Physics of Color

Physics of Color Alej Garcia Dept. Physics SJSU www.algarcia.org Spectrum of Visible Light Wavelengths & Photons Particles of light, called photons, each have a wavelength. Red Photon Yellow Photon Green Photon Blue Photon Prism Spectrum Additive Color Wheel There are No Photons of These Colors R M Y Spectral Colors B C G Red Yellow Green Cyan Blue Magenta Adding Color Lights Stream of red & green photons looks same as yellow photons (metamerism) or Eye to Brain Theatrical lighting Notice overlap of red, green, & blue is seen as white light Newton’s Color Wheel Prism spectrum is a straight line, so why did Isaac Newton describe color using a circular wheel? Trichromatic Theorists Thomas Young (1773-1829) English physicist Hermann von Helmholtz (1821-1894) German physicist James Clerk Maxwell (1831-1879) Scottish physicist Simple Trichromatic Theory Cyan & Blue photons excite me Yellow, Green & Cyan photons excite me Yellow & Red photons excite me MOE CURLY Inside your eye there are three receptors LARRY Trichromatic: Seeing Yellow Cyan & Blue photons excite me. Yawn. Yellow, Green & Cyan photons excite me. I’M EXCITED CURLY Yellow & Red photons excite me. I’M EXCITED OR MOE LARRY Yellow seen when Curly and Larry excited, either by yellow photons or red & green photons. 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. Trichromatic: Seeing Magenta Cyan & Blue photons excite me. I’M EXCITED Yellow, Green & Cyan photons excite me. Yawn. CURLY Yellow & Red photons excite me. I’M EXCITED MOE LARRY Magenta is seen by eye when Moe and Larry excited, which no single type of photon can achieve. Maxwell Color Disk Disk painted half red, half blue looks magenta when rapidly spinning. 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 Trichromatic: Seeing White Cyan & Blue photons excite me. I’M EXCITED Yellow, Green & Cyan photons excite me. I’M EXCITED CURLY Yellow & Red photons excite me. I’M EXCITED MOE LARRY White seen when all three very excited Gray seen when all three less excited Trichromatic: Color Blindness Cyan & Blue photons excite me. Yawn. Red , Yellow, Green & Cyan photons excite me. I’M EXCITED CURLY OR MOE Color blindness occurs if the eye is missing one of the three receptors. Other receptors try to compensate but cannot distinguish some colors. LARRY Do I see red or green? 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. Incidence (%) Classification Anomalous Trichromacy Protanomaly (Red-cone weak) Deuteranomaly (Green-cone weak) Tritanomaly (Blue-cone weak) Dichromacy Protanopia (Red-cone absent) Deuteranopia (Green-cone absent) Tritanopia (Blue-cone absent) Males 6.3 1.3 5.0 0.0001 2.4 1.3 1.2 0.001 Females 0.37 0.02 0.35 0.0001 0.03 0.02 0.01 0.03 29 or 70? 21 or 74? Rod Monochromacy (no cones) 0.00001 0.00001 Trichromatic: After-Image Yawn. Yawn. I’M EXCITED! MOE CURLY LARRY Then stare at WHITE ME TOO! Me too, but tired. First stare at RED Moe and Curly are excited so what color is seen? CYAN (light blue-green) I’M EXCITED! MOE CURLY LARRY Negative After-image Stare, unfocused, at the red cross for 10 seconds then look at white wall Negative After-image Cyan Negative After-image Stare, unfocused, at the flag for 10 seconds then look at white wall Negative After-image Cyan Magenta Yellow Additive Complements LARRY R MOE C CURLY After-image of red is cyan because Larry gets tired so when white light excites all three Stooges, Moe & Curly stronger than Larry. Cyan = White - Red Trichromatic: Opponency Oh, Shut The F*@% Up! Yellow, Green & Cyan photons excite me. I’M EXCITED CURLY Yellow & Red photons excite me. I’M EXCITED MOE Shine Red & Green photons (or Yellow photons) LARRY Yellow seen when Curly and Larry excited, which can annoy Moe, who then opposes them. 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 Color Vision in the Eye Three types of cones (color) One type of rod (B/W only) CIE Hue-Saturation Diagram Eye is not a perfect optical instrument. Color “wheel” is actually distorted cone shape. CURLY % 50% Larry 50% Curly 0% Moe Rim is full saturation, center is white 33% Larry 33% Curly 33% Moe LARRY % Hue, Saturation, Value Color wheel is not a single wheel but stack of wheels that range in value. Why Yellow & Orange are Special Cone Sensitivity CIE color “wheel” Green Blue Red 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 Spectral Reflectance Curves When white light shines on a colored object, some photons absorbed, others reflected by the object’s surface. Name That Pigment 100% BLUE GREEN RED BLUE GREEN RED BLUE GREEN RED 0% Titanium White Cadmium Red Burnt Sienna Name That Pigment 100% BLUE GREEN RED BLUE GREEN RED BLUE GREEN RED 0% Phthalocyanine Green Cadmium Yellow Cobalt Blue Artist’s Handbook These spectral reflectance curves and those of many other standard pigments are found in Mayer’s book. Pigment Value & Light Source Shine colored lights on pigments to see how values change Pigment Value & Light Source Paint your color grid under bright, natural light (no lava lamps) Reflectance of Pigment Mixtures 100% BLUE GREEN RED 80% Cadmium Red 60% 40% Cobalt Blue 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. 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%.) 20% Mixture 400 500 600 Nanometers 700 NOTE: This only gives approximate results; full theory more complex! Mixing to a Color: Near vs. Far Difficult to hit a target color by mixing two distant colors (pigments A and B). C Actual Mix B Target D A Easier to hit a target color by mixing two nearby colors (pigments C and D). Green vs. Yellow as Primary GREEN YELLOW Dots indicate pure pigment (Dana poster) Curved lines are mixtures of pigments. Vertical bars indicate value. WHITE CYAN RED MAGENTA BLUE Note that mixing green & red passes near the white/black spot. Mixing Pigments with White GREEN YELLOW WHITE CYAN Mixing paint pigments with Titanium White can cause shifts in hue RED MAGENTA BLUE Some pigments even become more saturated when mixed with a bit of white 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