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The Visual System and Visual Performance
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The Visible Spectrum
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3 vision.ppt Anatomy of the Eye Illustration by Mark Ericksen, St. Luke’s Cataract and Laser Center, StLukesEye.com
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4 vision.ppt The Eye (2) lCornea Protection Focusing lAqueous Humor Shape Nutrition lIris Light control Focusing
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5 vision.ppt The Eye (3) lLens Focusing Accommodation lVitreous Humor Shape lRetina Rods: black & white, night vision Cones: color, day vision Fovea: sharpest vision (concentration of cones)
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6 vision.ppt The Eye (4) lOptic Nerve Nerve signals to brain Optic Disk: blind spot lEye Muscles Eye movement Convergence
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7 vision.ppt NEAR AND FARSIGHTED EYE
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8 vision.ppt DENSITY OF RODS AND CONES
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9 vision.ppt Visual Performance lBrightness lVisual Angle lVisual Acuity lColor lVisual Field
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10 vision.ppt Brightness lRelative amount of light reflected from an object produces a sensation of lightness or brightness. lBrightness is related to the luminance of light as well as a subjective response to color
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11 vision.ppt Luminous Intensity
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12 vision.ppt Luminous Flux
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13 vision.ppt Illuminance
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14 vision.ppt Illuminance v. Luminance lIllumination/Illuminance: The amount of light striking any point on the inside surface of a sphere surrounding the light source (Luminous flux/unit area) Foot candle: 1 lumen/square foot Lux: 1 luman/square meter lLuminance: The amount of light per unit area leaving (reflected from) a surface Foot Lamberts: 1 lumen/square foot Candelas/square meeter
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15 vision.ppt Luminance Note: 1 foot-Lambert (ft-L) = 0.929 mL, so 1 ft-L ~ 1 mL.
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16 vision.ppt Luminance (2) lThreshold of detectability 1 x 10 -6 mL lThreshold of pain 3 x 10 4 mL lLimits to discriminability 3 - 4 levels
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17 vision.ppt Lightness
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18 vision.ppt Visual Angle (minutes of arc) Visual Angle = tan -1 H/D
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19 vision.ppt Visual Angle of Familiar Objects ObjectDistanceVisual Angle Sun 93,000,000 mi30’ Moon240,000 mi30’ Quarterarm’s length2° Quarter90 yd1’ Quarter3 mi1” Lowercase pica typereading distance13’
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20 vision.ppt Cumulative Probability of Detection
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21 vision.ppt
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22 vision.ppt Minimum Separable Acuity lAlso called gap resolution lSmallest space eye can detect between parts of a target (visual object).
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23 vision.ppt Minimum Separable Acuity as Function of Contrast
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24 vision.ppt Minimum Perceptible Acuity lAlso called spot detection. lEye’s ability to detect smallest possible target.
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25 vision.ppt Minimum Perceptible Acuity as Function of Contrast and Background Luminance
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26 vision.ppt Vernier Acuity lSmallest lateral displacement of one line from another that can be detected.
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27 vision.ppt Vernier Acuity as Function of Background Luminance
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28 vision.ppt Color lAttributes hue: red, green, blue … saturation: vividness of hue brightness: luminance lRelative discrimination thousands of distinct colors lAbsolute discrimination 24 distinct colors recommended: 9
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Visual Field
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30 vision.ppt Visual Impairments Myopia :Nearsightedness Hyperopia :Farsightedness Presbyopia :Loss of accommodation Night Blindness :Reduced rod vision Color Blindness :Inability to discriminate Tunnel Vision : Reduced field of view
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31 vision.ppt Other Factors Affecting Visual Performance lContrast: optimum level exists lIllumination: optimum level exists lTime: positive relationship lLuminance Ratio: see contrast
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32 vision.ppt Other Factors Affecting Visual Performance (2) lGlare: negative relationship lMovement: negative relationship lAge: negative relationship lDrugs: some drugs impair vision
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