Physics of Color

1. Physics of Color Alej Garcia Dept. Physics SJSU www.algarcia.org

2. Spectrum of Visible Light

3. Red Photon Wavelengths & Photons Particles of light, called photons, each have a wavelength. Yellow Photon Green Photon Blue Photon Prism Spectrum

4. There are No Photons of These Colors Spectral Colors Additive Color Wheel R M Y Red Yellow Green Cyan Blue Magenta G B C

5. Adding Color Lights Stream of red & green photons looks same as yellow photons (metamerism) YELLOW Eye to Brain or Notice overlap of red, green, & blue is seen as white light Theatrical lighting

6. Newton’s Color Wheel Prism spectrum is a straight line, so why did Isaac Newton describe color using a circular wheel?

7. Trichromatic Theorists James Clerk Maxwell (1831-1879) Scottish physicist Thomas Young (1773-1829) English physicist • Hermann von Helmholtz • (1821-1894) • German physicist

8. Simple Trichromatic Theory Yellow & Red photons excite me Yellow, Green & Cyan photons excite me Cyan & Blue photons excite me MOE LARRY CURLY Inside your eye there are three receptors

9. Trichromatic: Seeing Yellow Yellow & Red photons excite me. I’M EXCITED Yellow, Green & Cyan photons excite me. I’M EXCITED Cyan & Blue photons excite me. Yawn. CURLY MOE OR LARRY Yellow seen when Curly and Larry excited, either by yellow photons or red & green photons.

10. Seeing Yellow Sodium lamps emit pure yellow photons Color monitor can only emit red, green, and blue (RGB); creates other colors by selectively turning RGB pixels on or off.

11. Trichromatic: Seeing Magenta Yellow & Red photons excite me. I’M EXCITED Cyan & Blue photons excite me. I’M EXCITED Yellow, Green & Cyan photons excite me. Yawn. CURLY MOE LARRY Magenta is seen by eye when Moe and Larry excited, which no single type of photon can achieve.

12. Maxwell Color Disk Disk painted half red, half blue looks magenta when rapidly spinning.

13. Mixing Blue & Red Paint Mixing paint or ink is different from adding colors together by light. Mix of blue and red paint produces a blackish brown

14. Trichromatic: Seeing White Yellow & Red photons excite me. I’M EXCITED Cyan & Blue photons excite me. I’M EXCITED Yellow, Green & Cyan photons excite me. I’M EXCITED CURLY MOE LARRY White seen when all three very excited Gray seen when all three less excited

15. Trichromatic: Color Blindness Red , Yellow, Green & Cyan photons excite me. I’M EXCITED Cyan & Blue photons excite me. Yawn. Color blindness occurs if the eye is missing one of the three receptors. Other receptors try to compensate but cannot distinguish some colors. CURLY MOE OR LARRY Do I see red or green?

16. Color Blindness Weakness or absence of one of the three types of cones is the cause of color blindness, leading to a reduced ability to distinguish colors. 29 or 70? 21 or 74?

17. I’M EXCITED! Yawn. Yawn. Then stare at WHITE MOE LARRY CURLY Me too, but tired. I’M EXCITED! ME TOO! First stare at RED MOE LARRY CURLY Trichromatic: After-Image Moe and Curly are excited so what color is seen? CYAN (light blue-green)

18. Negative After-image Stare, unfocused, at the red cross for 10 seconds then look at white wall

19. Negative After-image Cyan

20. Negative After-image Stare, unfocused, at the flag for 10 seconds then look at white wall

21. Negative After-image Yellow Cyan Magenta

22. Additive Complements LARRY After-image of red is cyan because Larry gets tired so when white light excites all three Stooges, Moe & Curly stronger than Larry. R CURLY MOE C Cyan = White - Red

23. Trichromatic: Opponency Yellow & Red photons excite me. I’M EXCITED Yellow, Green & Cyan photons excite me. I’M EXCITED Oh, Shut The F*@% Up! CURLY MOE LARRY Shine Red & Green photons (or Yellow photons) Yellow seen when Curly and Larry excited, which can annoy Moe, who then opposes them.

24. Simultaneous Contrast Does the gray bar look slightly bluish? Are the two gray bars the same shade of gray? Yes, the presence of a nearby color affects perception of both hue and value, shifting both towards complement

25. Color Vision in the Eye Three types of cones (color) One type of rod (B/W only)

26. CIE Hue-Saturation Diagram Eye is not a perfect optical instrument. Color “wheel” is actually distorted cone shape. Rim is full saturation, center is white CURLY % 50% Larry 50% Curly 0% Moe 33% Larry 33% Curly 33% Moe LARRY %

27. Hue, Saturation, Value Color wheel is not a single wheel but stack of wheels that range in value.

28. Why Yellow & Orange are Special Red Green CIE color “wheel” Blue Cone Sensitivity Wavelength Peak sensitivities of green and red cone are close together, so we easily separate colors in this range, probably to spot ripe fruit and … La Victoria Hot Sauce

29. Spectral Reflectance Curves White Red When white light shines on a colored object, some photons absorbed, others reflected by the object’s surface.

30. BLUE GREENRED 100% 0% Name That Pigment BLUE GREENRED BLUE GREENRED Titanium White Burnt Sienna Cadmium Red

31. BLUE GREENRED 100% 0% Name That Pigment BLUE GREENRED BLUE GREENRED Phthalocyanine Green Cobalt Blue Cadmium Yellow

32. Artist’s Handbook These spectral reflectance curves and those of many other standard pigments are found in Mayer’s book.

33. Pigment Value & Light Source Shine colored lights on pigments to see how values change

34. Pigment Value & Light Source Paint your color grid under bright, natural light (no lava lamps)

35. Mixture Reflectance of Pigment Mixtures BLUE GREENRED 100% 80% 60% 40% 20% Take a mixture of equal parts cadmium red and cobalt blue. The mixture reflectance profile is defined as the geometric mean (square root of the product) of their separate reflectances for every wavelength in the spectrum. Cadmium Red Cobalt Blue For example, if cobalt blue reflects 20% of a specific blue wavelength (say 500nm), and cadmium red reflects only 5%, then their mixture will reflect roughly 10% of the 500nm light. (The product 20% x 5% = 100%, the square root of 100% is 10%.) • 500 600 700 • Nanometers NOTE: This only gives approximate results; full theory more complex!

36. Ideal mix C D Actual Mix Mixing to a Color: Near vs. Far Difficult to hit a target color by mixing two distant colors (pigments A and B). A Target Easier to hit a target color by mixing two nearby colors (pigments C and D). B

37. Green vs. Yellow as Primary Dots indicate pure pigment (Dana poster) Curved lines are mixtures of pigments. Vertical bars indicate value. GREEN YELLOW WHITE Note that mixing green & red passes near the white/black spot. RED CYAN MAGENTA BLUE

38. Mixing Pigments with White Mixing paint pigments with Titanium White can cause shifts in hue GREEN YELLOW WHITE CYAN RED Some pigments even become more saturated when mixed with a bit of white MAGENTA BLUE

39. Why Paint a Color Grid? “The results of mixing colored paints are sufficiently complicated so that no fully reliable theory has yet been developed. For the artist, there is no choice but to be fully familiar with the mixing properties of the paints on the palette.” Light and Color in Nature and Art S. Williamson and H. Cummins