• Exam 2: November 8 th
– 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)

• What is color?
– Color vocabulary
• Color mixtures
– Intensity-distribution curves
• Specifying colors
– Hue, saturation and brightness
– Color trees
– RGB color specification
– Chromaticity

• 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

• 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)

• 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

• 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
PINK

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

• 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:

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

• Because one color may have several intensitydistribution 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

• Hue corresponds to the main color or color name
• Specified by the dominant wavelength in an intensity-distribution curve
Orange
Brown (same hue)
Blue

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

• Saturation corresponds to the purity of a color, the absence of other wavelengths in the mixture
Orange (saturated)
Orange (desaturated)
(orange + white)
Blue (desaturated)

Which intensity distribution is the most saturated?
A B C

• Brightness corresponds to the sensation of overall intensity of a color
Bright white
Grey
Black

• Brightness corresponds to the sensation of overall intensity of a color
Orange
Dark Orange
Brown

“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

“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 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

• 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

Light area in a black and white picture
Dark area in a black and white picture
Reflectance curve of a magenta shirt
Reflectance curves of different parts of a photograph

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

• When selecting mirrors for laser experiments, you care how well the mirror reflects the wavelength of your laser.

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

hue saturation
• 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

• How can we describe the sensation of a mixture of lights?
blue light green light red light

• If we combine the red and the green lights, what do we get?
• We get something that looks yellow

NOT additive mixing

“Yellow”
Spectral yellow Spectral red +
Spectral green
Non-spectral red +
Non-spectral green
• All of these produce the sensation of “yellow”
• We need a simpler way of mixing 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

• 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.