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)

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 WHITEPINKBROWN

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? ABC

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? ABC

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 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

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 AB

Color Trees saturation hue lightness 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 lightgreen lightred 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 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.