vision - PowerPoint by zhangyun


									Spectral Color

• The amount of refraction depends on the index of refraction AND the
  index of refraction depends on the wavelength

• Instrument that measures the luminosity
  emitted at each wavelength (or the
  spectral power distribution)
Spectral power distributions
Retinal cells
       • Rods:
         – Black and white
         – Sensitive to low light
         – 120 million
       • Cones:
         – Less sensitive
         – Three types of cones
           which form color
         – 7 million
• At the fovea, each cone
  has its own optic nerve
• Outside the fovea,
  each nerve fiber is
  shared by about 80
  rods or cones
• This results in the
  highest-resolution part
  of our vision
• Some of the visual
  processing for each
  eye is done using the
  visual cortex of both
  the left and right side
  of the brain
• This offers redundancy
  in the case the brain is
• Visual cortex: processes the raw input from the
  optic nerve
• Visual thalamus: interprets the images produced
  by the visual cortex
• When light enters the eye, it hits the rhodopsin.
• Rhodopsin decomposes when it is exposed to
  light, changing into several intermediate
  compounds, but eventually (in less than a
  second) forms activated rhodopsin.
• This chemical causes electrical impulses that
  are transmitted to the brain and interpreted as
• Once the rhodopsin has
  been activated, it takes time
  to return to its original state.
• This recovery time increases
  with age.
             Dark adaptation
• Three processes to deal with low-light
  – Pupil dilates to collect more light
  – The visual center shifts its definition of “black”
    and changes its perceived contrast levels
  – Because less light is reaching retina, more
    receptors are in the ready phase (rhodopsin)
    rather than the bleached phase (activiated
    rhodopsin) and the retina becomes more
Color sensitivity of the eye
           Color perception
• The visual system’s best performance:
  – Can see about 1000 levels of light-dark
  – 100 levels of red-green
  – 100 levels of yellow-blue
• This means that the total number of colors
  we can see is about 1000 x 100 x 100 =
  10,000,000 (10 million).
• When our eyes are exposed to a color for a
  prolonged period, the rods & cones become
• When the eyes are then diverted to a blank
  space, these photoreceptors send out a weak
  signal and those colors remain muted.
• However, the surrounding cones that were not
  being excited by that color are still "fresh", and
  send out a strong signal.
• The signal is exactly the same as if looking at
  the opposite color, which is how the brain
  interprets it.
• The human eye can perceive scenes with a very
  high dynamic contrast ratio, around 1,000,000:1.
• Adaptation is achieved in part through iris
  dilation and slow chemical changes, which take
  some time.
• At any given time, the eye's static range is
  smaller, around 10,000:1. However, this is still
  generally higher than the static range achievable
  by most display technology
Contrast test
             Color constancy
• Color constancy is a feature of the human color
  perception system which ensures that the
  perceived color of objects remains relatively
  constant under varying illumination conditions.
• A green apple looks green to us at midday,
  when the main illumination is white sunlight, and
  also at sunset, when the main illumination is red.
  This helps us identify objects.
               Color systems
• Subtractive color: used in situations where
  wavelengths are absorbed
  – paint
  – printing
• Additive color: used in situations where
  wavelengths are emitted
  – lighting design
  – computer monitors
• Chroma: How pure a hue
  is in relation to gray
  Saturation: The degree of
  purity of a hue.
  Intensity: The brightness
  or dullness of a hue. One
  may lower the intensity by
  adding white or black.
  Luminance / Value: A
  measure of the amount of
  light reflected from a hue.
  Those hues with a high
  content of white have a
  higher luminance or value.
• Shade: A hue
  produced by the
  addition of black.
  Tint: A hue
  produced by the
  addition of white.
            Subtractive Color

• When mixing colors using paint, or through the
  printing process, the subtractive color method is
• Subtractive color mixing means beginning with
  white and ending with black.
• As one adds color, the result gets darker and
  tends to black.
  Transmission and Reflection of
          Colored Light

• The subtractive primaries combine to form
Halftone printing
Color halftoning with CMYK
          Four color printing
• cyan, magenta, yellow, and key (black)
• subtractive color model used in color
• ink is typically applied in the order of the
                 Additive Color

• When working on a computer, the colors seen on the
  screen are created with light using the additive color
• Additive color mixing begins with black and ends with
• As more color is added, the result is lighter and tends to
        Additive color mixing
• Any three colors of light
  that can be mixed to
  produce white light are
  called primary colors.
• Red, green, and blue
  are the most commonly
  used primary colors.
Filters are designed to pick out
 desired wavelengths of light
        Neutral density filter

• Neutral density filters reduce the
  transmission of all wavelengths equally.
           Spectral reflectance

Spectral reflectance curves show how much of
each wavelength of light is reflected by a surface.
• Spectral reflectance curves have been
  used to identify healthy and stressed
  populations of plants.
       Complementary colors

• Any two colors that can be added together
  to produce white are called
  complimentary colors.
Color systems
     • The visible spectrum
       consists of billions of
     • A monitor can display
     • A high quality printer
       is only capable of
       producing thousands.
Color wheel
Primary colors
   • Colors at their basic
   • Those colors that cannot
     be created by mixing
Secondary colors
 • Those colors achieved by a
   mixture of two primaries.
Tertiary colors
  • Those colors achieved by a
    mixture of primary and
    secondary hues.
Complementary colors
    • Those colors located
      opposite each other on a
      color wheel.
Analogous Colors
   • Those colors located close
     together on a color wheel.
       Munsell color system
• Ten basic hues
• For each hue, there are
  ten gradations
• Each color has a
  number specifying its
  chroma and value
Hues of the Munsell system

  • Each basic hue has the number 5
• Ring of hues
  found at
• Value = 6
• Chroma = 6
  Specifying a color in the Munsell
• A color is fully
  specified by three
• Hue: 5P, color in
  middle of purple band
• Value: 5
• Chroma: 10
Ostwald color system
Hues of the Ostwald system
    Commission International de
     L'Eclairage (CIE) system
• The theory is based on the fact that our
  eyes contain three different types of color
  receptors called cones.
• These three receptors respond differently
  to different wavelengths of visible light.
• This differing response of the three cones
  is measured in three variables X, Y, and
  Z in the CIE color model.
CIE color matching functions
Chromaticity diagram
• Read Ch 8
• Answer 5 questions
• Due next Monday, Apr 20
• Monday, April 6: Chaps 11 & 12
• Thursday, April 9: Chaps 13 & 14
• Friday, Apr 10 at 1:00 pm
• “Mathematics of Soap Bubbles” with Frank
  Morgan in Dickinson 225
• Monday, April 13: No class

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