color vision

• Light


  Travels in straight line with approx. speed of 299,792,458 m/s (in vacuum)
• Colors of visible light

  • red (longest wavelength and lowest frequency)
  • orange
  • yellow
  • green
  • blue
  • indigo
  • violet (shortest wavelength and highest frequency)

  
• Photons


  Mass-less particles that are electrical fields traveling through space
• Properties of Light


  • Wavelength – crest to crest or trough to trough
  • Amplitude – height of each crest or depth of each trough
  • Frequency – number of wavelengths within a period of time, measured in cycles per second
  • Energy – measured in electron volts

  
• Visible Spectrum


  The narrow portion within the electromagnetic spectrum that can be seen by the human eye
• Newton identified the ROYGBIV colors that make up the visible spectrum
• Optics documented Newton's discoveries from his experiments passing light through a prism – dispersion
• Effects of Color


  • Sparks our emotions
  • Influences our moods
  • Gives symbolic meaning
  • Influences our purchases
  • Used to define contours
  • Helps us remember objects
  • Helps us judge properties
  • Part of our experience
• Color (Definition 1)

  That aspect of things that is caused by different qualities of the light reflected or emitted by an object, definable by the observer in terms of: hue, lightness, and saturation

  
• Color (Definition 2)

  That characteristic of light by which an individual is made aware of the objects through the receptors of the eye, definable by the observer in terms of wavelength, luminance, and purity

  
• Qualities (Properties) of Color

  • Hue – the familiar color's name or family and directly linked to the wavelength
  • Brightness/Luminosity/Luminance/Value – the apparent intensity of color; the darkness (shade) or lightness (tint) of a color (0% lightness means black)
  • Saturation/Chroma – the purity of a color, dilution of hue by white (0% saturation means black)
  • Temperature – warm and cool colors

  
• Primary Colors

  • Red
  • Green
  • Blue
• Secondary Colors

  • Magenta (blue and red)
  • Yellow (green and red)
  • Cyan/Teal (blue and green)
• Factors Affecting the Color of Objects

  • Physics of the object relative to the environment – the spectrum of the incident light, reflectance properties (material and its physical and chemical state, the surface roughness, and geometric circumstances – light angle of incidence) of the surface
  • Characteristics of the perceiving eye (chromatic adaptation – ability to adjust to changes in illumination in order to preserve the appearance of object colors) and personal contextual cues (color constancy – ability to perceive colors as relatively constant over varying illuminations)

  
• Characteristics of the Human Eye

  • Cortical Level – color naming ability is present
  • Retinal Level - Long-cones, Medium-cones, Short-cones

  
• Color Vision


  Ability to discriminate various wavelengths independent of light intensity or the ability to discriminate among stimuli on the basis of hue, independent of brightness or any other cue

  
• Aspects of Color Vision

  • Physiological Aspect (Colorimetry) - Measure of visual function at the photoreceptor level, Color encoding, Retinal function
  • Psychological Aspect (Color Discrimination & Color Constancy) - Measure of cortical function at the later stages of visual processing, Color perception affected by surround stimulus, adaptation, and experience, Cortical function

  
• Factors Affecting Color Perception

  • Context/Chromatic Contrast/Chromatic Induction/Setting – The appearance of color is altered by introducing a second surrounding color
  • Prior exposure to Stimulus – processes of adaptation constantly adjust visual sensitivity according to the stimulus the observer is currently viewing
• Colorimetry


  The science and technology that quantifies and physically describes human color perception. It includes the perception of color by the human eye and brain, the origin of color in materials, and the physics of visible electromagnetic spectrum.

  
• The International Commission on Illumination (Commission Internationale de I'Eclairage – CIE) is a technical, scientific, and cultural non-profit organization whose objectives embraces fundamental subjects as vision, photometry, and colorimetry.
• Color Vision Supports 2 Distinct Visual Functions

  • Color Discrimination – ability to determine that two spectra differ or the ability to match or detect differences between
  • Color Constancy – ability to identify objects on the basis of their color appearance. The color of the object remains the same despite changes in the properties of the object and light.

  
• Cone Types and Their Spectral Sensitivity

  • S cones (Short-Wavelength, 440-450 nm, Blue) - Tritanope
  • M cones (Medium-Wavelength, 535-550 nm, Green) - Deuteranope
  • L cones (Long-Wavelength, 570-590 nm, Red) - Protanope
• Color Vision Process

  • Color Constancy/Chromatic Adaptation (surface-based processing) - A process of favorable or useful adjustment of sensory processes to compensate for changes in the spectral quality of light source in order to keep visual perception of color approximately constant. It also refers to the ability to perceive constant surface colors despite changes in illumination or the ability of the visual system to preserve the appearance of an object under a wide range of light sources.
  • Color Naming (category-based processing) - refers to the ability to identify colors based on memory, learning and categorization.

  
• Types of Color Mixing

  • Physical Color Mixing
  • Physiological - based on positive after-image
  • Juxtaposition - based on the positioning of colors, the arranging of different colors side-by-side to create optical effects in a viewer's eye

  
• Trichromatic Theory (Three Component Theory)


  States that there are 3 receptors in the retina that are responsible for color perception and the combination of these 3 colors produce all of the colors that are perceived.
• Trichromatic Theory - Key Contributors and Concepts

  • Thomas Young & Hermann von Helmholtz
  • Young in 1802 suggested that the eye contained different photoreceptor cells that were sensitive to different wavelengths of light in the visible spectrum
  • Helmholtz in mid-1800 suggested that the cone receptors of the eye were either short-wavelength (blue), medium-wavelength (green), or long-wavelength (red); and that the strength of the signals detected by the receptor cells determine how the brain interpreted color in the environment
  • Color sensations: Erythrolabe – red sensation, Chlorolabe – green sensation, Cyanolabe – blue sensation
  • There are three only (3) primary colors but many hues

  
• Opponent Process Theory


  States that the ability to perceive color is controlled by 3 receptor complexes with opposing actions. These 3 receptor complexes are the red-green complex, blue-yellow complex, and black-white complex.
• Current research suggests that the true pairings for these receptor complexes are actually blue-yellow, red-cyan, and green-magenta.
• The opponent color process works through a process of excitatory and inhibitory responses, with the two components of each mechanism opposing each other.
• A cell that was excited by wavelengths associated with green would be inhibited by wavelengths associated with red, and vice versa. One of the implications of opponent processing is that we do not experience greenish-reds or yellowish-blues as colors. Another implication is that this leads to the experience of negative afterimages.
• Complementary Color Theory


  States that each receptor pairing registers complementary colors – there is no white/black pairing. When complementary colors are added together, they make white.
• When you were staring at the red image, your brain got used to the red and suppressed the signals it was getting from red cells. When you the shifted your gaze to the white paper, your brain saw less red light as before and mentally "subtracted" red from what it as seeing. The green cells, however, hadn't been suppressed and could send full-strength signals. White "minus" red is green, hence why you saw a flash of green.
• Dual Process Theory


  The color vision system contains 2 stages: An initial trichromatic stage where trichromatic cone cells respond positively to one of three frequencies exhibited by photons arriving on their surface, and a later opponent-process stage where the 3 color channels are discovered by nearby opponent cells.

  
• Color Vision Classification


  • Normal – Trichromacy
  • Anomalous Trichromacy – color vision anomaly (Protanomaly, Deuteranomaly, Tritanomaly)
  • Dichromalous/Dichromacy/Dichromato

  psia – missing one group of cone cells (Protanopia, Deuteranopia, Tritanopia)

  • Achromalous/Achromacy/Achromatopsia – true color blindness (inability to see color or complete absence of color sensation)
  • Monochromalous/Monochromacy/Monochromato

  psia - Cone Monochromacy/Incomplete Achromatopsia (one functioning cone group), Rod Monochromacy/Complete Achromatopsia (no functioning cones)

  
• Most Common Color Vision Anomalies are Protanopia, Deuteranopia, and Tritanopia.

  
• Opponent cells

  • Tuned to luminosity are excited by the red, green, and blue color signals
  • Cg cells are excited by red and blue and inhibited by green
  • Cb cells are excited by red and green and inhibited by blue
• Protanopia


  Loss of red sensitivity
• Protanomaly


  Abnormal red-sensing pigment
• Deuteranopia


  Loss of green sensitivity
• Deuteranomaly


  Abnormal green-sensing pigment