Digital Color Basics

Rob SnyderJune 2011

Our Eyes are Light Detectors!

Our eyes have two types of photoreceptors called rods and cones. These cells are located in the retina layer at the back of the eye. Rods are used to see in very dim light and only show the world to us in black and white. This is why you see only black and white when you are outside after dark or in a dimly lit room. Cones allow us to see colors. They are not as sensitive as the rods so they only work in bright light.

There are three types of cones in the human eye.

Long wavelength cones have a peak detection of greenish-yellow. Medium wavelength cones have a peak detection of green .

Short wavelength cones detect principally blue and violet colors.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html#c1

Color processing that begins in retinas continues in the brain

Source: http://en.wikipedia.org/wiki/Color_vision

Analyzing Digital Images (ADI) computer software can be used to analyze colors detected by digital camera.

The Eye/Camera Analogy A digital camera can be compared and contrasted

with our eyes. It has a sensor that also detects the intensities of light in the red, green, and blue portions of the electromagnetic spectrum.

The ADI Software/Brain Analogy

ADI software can be compared and contrasted with our brain. The software interprets data it receives from three different sensors in a camera.

Our brain employs an opponent process where differences in the responses of cones are interpreted.

Red, Green, and Blue are commonly referred to as the primary colors of light

Note: Any 3 colors from three different regions of the visible spectrum can be primary colors.

There are many ways to show students how primary colors of light can be mixed.

http://mvh.sr.unh.edu/mvhinvestigations/red_green.htm

The University of New Hampshire’s Measuring Vegetative Health web site has interesting Quick Time movies.

Inexpensive floodlights also can be usedto mix colors of light.

Shining two primary colors of light of equal intensity onto a screen produces pure complementary colors.

cyan

magenta

yellow

Use a data table predict what colors of light must be mixed to produce there complementary colors.

ADI has a rectangle tool that can analyze what happens when blue and green floodlights shine onto a screen.

R = 2%, G = 70%, B = 62% Almost a pure cyan color.

A second data table can be used to record observations.

Red and Blue Floodlights produce a magenta color.

R = 62%, G = 25%, B = 60% A fairly good magenta color.

Red and Green floodlights produce a yellow color.

R = 63%, G = 51%, B = 34% Not a very pure yellow color.

Red, Green, and Blue floodlights produce a white color.

R = 60%, G = 58%, B = 59% Not very white.

Red light has the lowest frequency in the visible spectrum. Green light has a middle frequency. Blue light has a higher frequency.

Light from the different frequency regions can combine to form white light. Therefore, primary colors of light are referred to as additive primary colors.

Any three colors from each of the three frequency regions of the visible portion of the electromagnetic spectrum can be additive primary colors.

Students are often introduced primary colors by mixing blue, red, and yellow paints or food dyes.

As is true with light, other colors can be primary colors.

Cyan, Magenta, and Yellow are a set of primary pigments often used by artists and in printing.

What happens if you mix colors of paint or dye?

Some of you will mix blue, red, and yellow acrylic paints from Michaels ($1.49 a bottle).

Some of you will mix cyan, magenta, and yellow acrylic paints from a art supply store ($7 to $10 a tube).

Some of you will mix blue, red, and yellow food dye.

The Analyze Paint and Dye Colors document describes how to:

Paint a swatch of each primary color on art paper.Mix the three primary colors together and paint a

swatch on art paper.Take a photograph of the swatches of paint or dye

color.Upload the photograph to a file on a computer.Open ADI software.Upload the photograph of colors to ADI software.Use the rectangle tool to analyze the colors.

What did you discover when you used the rectangle tool to analyze paint or dye colors?.

What did you discover about yellow paint or dye?.

R = 62%, G = 45%, B = 0% Not quite a pure yellow.

Primary colors of paints and pigments are called subtractive primary colors.

Red paint absorbs blue and green light. Blue paint absorbs red and green light. Yellow paint absorbs blue and violet. Mixing these three primary colors of paint results in a muddy dark brown color. This means that not all wavelengths of incident light were absorbed.

Cyan absorbs red light. Yellow absorbs blue light. Magenta absorbs green light. A mix of equal amounts of each of these primary colors results in a black color. That means that all wavelengths of incident light were absorbed.