1 Ch.4: Cameras photography How cameras work; camera settings; Taking good pictures; Ch.5: Human Eye & Vision Eye & camera: similarities & differences How eye works & how image is formed; Ch.6: Optical instruments & vision correction Vision problems and correction; How eyeglasses & optical lenses work; Microscope and magnifying glass; Telescopes; Ch.7: Image processing by eye Lateral inhibition & temporal response; Illusions & afterimages. Summary of the material to be tested in the exam #2: Chapters 4-7 Exam: 11 multiple choice questions; Problems (2-3 per chapter); 1 Extra-credit problem. Nothing on Chapter 9 (color) Information/preparation: Exam assignment/solutions from 2007; Practicing problems: find answers; Reading Material; Help: additional office hours (F-521); Solutions will be posted on the web page soon after the exam;

An example of a possible exam question:

The instructor has eyeglasses of lens power -1.5D. Will he be able to see a student cheating in the opposite part of our lecture room? A. Yes; B. No. What if he forgets to bring his eyeglasses? A. Yes; B. No. Teaching assistant wears eyeglasses of lens power 2D. He came to do exam proctoring but forgot his eyeglasses. Will he be able to see a student cheating in the opposite part of our lecture room? A. Yes; B. No.

Chapter 8: Binocular vision & perception of depth: we skip this chapter now, but will learn the main points later (3D movies, etc.)

5 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) What happens when you add colored lights together (additive mixing)? Additive primaries (RGB) Spectral complements Partitive mixing of colored dots: TV How do color filters work (subtractive mixing) Water colors & printers inks Paints and pigments

What are spectral colors? Spectral colors are the color sensations we perceive in a rainbow or spectrum of a prism Demo with prism Other names for spectral colors Wavelength colors (because they each can be identified by one and only one wavelength) Monochromatic colors (mono means one, chromos means wavelength color) 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

Most colors that we see are mixtures of spectral (wavelength) colors. In order to specify a mixture of wavelength colors we need to describe how much of each one is present in the mixture The best way to do this is to plot a curve - the intensity-distribution curve Each point on the horizontal axis is a different wavelength color The vertical distance of the curve from x-axis tells us how much of that wavelength color is present in the mixture! You would see the intensity distribution if you passed this light through a prism! A pure spectral color would have an intensity distribution curve consisting of only one vertical line (at its wavelength) This is the intensity-distribution curve of light from a White fluorescent tube Most colors can be described by a (different) intensity-distribution curve

Intensity distributions for color filters

How we characterize colors: Hue, Saturation and Brightness (HSB, recall photoshop): what they mean in terms of intensity distribution curves? Hue is specified by the dominant wavelength color in the intensity- distribution curve Saturation is the purity of a color (absence of other wavelengths). The pure spectral colors are the most saturated 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 The two colors described by the two different intenstiy curves are called metamers

How is a reflectance curve different from an intensity distribution curve? Reflectance curve tells us how effectively part of a surface (a shirt, a part of a picture, a car, etc) reflects light of different wavelengths Reflectance curve of a magenta shirt (1 = 100%) Light and dark in terms of reflectance curves Light area in a black and white picture Dark area in a black and white picture magenta shirt

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

Hue, Saturation and Brightness (HSB): One way to use 3 numbers to specify a color instead of using an intensity-distribution curve 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

Continue Learning about perception of Color and color mixing

Exam # 2 was & extra credit assignments A. Trivial B. Relatively easy; C. Appropriate; D. Difficult; E. Overwhelmingly difficult; A. I will do extra credit projects to improve my grade (20points/project); B. I will not do extra credit projects;

Clicker Registration, etc. 8 students have not registered clickers!!! Clicker grades will be updated by Monday. Exam grades will be posted before Monday morning.

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 /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

Partitive mixing is another kind of additive color mixing but not achieved by superimposing colored lights! Instead, it works by putting small patches of colors next to each other. From a distance these colors mix just as though they were colored lights superimposed on each other Examples: Seurat pointillism Color TV and computer screens (Physics 2000) Photoshop example What is partitive mixing?

Subtractive mixing A color filter or colored object takes away certain wavelengths present in white light by absorbing them When light passes through a color filter or reflects from a colored object, 3 things can happen to the components of the light at each wavelength Transmission (light of a particular wavelength goes through and comes out the other side) Reflection (a particular wavelength is reflected) Absorption (a particular wavelength is soaked up by the filter or object and neither reflects nor is transmitted) Generally, different things happen at different wavelengths in the composition of the light for any particular filter or colored object Examples: Think of white light as a mixture of red, green and blue wavelengths An apple is red because when white light shines on it it absorbs (subtracts) blue and green wavelengths and reflects red wavelengths A colored filter is red because when white light shines on it absorbs (subtracts) blue and green wavelengths and transmits red wavelengths Superimposing filters and letting light go through the combination gives a very different result from superimposing the light passing through each of those filters separately It is a common misconception that a filter or an apple adds color to white light!

Color Filters

A colored filter subtracts colors by absorption. = Incident white light Only green gets through Cyan filter subtracts red Yellow filter subtracts blue

A colored filter subtracts certain colors by absorption and transmits the rest = Incident white light Magenta filter subtracts green Cyan filter subtracts red Only blue gets through

A colored filter subtracts colors by absorption. = Incident white light Magenta filter subtracts green Only red gets through Yellow filter subtracts blue

A Exam #2: Statistics/Results/Grades A A- B-,B, B+ C-, C, C+ Problems with grading: can be corrected within one week;

Exam # 2 was & extra credit assignments A. Trivial B. Relatively easy; C. Appropriate; D. Difficult; E. Overwhelmingly difficult; Extra credit projects to improve the grade (20points/project);

Plans for Today Plans: Finish Chapter #9 & continue with Chapter #10; Final Grade: Clicker questions – 5%; HWs – 20%; Exams – 75%; Demo on work of Displays and printers: use a microscope to see how different colors are obtained; Demo on spectral dependence of light coming from the display screen;

Clicker Registration, etc. 7 students have not registered clickers!!! Clicker grades will be updated by Thursday.

Demonstration

What is the effect of combining (sandwiching) different colored filters together? Rules for combining the subtractive primaries, cyan, yellow and magenta: White light passed through a cyan filter plus a magenta filter appears blue White light passed through a yellow filter plus a magenta filter appears red White light passed through a yellow filter plus a cyan filter appears green Why? cyan magenta yellow

A few words about the subtractive primaries cyan, yellow and magenta  Yellow is perceived in a very narrow band of wavelengths from 575 to 580 nm on the right edge of the horseshoe.  Cyan is perceived in a band of wavelengths from roughly 482 nm to 492 nm on the left edge of the horseshoe.  Magenta and its neighboring colors on the bottom straight part of the horseshoe cannot be found in the spectrum at any wavelength. However, in practice, any colored paper, picture, fabric or filter which is yellow or cyan is never a pure spectral color  Yellow, in practice, is always an additive mixture of the spectral green & red on either side of wavelength yellow  Cyan, in practice, is always an additive mixture of spectral blue & green on either side of wavelength cyan. Transmission distribution of filters Cyan is an additive mixture of mostly blues and greens Yellow is an additive mixture of mostly reds and greens

A transmittance curve shows what percent of each of the wavelengths in white light go through a filter Just as a reflectance curve shows what percent of each of the wavelengths in white light reflect from this magenta surface A transmittance curve shows what percent of each of the wavelengths are transmitted through this magenta filter Transmittance Reflectance

Non-ideal colored filter subtracts colors by absorption. Incident white light Orange-yellow gets through Non-ideal reddish filter subtracts some green and blue Transmittance Wavelength 100% Non-ideal yellow filter subtracts some blue, red and green Transmittance Wavelength 100%

Colored surfaces subtract certain colors by absorbing them, while reflecting others Magenta surface absorbs (subtracts) green. Green surface absorbs (subtracts) red and blue (magenta). White in Magenta out White in Green out

Green light on a magenta surface appears colorless because green is absorbed Magenta surface absorbs (subtracts) green. Green surface absorbs (subtracts) red and blue (magenta). Magenta light on a green surface appears colorless because magenta is absorbed Green in No color Magenta in No color

More examples of colored objects viewed under light which is not white Colored clothes or other colored object may change their apparent color and apparent lightness when viewed under light which is not white What does yellow look like under blue light? What does blue look like under blue light? What does yellow look like under yellow light? What does blue look like under yellow light? How can you predict the resulting color if you know the intensity distribution of the light source and the reflectivity of the colored clothes or other colored objects which are illuminated by that light source? Next slide explains how to do this

When looking at a colored object in a colored light source what is the resulting color? Rule: Multiply the intensity-distribution of the light source by the reflectance of the colored object to get the intensity distribution of the the illuminated object Example: Look at a magenta shirt in reflected light from a Cool White fluorescent tube. It appears grey (colorless) Confirm by multiplying the intensity distribution curve by the reflectance curve to get the new intensity distribution curve for the reflected light Cool white fluorescent bulb Magenta shirt How on earth do you multiply two curves? You multiply the two y-values at each x to get the new curve this number equals this number This number times How the shirt appears in this light

Printer's inks Reflected ray from top layer of printer's ink mixing additvely with transmitted- reflected-transmitted rays Magazine and newspaper images use subtractive mixing of cyan, magenta, yellow and black inks (CMYK color) plus the halftone process, which breaks up images into dots or other patterns The smaller the black dots the lighter the grey This enables additive mixing of the colored dots with each other and with the white paper to produce the lighter less saturated colors other than CMYK Paper beneath Printer's ink

Halftone Left: Halftone dots. Right: How the human eye would see this sort of arrangement from a sufficient distance or when they are small. Resolution: m easured in lines per inch (lpi) or dots per inch (dpi); for example, Laser Printer (600dpi)lines per inch

Color halftoning Three examples of color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern and finally how the human eye would observe the combined halftone pattern from a sufficient distance. Paper beneath Printer's ink

Demonstration

Color Liquid Crystal Displays (LCDs)

Concept Question: White is an equal mixture of red, green and blue. What is another metamer for white light? A. Red and cyan; B. Cyan, magenta and yellow; C. Blue and yellow; D. A,B, and C