What is Color?

Color is related to the wavelength of light. If a color corresponds to one particular wavelength, this is called spectral color.

=600 nm corresponds to a color, but naming it is not an exact science…. orange chrome, golden poppy, spectrum orange,

bitter sweet orange, oriental read, Saturn red, cadmium red orange, red orange…

A particular color sometimes corresponds to a range of wavelength Blue: 455 ~ 485 nm Green: 500 ~ 550 nm Yellow: 570 ~ 590 nm Red: ~ 625 nm

Most of the time, however, we see non-spectral colors. We can see a color even when the wavelength of the corresponding spectral color is absent.

There is more to color than meets the eye!

Intensity Distribution

Most of the colors we see are not spectral colors, but instead have a distribution of intensity in wavelength (composite color).

For example, a color green may contain all other colors but with the intensity peaked at the wavelength 500 nm.

Our eyes cannot distinguish the colors that are composite from spectral colors. (different

distributions may correspond to the same color!)

White light reflected from A greenish region.

Classification of colors

Hue: main color. The dominant wavelength in an intensity distribution curve.

Saturation: purity of the color. Measures how much dominant wavelength there is

compared to the amount of white mixed in. Saturated: all intensity fairly close to the dominant

wavelength (spectral color is 100%) White: complete unsaturated

Saturation

White: unsaturated saturated red

Less saturated red

Lightness of surfaces Lightness has to do with the percentage of

incident light reflected at each wavelength. (white: a lot of reflection; black: no reflection)

Color tree:

The same position has the same color independent of the spectral intensity! Trunk completely unsaturated. Out from the trunk,

the degree of saturation increases. The vertical axis is related to lightness (white at

the top and black at the bottom. The hue varies around the tree (green,blue…).

Simple color additive rules

Mixing the broad distributions of green and red yields yellow. Although the resulting spectral distribution is very different from spectral yellow (see 9.6g)

If one mixes roughly equal amounts of monochromatic green and red, the result also looks yellow, although the spectral yellow is completely absent. (see 9.6h)

Two colors which look alike even though they have different intensity distribution curves, are call metamers.

Mixing blue and green yield cyan. Mixing red and blue yields magenta. Add all color together yields a flat intensity

distribution which is white. (see 9.6i)

Complementary Colors

We know that

B+G+R = W We know also G+R = Y Therefore: Y+B = W. And we call Y and B

complementary colors. C is complementary to R M is complementary to G.

Some spectral colors have complementary spectral colors. For instance, the complementary of the orange (600nm) is bluish cyan.

Not all spectral colors have complementary spectral colors. For instance, the complementary of green has to be a double humped distribution.

Chromaticity Diagram

Question: if given spectral blue, green and red, can any color (hue, saturation, brightness) be matched by mixing them?

Almost! You cann’t match spectral cyan.

If you choose as your three colors: red (650nm), green (530nm), and blue (460nm) the relative amount of mix to get the spectral color is shown in Fig. 9.10

Another way is to see this is the horse-shoe type of diagram, with three colors at three different positions.

The straight-line between the two colors represents all the colors that one can get from the mixture of the two.

All the points in the triangle represents all the colors that one can make with three primary colors.

The spectral colors full outside of the triangle (negative mixing).

The complementary of any color is found by extending a straight line from that color through white and to the opposite side of the horseshoe.

CIE Chromaticity diagram

Invent three imaginary colors: x, y, z. Then using these colors, one can get all colors with positive amount of mixing. The imaginary x consisting of 150% red, and a

negative 50% green. For any color, the relative amounts add to 1.

However, all possible colors are still within the horse shoe inside of the triangle.

Relative amounts of [x],[y],and [z] needed to matchA give spectral color