Basic Color Theory Susan Farnand [email protected].

Basic Basic Color Color TheoryTheorySusan Farnand [email protected]

Tell me about color:

How would you define color?

How does color happen?Where does color

happen?

Why do things look colorful?

At least two things needed for color to happen:

A source…

…and a sensor

Visible Light SpectrumVisible Light Spectrum

400 nm

700 nm

Light Sources Compared

0

50

100

150

200

250

300

350

300 400 500 600 700 800

Wavelength, nm

Rel

ativ

e S

pec

tral

Po

wer

typical daylight

incandescent

mercury vapor lamp

6500K fluorescent

Basic Structure of the Human

Eye

Schematic of the Retina

PhotoreceptorsHorizontal, bipolar, and

amacrine cellsGanglion cellsIncoming light

Relative Spectral Sensitivity of Cones & Rods

0

50

100

350 450 550 650 750

Wavelength, nm

Rel

ativ

e S

ensi

tivi

ty

S rods M L

Rods Cones

S R M L

Spectral sensitivity

Color Mixing–Color Mixing–AdditiveAdditive

Adding light Mixing light sources Red, Green, Blue primaries

Additive Color Mixing:

Color Mixing–Color Mixing–AdditiveAdditive

Adding light Mixing light sources Red, Green, Blue primaries Can you think of examples of

systems using additive light

Color Mixing– Subtractive Removing light Mixing dyes, pigments or other colorants Cyan, magenta, & yellow primaries Can you think of examples of systems using subtractive light?

What happens to the What happens to the light?light?

Transmitted

Absorbed

Reflected

Subtractive Color Mixing

Measuring ColorMeasuring Color

Are these two patches the same?

Reflected Light

0

50

100

300 350 400 450 500 550 600 650 700 750 800

Wavelength, nm

Ref

lect

acn

e

Sensation of Color

Light Source

Object Observer

Light Sources Compared

0

50

100

150

200

250

300

350

300 400 500 600 700 800

Wavelength, nm

Rel

ativ

e S

pec

tral

Po

wer

typical daylight

incandescent

mercury vapor lamp

6500K fluorescent

Cone mosaic data from experiment by Hofer, Singer, and Williams (2005). Different sensations from cones with the same photopigment. Journal of Vision, 5(5):5, 444–454.

CIE Commission Internationale de

l’Eclairage (International Commission on Illumination), an international

organization that establishes and maintains standards of light and color. Its system of describing color is based on standardization of illuminants and

observers, not physical samples.

CIE Illuminants

0

50

100

150

200

250

300

300 400 500 600 700 800

Wavelength, nm

Rel

ativ

e S

pec

tral

Po

wer

Illuminant A

illuminant C

illuminant D50

illuminant D65

illuminant D75

Color Matching Experiment

Color Matching Functions

Color Matching Experiment

Color Matching Functions1931 Standard Observer

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

380 480 580 680

Wavelength, nm

CIE 2° x

CIE 2° y

CIE 2° z

Computing CIE Tristimulus Values

Illuminant Object Observer

The CIE tristimulus values X, Y, and Z are obtained by multiplying together the power (P) of a CIE standard illuminant, the reflectance (R) of the object, and the standard observer functions and then summing the products.

Computing CIE Tristimulus Values

The CIE tristimulus values X, Y, and Z are obtained by multiplying together the power of a CIE standard illuminant, the reflectance of the object, and the standard observer functions and then summing the products.

Doing the math…

Colorimetric MatchColorimetric Match

Colorimetric match is defined as when the tristimulus values for two stimuli match:

X1 = X2

Y1 = Y2

Z1 = Z2Sample 1 Sample 2

Metamerism

Two colored patches that have different reflectance curves may appear the same when viewed under one illuminant, but may appear different when viewed under a second illuminant.

0

2

4

6

8

10

350 450 550 650 750

Wavelength (nm)

Rela

tive

Sensi

tivi

tyPatch 1

Patch 2

Colorimetric MatchColorimetric Match

Colorimetric match is defined as when the tristimulus values for two stimuli match:

X1 = X2

Y1 = Y2

Z1 = Z2Sample 1 Sample 2

Chromaticity Values and the Chromaticity Diagram

The CIE tristimulus values X, Y, and Z are can be transformed to chromaticity values (x, y) and displayed on a unit plane is known as the chromaticity diagram.

Chromaticity values are calculated by:x = X/(X+Y+Z), y = Y/(X+Y+Z)

It is also possible to calculate a chromaticity for z, wherez=Z/(X+Y+Z). Since x+y+z = 1, it is redundant (i.e. z=1-x-y)

Chromaticity Diagram

Color Differences on the 1931 CIE Chromaticity Diagram