skip chap. 8 for now Chap. 9 Color (continued) Lecture 19 Tuesday, October 26 Next time: Chapter 10, start reading. Nov. 2: exam review Nov. 4: exam II There are computer problems with clicker registration. Be patient. 1 Ch. 9 - Color We are here Spectral and non-spectral colors Intensity distribution curve Intensity, hue, saturation Additive primaries: R, G. B. Subtractive primaries: C, M, Y Hair, skin, and eye color CIE diagram Lighting, painting and printing Start Chapter 10, color perception. 2
CIE diagram (International Commission on Illumination) This is a color triangle with Green at the top, Blue at lower left, Red at (lower) right x,y positions on the color triangle represent all the colors that can be reproduced by adding red, green and blue. Limitation: darkness is missing. To be complete you must also specify lightness or brightness. 3 White: X = 1/3 Y = 1/3 Less saturated colors are near the center Spectral color wavelengths are at the left and right edges Non spectral colors are at this edge 4
Projection (additive) TV systems can only reproduce colors inside the triangle between the 3 colors of the sources (dots). 5 Comparison of old TV phosphors with LEDs (light emitting diodes) 6
Properties of the CIE diagram The spectral colors are on the left and right edges Equal mixing of two spectral colors on the edge makes the color half way between these two colors (on the line joining the two colors.) Where is magenta on the diagram? It is half way across the bottom, which is an equal mixture of red and blue. Where is cyan? What s great about this diagram: two numbers, x and y, can be used to describe a color. What s missing from the C.I.E. diagram? Because the colors are adjusted to add to 100%, dark colors (that total less than 100%) aren t represented, for example, black and brown. 7 Alternate diagram: Munsell s color tree: Hue, value and chroma an alternate way to indicate color that includes brightness Compare to: 8
Ink transmission curves Magenta = white green 100% 400 nm 500 nm 600 nm 700 nm 9 Ink transmission curves Cyan = white red 100% 400 nm 500 nm 600 nm 700 nm 10
Ink transmission curves Yellow ink (unsaturated) = white blue (contains green, yellow, orange and red) 100% 400 nm 500 nm 600 nm 700 nm 11 Additive and subtractive color mixing Why does one figure have a black interior and the other is white? 12
Ch. 9 - Color We are here Spectral and non-spectral colors Intensity distribution curve Intensity, hue, saturation Additive primaries: R, G. B. Subtractive primaries: C, M, Y Hair, skin, and eye color CIE diagram Lighting, painting and printing 13 Methods of adding colors 1. Addition of illumination: stage lighting and 3-color TV projector. Projected colors overlap. 2. Partitive mixing: closely spaced dots of colors. TV screens, laptop screens, pointillist paintings, tight textile weaves, some printing 3. Time mixing: a rotating color wheel. It s hard to find examples. 4. Binocular mixing: different color to each eye. The colors "blend" in the brain. 14
Partitive mixing is placing colors next to one another so that they are merged in the eye. Examples: Pointillist painting LCD screens Old TV screens (CRT) Plasma TVs Detail from Circus Sideshow (or Parade de Cirque) (1889) showing pointillism Georges Seurat 15 Georges Seurat, Sunday Afternoon on the Island of La Grande Jatte, 1884-1886, The Art Institute of Chicago. 16
Sources of light, color balance Color balance important for movies and stage lighting. White light should contain equal amounts R, G and B but often does not. Examples of odd (unbalanced) lighting: Candles and campfires contain an excess of red. So do sunsets and tungsten bulbs. Some street lamps (low pressure sodium, for example) contain only a few wavelengths. All other colors are missing. When red is missing, red lipstick looks black. Demo: blue light with red, white, and blue papers 17 Color temperature The temperature of light bulb determines the relative amount of red. Cooler bulbs have more red than hotter bulbs. A lamp can be characterized by its temperature Sun, 5000 K (has a lot of blue) Photoflood lamp, 3200 K, an approximation of sunlight Tungsten lamp, 2850 K, looks orange, less blue. Candle, 1800 K, distinctly red, almost no blue. Color film is balanced for daylight. Photos made by candlelight or ordinary tungsten lamps will look unnaturally red. Demo: blue and red filter with dimmed bulb 18
Ch. 9 - Color Spectral and non-spectral colors Intensity distribution curve Intensity, hue, saturation Additive primaries: R, G. B. Subtractive primaries: C, M, Y Hair, skin, and eye color CIE diagram Lighting, painting and printing We are here 19 Printing Ink jet printers use four inks: cyan, magenta, yellow, and black. These are often called CMYK printers, with K for black. Ink on the paper acts as a filter for light reflected from paper. Combinations of C, M, and Y can make all colors by filtration. Black is needed for darker blacks. High brightness paper is needed for good pictures. Six color printing adds additional inks to CMYK. Some use red and blue, others use light cyan and light magenta. Canon has an 11 ink printer. Watercolors work the same way. Putting on more results in more light being absorbed, so a brighter blue requires less watercolor and a darker one more. 20
Recall that inks act like filters A colored filter subtracts certain colors by absorption and transmits the rest = Incident white light Magenta filter subtracts green Cyan filter subtracts red Only blue gets through Halftone printing Halftones (black and white): The printing plate is covered with dots of different size with the bigger dots putting more ink than the smaller dots. Halftones (color) There is a different halftone printing plate for each of the subtractive primaries. High quality color printing (National Geographic, art books) use more than four inks. Print resolution is measured in dots per inch. More dots/inch = more detail. Typical inkjet: 600 dots per inch but 1200 to 4800 dots per inch (dpi) is possible 22
Halftone printing black and white Size of dots determines amount of black ink. Newspapers had 85 lines of dots per inch. Wikipedia Magnified view First half tone 1873 23 Halftone printing - color Where would I see this? Sunday comics. 24
Halftone printing - color Cyan = -red Magenta = -green Yellow = -blue This is the final printed product of adding the 4 above images. Magazines use this method. Wikipedia 25 Zoom in on a color half tone. 26
End - Ch. 9 - Color Spectral and non-spectral colors Intensity distribution curve Intensity, hue, saturation Additive primaries: R, G. B. Subtractive primaries: C, M, Y Hair, skin, and eye color CIE diagram Lighting, painting and printing We are here 27 Paint Paints: consist of pigment particles in a liquid vehicle. The vehicle may dry to a glossy or matte finish. The vehicle may be clear or it may contain dye. Dye is colored at the molecular level, no solid particles. Pigment flakes may be partially transparent and work by selective absorption or they may be opaque and work by selective reflection. The size of the particle makes a difference, with smaller particles transmitting more and absorbing less. Paints appear different (often brighter) when wet because a wet surface is glossy. 28
Surfaces, glossy and matte Photographs can have a glossy or matte finish and so can house paint. Paint can also be semi-gloss (looks frosted ). Glossy surfaces are smooth and part of the light is reflected specularly (like from a mirror) and part is reflected diffusely. Glossy surfaces look brighter. Wet surfaces tend to be glossy, so all paints are glossy when they go on and may become matte as they dry. 29 Chapter 10: Color perception We are here Trichromacy Metamers Psychological primaries Channels Color deficiency Animal vision Temporal processing 30
Trichromacy You have 3 kinds of color receptors (cones) Wavelengths: Short Intermediate Long We know this because we can find the wavelengths absorbed by the cones. Fig. 10.3 in textbook. 31 Metamers (again) Yellow (600) looks like red (650) + green (550) No mixture will look like blue or red. 32
Why blue+yellow looks white S+I+L stimulated is the same stimulation as from white light 33 How colors are perceived Blue excites S receptors Cyan excites S + I Green excites I mostly Yellow excites I + L Orange excites I + L Red excites L 34
Sensitivity of cones http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=rgcb 35