• Exam 2: November 8th

– If you will need accommodations, please make sure you have documentation from the University Office of Disability Services

– It will cover Chapter 4 (cameras) through Chapter 9.5 (color mixing)

Chapter 9: Color• What is color?

– Color vocabulary

• Color mixtures– Intensity-distribution curves

• Specifying colors– Hue, saturation and brightness– Color trees– RGB color specification– Chromaticity

What is Color?

• Colors are sensations produced in our eye and brain• Color is a psychological phenomenon

• Color is not a property of light• The sensations of colors that we see cannot be

described by physics• We may not all see color in the same way

What is Color?• However there are a number of different

quantitative scientific ways of describing the properties of light which give our brain the sensation of a particular color

• These descriptions are needed by many different communities:– Art world, TV and computer technology, journalism

printing, film industry, scientific communities, agriculture (when to pick apples)

Color Vocabulary• Spectral colors are the color sensations we

perceive in a rainbow or spectrum of a prism• They are also known as:

• Wavelength colors (because they each can be identified by one and only one wavelength)

• Monochromatic colors (mono means one, chromos means wavelength color)

400 nm 700 nm

Color Vocabulary

• Most colors that we see are not spectral

• We are able to distinguish more than a million different colors

• Most of these are mixtures of different spectral colors

WHITE PINK BROWN

Color Mixing

For most of you, everything you know about color mixing was from this:

Color Mixtures

• How can we describe these mixtures of colors ?

• We can specify the amount of each color present in the mixture, and plot the result: this is called an intensity-distribution curve

• You may recognize this from our discussion of light bulbs:

Intensity-Distribution Curves

Both of these lights would appear white, so we can see there may be several intensity distributions that generate the same color

Specifying Colors

• Because one color may have several intensity-distribution curves, we don’t need the entire curve to specify a color.

• We can specify a color using just three numbers:

1. Hue 2. Saturation 3. Brightness

Specifying Colors: Hue

• Hue corresponds to the main color or color name

• Specified by the dominant wavelength in an intensity-distribution curve

Orange

Brown (same hue)

Blue

Concept Question: Hue

Which intensity distribution does not have the same hue as the other two?

A B C

Specifying Colors: Saturation

• Saturation corresponds to the purity of a color, the absence of other wavelengths in the mixture

Blue (desaturated)

Orange (desaturated)(orange + white)

Orange (saturated)

Concept Question: Saturation

Which intensity distribution is the most saturated?

A B C

Specifying Colors: Brightness

• Brightness corresponds to the sensation of overall intensity of a color

Bright white

Grey

Black

Specifying Colors: Brightness

• Brightness corresponds to the sensation of overall intensity of a color

Orange

Brown

Dark Orange

Intensity Curves for “Yellow”

• Here is an intensity distribution curve which gives us the sensation of yellow

• Here is a different intensity distribution curve which also gives us the same sensation of yellow

• The two colors described by the two different intensity curves are called metamers

Intensity Curves for “Yellow”

• Notice that in the second graph, the wavelength corresponding to spectral yellow is completely absent.

• It is not always obvious from the intensity curve what sensation will be produced by the light (what color it will “look” like)

Brightness vs. Lightness

• Brightness is the overall light level of a scene, and is related to the intensity of the light source

• Lightness is a property of a material surface and how much light it reflects

Brightness and Reflected Light

• The brightness of a part of a surface or small region of a picture depends on– the brightness of the light source (at each wavelength) – how much light that part of the surface reflects (at each

wavelength)

• Reflectance curves tell us how effectively part of a surface (a shirt, a part of a picture, a car, etc) reflects light of different wavelengths

Reflectance Curves

Light area in a black and white picture

Dark area in a black and white picture

Reflectance curve of amagenta shirt

Reflectance curves of differentparts of a photograph

Earth-orbiting satellites take pictures of Earth at different wavelengths to determine the health of crops and water

Mirror Reflectance• When selecting mirrors for laser experiments, you

care how well the mirror reflects the wavelength of your laser.

Mirror Reflectance

Which mirror is a better choice if the laser wavelength is 500 nm?Mirror “A” will reflect more light, so is a better choice

A B

Color Trees

saturationsaturation

huehue

light

ness

light

ness

• Color tree (e.g. Fig. 9.5 in book)• Moving up the tree increases the

lightness of a color• Moving around a circle of given radius

changes the hue of a color• Moving along a radius of a circle changes

the saturation (vividness) of a color• These three coordinates can be described

in terms of three numbers

Additive Color Mixing

• How can we describe the sensation of a mixture of lights?

blue light green light red light

Combining Intensity Distribution Curves

• If we combine the red and the green lights, what do we get?

• We get something that looks yellow

Mixing Colors

NOT additive mixingNOT additive mixing

Intensity Curves for “Yellow”

• All of these produce the sensation of “yellow”

• We need a simpler way of mixing colors

Spectral yellow Spectral red +Spectral green

Non-spectral red +Non-spectral green

Additive Primary Colors

• Many (but not all) colors can be described in terms of the relative intensities of a light mixture of a certain wavelength red, wavelength green and wavelength blue lights• 650-nm red• 530-nm green• 460-nm blue

• These are called the additive primaries

650-nm red

530-nm green

460-nm blue

Note on Primary Colors

• Why aren’t the primary colors red blue and yellow?

• You want to select primary colors that allow you to combine them to create the largest possible range of colors, called “gamut”

• Different ways of mixing colors have different sets of primaries. We will return to this several times.