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:http://www.colorado.edu/physics/phys1230/phys1230_fa08/Exams.htm

• 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:

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 ColorChapter 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

Violet Blue Green YellowYellow Orange Red Infrared

400 nm 460 nm 530 nm 575 nm575 nm 600 nm 650 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

BrightnessOrange

Brown (same)

Hue Saturation

Blue Blue

Desaturatatedorange = 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 takepictures of Earth at differentwavelengths 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

yellow650-nm red530-nm green530-nm green

460-nm blue

http://www.colorado.edu/physics/2000/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

magenta

greengreenyellow

blue

Additive mixing of colored light primaries

Blue added togreen = cyan.

Green added tored = yellow.

Red added toblue = 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 addingtwo 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 appleadds color to white light!

Color Filters

A colored filter subtracts colors by absorption.

=

Incident white light Only greengets

through

Cyanfilter subtracts

red

Yellowfilter subtracts

blue

A colored filter subtracts certain colors by absorption and transmits

the rest

=

Incident white light Magentafilter subtracts

green

Cyanfilter subtracts

red

Only bluegets

through

A colored filter subtracts colors by absorption.

=

Incident white light Magentafilter subtracts

green

Only redgets

through

Yellowfilter subtracts

blue

A

Exam #2: Statistics/Results/Grades

AA-

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 mixtureof mostly blues and greens

Yellow is an additive mixtureof 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

Tra

nsm

itta

nce

Ref

lect

ance

Non-ideal colored filter subtracts colors by absorption.

Incident white light Orange-yellowgets

through

Non-ideal reddishfilter subtracts some

green and blue

700500 600400

Transmittance

Wavelength

100%

Non-ideal yellowfilter subtracts someblue, red and green

700500 600400

Transmittance

Wavelength

100%

Colored surfaces subtract certain colors by absorbing them, while

reflecting others

Magenta surfaceabsorbs (subtracts)

green.

Green surfaceabsorbs (subtracts)

red and blue (magenta).

White inMagenta out

White inGreen out

Green light on a magenta surface appears colorless because green is

absorbed

Magenta surfaceabsorbs (subtracts)

green.

Green surfaceabsorbs (subtracts)

red and blue (magenta).

Magenta light on a green surface

appears colorless because magenta is

absorbedGreen in

No color

Magenta in

No colo

r

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 bulbMagenta shirt

How on earth do you multiply two curves?

You multiply the two y-valuesat each x to get the new curve

this number this number

equals this number equals this number

This number timesThis number times

How the shirtappears 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: measured in lines per inch (lpi) or dots per inch (dpi); for example, Laser Printer (600dpi)

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