Published on

Published in: Technology, Health & Medicine
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide


  1. 1. Color 1 Color To understand how to make realistic images, we need a basic understanding of the physics and physiology of vision. Here we step away from the code and math for a bit to talk about basic principles. 2 1
  2. 2. Basics Of Color • elements of color: 3 Basics of Color • Physics: – Illumination • Electromagnetic spectra – Reflection • Material properties • Surface geometry and microgeometry (i.e., polished versus matte versus brushed) • Perception – Physiology and neurophysiology – Perceptual psychology 4 2
  3. 3. Electromagnetic Spectrum 5 Theory of color 6 3
  4. 4. Theory of color (Goethe) 7 Theory of color (Goethe) 8 4
  5. 5. White Light • Sun or light bulbs emit all frequencies within the visible range to produce what we perceive as the quot;white lightquot; 9 Sunlight Spectrum 10 5
  6. 6. White Light and Color • when white light is incident upon an object, some frequencies are reflected and some are absorbed by the object • combination of frequencies present in the reflected light that determines what we perceive as the color of the object 11 Reflection 12 6
  7. 7. Physiology of Vision • The eye: • The retina – Rods – Cones • Color! 13 Photoreceptors 14 7
  8. 8. 15 Physiology of Vision • The center of the retina is a densely packed region called the fovea. – Cones much denser here than the periphery 16 8
  9. 9. Trichromacy • three types of cones – L or R, most sensitive to red light (610 nm) – M or G, most sensitive to green light (560 nm) – S or B, most sensitive to blue light (430 nm) – color blindness results from missing cone type(s) 17 Trichromats (with a special • Some animals – for example sensitivity to bees – have three types of cones. Two of the cones are ultraviolet) sensitive to quot;human visiblequot; wavelengths. The third cone is sensitive to colors in the ultraviolet range of the spectrum. This cone enables them to see colors that humans can’t see. They also perceive human- visible spectra in different hues than those that humans experience 18 9
  10. 10. • Most bird species (that have been studied) have Tetrachromats at least four types of cones. They are quot;tetrachromats.quot; Recent studies have confirmed tetrachromacy in some fish and turtles. Perhaps it is mammals, including humans, that have poor color vision! 19 • It has been suggested that some women are also Tetrachromats tetrachromats. One study suggested that 2-3% of the world's women may have the kind of fourth cone that gives a significant increase in color differentiation. • Another thing to consider: Some data suggests that the architecture of the human visual system (as well as that of many animals) is really tetrachromatic - but that this capacity is blocked. 20 10
  11. 11. • Many animals have only Dichromats two kinds of cones in their eyes. They are known as “dichromats.” It’s worth noting that the color-sensing pigment in these cones may be weak. Therefore, an animal – for example a dog - probably sees very weak colors. 21 Receptors in Retina • Receptors contain photopigments that produce electro-chemical response; our dynamic range of light, from a few photons to looking at the sun, is 1011 => division of labor among receptors • Rods (scotopic): only see grays, work in low-light/night conditions, mostly in periphery • Cones (photopic): respond to different wavelength to produce color sensations, work in bright light, densely packed near fovea, center of retina, fewer in periphery • Young-Helmholtz tristimulus theory1: 3 types of cones, each sensitive to all visible wavelengths of light, each maximally responsive in different ranges, often associated with red, green, and blue (although red and green peaks of these cones actually more yellow) • The three types of receptors can produce a 3-space of hue, saturation and value (lightness/brightness) • To avoid misinterpretations S (short), I (intermediate), L (long) often used instead 1Thomas Young proposed the idea of three receptors in 1801. Hermann von Helmholtz looked at the theory from a quantitative basis in 1866. Hence, although they did not work together, the theory is called the Young-Helmholtz theory since they arrived at the same conclusions. 22 11
  12. 12. Humans and Light • when we view a source of light, our eyes respond to – hue: the color we see (red, green, purple) • dominant frequency – saturation: how far is color from grey • how far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue) – brightness: how bright is the color • how bright are the lights illuminating the object? 23 Hue • hue (or simply, quot;colorquot;) is dominant wavelength – integration of energy for all visible wavelengths is proportional to intensity of color 24 12
  13. 13. Saturation or Purity of Light • how washed out or how pure the color of the light appears – contribution of dominant light vs. other frequencies producing white light 25 Intensity, Brightness • intensity : radiant energy emitted per unit of time, per unit solid angle, and per unit projected area of the source (related to the luminance of the source) • brightness : perceived intensity of light. One may lower the intensity by adding white or black. 26 13
  14. 14. Perception of Color • Human eyes can not differentiate between spectral yellow and any combination of red and green 27 Activity 28 14
  15. 15. Do all of the red squares look the same or do they look different? 29 30 15
  16. 16. 31 Metamers a given perceptual sensation of color derives from the stimulus of all three cone types • identical perceptions of color can thus be caused by very different spectra 32 16
  17. 17. Metamer Demo • tml 33 17