may affect red and blue-green or green and reddish-purple.
• Most color-deficient individuals are color-weak, not color-blind.
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Visual Acuity and Dynamic Visual Acuity
Visual acuity is defined as a measure of the ability of the
eye to distinguish subtle (very fine or small) detail.
Four factors major affect visual acuity:
Size, Time, Luminance, Contrast
Dynamic Visual Acuity
• The ability to discriminate detail in a moving target
• Important for some inspection tasks
• Not predicted by static visual acuity
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Eye Problems
• Myopia: Nearsightedness
• Hyperopia: Farsightedness
• Decrease in accommodation range
• Astigmatism: Unequal radii of curvature on two axes of
the eyeball
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Eye Defects
• Nearsightedness
– Myopia
• Farsightedness
– Hyperopia
• Astigmatism
– Caused by irregular cornea curve
• Presbyopia
– Loses its elasticity & thickening
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Eyeglasses
• Impact-resistant lenses can still break.
• Polycarbonate lenses are light but scratch easily.
• Chemical that reduce light may present a hazard in
welding areas.
• Provide protection that contact lenses do not.
• Contact lenses should not be work around chemical
fumes, vapors, splashes, or dusty atmospheres.
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Light Definitions
• Wavelength and polarization describe light.
• Lumens measure luminous power.
• Candelas measure luminous flux.
• Lux or foot candles measure illuminance.
• Luminance is a measure of the physical intensity of light.
• Brightness is a measure of the intensity of the sensation perceived by an observer.
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Light Definitions
• Light - electromagnetic radiant energy within the visible
spectrum between ultraviolet and infrared.
• Photometry – branch of optical physics concerned with
the measurement of light
• Photometer - an instrument that measures light
• Our interest: provide proper levels of lighting and
contrasts among objects in a workplace.
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Physics of Light
• Luminous flux - rate at which light energy is emitted in all
directions from a light source
o It is the power of the light source
o Units: lumen (lm)
o 683 Lm = 1W energy (if yellow-green light)
• Luminous intensity - luminous flux emitted in a given
direction
• Closely related to luminous flux
• Units: candela (cd)
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Physics of Light (cont.)
• Illuminance - luminous flux shining per unit area on a
surface
– Units: lux (lx) = 1 lumen per sq meter
• Luminance - amount of light reflected from a surface
– Units: cd/sq m
– Depends on
Illuminance - amount of light striking surface
Reflectance - depends on color and texture of surface
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Color
• Hue – related to
wavelength (red,
blue, yellow)
• Brightness – related
to intensity of light
• Saturation – related
to purity of light
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Criteria for Lighting a Task
• Have satisfactory visual performance.
• Minimize cost of the lighting.
• Have satisfactory esthetics.
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Illumination Cost
• Energy cost
• Fixture cost
• Lamp cost
• Labor cost is approximately 250 times the expense of
lighting.
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Influences on Visual Performance
• Individual differences
• Quantity of light
• Quality of light
• Task requirements
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Individual Differences
• Visual acuity, resistance to glare, and color discrimination
decline with age.
• Individuals differ greatly in accommodation and
convergence capabilities.
• Prolonged performance may result in fatigue and
headaches.
• Many people do not have the best possible correction.
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Quantity of Illumination
• Recommended amounts from IESNA (Illuminating
Engineering Society of North America).
• Sources are measured by cost, amount of light, and
quality.
• Luminaires (fixtures)
– Coefficient of utilization
– Direct or indirect
– Beam spread
– Re-locatability
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Windows
• Have capital and operating costs
• Are a source of glare
• Are not a practical source of illumination
• May admit air
• Pass noise and
distractions through
the wall
• Decrease privacy
• Permit a view
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Quality of Illumination
• Color
• Glare
• Orientation
• Esthetics
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Task Requirements
• Goal is to enhance the object.
– Increase size ( e.g., larger font, bring closer, optical aids /
magnification)
– Increase contrast (color contrast, brightness contrast, use
anti-camouflage)
– Increase time (move more slowly, have operator face
“upstream*, use stationary objects)
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Uniform Ceiling Lighting
• Lights the entire area uniformly.
• Allows flexibility in arranging machines and
workstations.
• Allows use of large lamps.
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Uniform Ceiling Lighting Guidelines
• Use low fixtures.
• Reuse the light.
• Use efficient fixtures.
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Energy Conservation Approaches
• Reduce lighting power:
– Luminous environment
– Physical environment
– Equipment selection
– Design and maintenance procedures
• Reduce lighting time:
– Occupancy
– Cleaning
– Daylight
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Lighting for VDT Areas
• Lighting must be designed for vertical screens, vertical paper, and horizontal paper.
• If lighting will be uniform, design for the screen.
• If task lighting is used, put high illumination on the document and low on the screen.
• In a paperless environment, use low ambient light and indirect lighting.
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Reducing VDT Screen Glare
• Reduce light from the source:
– Reduce light from windows.
– Reduce ceiling glare.
– Reduce other brightness sources.
• Use barriers to intercept the light.
• Change the workstation:
– Tilt the screen.
– Change workstation orientation.
– Consider screen treatments.
– Change screen/character background.
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Inspection
• Provide adequate amount of light.
• Specify appropriate color of light.
• Adjust contrast to bring out shape or surface
characteristics.
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Warehouse Aisle Lighting
• Use High Intensity Discharge (HID) luminaries with high
racks.
• Use a high-reflectance floor.
• Mount the luminaires above the aisle.
• Consider mounting fixtures on tracks.
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Emergency Lighting
• Provide minimum illuminance levels from IESNA
(Illuminating Engineering Society of North America).
• For stairs, use more than one luminaire and photo-
luminescent paint.
• Provide adequate power sources for emergency
lighting.
• Consider solutions to re-strike problems.
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Security Lighting
• Target hardening
• Offense vs. defense
• Site considerations
• Public spaces
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Typical Reflectance Values
Reflectance
0.80 - 0.90
0.75 - 0.90
0.60 - 0.70
0.03 - 0.05
Object
Mirrored glass
White matte paint
Aluminum paint
Black painted object
Reflectance
0.85
0.65
0.35
0.08
Color
White
Light green
Medium blue
Dark blue
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Typical Reflectance Values
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Recommended Illumination Level
Given: A workplace has a reflectance of only 25% for a task that
is performed by a 45-year old female worker. The task includes
visual elements involving small objects. Speed and accuracy
requirements are judged to be critical.
Determine: What is the recommended illumination level?
Solution:
Work best fits into category E, that is illumination level is 750 lx
Age correction: 0
Speed and accuracy requirement: +1
Reflectance: +1
Total: +2
Then the recommended illumination level is 750*1.3=975 lx.
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End of Chapter