How do we see color?
• There is only one type of rod. It can only
tell the intensity of the light, not its color.
• Because the cones can differentiate colors,
there must be more than one type of cone.
• Thomas Young (1801) (this is the Young of
double slit experiment fame) postulated three
types of cones --• Trichromacy, based on there being three
attributes of color: hue, saturation,
lightness. (need three inputs to get three outputs)
If we just had rods…
How would we tell a small amount of light at l1 from a
large amount of light at l2?
We would see in black and white
- as we do under scotopic conditions
Helmholtz three cone types
• Hermann von Helmholtz (1821-1894)
postulated three response curves for the
three types of cones to explain Young’s
trichromacy:
– S-cones: has its best response to short
wavelengths of light
– L-cones: …. to long wavelengths of light
– I-cones: ….. to the intermediate wavelengths of light.
• Different colors correspond to different
patterns of response in these cone types.
Determine the response curves
• Since every wavelength of the visible light
is perceived as a different color, the
response curves of the three cone types
must overlap. Why is this true?
• For white light, all cones respond equally.
• From the region of spectral colors without
spectral complementaries (greens from 495
to 565 nm), we can determine where L & I
and S & I responses cross.
C.I.E. Chromaticity Diagram
S
I
L
I > S and L then complement must have S & L > I
(because green + purple (magenta) = white
Igreen + Ipurple = Sgreen+Spurple=Lgreen+Lpurple)
Can’t be done with a single wavelength
Purple (magenta) is a double humped distribution
• The ability for hue discrimination depends
on the wavelength. From this, one can find
the steep rise and falling segments of the
response curves.
nm
• The cone responses can be measured
directly from microspectrophotometry.
– Measure the amount of light absorbed by the
cones as a function of wavelength.
– Only three types of responses have been found!
Corresponds to three types of cones
(trichromacy).
(short)
(intermediate)
(long)
Response curves and color mixing
• The information contained in the
chromaticity diagram is consistent with that
of the response curves.
• Pick two colors that are complements. When
they combine to produce white, they must be
exciting the S, I, and L cones about equally,
just like broadband white.
Bees are Trichromatic also
Chromaticity Diagram for Bees
Four Psychological Primaries
• When we additively mix red and green, we
don’t see reddish green; we see yellow.
• A subtractive mixture of cyan and yellow
gives green, not yellowish cyan.
• Thus to tell what colors look like, we need
four primaries: blue, green, yellow and red.
Any hue can be verbally described as a
combination of them.
Color Opponents
• Using the psychological primaries, we can
name all spectral colors.
– Red and green are opposite colors; they never
appear at the same wavelength.
– So are yellow and blue.
• We call them opponents.
• The opponent color theory was put forward
by Ewald Hering (1866-1948) and was in
competition with trichromacy.
Color Afterimage
+
+
Stare at the plus on the left side for 30 s. Then quickly move your gaze to the plus
on the right side. You should see yellow and magenta circles.
Color Cancellation
• If a color is too bluish, it can be made less
bluish by adding yellow. The amount of
yellow that one adds to cancel the blue
entirely determines the blueness of the
original color.
• The yellowness and blueness, and redness
and greenness can be used to describe a
color quantitatively.
Opponents:
Green:red and Blue:yellow
Which is correct :
trichromacy or opponent
processing?
Biology : trichromacy
Psychology: opponent processing
Answer: 3 cone types are wired in the retina to
give opponent processing!
Opponent Processing
• The responses of the L,I,S cones are
processed by three opponent channels:
yellow-blue, red-green, and white-black. It
is the latter information which is passed to
the brain.
•
S
I
L type of cones
y-b
r-g
+
+
-
+
+
w-bk +
+
+
contribution to the signal
Opponent Processing
S
-
+
I
+
Y-Blue
Chromatic channel
+
L
+
+
W-Black
Luminance channel
- to brain -
+
-
+
R-G
Chromatic channel
Opponent Process Curves
Simultaneous Color Contrast
• Imbed a gray region among yellow and
green. The yellow region makes the gray
look bluish and the green region makes the
gray reddish. Thus a region of the retina
contains spatial opponency -- lateral
inhibition of color.
– The center is r-g, the surrounding is g-r.
– Used widely in impressionists’ paintings.
Simultaneous Color Contrast
Trichromacy and Opponent
Color Theory
• We now appear to have a consistent picture
of how signals from the three types of cones
are combined into r-g and y-b opponent
chromatic visual channels.
• The two competing theories of color vision
both give part of the story.
Color Constancy
• Objects tend to retain the same perceived
color even though the coloration of the
overall illumination may change.
– A biological necessity.
– Comes from lateral inhibition.
• The color constancy is nor perfect. It
depends on the state of adaptation.
Negative afterimage
• When you are adapted to yellow, you will
see blue in a white area.
• When you look at yellow for a while, the L
and I cones are excited and become tired.
So when you avert your gaze to white, the S
cones respond more than L and I cones.
This gives you an impression of blue!
• Illusion: Bidwell’s disk.
Greener than Green
Positive afterimage
• Movies and TV.
• The positive afterimage retains its original
color.
– Since the cones recover at different rate, you
may experience a change of the color with time.