Graphics Programs Reference
In-Depth Information
taught me about ROY-G-BIV, a mnemonic aid to remembering the order
of colors in the spectrum: Red-Orange-Yellow-Green-Blue-Indigo-
Violet, with a continuum of
hues
between each.
Most colors are not the pure spectral hues we see in the rainbow. We
see these pure spectral colors only rarely from devices like lasers. Instead,
most colors we experience are a mixture of many wavelengths at differ-
ent intensities. A very important term to understand when discussing
color is the
illuminant
. An illuminant is a description of a real or imagi-
nary light source described by what scientists call a
spectral power distri-
bution
curve (SPD). An SPD is a graph of intensity for each wavelength
in the visible spectrum. Defined this way, illuminants are an absolute,
unambiguous measurement of a light source.
Another important term is
spectrophotometry
. When you shine an
illuminant on a patch of color and measure the light reflected back you
have an exact map of the color in the form of a curve. This type of
spectrophotometric data, often referred to as the Spectral Data, is very
important to color management. The device used to take this kind of
measurement is called a
spectrophotometer
.
The phenomenon of “color” exists only inside our brain, a sensation
in the mind created by various frequencies of light falling upon the retina.
A red apple does not emit red light. Rather, it absorbs all the shorter
wavelengths of light shining on it and reflects the longer wavelengths. A
receptor in the retina that is sensitive to longer wavelengths is stimulated
and sends a signal to the visual cortex. The shorter wavelength receptors
do not send a signal. The visual cortex processes this pattern of signals
and associates them with a sensation of color. Another part of the brain
associates that sensation with the word “red” so you can describe the
color using language.
It is important to note that due to the primitive nature of our visual
system (we have only three color receptors), many very different mixes
of frequencies produce the exact same sensation in our brain. This means
that two colors with very different spectral properties may look identi-
cal. This is what allows us to simulate colors on a printed page or on a
computer screen using very different methods.
Many years after Sir Isaac, other scientists discovered that they could
simulate many colors with only three
primaries
: red, green, and blue.
These are known as additive primaries of light. If we start with none of
these colors, we have no light, and thus black. As we begin adding certain
proportions of red, green, and blue light, we can simulate most colors.
When we have equal amounts of each, we perceive neutral colors (from
dark gray to white) as seen in Fig. 1-5. We are adding the colors together,
thus the term
additive primaries
.
CMY (Cyan/Magenta/Yellow) is a cousin of RGB, known as the
sub-
tractive primaries
. We start with white, perhaps something like paper, and
add density, using CMY colorant (inks, dyes, toner, or pigments) until we
reach black as seen in Fig. 1-5. This is the opposite of the additive RGB
Definition
Hue:
The pure spectral
colors of the rainbow.
Definition
Primary colors:
Basic
color building blocks used
to create other colors. The
most common primaries
are red, green, and blue,
the additive primaries of
light. Another common
primary system is cyan,
magenta, and yellow.
Referred to as the
subtractive primaries, these
colors of pigment each
subtract one of the
additive primaries from
white light. Note that not
all color systems
necessarily use red, green,
and blue as primary colors.
Green is not a primary
color when mixing paint,
but yellow, red, and blue
are.
