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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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