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# PowerPoint Presentation - Physics 1230: Light and Color Chapter 1

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```Physics 1230: Light and Color
Chapter 10
вЂў Chapter 10: Color
Perception
вЂў How we see color
вЂў Three types of cones - each
with different responses at
all wavelengths
вЂў Color matching
вЂў Opponent processing
вЂў Color blindnesses
(deficiencies in seeing
color)
вЂў Spatial color processing in
receptive fields
1
We have three different kinds of cones вЂ” whose responses
are mainly at short, intermediate and long wavelengths
вЂў s-cones absorb short wavelength light best,
with peak response at 450 nm (blue)
вЂў L-cones absorb long wavelength light best,
with peak response at 580 nm (red)
вЂў i-cones absorb intermediate wavelengths
best, with peak response at 540 nm (green)
вЂў 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.
вЂў 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
mixture of blue and
yellow?
вЂў A. Green;
вЂў B. Red;
вЂў C. White;
вЂў D. Black;
вЂў E. Orange;
вЂў We learned this in
Chapter 9вЂ¦ Why do
we still need this?
Lig ht color
Brightn ess
46 0 nm blue
1
57 5 nm ye llow
1 .6 6
M ixtur e ( perc eiv ed as whit e )
S -cone r es p onse
60
0
60 + 0 = 6 0
I-cone r es p onse
5
1 .66 x 3 3
5+1 .66 x 33 = 6 0
L -co n e r es p onse
2
1 .66 x 3 5
2+1 .66 x 35 = 6 0
Examples of two different ways we see white
вЂў Our sensation of color depends on how much total s, i
& L cone response occurs due to a light intensitydistribution
вЂў Multiply the intensity distribution curve by each
response curve to determine how much total S, i,
and L response occurs
вЂў We experience the sensation white when we have
equal total s, i & L responses
вЂў There are many ways this can occur!!
вЂў E.g., when broadband light enters our eye
вЂў Another way to experience white is by viewing a
mixture of blue and yellow
вЂў E.g., 460 nm blue of intensity 1 and 575 nm
yellow of intensity 1.66
вЂў The blue excites mainly s-cones but also a
bit of i-cones and a bit of L-cones
вЂў The yellow excites i-cones and (slightly
more) L-cones but no s-cones
вЂў 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
вЂў Our sensation of yellow depends on a special s, i &
L cone response
вЂў We experience the sensation yellow when 575 nm
light reaches our eyes
вЂў What really gives us the sensation of yellow is
the almost equal response of i and L cones
together with no s-cones!!
вЂў Another way to experience yellow is by seeing
overlapping red & green lights
вЂў E.g., 530 nm green of intensity 1 and 650
nm red of intensity 2.15
вЂў The green excites mainly i-cones but also
L-cones, while the red excites mainly Lcones but also i-cones
вЂў 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
вЂў 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
вЂў Missing one type of cone results in one type
of color-blindness
вЂў E.g., someone whose L-cone is missing
will not see colors correctly
вЂў 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
вЂў 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
вЂў All colors they see can be obtained by
mixing only 2 different wavelength lights
вЂў This type of color-blindedness is called
protanopia (a kind of dichromacy)
вЂў 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?
вЂў Missing 2 or all 3 type of cones results
in a different (rare) type of colorblindness called monochromacy
вЂў Cone monochromats have only one type
of cone (s, i or L).
вЂў Rod monochromats have no cones and
have difficulty seeing with their rods
under bright light (photopic) conditions
вЂў Monochromats can match a light of any
color by varying the intensity of only
one spectral (wavelength) light
вЂў They are truly color-blind because they
cannot distinguish any wavelength color
from any other
вЂў They see in blacks, whites and greys
вЂў 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
вЂў Can rod
monochromats
distinguish red color
from green color?
вЂў A. Yes
вЂў B. No
вЂў C. Only during a
bright day;
вЂў D. Only during gthe
night;
The four psychological primaries
primaries (RGB) and the
subtractive primaries (CMY)
there is another set of (4)
primary colors, called the
psychological primaries
п‚§
п‚§
п‚§
п‚§
Blue
Green
Yellow
Red (really closer to magenta)
вЂў These hues can be used to
describe all other hues.
вЂў All hues can be verbally described as
combinations of these colors. For
example,
вЂў
вЂў
вЂў
вЂў
Yellowish red
Greenish yellow
Bluish green
Bluish red
вЂў BUT we don't recognize hues such as
вЂў Reddish green
вЂў Yellowish blue
вЂў Red and green are opponent hues
вЂў 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
вЂў We don't need orange, purple or pink:
вЂў orange can be thought of as yellow-red
вЂў purple can be thought of as red-blue
вЂў 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
вЂў The endpoints of the yellow line are 580
nm "unique" yellow and 475 nm
"unique" blue
вЂў 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.
вЂў 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.
вЂў We call this perception our y-b
channel
вЂў Yellow & blue are opponents
вЂў Moving parallel to the red line from
500 nm green towards nonspectral
red we see more redness in each color
and less greenness.
вЂў We call this perception our r-g
channel
вЂў Red and green are opponents
вЂў 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?
вЂў 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)
вЂў 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)
вЂў "wiring" is the following:
вЂў When light falls on the L-cones they tell all 3
neural cells to increase the electrical signal they
send to the brain
вЂў 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
вЂў 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
пЂ­
++
neural cell
for y-b
chromatic
channel
i-cone
L-cone
+ пЂ­ +
+ ++
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?
вЂў The neural cell for the y-b chromatic
channel has its signal
s-cone
i-cone
L-cone
вЂў inhibited when (bluE) light excites the
s-cone
INTERPRETED AS BLUE
вЂў enhanced when light excites the i & L
cones
INTERPRETED AS YELLOW
вЂў The neural cell for the r-g chromatic
channel has its signal
вЂў
вЂў inhibited when (green) light falls on the
пЂ­ ++ + пЂ­ +
i-cone
INTERPRETED AS GREEN
neural cell
neural cell
вЂў 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
вЂў The neural cell for the y-b chromatic
channel has its signal
s-cone
i-cone
L-cone
вЂў inhibited when (bluE) light excites the
s-cone
INTERPRETED AS BLUE
вЂў enhanced when light excites the i & L
cones
INTERPRETED AS YELLOW
вЂў The neural cell for the r-g chromatic
channel has its signal
вЂў
вЂў inhibited when (green) light falls on the
пЂ­ ++ + пЂ­ +
i-cone
INTERPRETED AS GREEN
neural cell
neural cell
вЂў 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)
вЂў Monochromacy (can match any colored light
with any 1 spectral light by adjusting
вЂў
intensity)
вЂў Either has no cones (rod monochromat)
or has only 1 of the 3 types of cones
working (cone monochromat).
вЂў Sees ony whites, greys, blacks, no hues
вЂў Dichromacy (can match any colored light
with 2 spectral lights of different intensities of
(rather than the normal 3)
вЂў L-cone function lacking = protanopia
вЂў i-cone function lacking = deuteranopia
вЂў s-cone function lacking = tritanopia
вЂў 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)
вЂў Protanomaly
вЂў Shifted L-cone response curve
вЂў Deuteranomaly (most common)
вЂў Shifted i-cone response curve
вЂў Confusion between red and green.
вЂў Tritanomaly
вЂў Yellow-blue problems: probably
defective s-cones
вЂў Neuteranomaly
вЂў ineffective r-g channel
Visualizing dichromacy:
protanopia
вЂў No L-cone function
вЂў See yellows & blues instead of reds & greens
вЂў Neutral hue pts. below 500 nm & nonspectral magenta; neutral line close to r-g
line in chromaticity diagram (effectively
missing)
вЂў 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
вЂў i-cone function lacking
вЂў Like protanopes, they see yellows &
blues instead of reds & greens
вЂў Neutral hue points near 500 nm and
non-spectral purple
вЂў Neutral hue line close to the line
joining the neutral points in the
chromaticity diagram
вЂў Hence, like protonopes, deuteranopes
don't distinguish green from red very
well
Visualizing dichromacy:
tritanopia
вЂў s-cone function lacking
вЂў
They see reds and greens instead of blues
and yellows
вЂў Neutral hue points at 570 nm and blue
purple; neutral hue line between neutral
hue points in the chromaticity diagram
which is effectively missing
вЂў
Hence, they don't distinguish blues and
yellows very well
вЂў Tetartanopes lack the y-b channel
вЂў
See similarly to tritanopes
Take the color blindness test
вЂў The color blindness test consists
of a set of five charts. Each chart
shows a number in one color on
a different backgound color.
вЂў 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.
вЂў 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
вЂў Double opponent receptive fields in
our retina
вЂў are responsible for lateral
inhibition, just like light-dark
receptive fields we have studied
вЂў enable us to notice sharp color
boundaries in the same way that
light-dark receptive fields allowed
us to notice sharp light-dark
boundaries
вЂў 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
вЂў are responsible for color constancy
in the same way that light-dark
receptive fields were responsible for
lightness constancy
вЂў consist of photoreceptors in a
center-surround geometry, all
pooled to one final neural cell
(ganglion cell)
вЂў There are two types of doubleopponent receptive fields (each
paired with its own neural cell)
вЂў The r-g receptive field and cell
вЂў The y-b receptive field and cell
Receptive field of a double-opponent
cell of the r-g type
вЂў 2 different ways to INCREASE the
signal the ganglion cell sends to brain
вЂў Red light falling on cones in center
of receptive field attached to
ganglion cell
вЂў Green light on surround
вЂў 2 different ways to decrease the
signal the ganglion cell sends to the
brain
вЂў Red light on surround
вЂў Green light on center
вЂў Electrical signal to brain from ganglion
cell is at ambient level when no light is
on center or surround
вЂў When signal to brain is
INCREASEDwe interpret that as red
вЂў 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
вЂў A double-opponent cell differs
from a single opponent cell
вЂў In both of them R in the center
increases the signal
вЂў In a single-opponent cell G in
surround would inhibit signal,
whereas in double-opponent cell
G enhances
вЂў In a double-opponent cell
вЂў R in center enhances signal
(ganglion cell signals red)
вЂў G in surround enhances signal
(ganglion cell signals red)
вЂў R in surround inhibits signal
(ganglion cell signals green)
вЂў G in center inhibits signal
(ganglion cell signals green)
Fictional cell
real cell
Concept Question:
вЂў 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:
вЂў 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:
вЂў 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+y-
y+b-
Concept Question:
вЂў 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
вЂў Look at the grey squares in your
peripheral vision
вЂў Does the grey square
surrounded by yellow appear to
take on a tint?
вЂў What color is it?
вЂў Repeat for the grey squares
surrounded by
вЂў Blue
вЂў Green
вЂў Red (pink)
Color constancy depends on doubleopponent processing
вЂў 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
вЂў This is because our double-opponent
receptiive fields compare neighboring
colors and are not very sensitive to an
overall change in color
вЂў Color constancy developed in the
evolution of mankind so that we could
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
вЂў Here are the enhanced edges
channel
вЂў Note the edges that separate a
yellowish from a bluish color are
enhanced the most
вЂў Here are the enhanced edges
channel
вЂў Note the edges that separate a
reddish from a greenish color are
enhanced the most
вЂў Here are the enhanced edges
achromatic channel
вЂў 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
response.
вЂў
In successive lightness contrast, a
signal received at a later time inhibits
response in the receptive field.
вЂў Try it in home;
```
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