Chapter 9: Color• Color mixtures

– Additive Mixing– Partitive Mixing– Subtractive Mixing

• Colored Inks and Paint– Watercolors– CMYK Printing– Halftones

Additive Primary Colors

cyan

magenta

yellow

650-nm red

530-nm green

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) Greenb) Redc) Yellowd) Bluee) 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.

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Colored Filters

• What happens if we layer colored filters? • Filters subtract light by absorption or reflection

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Incident white light Magenta filter subtracts green

Cyan filter subtracts red

Only bluegetsthrough

Colored Filters

• Another example:

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Incident white light Magentafilter subtractsgreen

Only redgetsthrough

Yellowfilter subtractsblue

Concept Question: Colored Filters

What color makes it through both the cyan and yellow filters?

a) Redb) Bluec) Green

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Incident white light

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

A magenta colored surface absorbs (subtracts) green and reflects red and blue, thus appeared to be magenta

White in Magenta out

Subtractive Mixing: Reflection

White in Green out

A green surface absorbs (subtracts) red and blue (magenta), and reflects green

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 QuestionThe 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) Magentab) Redc) Blued) 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, andis common in newspaper and magazineprinting

Halftones

When ink dots overlap: subtractive mixingInk dots next to each other, partitive/additive mixing