Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 21

2 Chapter 9: Color How to identify and classify color Wavelength colors Intensity distribution curve Hue, saturation and brightness "Color tree" or "cylinder" Chromaticity diagrams and how to use them Physics 1230: Light and Color Chapter 9 (with some material from Chapter 8)

Continue Learning about perception of Color and color mixing

Primaries = cone responses = “red”, “blue”, “green” Cones are responsive to range of wavelengths red (Long), green (Medium) and blue (Short) Stimulate more than one cone type = various colors (purple, yellow…)

What are spectral colors? Spectral colors are the color sensations we perceive in a rainbow or spectrum of a prism Other names for spectral colors Wavelength colors Monochromatic colors Examples: 650 nm red, 530 nm green, 460 nm blue We are able to distinguish more than a million different colors Most colors that we see are not spectral colors. Examples: White Pink Brown VioletBlueGreenYellow OrangeRedInfrared 400 nm460 nm530 nm 575 nm 600 nm650 nm

Intensity distributions for color filters

How we characterize colors: Hue, Saturation and Brightness (HSB, recall photoshop): Hue is specified by the dominant wavelength color in the intensity- distribution curve Saturation is the purity of a color (absence of other wavelengths). Brightness refers to the sensation of overall intensity of a color Bright white Grey Black Brightness Orange Brown (same) HueSaturation Blue Desaturatated orange = saturated orange + white

The same color sensation can often be produced by 2 or more different intensity distribution curves 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

Hue, Saturation and Brightness (HSB): 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 Photoshop: uses H, S and B lightness hue saturation

Red, green and blue (RGB): RGB is another way to use 3 numbers to specify a color instead of using an intensity-distribution curve or HSB In addition to using Hue, Saturation and Brightness (HSB); 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 The mixing of the additive primaries is called additive mixing Additive mixing is usually done by mixing primary color lights with different intensities but there are other ways to be discussed later Demonstrate with Physics 2000 cyan magenta yellow 650-nm red 530-nm green 460-nm blue 0/tv/colortv.html

Complementary additive colors Definition of complementary color (for additive mixtures): The complement of a color is a second color. When the second color is additively mixed to the first, the result is white. Blue & yellow are complementary B + Y = W. Green & magenta are complementary G + M = W Cyan and red are complementary C + R = W Magenta is not a wavelength color— it is not in the rainbow There is at most one wavelength complementary color for each wavelength color (Fig 9.9) white cyan red magentagreen yellow blue

Additive mixing of colored light primaries Blue added to green = cyan. Green added to red = yellow. Red added to blue = magenta.

Complementary colored lights (additive mixing) Blue (primary) and yellow. Green (primary) and magenta. Red (primary) and cyan.

Chromaticity diagrams: Yet another way to represent colors by (3) numbers The chromaticity diagram is in many ways similar to a color tree A chromaticity diagram has a fixed brightness or lightness for all colors Wavelength colors are on the horseshoe rim but non-wavelength colors like magenta are on the flat part of the rim Inside are the less saturated colors, including white at the interior less saturated colors saturated wavelength colors saturated non-wavelength colors

Different lightnesses are on other chromaticity diagram "slices" along the trunk of a chromaticity "tree" Lightness (z) hue saturation hue saturation Chromaticity "tree" 3 numbers (x, y, z) specify a color color tree x y

We will now learn the many uses of a chromaticity diagram To identify colors with three numbers To predict the results of additive mixing To understand complementary colors To find the dominant hue of a color

Using the chromaticity diagram to identify colors The numbers that we use to identify a color are its x-value and y-value inside the diagram and a z-value to indicate its brightness or lightness x and y specify the chromaticity of a color Example: Apple pickers are told around the country that certain apples are best picked when they are a certaim red (see black dot) Since the chromaticity diagram is a world standard the company can tell its employees to pick when the apples have chromaticity x = 0.57 y = 0.28 The "purest" white is at x = 0.33 and y = 0.33 Chromaticity diagram can be related to colors in Photoshop

Using the chromaticity diagram to understand the result of additive mixing of colors An additive mixture of two wavelength colors lies along the line joining them Example: The colors seen by mixing 700 nm red and 500 nm green lie along the line shown Where along the line is the color of the mixture? Answer depends on the relative intensities of the 700 nm red and the 500 nm green. Here is what you get when the green is much more intense than the red (a green) Here is what you get when the red is much more intense than the green (a red) Here is what you get when the red is slightly more intense than the green (a yellow) Note — this works for adding two colors in middle also!

Using the chromaticity diagram to understand complementary colors The complement to any wavelength color on the edge of the chromaticity diagram is obtained by drawing a straight line from that color through white to the other edge of the diagram Example: The complement to 700 nm red is 490 nm cyan Example: The complement to green is magenta - a non- wavelength color

Using the chromaticity diagram to find the dominant hue of a color in the interior of the diagram To find the dominant hue of the color indicated by the black dot Draw st. line from white through the point to get dominant wavelength, and hence, hue (547 nm green) Works because additive mixture of white with a fully-saturated (wavelength) color gives the desaturated color of the original point

Announcements Good job on the 2 nd Midterm Exam; Exam Results Posted on Wednesday; Can ask the grader on Thursday; HW #6 assigned today; Finish Chapter 9 today; Science Lab Tour on Thursday; Move to Chapters 10, 11, and 13 after the Fall break;

Science Lab Tour Meet on Thursday in Class at the regular time; We will have demonstrations in the classroom; Class will be split to ~4-5 groups to visit the labs; Demonstrations available in the classroom; Cool things to try and learn about in the lab (like moving things with light without touching);