Chapter 9: Color
• Color mixtures
– Additive Mixing
– Partitive Mixing
– Subtractive Mixing
• Colored Inks and Paint
– Watercolors
– CMYK Printing
– Halftones
Additive Primary Colors
530-nm green
yellow
650-nm red
cyan
magenta
460-nm blue
Additive Mixing: Yellow
Display Pixels: Partitive Mixing
• What it instead, we make the different colors
coming from separate, very small, very closely
spaced points
• You eye cannot see them as separate sources, so
the colors mix and you see the same color. This is
called partitive mixing
Concept Question: Partitive Mixing
A computer pixel is composed
of red, green and blue lights. If
the red and green ones are
illuminated, what color does it
appear from a distance?
a)
b)
c)
d)
e)
Green
Red
Yellow
Blue
Magenta
Un-illuminated pixel
Pixel with red and green
lights illuminated
Partitive Mixing: Pixels
Subtractive Mixing
• In additive mixing, we added the wavelengths that
were hitting the eye: say red light and green light
• What about things like filters and dye pigments?
• These mix by subtractive mixing
Subtractive Mixing
• Suppose we have a range of wavelengths hitting some
object:
• Three things can happen to each wavelength of light:
– Reflection: that particular wavelength bounces off the object
– Transmission: that wavelength passes through the object
– Absorption: that wavelength is soaked up by the object
• In general different things happen at different
wavelengths
Colored Filters
• Recall that magenta is the additive mixture of
blue and red
• If a light looks magenta, it means that red and
blue light is reaching our eyes, thus a magenta
filter must transmit blue and red light, and
subtract, by reflection or absorption, green light.
=
Colored Filters
• What happens if we layer colored filters?
• Filters subtract light by absorption or reflection
=
Incident white light
Magenta filter
subtracts green
Cyan filter
subtracts red
Only blue
gets
through
Colored Filters
• Another example:
=
Incident white light
Magenta
filter subtracts
green
Yellow
filter subtracts
blue
Only red
gets
through
Concept Question: Colored Filters
=
Incident white light
What color makes it through both the cyan and yellow
filters?
a) Red
b) Blue
c) Green
Subtractive Mixing: Primaries
• The subtractive primaries
are
– Cyan
– Magenta
– Yellow
• In subtractive mixing,
combining complementary
colors produces black
Subtractive Mixing: Reflection
• Colored filters subtract some colors and transmit
others.
• Subtractive mixing can also occur when a colored
surface absorb some colors and reflect others
White in
Magenta out
A magenta colored surface
absorbs (subtracts) green
and reflects red and blue,
thus appeared to be
magenta
Subtractive Mixing: Reflection
A green surface absorbs (subtracts) red and blue
(magenta), and reflects green
White in
Green out
Subtractive Mixing: Reflection
• The colors of light that are reflected from a
surface mix additively when the hit your retina.
• The subtractive part of the mixing is the light
hitting the surface and part of it being absorbed
or transmitted.
• A surface that reflects both red and green light
will appear yellow by additive mixing of the red
and green light that reaches your eyes
Earth-orbiting satellites
take pictures of Earth
at different
wavelengths to
determine the health
of crops and water
Subtractive Mixing: Real Filters
• Real filters are non-ideal, they transmit a range of
wavelengths, not just one. We can specify which
wavelengths using an intensity distribution curve.
Magenta Filter Transmission
Cyan Filter Transmission
Subtractive Mixing: Colored Light
• We have so far been assuming that we are
illuminating our colored filters and surfaces with
uniform white light.
• But most light is not uniform white, or even white
at all.
• How can we figure out what objects will look like in
non-ideal or non-uniform light?
Colored Light: Example
Fluorescent light emission
Reflectance of a magenta shirt
Combining Intensity Curves
To combine two
intensity curves,
you multiply the
curves at each point
to get the combined
curve
When illuminated
by this fluorescent
light, this magenta
shirt will appear
gray (colorless)
Combining Intensity Curves: Example
Concept Question
The black line is the intensity
distribution curve of a “daylight”
CFL.
The purple line is the reflectance
curve of a magenta colored
surface.
What color does the surface
appear to be when illuminated
with this light?
a) Magenta
b) Red
c) Blue
d) Purple
Color Mixing: Ink and Paint
• Color mixing with ink and pigment is in general, a
complex mixture of additive and subtractive
mixing.
• Light rays hitting paint or ink on a piece of paper
can interact with the pigments in several ways
Color Mixing: Ink and Paint
• Some light is reflected from the surface
• Some light passes through the ink and reflects
from the paper underneath, while the ink absorbs
some wavelengths
• The rays from these two processes combine in
the eye, mixing additively
Color Mixing: Ink and Paint
• This process only occurs if the ink or paint is at
least semi transparent, such as watercolors and
some printing inks.
Watercolors
• Watercolor is a challenging medium, because the transparent
colors have to be layered carefully to avoid a muddy looking
appearance
• As light passes through each color layer, more wavelengths are
subtracted, just like layering a set of filters
• The light also reflects off the paper underneath, so the
reflectance curve of the paper itself also subtracts some colors.
Process Ink Printing
• The transparent inks used to print colored documents
and packaging are called “process inks”
• A printer wants to use the best primaries possible to
create the largest number of colors from the smallest
number of printing plates.
• Each separate color requires a separate printing plate
or roller
Process Ink Printing
• The printing primaries are
–
–
–
–
Cyan
Magenta
Yellow
Black
• Black is necessary because the printing inks are not
ideal, and combining them together does not produce
a true black, but a slightly color-tinged black.
• This is also called the “CMYK” color system
Non-ideal Inks
• The reason inks cannot be ideal is that they cannot be
too saturated.
• A narrow reflectance curve (more saturated) reflects
less overall light, so saturated colors would be very
dark
CMYK Printing
CMYK Printing
CMYK Printing
• We have seen that we can generate a range of colors
by subtractive mixing our CMYK primaries
• But what it we want to print lighter, less saturated
colors?
• Diluting the inks would require a new printing plate
for each color.
CMYK Printing: Halftones
• To avoid having to add even more printing plates or
rollers, printers use a variation of partitive mixing to
generate lighter colors
• They simply print less ink in a given area
• This is called “half-tone“ printing, and
is common in newspaper and magazine
printing
Halftones
When ink dots overlap: subtractive mixing
Ink dots next to each other, partitive/additive mixing