1. Basic Color Theory Susan Farnand farnand@cis.rit.edu

2. Tell me about color: How would you define color? How does color happen? Where does color happen? Why do things look colorful?

3. At least two things needed for color to happen: A source…

4. …and a sensor

5. Visible Light Spectrum 400 nm700 nm

6. Light Sources Compared

7. Basic Structure of the Human Eye

8. Schematic of the Retina Photoreceptors Horizontal, bipolar, and amacrine cells Ganglion cells Incoming light

9. Relative Spectral Sensitivity of Cones & Rods S rods ML

10. RodsCones S R M L Spectral sensitivity

11. Color Mixing–Additive  Adding light  Mixing light sources  Red, Green, Blue primaries

12. Additive Color Mixing:

13. Color Mixing–Additive  Adding light  Mixing light sources  Red, Green, Blue primaries  Can you think of examples of systems using additive light

14. Color Mixing– Subtractive  Removing light  Mixing dyes, pigments or other colorants  Cyan, magenta, & yellow primaries  Can you think of examples of systems using subtractive light?

15. What happens to the light? Transmitted Absorbed Reflected

16. Subtractive Color Mixing

17. Measuring Color Are these two patches the same?

18. Reflected Light

19. Sensation of Color Light Source Object Observer

20. Light Sources Compared

21. Cone mosaic data from experiment by Hofer, Singer, and Williams (2005). Different sensations from cones with the same photopigment. Journal of Vision, 5(5):5, 444–454.

22. CIE Commission Internationale de l’Eclairage (International Commission on Illumination), an international organization that establishes and maintains standards of light and color. Its system of describing color is based on standardization of illuminants and observers, not physical samples.

23. CIE Illuminants

24. Color Matching Experiment

25. Color Matching Functions

26. Color Matching Experiment

27. Color Matching Functions 1931 Standard Observer

28. Computing CIE Tristimulus Values Illuminant Object Observer The CIE tristimulus values X, Y, and Z are obtained by multiplying together the power (P) of a CIE standard illuminant, the reflectance (R) of the object, and the standard observer functions and then summing the products.

29. Computing CIE Tristimulus Values The CIE tristimulus values X, Y, and Z are obtained by multiplying together the power of a CIE standard illuminant, the reflectance of the object, and the standard observer functions and then summing the products. Doing the math…

30. Colorimetric Match Colorimetric match is defined as when the tristimulus values for two stimuli match: X 1 = X 2 Y 1 = Y 2 Z 1 = Z 2 Sample 1Sample 2

31. Metamerism Two colored patches that have different reflectance curves may appear the same when viewed under one illuminant, but may appear different when viewed under a second illuminant.

32. Colorimetric Match Colorimetric match is defined as when the tristimulus values for two stimuli match: X 1 = X 2 Y 1 = Y 2 Z 1 = Z 2 Sample 1Sample 2

33. Chromaticity Values and the Chromaticity Diagram The CIE tristimulus values X, Y, and Z are can be transformed to chromaticity values (x, y) and displayed on a unit plane is known as the chromaticity diagram. Chromaticity values are calculated by: x = X/(X+Y+Z), y = Y/(X+Y+Z) It is also possible to calculate a chromaticity for z, where z=Z/(X+Y+Z). Since x+y+z = 1, it is redundant (i.e. z=1-x-y)

34. Chromaticity Diagram

35. Color Differences on the 1931 CIE Chromaticity Diagram