Color Vision
Lecture 10 Chapter 5, Part A
Jonathan Pillow Sensation & Perception (PSY 345 / NEU 325) Princeton University, Fall 2017
1 Exam #1: Thursday 10/19
Format: multiple-choice, fill-in-the-blank, & short answer
What to study: - all material from lectures & slides - precept readings (basic gist & findings of each article)
(If something appeared only in the book, and not at all in class or precept or slides, you can probably safely ignore it)
Review session: tonight @ 7:00 in PNI A30
2 2015 called….
Grey-and-green, or pink-and-white?
3 4 5 • color vision has evolutionary value
• lack of color vision ≠ black & white
6 Basic Principles of Color Perception
The book says:
“Color is not a physical property but a psychophysical property”
7 Basic Principles of Color Perception
• Most of the light we see is reflected • Typical light sources: Sun, light bulb, LED screen • We see only part of the electromagnetic spectrum(between 400 and 700 nm). Why??
8 Basic Principles of Color Perception • Why only 400-700 nm?
(Pomerantz, Rice U.)
Suggestion: unique ability to penetrate sea water
9 Basic Principles of Color Perception
Q: How many numbers would you need to write down to specify the spectral properties of a light source?
A: It depends on how you “bin” up the spectrum • One number for each spectral “bin”:
20 17 16 15
13 12 10 example: 13 5 bins
energy 0 0 0 0 0
10 Basic Principles of Color Perception
Device: hyper-spectral camera - measures the amount of energy (or number of photons) in each small range of wavelengths - can use thousands of bins (or “frequency bands”) instead of just the 13 shown here
20 17 16 15
13 12 10 5
energy 0 0 0 0 0
11 Basic Principles of Color Perception
Some terminology for colored light: spectral - referring to the wavelength of light the illuminant - light source power spectrum - this curve. Description of the amount of 20 energy (or power) 17 16 at each frequency 15
13 12 10 5
energy 0 0 0 0 0
12 Basic Principles of Color Perception
an illuminant with most power at long wavelengths (i.e., a reddish light source)
13 measurements of power spectrum (example) energy
13 Basic Principles of Color Perception
an illuminant with most power at medium wavelengths (i.e., a greenish light source)
13 measurements of power spectrum (example) energy
14 Basic Principles of Color Perception
an illuminant with most power at long wavelengths (i.e., a blueish light source)
13 measurements of power spectrum (example) energy
15 Basic Principles of Color Perception
an illuminant with power at all visible wavelengths (a neutral light source, or “white light”)
13 measurements of power spectrum (example) energy
16 Q: How many measurements of this same spectrum does the human eye take (in bright conditions?)
17 Q: How many measurements of this same spectrum does the human eye take (in bright conditions?)
A: Only 3! One from each cone class
534 420 564 cone types S = short (blue) M = medium (green) L = long (red) photoreceptorresponse
Color vision: Relies entirely on comparison of responses from three cone types!
18 absorption spectrum - describes response (or “light absorption”) of a photoreceptor as a function of wavelength
534 420 564 photoreceptorresponse
could also call this “sensitivity”
19 A single photoreceptor doesn’t “see” color; it gives greater Problem: response from a single cone is ambiguous response to some frequencies than others
single cone absorption spectrum
20 Problem: response from a single cone is ambiguous
single cone absorption spectrum • All the photoreceptor gives you is a “response”
10 spikes • Can’t tell which light frequency gave rise to this response (blue or orange)
21 Problem is actually much worse: can’t tell a weak signal at the peak sensitivity from a strong signal at an off-peak intensity
single cone absorption spectrum +2 • All three of these lights give the same response from this spectral power spectral cone 10 spikes +1 cone respone = aborption spectrum x +0.5 light intensity
22 Problem of univariance: infinite set of wavelength+intensity combinations can elicit exactly the same response
single cone absorption spectrum +2 spectral power spectral 10 spikes +1
+0.5
23 So a single cone can’t tell you anything about the color of light!
Colored stimulus
Response of your “S” cones
24 cone responses: 40 175 240
1
0.8 percept
0.6
0.4
sensitivity 0.2
0 400 450 500 550 600 650 700
illuminant #1
Metamers #4 #3 - Illuminants that are physically distinct but
energy #2 perceptually indistinguishable wavelength
25 written as a linear algebra problem (if that’s meaningful to you)
S = M L
cone cone responses absorption spectra illuminant spectrum • cone sensitivities define a 3D subspace of color perception • metamers differ only in the null space!
26 Implication: many things in the natural world have different spectral properties, but look the same to us.
But, great news for the makers of TVs and Monitors: any three lights can be combined to approximate any color.
Single-frequency spectra produced by (hypothetical) monitor phosphors illuminant #1
Monitor phosphors produce “metameric match” to illuminant #1
energy (or any other possible illuminant). wavelength
27 Close-up of computer monitor, showing three phosphors, (which can approximate any light color)
28 Spectra of typical CRT monitor phosphors
29 This wouldn’t be the case if we had more cone classes.
hyperspectral marvel: mantis shrimp (stomatopod)
• 12 different cone classes • sensitivity extending into UV range
• No surprise that they never invented color TV!
30 Real vs. Conterfeit $$
Output of hyper-spectral camera (colorized artificially)
31 32 3 “primary” lights
any color can be made by combining three suitable lights...
R G B How did they figure this out?
33 James Maxwell: color-matching experiment
Given any “test” light, you can match it by adjusting the intensities of any three other lights (2 is not enough; 4 is more than enough)
34 Cone responses entirely determine our color percepts:
S M L 100 100 100 50 50 50 0 0 0
100 0 0 0 100 0 0 0 100
100 100 0 “non-spectral hues” 0 100 100 • percept couldn’t be produced by any single- 100 0 100 wavelength light
35 Color space: A three-dimensional space that describes all possible color percepts.
Several ways to describe this space:
• RGB color space: Defined by the outputs of Long, Medium, Short wavelength (or R, G, B) lights.
• HSB color space: Defined by hue, saturation, and brightness ! Hue: The chromatic (color) aspect of light ! Saturation: The chromatic strength of a hue ! Brightness: The distance from black in color space
36 2D slice of color space
• hue around the edge • saturation increasing from center to edge • brightness not shown normalizedresponseM
normalized L response
37 Color picker
38 Trichromatic color vision: (Young & Helmholtz theory)
- three lights needed to make a specific color percept, due to use of 3 distinct cones with different sensitivities
- colors uniquely defined by combinations of cone activations
39 Late 17th Century: Isaac Newton
“The rays themselves, to speak properly, are not coloured”
40 Newton’s Spectrum: R O Y G B I V Newton’s Theory: seven kinds of light -> seven kinds of photoreceptor
41 However, this doesn’t quite explain everything
42 43 44 Opponent color theory:
- perception of color is based on the output of three channels, each based on an opponency between two colors
Opponent Channels:
• L-M (red - green) • S - (L+M) (blue -yellow) • L+M - (L+M) (black - white )
45 fields with “color-opponency” with fields receptive center-surround have Cells Ganglion Retinal Some M M M L M M M Σ • • Carries info about red vs.red green about info Carries Red-Green 46 ( L - response M ) Color-Opponent cell cell Color-Opponent space Some Retinal Ganglion Cells have center-surround receptive fields with “color-opponency”
Σ response
space
L L L M L L L • Red-Green (M-L) Color-Opponent cell • Carries info about red vs. green
47 Some Retinal Ganglion Cells have center-surround receptive fields with “color-opponency”
Σ response
space
L M M S L L M • Blue-Yellow (S-(M+L)) Opponent cell • Carries info about blue vs. yellow
48