4 - Color Mixing

1. 4 - Color Mixing

2. Spectral Power Distribution (SPD) Perfect absorber at all wavelengths = “Blackbody” (if T = 0 K)

3. Blackbody with T = 3000 K

4. Units? W/(m2 Hz steradian) W/(m2 m ster) W/(m2m ster) W/(m2 nm ster) etc.

5. Color Perception & Color Mixing • Hue– the color • Saturation– how deep the color is. Higher saturation means less “whiteness” • Brightness– the intensity of the light

6. Additive Color Mixing What does the human eye see when mixing roughly “equal” amounts of Red, Yellow, Green, Cyan, Blue, and Magenta? G + R = Y B + R = M B + G = C B + G + R = W (unequal amounts will produce other hues)

7. Primary Colors Any 3 colors that add up to make white light can be “primary” colors, but RGB seems to produce the widest range of other colors, so we usually refer to them as THE primary colors for additive mixing of light. Example: M + Y = (B + R) + (G + R) = (B + G + R) + R = W + R

8. Newton’s Color Wheel originally contained red, orange, yellow, green, blue, indigo, and violet.

9. To do correctly, we need to quantify “brightness” saturation

10. Different Types of Color Addition (colored lights) Superposition Partitive Mixing - close spatially Visual Persistence - close temporally

11. Subtractive Color Mixing (paints, filters, “what’s left?”)

12. Rule for combining filters:

13. Result of passing light with a given SPD through a filter (or series of filters):

14. Filter Types

15. Kodak Wratten 44A Filter High transmission We see this but not this

16. Spectral “Purity” • “Bandpass” -  (such as Full-Width-Half-Maximum) • “Spectral Resolution” - R = / • “Nonspectral” R ~ a few • “Spectral” R ~ 10 • “Monochromatic” R > 100 Sun with“H” filter,  = 656.3 nm, R~10,000 Neutral Density Filters - These have “flat” transmission curves, or R << 1

17. Building colored images from filtered ones

19. Colored Pigments (Paints) Relies on combinations of additive and subtractive color mixing. When layered, can depend in transmission, reflection, partitive color mixing, etc.