• The amount of refraction depends on the index of refraction AND the
index of refraction depends on the wavelength
• Instrument that measures the luminosity
emitted at each wavelength (or the
spectral power distribution)
Spectral power distributions
– Black and white
– Sensitive to low light
– 120 million
– Less sensitive
– Three types of cones
which form color
– 7 million
• At the fovea, each cone
has its own optic nerve
• Outside the fovea,
each nerve fiber is
shared by about 80
rods or cones
• This results in the
of our vision
• Some of the visual
processing for each
eye is done using the
visual cortex of both
the left and right side
of the brain
• This offers redundancy
in the case the brain is
• Visual cortex: processes the raw input from the
• Visual thalamus: interprets the images produced
by the visual cortex
• When light enters the eye, it hits the rhodopsin.
• Rhodopsin decomposes when it is exposed to
light, changing into several intermediate
compounds, but eventually (in less than a
second) forms activated rhodopsin.
• This chemical causes electrical impulses that
are transmitted to the brain and interpreted as
• Once the rhodopsin has
been activated, it takes time
to return to its original state.
• This recovery time increases
• Three processes to deal with low-light
– Pupil dilates to collect more light
– The visual center shifts its definition of “black”
and changes its perceived contrast levels
– Because less light is reaching retina, more
receptors are in the ready phase (rhodopsin)
rather than the bleached phase (activiated
rhodopsin) and the retina becomes more
Color sensitivity of the eye
• The visual system’s best performance:
– Can see about 1000 levels of light-dark
– 100 levels of red-green
– 100 levels of yellow-blue
• This means that the total number of colors
we can see is about 1000 x 100 x 100 =
10,000,000 (10 million).
• When our eyes are exposed to a color for a
prolonged period, the rods & cones become
• When the eyes are then diverted to a blank
space, these photoreceptors send out a weak
signal and those colors remain muted.
• However, the surrounding cones that were not
being excited by that color are still "fresh", and
send out a strong signal.
• The signal is exactly the same as if looking at
the opposite color, which is how the brain
• The human eye can perceive scenes with a very
high dynamic contrast ratio, around 1,000,000:1.
• Adaptation is achieved in part through iris
dilation and slow chemical changes, which take
• At any given time, the eye's static range is
smaller, around 10,000:1. However, this is still
generally higher than the static range achievable
by most display technology
• Color constancy is a feature of the human color
perception system which ensures that the
perceived color of objects remains relatively
constant under varying illumination conditions.
• A green apple looks green to us at midday,
when the main illumination is white sunlight, and
also at sunset, when the main illumination is red.
This helps us identify objects.
• Subtractive color: used in situations where
wavelengths are absorbed
• Additive color: used in situations where
wavelengths are emitted
– lighting design
– computer monitors
• Chroma: How pure a hue
is in relation to gray
Saturation: The degree of
purity of a hue.
Intensity: The brightness
or dullness of a hue. One
may lower the intensity by
adding white or black.
Luminance / Value: A
measure of the amount of
light reflected from a hue.
Those hues with a high
content of white have a
higher luminance or value.
• Shade: A hue
produced by the
addition of black.
Tint: A hue
produced by the
addition of white.
• When mixing colors using paint, or through the
printing process, the subtractive color method is
• Subtractive color mixing means beginning with
white and ending with black.
• As one adds color, the result gets darker and
tends to black.
Transmission and Reflection of
• The subtractive primaries combine to form
Color halftoning with CMYK
Four color printing
• cyan, magenta, yellow, and key (black)
• subtractive color model used in color
• ink is typically applied in the order of the
• When working on a computer, the colors seen on the
screen are created with light using the additive color
• Additive color mixing begins with black and ends with
• As more color is added, the result is lighter and tends to
Additive color mixing
• Any three colors of light
that can be mixed to
produce white light are
called primary colors.
• Red, green, and blue
are the most commonly
used primary colors.
Filters are designed to pick out
desired wavelengths of light
Neutral density filter
• Neutral density filters reduce the
transmission of all wavelengths equally.
Spectral reflectance curves show how much of
each wavelength of light is reflected by a surface.
• Spectral reflectance curves have been
used to identify healthy and stressed
populations of plants.
• Any two colors that can be added together
to produce white are called
• The visible spectrum
consists of billions of
• A monitor can display
• A high quality printer
is only capable of
• Colors at their basic
• Those colors that cannot
be created by mixing
• Those colors achieved by a
mixture of two primaries.
• Those colors achieved by a
mixture of primary and
• Those colors located
opposite each other on a
• Those colors located close
together on a color wheel.
Munsell color system
• Ten basic hues
• For each hue, there are
• Each color has a
number specifying its
chroma and value
Hues of the Munsell system
• Each basic hue has the number 5
• Ring of hues
• Value = 6
• Chroma = 6
Specifying a color in the Munsell
• A color is fully
specified by three
• Hue: 5P, color in
middle of purple band
• Value: 5
• Chroma: 10
Ostwald color system
Hues of the Ostwald system
Commission International de
L'Eclairage (CIE) system
• The theory is based on the fact that our
eyes contain three different types of color
receptors called cones.
• These three receptors respond differently
to different wavelengths of visible light.
• This differing response of the three cones
is measured in three variables X, Y, and
Z in the CIE color model.
CIE color matching functions
• Read Ch 8
• Answer 5 questions
• Due next Monday, Apr 20
• Monday, April 6: Chaps 11 & 12
• Thursday, April 9: Chaps 13 & 14
• Friday, Apr 10 at 1:00 pm
• “Mathematics of Soap Bubbles” with Frank
Morgan in Dickinson 225
• Monday, April 13: No class