Transcript Physics 1230: Light and Color Chapter 10 • Chapter 10: Color Perception • How we see color Clicker grades updated!!! • Three types of cones -

Physics 1230: Light and Color
Chapter 10
• Chapter 10: Color
Perception
• How we see color
Clicker grades updated!!!
• Three types of cones - each
with different responses at
all wavelengths
• Color matching
• Opponent processing
• Color blindnesses
(deficiencies in seeing
color)
• Spatial color processing in
your retina• Double opponent with
receptive fields
1
We have three different kinds of cones — whose responses
are mainly at short, intermediate and long wavelengths
• s-cones absorb short wavelength light best,
with peak response at 450 nm (blue)
• L-cones absorb long wavelength light best,
with peak response at 580 nm (red)
• i-cones absorb intermediate wavelengths
best, with peak response at 540 nm (green)
• Light at any wavelength in the visual
spectrum from 400 to 700 nm will excite
these 3 types of cones to a degree
depending on the intensity at each
wavelength.
• Our perception of which color we are
seeing (color sensation) is determined by
how much S, i and L resonse occurs to
light of a particular intensity distribution.
Rule: To get the overall response of each type of
cone, multiply the intensity of the light at each
wavelength by the response of the cone at that
wavelength and then add together all of the
products for all of the wavenumbers in the
intensity distribution
L-cones
i-cones
s-cones
Spectral response of cones in typical human eye
Concept Question
• What is an additive
mixture of blue and
yellow?
• A. Green;
• B. Red;
• C. White;
• D. Black;
• E. Orange;
• We learned this in
Chapter 9… Why do
we still need this?
Light color
Brightness
460 nm blue
1
575 nm yellow
1.66
Mixture ( perceived as white)
S-cone response
60
0
60 + 0 = 60
I-cone response
5
1.66 x 33
5+1.66 x 33 = 60
L-cone response
2
1.66 x 35
2+1.66 x 35 = 60
Examples of two different ways we see white
• Our sensation of color depends on how much total s, i
& L cone response occurs due to a light intensitydistribution
• Multiply the intensity distribution curve by each
response curve to determine how much total S, i,
and L response occurs
• We experience the sensation white when we have
equal total s, i & L responses
• There are many ways this can occur!!
• E.g., when broadband light enters our eye
• Another way to experience white is by viewing a
mixture of blue and yellow
• E.g., 460 nm blue of intensity 1 and 575 nm
yellow of intensity 1.66
• The blue excites mainly s-cones but also a
bit of i-cones and a bit of L-cones
• The yellow excites i-cones and (slightly
more) L-cones but no s-cones
• The result is an equal response of s-cones, icones and L-cones (details)
Spectral response of cones in typical human eye
1.66
1
0
460 nm blue of
intensity 1
575 nm yellow
of intensity 1.66
Light color
Brightness
S-cone response
I-cone response
L-cone response
530 nm green
1
negligible
41
How does a normal
person
see yellow
when only 28red
650 nm red
2.15
negligible
2.15 x 2
2.15 x 9
and asgreen
are superimposed?
Mixture
(perceived
yellow ) lights
negligible
41 +2.15 x 2 =45
28 +2.15 x 9 =47
575 nm yellow
1.35
negligible
• Our sensation of yellow depends on a special s, i &
L cone response
• We experience the sensation yellow when 575 nm
light reaches our eyes
• What really gives us the sensation of yellow is
the almost equal response of i and L cones
together with no s-cones!!
• Another way to experience yellow is by seeing
overlapping red & green lights
• E.g., 530 nm green of intensity 1 and 650
nm red of intensity 2.15
• The green excites mainly i-cones but also
L-cones, while the red excites mainly Lcones but also i-cones
• The total respone of s & i-cones due to the
spectral green and red is the same as the
total response due to spectral yellow
• In general need 3 wavelength lights to mix to
any color
1.35 response
x 33 = 45of cones
1.35
x 35 = 47
Spectral
in typical
human eye
650 nm red
575 nm yellow of intensity
of intensity 1.35 2.15
530 nm green
1 of intensity 1
2.15
0
What happens if a person is missing one type of cone?
Spectral response of cones in protanopic eye
• Missing one type of cone results in one type
of color-blindness
• E.g., someone whose L-cone is missing
will not see colors correctly
• They will see white or grey when a single
wavelength 495 nm is present because light
at 495 nm excites S & i cones equally no
matter what its intensity
• They will also be able to see white by
mixing any 2 wavelength lights with the
correct intensities so that the S and i cones
respond equally
• All colors they see can be obtained by
mixing only 2 different wavelength lights
• This type of color-blindedness is called
protanopia (a kind of dichromacy)
• Dichromats can match any light color by
mixing only 2 wavelength lights
What happens if a person is missing 2 (or all 3)
types of cones?
• Missing 2 or all 3 type of cones results
in a different (rare) type of colorblindness called monochromacy
• Cone monochromats have only one type
of cone (s, i or L).
• Rod monochromats have no cones and
have difficulty seeing with their rods
under bright light (photopic) conditions
• Monochromats can match a light of any
color by varying the intensity of only
one spectral (wavelength) light
• They are truly color-blind because they
cannot distinguish any wavelength color
from any other
• They see in blacks, whites and greys
• Trichromats (those with
trichromacy) possess all 3 types
cones, but either have shifted
response curves for one or more of
those cones or else have a problem
with opponent processing (to be
discussed next)
Concept question
• Can rod
monochromats
distinguish red color
from green color?
• A. Yes
• B. No
• C. Only during a
bright day;
• D. Only during gthe
night;
The four psychological primaries
• In addition to the additive
primaries (RGB) and the
subtractive primaries (CMY)
there is another set of (4)
primary colors, called the
psychological primaries




Blue
Green
Yellow
Red (really closer to magenta)
• These hues can be used to
describe all other hues.
• All hues can be verbally described as
combinations of these colors. For
example,
•
•
•
•
Yellowish red
Greenish yellow
Bluish green
Bluish red
• BUT we don't recognize hues such as
• Reddish green
• Yellowish blue
• Red and green are opponent hues
• Yellow and blue are opponent hues
We can verify color naming of hues in terms of the
psychological primaries on the chromaticity diagram
All of the hues can be named
qualitatively by how much green, red,
blue or yellow is "in" them
• We don't need orange, purple or pink:
• orange can be thought of as yellow-red
• purple can be thought of as red-blue
• pink has the same hue as red but differs
only in lightness
We can break up the diagram into 4
different regions by drawing two lines
whose endpoints are the psychological
primary hues
• The endpoints of the yellow line are 580
nm "unique" yellow and 475 nm
"unique" blue
• One endpoint of the red line is 500 nm
"unique" green and the other is "red"
(not unique or spectral - really more like
magenta)
Greenness &
yellowness
Redness &
yellowness
What is meant by the opponent nature of red vs green
(r-g) perception and of yellow vs blue (y-b) perception.
• Viewing a progression of colors in
the direction of the yellow line from
475 nm blue towards 580 nm yellow,
we see more yellowness of each color
and less blueness.
• We call this perception our y-b
channel
• Yellow & blue are opponents
• Moving parallel to the red line from
500 nm green towards nonspectral
red we see more redness in each color
and less greenness.
• We call this perception our r-g
channel
• Red and green are opponents
• The lines cross at white, where both
y-b & r-g are neutralized
Greenness &
yellowness
Redness &
yellowness
How might the three types of cones be "wired" to neural
cells to account for our perception of hues in terms of two
opponent pairs of psychological primaries r-g and y-b?
• The 3 kinds of cones are related to r-g and y-b by
the way they are connected to neural cells (such as
ganglion cells)
• Cones of each kind are attached to 3 different
neural cells which control the two chromatic
channels, y-b and r-g, and the white vs black
channel called the achromatic channel (lightness)
• "wiring" is the following:
• When light falls on the L-cones they tell all 3
neural cells to increase the electrical signal they
send to the brain
• When light falls on the i-cones they tell the r-g
channel cell to decrease (inhibit) its signal but tell
the other cells to increase their signal
• When light falls on the s-cones they tell the y-b
channel cell to decrease (inhibit) its signal but tell
the other cells to increse their signal
s-cone

++
neural cell
for y-b
chromatic
channel
i-cone
L-cone
+  +
+ ++
neural cell
for r-g
chromatic
channel
Electrical signal to brain
neural cell
for w-blk
achromatic
channel
How can this "wiring" work to produce the chromatic
channels?
• The neural cell for the y-b chromatic
channel has its signal
s-cone
i-cone
L-cone
• inhibited when (bluE) light excites the
s-cone
INTERPRETED AS BLUE
• enhanced when light excites the i & L
cones
INTERPRETED AS YELLOW
• The neural cell for the r-g chromatic
channel has its signal
•
• inhibited when (green) light falls on the
 ++ +  +
i-cone
INTERPRETED AS GREEN
neural cell
neural cell
• enhanced when light excites the s and
for y-b
for r-g
L cone
chromatic
chromatic
INTERPRETED AS MAGENTA
channel
channel
(Psychological red)
The neural cell for the achromatic channel
has its signal enhanced when light excites Electrical signal to brain
any of the cones
+ ++
neural cell
for w-blk
achromatic
channel
We learned: how cone-neural cell "wiring"
works to produce the chromatic channels
• The neural cell for the y-b chromatic
channel has its signal
s-cone
i-cone
L-cone
• inhibited when (bluE) light excites the
s-cone
INTERPRETED AS BLUE
• enhanced when light excites the i & L
cones
INTERPRETED AS YELLOW
• The neural cell for the r-g chromatic
channel has its signal
•
• inhibited when (green) light falls on the
 ++ +  +
i-cone
INTERPRETED AS GREEN
neural cell
neural cell
• enhanced when light excites the s and
for y-b
for r-g
L cone
chromatic
chromatic
INTERPRETED AS MAGENTA
channel
channel
(Psychological red)
The neural cell for the achromatic channel
has its signal enhanced when light excites Electrical signal to brain
any of the cones
+ ++
neural cell
for w-blk
achromatic
channel
More systematic descriptions
of color-blindedness (no need
to memorize terminology)
• Monochromacy (can match any colored light
with any 1 spectral light by adjusting
•
intensity)
• Either has no cones (rod monochromat)
or has only 1 of the 3 types of cones
working (cone monochromat).
• Sees ony whites, greys, blacks, no hues
• Dichromacy (can match any colored light
with 2 spectral lights of different intensities of
(rather than the normal 3)
• L-cone function lacking = protanopia
• i-cone function lacking = deuteranopia
• s-cone function lacking = tritanopia
• no y-b channel but all 3 cones OK =
tetartanopia
Anomalous trichromacy (can match
any colored light with 3 spectral lights
of different intensities as in normal
vision, but still have color perception
problems)
• Protanomaly
• Shifted L-cone response curve
• Deuteranomaly (most common)
• Shifted i-cone response curve
• Confusion between red and green.
• Tritanomaly
• Yellow-blue problems: probably
defective s-cones
• Neuteranomaly
• ineffective r-g channel
Visualizing dichromacy:
protanopia
• No L-cone function
• See yellows & blues instead of reds & greens
• Neutral hue pts. below 500 nm & nonspectral magenta; neutral line close to r-g
line in chromaticity diagram (effectively
missing)
• As move in direction of black arrows all
colors aligned with white arrows only have
different yellowness and blueness, not
different greenness or redness
Spectral response of cones in protanopic eye
Visualizing dichromacy:
deuteranopia
• i-cone function lacking
• Like protanopes, they see yellows &
blues instead of reds & greens
• Neutral hue points near 500 nm and
non-spectral purple
• Neutral hue line close to the line
joining the neutral points in the
chromaticity diagram
• Hence, like protonopes, deuteranopes
don't distinguish green from red very
well
Visualizing dichromacy:
tritanopia
• s-cone function lacking
•
They see reds and greens instead of blues
and yellows
• Neutral hue points at 570 nm and blue
purple; neutral hue line between neutral
hue points in the chromaticity diagram
which is effectively missing
•
Hence, they don't distinguish blues and
yellows very well
• Tetartanopes lack the y-b channel
•
See similarly to tritanopes
Take the color blindness test
• The color blindness test consists
of a set of five charts. Each chart
shows a number in one color on
a different backgound color.
• People with normal color vision
will have no problem seeing the
numbers on the charts, but
people with color blindness will
see only random colored dots.
• Seventy-five percent of color
blind people have poor green
perception. Of the remaining,
24% have poor red perception,
and one percent are affected by
a rare tritan type.
The opponency of red and green and of yellow and blue
can be understood in terms of special receptive fields in
our retina called double-opponent receptive fields
• Double opponent receptive fields in
our retina
• are responsible for lateral
inhibition, just like light-dark
receptive fields we have studied
• enable us to notice sharp color
boundaries in the same way that
light-dark receptive fields allowed
us to notice sharp light-dark
boundaries
• exaggerate colors on either side of
an opponent color boundary in the
same way that light-dark receptive
fields exaggerated the lightness or
darkness on either side of the
boundary
• are responsible for color constancy
in the same way that light-dark
receptive fields were responsible for
lightness constancy
• consist of photoreceptors in a
center-surround geometry, all
pooled to one final neural cell
(ganglion cell)
• There are two types of doubleopponent receptive fields (each
paired with its own neural cell)
• The r-g receptive field and cell
• The y-b receptive field and cell
Receptive field of a double-opponent
cell of the r-g type
• 2 different ways to INCREASE the
signal the ganglion cell sends to brain
• Red light falling on cones in center
of receptive field attached to
ganglion cell
• Green light on surround
• 2 different ways to decrease the
signal the ganglion cell sends to the
brain
• Red light on surround
• Green light on center
• Electrical signal to brain from ganglion
cell is at ambient level when no light is
on center or surround
• When signal to brain is
INCREASEDwe interpret that as red
• When signal to brain is decreased we
interpret that as green
signal to brain
We can summarize this by just showing the center &
surround of the receptive field and indicating the effect of
red (R) and green (G) on each
• A double-opponent cell differs
from a single opponent cell
• In both of them R in the center
increases the signal
• In a single-opponent cell G in
surround would inhibit signal,
whereas in double-opponent cell
G enhances
• In a double-opponent cell
• R in center enhances signal
(ganglion cell signals red)
• G in surround enhances signal
(ganglion cell signals red)
• R in surround inhibits signal
(ganglion cell signals green)
• G in center inhibits signal
(ganglion cell signals green)
Fictional cell
real cell
Concept Question:
• What is effect of red
light falling on both
the center AND
surround?
a) No color
b) Sensation of red
c) Sensation of green
d) Sensation of
yellow
Concept Question:
• What is effect of
green light falling on
surround only?
a) No color
b) Sensation of red
c) Sensation of green
d) Sensation of
yellow
Concept Question:
• What is the effect of
green light falling on
surround and red light
falling on the center of
the receptive field?
a) No color
b) Sensation of red
c) Sensation of green
d) Sensation of
yellow
Here is an illustration of the effect of red or green light
falling in various combinations on the center or
surround of a double-opponent r-g cell
Strongest
signal
(interpreted
as red)
Weakest
signal
(interpreted
as green)
Note, you would
still "see" red if
the center were
grey!
Note, you would
still "see" green
if the center
were grey!
No change in
signal (color
not noticed)
No change in
signal (color
not noticed)
y-b double-opponent receptive fields and cells work the
same way
Strongest
signal
(interpreted
as yellow)
Weakest
signal
(interpreted
as blue)
Note, you would
still "see" yellow
if the center
were grey!
Note, you would
still "see" blue if
the center were
grey!
No change in
signal (color
not noticed)
No change in
signal (color
not noticed)
b+yy+b-
Concept Question:
• What is the effect of
blue light falling on
surround of receptive
field only?
a) No color
b) Sensation of blue
c) Sensation of green
d) Sensation of
yellow
e) Sensation of red
Here is an optical illusion which can be explained by
double-opponent retinal fields and cells
• Look at the grey squares in your
peripheral vision
• Does the grey square
surrounded by yellow appear to
take on a tint?
• What color is it?
• Repeat for the grey squares
surrounded by
• Blue
• Green
• Red (pink)
Color constancy depends on doubleopponent processing
• Color constancy means we see the
proper colors of a picture or scene or
object relatively correctly even though
the overall illumination may change its
color
• This is because our double-opponent
receptiive fields compare neighboring
colors and are not very sensitive to an
overall change in color
• Color constancy developed in the
evolution of mankind so that we could
recognize colorful things in broad
daylight, late afternoon, and early
evening
No change in
signal (color
not noticed)
No change in
signal (color
not noticed)
Illustration of how the three opponency channels work
in your perception of the design below
• Here are the enhanced edges
resulting from your y-b chromatic
channel
• Note the edges that separate a
yellowish from a bluish color are
enhanced the most
• Here are the enhanced edges
resulting from your r-g chromatic
channel
• Note the edges that separate a
reddish from a greenish color are
enhanced the most
• Here are the enhanced edges
resulting from your wt-blk
achromatic channel
• Compare with the way a photocopy
machine would see the design
The artist Van Gogh knew how to use the opponency of
yellow and blue to enhance each of them
Note also that we use yellow letters
against a blue background in these
notes for emphasis, although we
prefer white in general. Red would
be less effective than yellow because
it is not an opponent to blue
Negative afterimages occur when you stare at an
image for a long time without moving your eyes
1
Conditions for negative afterimages
o Prolonged stimulation by an image on
the retina adapts or desensitizes part
of retina.
o That part of retina has a weaker
response to subsequent to stimulation.
o Demo Fig. 7.16
2
Negative afterimages are a temporal
version of lateral inhibition.
o In simultaneous lightness contrast, a
signal received at a different place in
your receptive field inhibits
response.
•
In successive lightness contrast, a
signal received at a later time inhibits
response in the receptive field.
• Try it in home;