Chapter 3:

Colorimetry

How to measure or specify color?

Color dictionary?

Colorimetry

• Measurement of color and its faithful reproduction are important! Eye can only detect ~150 different colors.

• Appearance of color is determined by:

1. Hue: Main color characterized by dominant wavelength D.

2. Saturation: Purity of color characterized by width of the spectral curve.

3. Brightness: Overall intensity characterized by height of the spectral curve.

Intensity

(nm)D

Newton’s Color Wheel

• Simple colorimetric system that summarizes results from additive color mixing.

Original Modified to account for the purples

Newton’s Barocentric Color System

Features:

• Hues: 8 unequal segments.

• Saturation: Distance from the center. Purest colors lie on the periphery, center is white.

• Complementary colors lie diametrically opposite.

Review

Using Newton’s system predict the results of the following mixtures of colored light:

a) 2 Parts red and 1 part green.

b) Equal amounts of red, yellow and indigo.

c) Equal amounts of yellow and blue

3 parts unsaturated red-orange

White

Unsaturated green

Colorimetry

• Two approaches in use today:

Analyze a spectrum and assign numbers

(Tristimulus Value)

CIE System

Two spectra with same tristimulus values have

same color

Use standard color samples

Munsell, Ostwald systems

Samples arranged in many different ways

Color Matching

• Metamerism: Basis of color reproduction techniques.

• Metamers are two (or more) colors that appear identical, but have different spectra.

• Example: Metameric whites…Intensity

400 700 (nm)500 600

Intensity

400 700 (nm)500 600

B: 480 nmB: 480 nm Y: 580 nmY: 580 nm

http://www.cs.rit.edu/~ncs/color/a_game.html

http://www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/cs/exploratories/applets/spectrum/metamers_guide.html

Color Matching (Contd.)

• Color matching by a set of 3 additive primaries:

IR = Intensity of red primary of wavelength R

IG = Intensity of green primary of wavelength G

IB = Intensity of blue primary of wavelength B

• Note: Choice of R, G, and B is arbitrary.

• Let U be unit intensity of any wavelength

• Question: Can we match any in hue, saturation, and brightness by adding IR, IG, and IB?

• Answer: Any color, except saturated ones!

e.g. R + G = Unsaturated Y

• However, Unsaturated Y = Saturated Y + Little B!http://www.physics.utoledo.edu/~lsa/_color/21_chroma.htm

Color Matching (Contd.)

• Pure colors can be matched if one is allowed to subtract colors.

• Example: Saturated Y = R + G - Little B

• Therefore, for any wavelength

• One of IR, IG, or IB is negative, depending on the choice of

• Disadvantages:

• For many spectra, one of the primaries is negative.

• Choice of primaries is arbitrary.

UIR + IG + IB

Color Matching (Contd.)

• Calculate the relative intensities needed to match 500 nm.

• Answer: IR = -0.12; IG = 0.37 ; IB = 0.2

http://users.starpower.net/jworthey/jimtalk2004nov.html

The CIE System (1931)

• “Imaginary” primaries that can be derived from any system of real primaries. No negative numbers.

• Green primary has a curve that matches the visual sensitivity of the human eye. Apparent brightness is represented.

• Tristimulus values (X, Y, Z): Are the relative amounts of R, G, and B primaries needed to match a given spectrum. Value Y also represents the total brightness of light.

• Metameric pairs have the same (X, Y, Z) values.

COMMISSION INTERNATIONALE DE  L'ECLAIRAGEINTERNATIONAL COMMISSION ON ILLUMINATION

Color Matching Functions for the XYZ System

Tris

timul

us V

alue

Matches the visual sensitivity curve

ZYX

Yy

ZYX

Xx

:sCoordinatety Chromatici

Chromaticity Coordinates

• Tristimulus values: (X, Y, Z)

• Define coordinates:

• Note: x + y + z =1

• Hue and saturation are specified by (x, y) and brightness is specified by the tristimulus value Y.

• Complete description of a spectrum is given by:

(x, y, Y) or (D, p, Y)

ZYX

Zz

ZYX

Yy

ZYX

Xx

CIE Chromaticity Diagram

Graphical representation of chromaticity coordinates (x, y).

Spectrum Locus: All visible colorsare contained within this horse-shoe shaped curve.

Equal Energy WhiteE(1/3, 1/3)

Purple Line: Produced By mixing R & B.

CIE Chromaticity Diagram Features

•Colors on the spectrum locus are the most saturated.

•Colors closer to the center are unsaturated.

•Complementary colors lie on opposite sides of E.

•Dominant wavelength D of a source S is found by intersection of the line joining E, S, and the spectrum locus.

•Purity (or saturation) is given by: 100x DELength

SE Length Purity%

E

S

D

Review Problem

Tristimulus values of a spectrum are:

X = 11, Y = 7, Z = 4

• Find:

a) Color coordinates

b) Color region

b) Dominant wavelength

c) Purity

d) Complementary wavelength

(0.5, 0.32)

650 nm

About 41%

494 nm

Pink

Review Problem

• Estimate the complementary wavelength of each of the following:

• 460 nm

• 600 nm

• 520 nm

570 nm

Mixture of red (700 nm) and violet (400 nm)

488 nm

Colored Surface Analysis

• Reflected color depends on:

• Incident light spectrum

• Reflection characteristics of the surface

• Note: A surface could have different chromaticities depending on the light source.

• Standard sources adopted by the CIE:

SourceSourceColor Color

CoordinateCoordinatess

Color Color TemperatuTemperatu

rere

AA

(Incandescent (Incandescent Lamp)Lamp)

(0.43, 0.4)(0.43, 0.4) 2854 K2854 K

BB

(Noon Sunlight)(Noon Sunlight)(0.35, 0.35)(0.35, 0.35) 4870 K4870 K

CC

(Average (Average Daylight)Daylight)

(0.3, 0.32)(0.3, 0.32) 6700 K6700 K

Colored Surface Analysis (Contd.)

• Dominant wavelength and purity are found by replacing E by the source type (A, B, or C).

• Brightness is specified by “Y”. (Note: Same (x, y) pair can have different Y!)

• Yw : When a standard source illuminates a white surface.

• Ys : When the same source illuminates a given surface.

100x Y

YY

w

s

Review Problem

Following are the tristimulus values when a CIE source C, illuminates a

White surface: (35, 35, 30)

Colored surface: (30, 28, 10)

What are the chromaticity coordinates?

What is the dominant wavelength?

What is the purity?

What is the reflected brightness?

583 nm

~ 60%

(0.44, 0.41)

80%

The CIE System

• Reduces a spectrum to a set of numbers:

(X, Y, Z) or (x, y, Y) or (D, p, Y)

• Visual sensation can be different even if the set of numbers is the same!

• Under a given set of conditions, two different spectra with identical tristimulus values will appear identical.

• CIE system can only be used for color matching.

Application of the CIE System

What is all thisColor Management stuff anyway?

The problem with color is that different devices make it in different ways. Monitors produce light by

exciting red-, blue- and green-emitting phosphors by bombarding them with an electron beam. This is

called additive color. Printers use subtractive color. Light scattered from the paper makes it look white. Ink on the paper subtracts or absorbs certain colors and the remaining ones bounce off the paper back at you. For example, if the ink subtracts cyan and

magenta, yellow is what you see. It also means that if you change the lighting white will look more

yellowish or more bluish and if you print on pink paper getting any white is impossible. The end

result of all this is that the range of color devices such as a monitor, or a printer, can display is not the

same. The official name for this range is the color gamut of a device.

The Munsell Color Notation System (1915)

• Color classification is based on three perceivable qualities:

1. Hue: Vertical planes, each hue subdivided into segments.

2. Value: Brightness on a scale of 0 (Black) to 10 (White).

3. Chroma: Saturation on a scale of 0 (neutral) to 12 (saturated).

Notation example: Notation example: 5R5R 3 / 14 3 / 14

Is the color Is the color designation designation of red in the of red in the

US flagUS flag

http://www.learn.londonmet.ac.uk/packages/synthlight/visuals/chapter1/munsell.html

The Ostwald Color System (1917)

• Variables for a given hue are colored pigment (C), white pigment (W), and black pigment (B)

• C, W, B form the corners of a triangle and colors within are additive mixtures. B + W + C = 1

1. Isotones: Colors parallel to WC. Fixed B content.

2. Isotints: Colors parallel to BC. Fixed W content.

3. Isochromes: Colors parallel to BW. Fixed C content.

Comparison

• Advantage: Color grading and specification is easily done by direct perceptual comparison.

• Disadvantage: Color chips fade in time.

Munsell System

•Specific notation replaces loosely stated or unrelated color terms.•Room for newly discovered colors.

Ostwald System

•Useful for mixing paints and pigments.•Need to add a new page for newly discovered colors.