Summary of the material to be tested in
the exam #2: Chapters 4-7
• Ch.4: Cameras photography • Exam:
•How cameras work;
•camera settings;
•Taking good pictures;
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• 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.
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
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•
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•
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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;
1
An example of a possible exam question:
An example of a possible exam question:
• The instructor has eyeglasses
•
Teaching
assistant
wears
of lens power -1.5D. Will he
eyeglasses of lens power
be able to see a student
2D.
He
came
to
do
exam
cheating in the opposite part
proctoring but forgot his
of our lecture room?
eyeglasses. Will he be
•A. Yes;
able to see a student
•B. No.
cheating in the opposite
part
of
our
lecture
room?
• What if he forgets to bring
his eyeglasses?
•A. Yes;
•B. No.
•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.)
Physics 1230: Light and Color
Chapter 9 (with some material from Chapter 8)
• 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
• 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
5
What are spectral colors?
Violet
Blue
Green
Yellow
Orange
Red
400 nm
460 nm
530 nm
575 nm
600 nm
650 nm
• 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
Infrared
• We are able to distinguish more
than a million different colors
• Most colors that we see are not
spectral colors. Examples:
• White
• Pink
• Brown
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 intensitydistribution 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
Brightness
Bright white
Grey
Black
Hue
Saturation
Desaturatated
orange = saturated
Brown (same) orange + white
Orange
Blue
Blue
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?
magenta shirt
• 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
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)
• Photoshop: uses H, S and B
hue
saturation
lightness
• 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
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
http://www.colorado.edu/physics/
2000/tv/colortv.html
530-nm green
cyan
yellow
650-nm red
magenta
460-nm blue
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)
yellow
green
white
cyan
magenta
blue
red
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"
saturation
color tree
Lightness (z)
hue
Chromaticity "tree"
3 numbers (x, y, z)
specify a color
y
x
hue
saturation
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 nonwavelength 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
What is partitive
mixing?
• 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
Subtractive mixing
It is a common misconception that a filter or an apple
adds color to white light!
• Examples:
• A color filter or colored object takes
• Think of white light as a mixture of
away certain wavelengths present in
red, green and blue wavelengths
white light by absorbing them
• An apple is red because when white
• When light passes through a color filter
light shines on it it absorbs (subtracts)
or reflects from a colored object, 3 things
blue and green wavelengths and
can happen to the components of the light
reflects red wavelengths
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
• 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
Color Filters
A colored filter subtracts
colors by absorption.
=
Incident white light
Cyan
filter subtracts
red
Yellow
filter subtracts
blue
Only green
gets
through
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
Yellow
filter subtracts
blue
Only red
gets
through
Exam #2: Statistics/Results/Grades
• Problems with grading: can be
corrected within one week;
C-, C, C+
B-,B, B+
AA
A
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 & • Demo on work of Displays
continue with Chapter #10;
and printers: use a
microscope to see how
different colors are
• Final Grade: Clicker
obtained;
questions – 5%; HWs –
• Demo on spectral
20%; Exams – 75%;
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
yellow
magenta
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.
Transmission distribution of filters
• 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.
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
• A transmittance curve shows
what percent of each of the
wavelengths are transmitted
through this magenta filter
Transmittance
Reflectance
• Just as a reflectance curve
shows what percent of each of
the wavelengths in white light
reflect from this magenta
surface
Non-ideal colored filter
subtracts colors by absorption.
Incident white light
Non-ideal reddish
filter subtracts some
green and blue
Transmittance
100%
Non-ideal yellow Orange-yellow
filter subtracts some
gets
blue, red and green
through
Transmittance
100%
400 500 600 700
Wavelength
400 500 600 700
Wavelength
Colored surfaces subtract certain
colors by absorbing them, while
reflecting others
White in
Magenta out
Magenta surface
absorbs (subtracts)
green.
White in
Green out
Green surface
absorbs (subtracts)
red and blue (magenta).
Green light on a
magenta surface
appears colorless
because green is
absorbed
Green in
Magenta surface
absorbs (subtracts)
green.
Magenta light on a
green surface
appears colorless
because magenta is
absorbed
Magenta in
Green surface
absorbs (subtracts)
red and blue (magenta).
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
This number times
How the shirt
appears in this light
equals this number
Magenta shirt
this number
How on earth do you
multiply two curves?
You multiply the two y-values
at each x to get the new curve
Printer's inks
•
•
•
Reflected ray from top layer of printer's
ink mixing additvely with transmittedreflected-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
Printer's ink
Paper beneath
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
Printer's
ink
Paper beneath
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.
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