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Document info 8.
ColorTuesday, 9/19/2006
Physics 158Peter Beyersdorf
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Class Outline
What is color?
How do we perceive color?
Color generation
Additive processes
Subtractive processes
The gamut
Interference, dispersive and scattering processes
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What is color?
Color is an aspect of human perception
Most humans perceive visible light of decreasing wavelengths as red, orange, yellow, green, blue and violet
light with a wavelength of 589 nm will appear yellow
There are other ways to create color though
light with two components of wavelengths 632 nm and 532 nm will also be perceived as yellow, even though the individual components would be perceived as red and green respectively
The wavelength of light is to color as …
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Perception of Color
What color are the center squares?
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What color are the center pieces?
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Perceiving Color
The retina of the eye has rods and cones that detect light
Rods are highly sensitive to all visible wavelengths of light (i.e. they see B&W)
Cones are sensitive to different spectral regions so they can see color, but they are lower resolution and require more light
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Spectral Responses
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response of green cones
response of blue cones
response of red cones
response of rods
lens of the eye
typical red phosphor
typical green phosphor
typical blue phosphor
HeNe Laser
Doubled Nd:YAG laser
Blue Ar ion laser line
Cyan pigment
Magenta Pigment
Yellow pigment
reflection spectraabsorption spectra emission spectra
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Perceiving Color
The signals from the rods and cones provide 4 pieces of information that get processed into a color by our brain R, G, B, W
There are only 3 linearly independent signals since W=R+G+B
7R,G & B combinations + 0.67 W R,G & B combinations + 0.33 W R,G & B combinations + 0 W
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Color Generation
A range of colors can be created by adding light of various colors together (as in an LCD or CRT display)
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A range of colors can be created by subtracting light from white light (as in painting or printing)
R+G→YG+B→CB+R→M
B+R+G→W
W-B→YW-R→CW-G→M
W-(R+G+B)→Bk
W-Y→?W-C→?W-M→?
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Color Generation
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A butterfly wing’s color comes from interference
Colors in a soap bubble come from interference
Colors of a rainbow are separated from white light by dispersion in water
The blue sky and the red sunset are due to scattering of short wavelengths by the
atmosphere
Colors from a prism are separated by white light by dispersion in glass
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Soap Bubble Example
Find the last color seen at the top of a soap bubble before it pops
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Er ! rtopEi + t2toprbottomEieik0(2nd/ cos !t)
!OPL = 2nd/ cos !t
dn
θt
The reflected field is maximized when k02nd/cosθt=π+2πm (why not just 2πm?)
As d→0 the last reflected maximum occurs for blue light which has the higherst value for k0=2π/λ since λ is the shortest of the visible wavelengths
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Color Space and Gamut
The gamut of colors that can be produced by adding light depends on how well the various components can selectively excite the various cones without cross talk.
The spectral sensitivity of the cones overlap so it is impossible to have perfectly separated signals, however a laser or other single frequency (monochromatic) can come as close to this as possible.
A color space chart can be used to illustrate the meaning of the color gamut. This 2D chart is part of a larger 3D color space
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Color Space and Gamut
12image credits: Adobe
When color information is passed from one device to another it must be interpreted in terms of the gamut of the new device. IT may not be possible (or desirable) to exactly reproduce the color on the new device
Different imaging devices use different color spaces, RGB with various phosphors, CMYK with different pigments, etc.
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Example of Color Spaces
“ColorSync” profiles stored on my computer
13Note: See Apple Colorsync Utility (Profiles, Luv) as an example of colorspaces or the “display” control panel to try different colorspaces on a Mac’s display
NTSC television standard Canon MP780 CMYK color printer
KODAK photo CD standard
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RGB, HSV, CMYK, etc.
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Colors can be specified in a variety of ways:
RGB - values for the Red, Green, and Blue light components added together
CMYK - values of Cyan, Magenta, Yellow and blacK subtracted from white
HSB - Hue (angular position along color space), Saturation (radial position in color space) and Brightness (amount of light)
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Sony’s RGBE CCD
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Color in Television
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What is the test pattern of colorbars for?
How could you ensure the R, G, and B guns are calibrated to produce equal intensity?
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Colorbars for Calibration
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NTSC video
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Video signal is constrained
6 MHz channel separation and 2 MHz channel buffers
Backwards compatible with “old” Black and White displays
300 lines of horizontal luminescence resolution
about 50 lines of chromatic resolution
Video signal components
Y Luminance = (Red x 0.30) + (Green x 0.59) + (Blue x 0.11)
Q Signal = (Red x 0.21) - (Green x 0.52) + (Blue x 0.31)
I Signal = (Red x 0.60) - (Green x 0.28) - (Blue x 0.32)
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ATSC video
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Video signal is constrained
6 MHz channel separation - very little channel buffer necessary due to modern signal separation techniques
up to 1920x1080 pixel resolution in MPEG 2 encoded signal
MPEG-2 compression algorithm
Separate signal into luminecence (Y) and 2 chromatics signals (Cb, Cr)
Typical encoding has 4 8x8 pixel Y-blocks per “macroblock” and one each of the Cb and Cr-blocks. This is called 4:2:0 format
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Color SubsamplingMost compression formats subsample chromatic information, since the eye is less sensitive to color than to brightness
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The effects of color subsampling on colorbars in DV encoded video
4:1:1 sampling detail
4:2:0 sampling detail
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“Digital Color”
“DLP” chips use filtered white light to generate color, the intensity of which is set by the duty cycle of the movable micro-mirrors on the chip, allowing “digital color”
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Summary
Color is a consequence of human perception
Additive coloring mixes colors from the periphery of a color space to generate colors within that color space
Red, Green and Blue form a basis that captures much of the color space
Subtractive coloring removes colors from white light
Cyan, Magenta and Yellow form a complementary basis to Red, Green and Blue additive colors
The limited sensitivity and resolution of the human eye to color has lead to various techniques to compress visual information by reducing the amount of chrmoatic data
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