Physics of Color Alej Garcia Dept. Physics SJSU

Spectrum of Visible Light

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

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

Adding Color Lights Stream of red & green photons looks same as yellow photons (metamerism) Theatrical lighting or YELLOW Eye to Brain white 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 ( ) English physicist Hermann von Helmholtz ( ) German physicist James Clerk Maxwell ( ) Scottish physicist

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

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. Yellow seen when Curly and Larry excited, either by yellow photons or red & green photons. MOE CURLY LARRY OR

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

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

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

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

Color Blindness Classification Incidence (%) MalesFemales 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) Rod Monochromacy (no cones) or 70? 21 or 74? 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.

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

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

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

Simultaneous Contrast 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 Does the gray bar look slightly bluish?

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. Rim is full saturation, center is white LARRY % CURLY % 50% Larry 50% Curly 0% Moe 33% Larry 33% Curly 33% Moe

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

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

Pigment Reflectance Curves Quinacridone rose (PV19) Quinacridone magenta (PR122) Manganese violet (PV16)

Name That Pigment Titanium White Cadmium Red Burnt Sienna BLUE GREEN RED 0% 100% BLUE GREEN RED

Name That Pigment Phthalocyanine Green Cadmium Yellow Cobalt Blue BLUE GREEN RED 0% 100% BLUE GREEN RED

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 BLUE GREEN RED 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. NOTE: This only gives approximate results; full theory more complex! 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%.) Cadmium Red Cobalt Blue Nanometers Mixture 100% 80% 60% 40% 20%

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

Reflectance of Pigment Mixtures Take a mixture of equal parts ultramarine blue and cadmium red deep. 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 ultramarine blue reflects 80% of a specific blue wavelength (say 480nm), and cadmium red deep reflects only 8%, then their mixture will reflect roughly 25% of the 480nm light. (The product 8% x 80% = 640%, the square root of 640% is 25%.) NOTE: This only gives approximate results; full theory more complex!

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

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