• Homework Set 8:
Due Monday, Nov. 18
From Chapter 9:
P10, P22, P26, P30, PH3,
From Chapter 10:
P4, P5, P9
How do we see color?

There is only one type of rods. It can only
tell the intensity of the light, not its color.
One cannot see color under scotopic conditions
Three types of cones?
Because the cones can differentiate
colors, there must be more than one type
of cones.
 Thomas Young (1801) postulated three
types of cones --- Trichromacy, based on
the three attributes of color: hue,
saturation, lightness.

 need
three inputs to get three outputs
Helmholtz three cones

Von Helmholtz postulted three response
curves for the three types of cones:
 S-cones:
has the best response to short
wavelength of light
 L-cones: …. to long wave length of light
 I-cones: ….. to the intermediate wavelength of light.

Different colors correspond to different
patterns of responses in these cones.
No two colors produce the same response from
the three cones.
Helmholtz hypothetical spectral response curves of human
photoreceptors.
Determine the response curves

Since every wavelength of the visible light
has different color, the response curves
must overlap.
 The
response curves are generally broad and
cover a range of frequencies
 If there were a region of the spectrum where
cones of only one type responded, one
cannot discriminate the colors based on the
wave length along
Determine the response curves

For white light, all cones respond equally.
 All
wavelengths contribute equally to broadband white light.
 An additive mixture of two complementary
lights can also yield white.

From the region of spectral colors without
spectral complementary, we determine
where L & I and S & I responses cross.
The ability for hue discrimination depends
on the wavelength. From this, one can find
the steep rise and falling segments of the
response curves.
 The cone responses can be measured
directly from microspectrophotometry.

 Measuring
the amount of light absorbed by
the cones when it passes through it.
 Only three types of responses have been
found!
Spectral absorptions by 3-cone types
Consistency between Response
curves and color mixing



Consider matching 490-nm spectral cyan
which excites the S and I-cones equally and a
little bit of L-cones.
Use 460-nm blue and 530-nm green and 650nm red to match cyan. The blue and green
must be roughly the same amount. However,
this mixture excite the L-cones too much, we
need anti-red to de-excite the L-cones.
The information contained in the chromaticity
diagram is consistent with that of the response
curves.
Four Psychological Primaries
When we mix red and green, we don’t see
reddish green; we see yellow.
 When we mix cyan and yellow, we don’t
see yellowish cyan, we see green.
 Thus to tell what colors look like, we need
four primaries: blue, green, yellow and red.
Any hue can 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, was in
competition with trichromacy.

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.

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
Y-b
signal
R-g
+
I
+
L
+
-
+
type of cones
contribution to the
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.
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 the white.
 When you see too much yellow, the L and
I cones excited and tired. So when you
see the white, the S cones respond more
than L and I cones. This gives you a
feeling of blue!
 Illusion: Bidwell’s disk.

Positive afterimage
Movies and TV.
 The positive afterimage retains its original
color.

 Since
the cones recover at different rate, you
may experience the change of the color with
time.