Color Addition
Color perception,
like sound perception, is a complex subject involving the
disciplines of psychology, physiology, biology, chemistry and physics. When you
look at an object and perceive a distinct color,
you are not necessarily seeing a single frequency of light. Consider for
instance that you are looking at a shirt and it appears purple to your eye. In
such an instance, there may be several frequencies of light striking your eye
with varying degrees of intensity. Yet your eye-brain system interprets the
frequencies that strike your eye and the shirt is decoded by your brain as
being purple.
Primary Colors of Light
The subject of color perception
can be simplified if we think in terms of primary colors of
light. We have already learned that white is not a color at all, but rather the presence of all the
frequencies of visible light. When we speak of white light, we are referring to
ROYGBIV - the presence of the entire spectrum of visible light. But combining
the range of frequencies in the visible light spectrum is not the only means of
producing white light. White light can also be produced by combining only three
distinct frequencies of light, provided that they are widely separated on the
visible light spectrum. Any three colors (or
frequencies) of light that produce white light when combined with the correct
intensity are called primary colors of light. There are a variety of sets of primary colors.
The most common set of primary colorsis red (R),
green (G) and blue (B). When red, green and blue light are mixed or added
together with the proper intensity, white (W) light is obtained. This is often
represented by the equation below:
R + G + B = W
In fact, the mixing together (or addition) of two or three of
these three primary colors of light with
varying degrees of intensity can produce a wide range of other colors. For this reason, many television sets and computer
monitors produce the range of colors on the
monitor by the use of red, green and blue light-emitting phosphors.
The addition of the primary colors of light
can be demonstrated using a light box. The light box illuminates a screen with
the three primary colors - red (R), green
(G) and blue (B). The lights are often the shape of circles. The result of
adding two primary colors of light is
easily seen by viewing the overlap of the two or more circles of primary light.
The different combinations of colors produced
by red, green and blue are shown in the graphic below. (CAUTION: Because
of the way that different monitors and different web browsers render the colors on the computer monitor, there may be slight
variations from the intended colors.)
Color Addition Rules
These demonstrations with the color box
illustrate that red light and green light add together to produce yellow (Y)
light. Red light and blue light add together to produce magenta (M) light.
Green light and blue light add together to produce cyan (C) light. And finally,
red light and green light and blue light add together to produce white light.
This is sometimes demonstrated by the following color equations
and graphic:
|
R + B = M G + B = C |
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Yellow (Y), magenta (M) and cyan (C) are sometimes referred to as secondary colors of light since they
can be produced by the addition of equal intensities of two primary colors of light. The addition of these three
primary colors of light with varying
degrees of intensity will result in the countless other colors that we are familiar (or unfamiliar) with.
Complementary Colors of Light
Any two colors of light
that when mixed together in equal intensities produce white are said to be complementary colors of each other. The complementary color of red light is cyan light. This is reasonable
since cyan light is equivalent to a combination of blue and green light; and
blue and green light when added to red light will produce white light. Thus,
red light and cyan light (which is equivalent to blue + green light) represent
a pair of complementary colors of light;
they add together to produce white light. This is illustrated in the equation
below:
R + C = R + (B + G) = White
Each primary color of
light has a secondary color of light as its
complement. The three pairs of complementary colors are
listed below. The graphic at the right is extremely helpful in identifying
complementary colors. Complementary colors are always located directly across from each
other on the graphic. Note that cyan is located across from red, magenta across
from green, and yellow across from blue.
|
Red and Cyan Green and Magenta Blue and Yellow |
|
The production of various colors of light
by the mixing of the three primary colors of
light is known as color addition. The color addition principles
discussed on this page can be used to make predictions of the colors that would result when different colored lights are mixed. In the next part of Lesson
2, we will learn how to use the principles of color addition to determine why different objects look
specific colors when illuminated with
various colors of light.
Check Your Understanding
1. Two lights are arranged above a white sheet of paper. When
the lights are turned on they illuminate the entire sheet of paper (as seen in
the diagram below). Each light bulb emits a primary color of
light - red (R), green (G), and blue (B). Depending on which primary color of light is used, the paper will appear a
different color. Express your understanding
of color addition by determining the color that the sheet of paper will appear in the
diagrams below.
R
+ G ---> Yellow
R
+ B ---> Magenta
B
+ G ---> Cyan
2. Suppose that light from a magenta spotlight and light from
a yellow spotlight are mixed together, will white light be produced? Explain.
Answer: No
The magenta
spotlight can be thought of as a combination of red and blue light in equal
intensities and the yellow spotlight is equivalent to a combination of red and
green light in equal intensities. Observe the double abundance of red.
Combining the light from the magenta and yellow spotlights will produce a
whitish-red color - that is, pink.