Chapter 22: Illumination and Vision

Transcription

1 Chapter 22: Illumination and Vision Learning Outcomes 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, 1

2 Normal Vision Islamic University of Gaza -Palestine Red-Green Colorblindness Color Sensitivity of Human Eye Relative response Wavelength (nm) Blue Cyan Green Red 2

3 Object is seen In a straight line Direction of beam is changed Light enters our eyes Shiny surfaces better than dull How we see things? Direction of a ray can be indicated by an arrow Object nearer light Formed by opaque objects Object nearer surface How do we see things? 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 3

4 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. How we see objects? 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. 4

5 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. How we see objects? (The Eye Cont.) 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 5

6 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 6

7 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 unrelated to 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. 7

8 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. 8

9 Anatomy and physiology of the human eye Cornea Iris Pupil Retina Lens Optic Nerve 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 9

10 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 10

11 External Anatomy of Eye 21 Eyelashes & Eyebrows Eyeball = 1 inch diameter 5/6 of Eyeball inside orbit & protected 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 11

12 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. 12

13 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 13

14 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. 14

15 Accommodation It is the process of adjusting the shape of the lens so that the external image fall exactly on the retina Accommodation 15

16 AccommodationAbnormalities Myopia Hyperopia Astigmatism: the cornea is irregular irregular pattern of vision Presbyopia: stiffening of the lens occurring with aging increased difficulty with near vision Accommodation Abnormalities 16

17 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 17

18 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 red and green makes yellow Adding all three makes white again 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 Additiveprimary colors are red, blue, and green because these colors produce the highest number of different colors. 18

19 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. 19

20 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 colorblind. 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 20

21 Eye Problems Myopia: Nearsightedness Hyperopia: Farsightedness Decrease in accommodation range Astigmatism: Unequal radii of curvature on two axes of the eyeball Visual Acuity Problems 21

22 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. 22

23 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 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. 23

24 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) 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 24

25 Color Hue Brightness Saturation Criteria for Lighting a Task Have satisfactory visual performance. Minimize cost of the lighting. Have satisfactory esthetics. 25

26 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 26

27 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 27

28 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 28

29 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. 29

30 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 30

31 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. 31

32 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. 32

33 Emergency Lighting Provide minimum illuminance levels from IESNA (Illuminating Engineering Society of North America). For stairs, use more than one luminaire and photoluminescent 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 33

34 Typical Reflectance Values Object Mirrored glass White matte paint Aluminum paint Black painted object Color White Light green Medium blue Dark blue Reflectance Reflectance Typical Reflectance Values 34

35 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. End of the Chapter 35