Announcements 4/9/12
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Prayer
HW 25 due on Tuesday
HW 26 due on Wednesday (but it’s very likely
Claira won’t pick it up until Thursday night.
Shhh!)
Frank & Ernest
Reading Quiz
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How many types of cone cells are there in
humans?
a. 1
b. 2
c. 3
d. 4
e. 5
Reading Quiz
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The functions r(l), g(l), and b(l) are called
the:
a. color adding functions
b. color displaying functions
c. color matching functions
d. color realizing
e. color subtracting functions
The Goal
Visible Spectrum
From Wikipedia, “Visible Spectrum”
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“All the colors of the rainbow…”  Where is
brown??
What’s a “luminescence spectrum” that you
might measure?
Cone cells
From Wikipedia,
“Color Vision”
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“Short”
“Medium”
“Long”
Color blindness
Test for tritanopia
Test for deuteranopia
From Wikipedia,
“Color Blindness”
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tritanomaly – S cones mutated (rare)
tritanopia – lacks S cones (<1% of males)
deuteranopia – lacks M cones (1% of males)
deuteranomaly – M cones mutated (6% of males;
0.4% of females)
protanomaly – L cones mutated (1% of males)
protanopia – lacks L cones (don’t know %)
Primary Colors
From Wikipedia,
“RGB Color Model”
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How the primary song should go
“Additive color mixing” – demos
Subtractive colors - demo
Components of R, G, B
From Wikipedia,
“RGB Color Model”
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Plot 3 components in 3D “color space”
What are R, G, B?
From Wikipedia,
“Primary Color”
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The spectra of R, G, and B phosphors from a
standard CRT (i.e. non LCD) computer monitor
Could also, e.g., have R = sharp peak at 635 nm, G
= sharp peak at 532 nm, B = sharp peak at 447
A Problem
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R = sharp 635 nm, G = sharp 532 nm, B = sharp 447 nm
What happens if you want to get, say, orange = 580 nm. Can you
mix R, G and B to get this?
 532 nm will excite some S!
 580 nm alone will never excite S!
1920’s Experiments
red source = 700 nm green source = 546.1 nm blue source = 435.8 nm
Important results:
1. Human eye response can (mostly)
be described by 3 parameters
2. Human eye response is (mostly)
linear
to get 580 nm orange, need
some “negative” blue
From Wikipedia, “1931 Color Space” (also in P&W)
What is a “Color Space”?
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How strong of r, g, and b, lights would you
need to match a light that is not a delta
function?
Compare: what would you get for the
response of a detector that has, say, g-bar as
its response curve?
1931 Color Matching Functions
From Wikipedia,
“CIE 1931 Color Space”
Human eye response again
Properties of these functions:
• all are positive
• z-bar = very close to S cones, very close to previous b-bar
• y-bar = matches intensity response of eye, very close to M cones
• x-bar = chosen so that white is equal parts x-bar, y-bar, z-bar
1931 Color Matching Functions
X   I (l ) x (l ) d l
R   I (l ) r (l ) d l
etc.
etc.
Worked Example
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X=
Y=
Z=
Normalize (because “color” should not depend on overall intensity)
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x=
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y=
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z=