Illumination and Vision [Kompatibilit?tsmodus]site.iugaza.edu.ps/aschokry/files/2014/01/Illumination-and-Vision.pdf · to the eyeglass prescription required to correct vision. Instead,

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Islamic University of Gaza - Palestine

Chapter 22: Illumination and Vision


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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Islamic University of Gaza - PalestineNormal Vision

Red-Green Colorblindness


Color Sensitivity of Human Eye

400 460 530 650600 700500

Wavelength (nm)

Rel

ativ

e re

spon

se

Blue Cyan Green Red

490

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How do we see things?

How we see things?

In a straight line

Light enters our eyes

Direction of a ray can be indicated by an arrow

Object is seen

Direction of beam is changed

Shiny surfaces better than dull

Formed by opaque objects

Object nearer light

Object nearer surface


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.


When you look at any object•Light waves from that object enter the eye first throughthe cornea, which is the clear dome at the front of the eye

• Light waves progress through the pupil, the circularopening in the center of the colored iris

• Immediately behind the iris (and pupil) is the crystallinelens, and light passes through that also.

How we see objects?

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How we see objects? (The Eye)

• Light waves are converged first by the cornea, then even more so by the crystalline lens, to a nodal point which is immediately behind the lens

• At the nodal point, the light waves (image) become reversed (turned backwards) and inverted (turned upside down)

• Light waves continue through the vitreous humor, the clear gel that makes up about 80% of the eye’s volume, and then back to a clear focus on the retina behind the vitreous

• The small, central area of the retina is the macula; it provides the best vision of any location in the retina.


• The light impulses are changed into electrical signals,then sent through the optic nerve along the visual pathway to the occipital cortex, or posterior (back), of the brain

• This is where the electrical signals are seen by the brain as a visual image

• When light entering the eye is bright enough, the pupils will get smaller (constrict) due to pupillary light response

How we see objects? (The Eye Cont.)

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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


Normal Vision

• Normal visual acuity is commonly referred to as 20/20 vision (even though acuity in normally sighted people is generally higher), the metric equivalent of which is 6/6 vision. At 20 feet or 6 meters

• 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, feet vision).

• Measurements:– Gap detection– Spot detection– Lateral displacement of two lines– Dynamic

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Normal Vision

• The precise distance at which acuity is measured is not important as long as the size of the type on the retina is the same. That size is specified as a visual angle, which is the angle, at the eye, under which the type appears. For 20/20 = 1.0 acuity, the size of a letter on the Snellen Chart or Landolt-C chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree). By the design of a typical type (like a Snellen Chart or Landolt-C), the critical gap that needs to be resolved is 1/5 this value, i.e., 1 arc min. The latter is the value used in the international definition visual acuity:

• Acuity = 1/gap size [arc min]


Normal Vision

• Acuity is a measure of visual performance and is unrelatedto the eyeglass prescription required to correct vision. Instead, an eye exam seeks to find the prescription that will provide the best corrected visual performance achievable. The resulting acuity may be greater or less than 20/20 = 1.0. Indeed, a subject diagnosed as having 20/20 vision will often actually have higher visual acuity because, once this standard is attained, the subject is considered to have normal (in the sense of undisturbed) vision and smaller types are not tested.

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Snellen Chart or Landolt-C chart or E - Chart


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

OpticNerve

Lens

Pupil

Iris

Cornea


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.


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


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.


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


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


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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Accommodation

• It is the process of adjusting the shape of the lens so that the external image fall exactly on the retina


Accommodation

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Accommodation Abnormalities

• Myopia• Hyperopia• Astigmatism: the cornea is irregular à irregular pattern

of vision• Presbyopia: stiffening of the lens occurring with aging à

increased difficulty with near vision


Accommodation Abnormalities

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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


The Pupillary Muscles

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Adding colours

• White light can be split up to make separate colours. These colours can be added together again.

• The primary colours of light are red, blue and green:

Adding blue and red makes magenta (purple)

Adding blue and green makes cyan (light blue)

Adding all three makes white

again

Adding red and green makes yellow


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 =YELLOWRED+ BLUE = MAGENTAGREEN + 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)


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.


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


Visual Acuity Problems

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Eye Defects

• Nearsightedness – Myopia

• Farsightedness – Hyperopia

• Astigmatism– Caused by irregular

cornea curve• Presbyopia

– Loses its elasticity & thickening


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

• Light travels VERY FAST – around 300,000 kilometres per second.

• At this speed it can go around the world 8 times in one second.

• 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.


Light Definitions

• Light - electromagnetic radiant energy within the visiblespectrum between ultraviolet and infrared.

• Photometry – branch of optical physics concerned withthe measurement of light

• Photometer - an instrument that measures light

• Our interest: provide proper levels of lighting andcontrasts among objects in a workplace.

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Physics of Light

• Luminous flux - rate at which light energy is emitted in alldirections from a light source

o It is the power of the light sourceo Units: lumen (lm)o 683 Lm = 1W energy (if yellow-green light)

• Luminous intensity - luminous flux emitted in a givendirection• Closely related to luminous flux• Units: candela (cd)


Physics of Light (cont.)

• Illuminance - luminous flux shining per unit area on asurface

– 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 surfaceReflectance - depends on color and texture of surface

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Color

• Hue• Brightness• Saturation


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.


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.


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


Quality of Illumination

• Color• Glare• Orientation• Esthetics

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Task Requirements

• Goal is to enhance the object.• Increase size (magnify)• Increase contrast (use anti-camouflage)• Increase time (use stationary objects)


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.


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.


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.


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.


Security Lighting

• Target hardening• Offense vs. defense• Site considerations• Public spaces

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Typical Reflectance Values

Reflectance0.80 - 0.900.75 - 0.900.60 - 0.700.03 - 0.05

ObjectMirrored glassWhite matte paintAluminum paintBlack painted object

Reflectance0.850.650.350.08

ColorWhiteLight greenMedium blueDark blue


Typical Reflectance Values

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Recommended Illumination LevelGiven: A workplace has a reflectance of only 25% for a task thatis performed by a 45-year old female worker. The task includesvisual elements involving small objects. Speed and accuracyrequirements 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: 0Speed and accuracy requirement: +1Reflectance: +1Total: +2Then the recommended illumination level is 750*1.3=975 lx.


End of the Chapter

Documents

Illumination and Vision [Kompatibilit?tsmodus]site.iugaza.edu.ps/aschokry/files/2014/01/Illumination-and-Vision.pdf · to the eyeglass prescription required to correct vision. Instead,