Color Vision Lecture 10 Chapter 5, Part A Jonathan Pillow - - PowerPoint PPT Presentation

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Color Vision Lecture 10 Chapter 5, Part A Jonathan Pillow - - PowerPoint PPT Presentation

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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

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Color Vision

Lecture 10 Chapter 5, Part A

Jonathan Pillow Sensation & Perception (PSY 345 / NEU 325) 
 Princeton University, Fall 2017

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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

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2015 called…. Grey-and-green, or pink-and-white?

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  • color vision has evolutionary value
  • lack of color vision ≠ black & white

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Basic Principles of Color Perception The book says: “Color is not a physical property but a psychophysical property”

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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??

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Basic Principles of Color Perception

  • Why only 400-700 nm?

Suggestion: unique ability to penetrate sea water

(Pomerantz, Rice U.)

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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”:

example: 13 bins

5 10 13 20 15 16 17 12

energy

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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

5 10 13 20 15 16 17 12

energy

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Basic Principles of Color Perception Some terminology for colored light:

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energy

the illuminant - light source spectral - referring to the wavelength of light power spectrum - this curve. Description of the amount of energy (or power) at each frequency

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Basic Principles of Color Perception

energy

an illuminant with most power at long wavelengths (i.e., a reddish light source)

13 measurements of power spectrum (example)

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Basic Principles of Color Perception

energy

an illuminant with most power at medium wavelengths (i.e., a greenish light source)

13 measurements of power spectrum (example)

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Basic Principles of Color Perception

energy

an illuminant with most power at long wavelengths (i.e., a blueish light source)

13 measurements of power spectrum (example)

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Basic Principles of Color Perception

energy

an illuminant with power at all visible wavelengths (a neutral light source, or “white light”)

13 measurements of power spectrum (example)

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Q: How many measurements of this same spectrum does the human eye take (in bright conditions?)

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Q: How many measurements of this same spectrum does the human eye take (in bright conditions?) A: Only 3! One from each cone class

photoreceptor response

420 534 564

S = short (blue) M = medium (green) L = long (red) cone types Color vision: Relies entirely on comparison

  • f responses from three

cone types!

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absorption spectrum - describes response (or “light absorption”) of a photoreceptor as a function of wavelength

photoreceptor response

could also call this “sensitivity”

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A single photoreceptor doesn’t “see” color; it gives greater response to some frequencies than others

single cone absorption spectrum

Problem: response from a single cone is ambiguous

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  • All the photoreceptor

gives you is a “response”

  • Can’t tell which light

frequency gave rise to this response (blue or

  • range)

single cone absorption spectrum

10 spikes Problem: response from a single cone is ambiguous

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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

spectral power

  • All three of these

lights give the same response from this cone

+2 +1 +0.5

10 spikes cone respone = aborption spectrum x light intensity

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single cone absorption spectrum

spectral power +2 +1 +0.5

Problem of univariance: infinite set of wavelength+intensity combinations can elicit exactly the same response 10 spikes

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So a single cone can’t tell you anything about the color of light! Colored stimulus Response of your “S” cones

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400 450 500 550 600 650 700 0.2 0.4 0.6 0.8 1

wavelength energy

240 175 40

cone responses:

Metamers

  • Illuminants that are

physically distinct but perceptually indistinguishable illuminant #1 #2 #3 #4

sensitivity

percept

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written as a linear algebra problem

(if that’s meaningful to you) = cone responses cone absorption spectra S M L illuminant spectrum

  • cone sensitivities define a 3D subspace of color perception
  • metamers differ only in the null space!

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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.

wavelength energy

illuminant #1 Single-frequency spectra produced by (hypothetical) monitor phosphors Monitor phosphors produce “metameric match” to illuminant #1 (or any other possible illuminant).

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Close-up of computer monitor, showing three phosphors, (which can approximate any light color)

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Spectra of typical CRT monitor phosphors

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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!

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Real vs. Conterfeit $$ Output of hyper-spectral camera

(colorized artificially)

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R G B 3 “primary” lights any color can be made by combining three suitable lights... How did they figure this out?

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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)

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Cone responses entirely determine our color percepts: S M L 100 100 100 100 50 100 50 100 50 100 100 100 100 100 100 “non-spectral hues”

  • percept couldn’t be

produced by any single- wavelength light

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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

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  • hue around the edge
  • saturation increasing

from center to edge

  • brightness not shown

2D slice of color space

normalized L response normalized M response

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Color picker

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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

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Late 17th Century: Isaac Newton

“The rays themselves, to speak properly, are not coloured”

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R O Y G B I V Newton’s Theory: seven kinds of light -> seven kinds of photoreceptor Newton’s Spectrum:

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However, this doesn’t quite explain everything

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Opponent Channels:

  • L-M (red - green)
  • S - (L+M) (blue -yellow)
  • L+M - (L+M) (black - white )

Opponent color theory:

  • perception of color is based on the output of three

channels, each based on an opponency between two colors

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response space

Some Retinal Ganglion Cells have center-surround receptive fields with “color-opponency”

  • Red-Green (L - M) Color-Opponent cell
  • Carries info about red vs. green

Σ L M M M M M M

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L

response space

Some Retinal Ganglion Cells have center-surround receptive fields with “color-opponency” Σ M L L L L L

  • Red-Green (M-L) Color-Opponent cell
  • Carries info about red vs. green

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M

response space

Some Retinal Ganglion Cells have center-surround receptive fields with “color-opponency”

  • Blue-Yellow (S-(M+L)) Opponent cell
  • Carries info about blue vs. yellow

Σ S M L L M L

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