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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