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It’s A Colorful Life

Dr. Larry Woolf

Larry.Woolf@ga.com

www.sci-ed-ga.org

General Atomics

Presented 3/24/07 to

BEWiSE students

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Why study color?• Color is multidisciplinary and interdisciplinary –

involving physics, chemistry, biology, technology, engineering, mathematics

• Color mixing is the basis for much color display technology

• A wide variety of models and methods are used, so it provides an interesting educational experience

• Color is colorful!• Most books are inconsistent/incorrect – don’t trust

everything you read! Provides interesting lesson in “truth.”

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Addition and Subtraction

• Suppose you are limited to numbers from 0 to 100.

• Starting at 0, how do you get to 70?

• Starting at 100, how do you get to 70?

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Bar chart addition

0

100

R G B

Suppose you start with 3 bar charts that are empty (all at 0)

By addition, how could you end up with this result?

0

100

R G B

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Bar chart subtraction

0

100

R G B

Suppose you start with 3 bar charts that are full (all at 100)

By subtraction, how could you end up with this result?

0

100

R G B

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What do you know about color?

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What is meant by “primary colors?”

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• You can make “all” other colors – (not really true but OK to say – 3 primary colors can actually produce

about 50% of the colors that can be seen)

• You can’t make a primary color by mixing

What is meant by “primary colors?”

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Using your colored films, let’s do the experiment: Are the primary colors red, yellow, blue?

• What colors can you make by mixing red, yellow and blue?

• What colors can you make by mixing cyan, magenta, and yellow?

• Which set of 3 produces the largest range of colors?

• Can you make any of these “primary colors” by mixing?

• What are likely candidates for the 3 primary colors? What cannot be the primary colors?

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Using your colored films, let’s do the experiment: Are the primary colors red, yellow, blue?

• What colors can you make by mixing red, yellow and blue films?– Mixing red and blue makes black– Mixing red and yellow makes red– Mixing yellow and blue makes black

• What colors can you make by mixing cyan, magenta, and yellow films?– Red, green, and blue

• Which set of 3 produces the largest range of colors?– Cyan, magenta, and yellow

• Can you make any of these “primary colors” by mixing?– Yes, you can make red by mixing magenta and yellow– Yes, you can make blue by mixing magenta and cyan

• What are likely candidates for the 3 primary colors? – Cyan, magenta, and yellow

• What cannot be the primary colors?– Red, yellow, and blue because you can make red and blue by mixing 2 other

colors and because you can’t generate a wide range of colors using red, yellow, and blue

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Let’s learn more about how we see color

Basic simplifying assumptions:1. The color we see results from light of that color entering our eye.2. This room is illuminated by uncolored (white) light

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Absorption of light by colored films

• Place C film over color wheel on white paper– C film absorbs what color of light?

• Place M film over color wheel on white paper– M film absorbs what color of light?

• Place Y film over color wheel on white paper– Y film absorbs what color of light?

• Place C, M, Y films on top of each other over color wheel on white paper– What happens? What does this mean?

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Absorption of light by colored films

• Place C film over color wheel on W paper– C film absorbs R light

• Place M film over color wheel on W paper– M film absorbs G light

• Place Y film over color wheel on W paper– Y film absorbs B light

• Place C, M, Y films on top of each other– All light (white light) is completely absorbed by the R

light absorber,G light absorber, and B light absorber

How can these observations be written mathematically? (R is red light, G is green light, and B is blue light and W is white light) See next page for guidance…

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Consider the cyan film on white paper

• When cyan film is placed on white paper…– What color light do you start with?– What color of light is subtracted?– What color light remains after the subtraction?– How can you write this mathematically?

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Color math

W W W C

W – R = C

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Consider the magenta film on white paper

• When magenta film is placed on white paper…– What color light do you start with?– What color of light is subtracted?– What color light remains after the subtraction?– How can you write this mathematically?

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Color math

W M

W – G = M

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Consider the yellow film on white paper

• When yellow film is placed on white paper…– What color light do you start with?– What color of light is subtracted?– What color light remains after the subtraction?– How can you write this mathematically?

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Color math

W Y

W – B = Y

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Place cyan, magenta, and yellow films on top of each other

• What happens and why?• How do you describe this mathematically and

pictorially?• What does white light consist of?

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Color math

W

W – R – G – B = 0

W = R + G + B

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Alternate model

W – R – G – B = 0

W = R + G + B

Each colored film subtracts a primary color of light: hence C,M,Y are called the subtractive primaries

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Place a cyan film over a magenta film

What color of light do you start with?

What colors of light are subtracted?

What color of light remains?

How can you describe this mathematically?

How can you describe this pictorially?

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Color math

(R +G +B) – R – G = B

B

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Now use an alternate pictorial model to show what happens:

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Alternate pictorial model

(R +G +B) -R = G +B

-G(G +B) = B

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What color results from each pair of colored film?

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What color results from these pair of colored film?

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What is the one big idea that determines color?

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• Color is determined by light absorption• More generally, you will learn in subsequent

physics classes the following big idea:

When light interacts with matter, it can be reflected, absorbed, or transmitted

What is the one big idea that determines color?

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Color mixing

• We found that mixing cyan and magenta films made a blue film

• Mixing cyan film and yellow film makes a green film

• Mixing yellow and magenta films makes a red film

Now let’s make a model that describes these results

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Color Wheel Model for Subtractive Colors

Y

M C

What colors are between each of the subtractive primaries?

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Color Wheel Model for Subtractive Colors

R

Y

M

B

G

C

Now let’s deconstruct the model in terms of cyan, magenta, and yellow components

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R

Y

M

B

G

C

Deconstruct the model in terms of cyan, magenta, and yellow components

Now, how could you make this “real?”

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Put them together and see what happens- Do you make a color wheel?

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Color Wheel Model for Subtractive Colors

R

Y

M

B

G

C

What are the limitations of this model?Does it show all the possible colors?Does this model explain how our eyes see color?

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So What?• Let’s see what subtractive color mixing is

good for:

• Look at the color gradient strips and overlay the C, M, Y, and K (K is the letter used to represent black) strips to make different colors. Can you make more colors than the original films?

• Take a look at the colored magazines using the handheld microscope.

• How are colored pictures made?

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Other color models

• Color Cube

• HSV (Hue/Saturation/Value) model– Color strips

• Each has same Hue• Each square on a strip differs in color

Saturation– Placing a K square under any color changes the

Value

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Let’s look at a cyan film from a different perspective

(R +G +B) -R = G +B

We see this color as cyan, so cyan light is entering our eye

So C = ?

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Let’s look at a cyan film from a different perspective

(R +G +B) -R = G +B

We see this color as cyan, so cyan light is entering our eye

So C = G + B

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Let’s look at a magenta film from a different perspective

(R +G +B) -G = R +B

We see this color as magenta, so magenta light is entering our eye

So M = ?

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Let’s look at a single colored film from a different perspective

(R +G +B) -G = R +B

We see this color as magenta, so magenta light is entering our eye

So M = R + B

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Let’s look at a yellow film from a different perspective

(R +G +B) -B = R +G

We see this color as yellow, so yellow light is entering our eye

So Y = ?

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Let’s look at a single colored film from a different perspective

(R +G +B) -B = R +G

We see this color as yellow, so yellow light is entering our eye

So Y = R + G

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We just developed the rules for mixing colors of light (additive

color mixing)!

• W = R + G + B• C = G + B• M = R + B• Y = R + G• R, G, B light sources used to generate wide range

of colors for color displays

Now let’s make a model that describes these results

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Let’s now confirm these rules for additive color mixing using 2 light sources (slide projectors)

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Color Wheel Model for Additive Colors

R

B

G

What colors lie between them?

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Color Wheel Model for Additive Colors

R

B

G

M

Y

C

The same as the color wheel for subtractive colors!

The color cube is also the same – just different primaries!

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Why was this slide used at the beginning of this presentation?

Bar chart addition

0

100

R G B

Suppose you start with 3 bar charts that are empty (all at 0)

By addition, how could you end up with this result?

0

100

R G B

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Why was this slide used at the beginning of this presentation? Bar chart subtraction

0

100

R G B

Suppose you start with 3 bar charts that are full (all at 100)

By subtraction, how could you end up with this result?

0

100

R G B

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How do colors of hot objects change with increasing temperature?

Increasing energy

Infra-red Visible Ultra-violetRed Green Blue

Blue

Increasingtemperature ofstar or object

Blac

Yellow

White

Cyan

e

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Infra-red Visible Ultra-violetRed Green Blue

Blue

Increasingtemperature ofstar or object

Black

Yellow

White

Cyan

Red

Application of additive color mixing

Increasing energy

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Now let’s look and see how colors are produced on a

computer monitor for another example of additive color mixing

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Blue sky

• The Sun is a source of white light.

• The sky scatters blue light a lot more than red or green light.

• Draw a diagram and explain the color of the Sun and sky

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Dark red

Light red

Light red spectrum

700nm 600nm 500nm 400nm

How are other colors made, such as lights and darks?

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Other Simple Color Models• Color cube

– Sold as a model for C, M, Y – subtractive color mixing

– Also a model for R, G, B – additive color mixing

– Note that color cube is a more complete model than the color wheel

– Distance between color coordinates quantifies color differences

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Distance between color coordinates

quantifies color differences • Use color wheel as an example

• Use color cube as an example

So both color and color differences can be quantified mathematically.

This is critical for making colored products and for determining color consistency, e.g. what color differences during production are acceptable.

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Theory of Color Vision and Implications

• Red (R), Green (G), and Blue (B) cones– R(L-long wavelength), G(M-mid wavelength), B(S-short wavelength)

• 3 cone responses are processed to become 3 color receptive responses– R+G yields brightness and perception of yellow

– R-G yields red or green response

– (R+G)-B = Y-B yields yellow or blue response

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R G B

Sensation of yellow and sensation of lightness

+

Sensation of red or green

-

Sensation of yellow or blue

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Diagram of Human Color Vision

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Theory of Color Vision and Implications

• We see 4 distinct colors: R, G, Y, B - basis for L*a*b* color space

• We do not see reddish greens or yellowish blues• The blue cone does not contribute to brightness so

images that only differ in brightness of blue are difficult to see. This is why blue is hard to see at night and why blue on black is a terrible choice for web pages. See example of colors printed at different lightnesses.

61M-250(3)4-17-01

A slice of the L* a* b* color system

62M-250(4)4-19-01

L*a*b* COLOR SPACE

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Let’s see what are the primary colors according to expert sources

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Authoritative approach

Webster’s New World Dictionary:

“color: the primary colors of paints, pigments, etc. are red, yellow, and blue, which, when mixed in various ways, produce the secondary colors (green, orange, purple, etc.)”

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The gray scale approach(neither black or white)

Art Fundamentals Theory and Practice:

“There are three colors, however, which cannot be created from mixtures; these are the hues, red, yellow, and blue. They are called the primary colors.

A mixture of the three primaries should theoretically result in white; actually this mixture produces a neutral grey which may be considered a darkened form of white.”

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The 2 correct answers approach

The Journal of Chemical Education:“… students should identify the three colors needed to produce all the others as red, blue, and yellow. Most artists call these the fundamental colors, The correct subtractive colors, used by printers, for example, are cyan, magenta, and yellow.”

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The loosely speaking approach

Hewitt’s Conceptual Physics

“For this reason, cyan, magenta, and yellow are called the subtractive primary colors. In painting or printing, the primaries are often said to be red, yellow, and blue. Here we are loosely speaking of magenta, yellow, and cyan.”

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Not much agreement or consistency in the meaning of

primary colors!Even the “experts” are confused!Let’s look at some other books and see how they discuss color

ideas.

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Conclusion

• Primary colors of painting are not R, Y, B; the primary colors of painting are C, M, Y

• Subtractive color mixing using CMY generates a wide variety of printed colors by absorbing light

• Additive color mixing using RGB generates a wide variety of colors of light

• Many models to describe additive and subtractive colors including color wheel, color cube

• Many ways to investigate additive and subtractive color mixing