1. ECE 638: Principles of Digital Color Imaging Systems
Lecture 2: Foundations of Color Science

2. Synopsis
Briefly trace development of our knowledge of color and vision; so that we can “discover” it in a logical manner.
Explore concepts of color matching that form basis for trichromatic theory of color and colorimetry
Develop mathematical framework for trichromatic theory

3. Early concepts of vision
Anatomy of eye
Emanation theory
Pinhole camera
The lens
Spectrum of white light
Trichromacy
Source: Robert M. Boynton, Human Color Vision, Optical Society of America, 1992, pp. 1-18

4. Robert Boynton
Text

5. Anatomy of the human eye
Early Greeks practiced dissection

6. Emanation theory
Empedocles (5th century BC) – the “eye is like a lantern”
Galen ( AD)
physiologist
the brain is “an organ where all sensations arrive, and where all mental images, and all intelligent ideas arise.
rays are discharged in the direction of the object, interact with the object, and return to the eye
His views remained influential for about 1500 years

7. Pinhole camera Abu Ali Mohammed Ibn Al Hazen (965 - 1039 AD)
Experimented with simple pinhole camera
Concluded that eye contained an “image” with a point-to-point correspondence with scene under observation
Leonardo da Vinci ( )
Developed notions of perspective
Proposed ray tracing
Hampered by hypothesis that image within eye must be right-side up

8. The lens Spectacles discovered by 1285
Used to improve pinhole camera by G. B. Della Porta in 1589
Johannes Kepler
Astronomer
Constructed and understood lenses
Concluded that image within eye formed on retina
Aranzi cut a hole in back of animal’s eye to observe retinal image in 1595

9. Spectrum of white light
Newton’s experiment with prism (1730)
Concluded that white light is a mixture of colored lights
Removed color from perceived object and placed it in rays reflected from the object

10. Trichromacy Color mixtures
Artisans and scientists experimented with them from ancient times
Are properties due to materials or due to perception?
Thomas Young proposed trichromatic theory in 1801
“As it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited; for instance to the three principal colours, red, yellow, and blue, that each of the particles is capable of being put in motion more or less forcibly by undulations differing less or more from perfect unison.”

11. Trichromacy (cont.) Young’s ideas were ignored for over 50 years
Helmholtz revived them in his Handbook of Physiological Optics first published from 1856 to 1866

12. Contrast this with development of calculus in 17th century (1600’s)*
*From Wikipedia

13. Or with development of the theory of gravitational forces in 17th century (1600’s)*
*From Wikipedia

14. Helmholtz’s three spectral sensivity curves (1924)

15. Simulated retinal mosaic
From Mark Fairchild, Color Appearance Models, Wiley.

16. Trichromacy (cont.)
“It is still impossible to the present day to show clear anatomical differences between the three cone types that are believed to exist in primates, or to extract their photo-pigments, which are believed responsible for absorption of light.” – Robert M. Boynton, Human Color Vision, 1992

17. If we cannot conclusively establish physiological properties of the human eye, how can we quantitatively work with color?
Answer: we treat the human visual system (HVS) as a block box, and characterize it by its response to color stimuli.

18. Input-output characterization of HVS
Present subject with specially designed visual stimuli under precisely controlled conditions
Physiology
Measure chemical, electrical, or magnetic changes
Examples: neural probes, functional magnetic resonance
Psychophysics
Measure cognitive response:
“I can see A”
“A matches B”
“A looks brighter than B”

19. Trichromatic theory of color
Follows directly from simple model for interaction of stimulus and three different types of receptors in the human eye.
Can be described in terms of linear vector spaces
Forms basis for colorimetry and design of many aspects of color imaging systems
Does not predict all visual phenomena

20. Sources
G. Wyszecki and W. S. Stiles, Color Science: Concepts, and Methods, Quantitative Data and Formulae, Wiley, 1982, pp

21. Gunter Wyszecki
*From Wikipedia

22. Results from Search: Stiles Color

23. Development of trichromatic theory
Color matching experiment
Grassman’s laws for color matching
Spectral models for color
Surface-illuminant interaction model
Sensor model
Reinterpretation of conditions for color match

24. Color matching experiment – setup

25. Color matching experiment - procedure
Test stimulus is fixed
Observer individually adjusts strengths of the three match stimuli to achieve a visual match between the two sides of the split field
Mixture is assumed to be additive, i.e. radiant power of mixture in any wavelength interval is sum of radiant powers of the three match stimuli in the same interval
To achieve a match with some test stimuli , it may be necessary to move one or two of the match stimuli over to the side where the test stimulus is located

26. Trichromatic generalization
“…over a wide range of conditions of observation, many color stimuli can be matched in color completely by additive mixtures of three fixed primary stimuli whose radiant powers have been suitably adjusted”
This statement is subject to a number of qualifications. See reference below for details
Source: G. Wyszecki and W. S. Stiles, p. 117.

27. Colorimetry
“The branch of color science concerned … with specifying numerically the color of a physically defined visual stimulus in such a manner that:
when viewed by an observer with normal color vision, under the same observing conditions, stimuli with the same specification look alike,…
stimuli that look alike have the same specification
The numbers comprising the specifications are continuous functions of the physical parameters defining the spectral radiant power distribution of the stimulus”
Source: G. Wyszecki and W. S. Stiles, p. 117.

28. Grassman’s laws of color matching (1853)
Definitions: means “ matches ”
is an additive mixture of and
Symmetry:
Transitivity:

29. Hermann Grassmann
text
*From Wikipedia

30. Grassman’s laws (cont.)
Proportionality:
where is a positive factor that scales the radiant power without changing relative spectral distribution
Additivity: Any two of
implies that

31. Limitations of Grassman’s laws
Grassman’s laws provide a basic description of the results of color matching experiments
But they don’t provide:
general basis for understanding why the laws hold
a theory that can easily describe a wider range of situations

32. Trichromatic model Assume simple characterizations for
illuminant
optical properties of surfaces
interaction of illuminant with surface
response of HVS to visual stimulus
All are based on spectral representations

33. Spectral representation of color
Show a drawing illustrating a source, reflective surface, and sensor. Indicate spectra of illuminant, surface reflectance, and reflected light.

34. Interaction of illuminant and surface
At each wavelength, characterize surface by ratio of power in reflected light to that incident on surface
Reflectance
Absorptance
Absorptance includes any light that is not reflected from front surface

35. Limitations of interaction model – angular dependence
Does not account for dependence of spectral reflectance on
angle between incident light and surface normal
angle between reflected light and surface normal
More general model includes both specular and body reflectance terms
Complete characterization requires bi-directional reflectance function (BRDF)

36. Limitations of interaction model – scattering
Model is only valid for:
bulk (macroscopic) behavior
Microscopic behavior in which there is no lateral scattering
More complex scattering models exist

37. Scattering example – Yule-Nielsen effect
Lateral scattering of light within paper causes light rays that enter white paper substrate to be trapped under colorant dots
Print appears darker than predicted by fractional area coverage of colorant
Ray is trapped under colorant dot

38. Limitations of interaction model – fluorescence
Model assumes that reflected light at any wavelength depends only on light incident at that wavelength
Does not describe interaction of light with surfaces that fluoresce
papers and fabrics containing whiteners or brighteners
human teeth