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Color Systems (Color Spaces)
HSV (hue, saturation,
value),
also known as HSB (hue, saturation, brightness) is often
used by artists because it is often more natural to think about a color
in terms of hue and saturation than in terms of additive or subtractive
color components. HSV is a transformation of an RGB colorspace, and its
components and colorimetry are relative to the RGB colorspace from which
it was derived.
HSL (hue,
saturation, lightness/luminance), also known as HLS
or HSI (hue, saturation, intensity) is quite similar to
HSV, with
"lightness" replacing "brightness". The difference is that the
brightness of a pure color is equal to the brightness of white,
while the lightness of a pure color is equal to the lightness of
a medium gray.
Hue, also called tone or tint is the property which distinguishes
one color from another. Saturation, also called chroma or
intensity, measures the purity of a color. Value, also called
lightness or brightnes, is a measure of how dark a color is.
RGB
(Red, Green Blue),
the system used in computer and video monitors, uses
additive color
mixing, because it describes what kind of light needs to be
emitted to produce a given color. Light is added together to create
form from out of the darkness. RGB stores individual values for red,
green and blue.
RGBA is RGB
with an additional channel, alpha, to indicate transparency. Common
color spaces based on the RGB model include
sRGB,
Adobe RGB
and
Adobe Wide Gamut RGB.
CMYK
(Cyan, Magenta, Yellow and Black),
also called process color or four color process, is the method of
reproducing colors for print. CMYK uses
subtractive color
mixing used in the printing process, because it describes what kind of
inks need to be applied so the light reflected from the substrate
and through the inks produces a given color. One starts with a white
substrate (canvas, page, etc), and uses ink to subtract color from white
to create an image.
CMYK stores
ink values for cyan, magenta, yellow and black. There are many CMYK
colorspaces for different sets of inks, substrates, and press
characteristics (which change the dot gain or transfer function for each
ink and thus change the appearance).
Hexachrome:
Each color space reproduces a different area on the visible spectrum
(see "Gamuts"). Because we create color in the RGB color space and print
in the CMYK space, there have been various efforts to make the CMYK
color gamut more closely reproduce the RGB gamus by adding additional
colors to the traditional four color process. Pantone has produced what
is becoming the standard, a six color printing process called "Hexachrome".
Hexachrome adds orange and green to the cyan, magenta, yellow and black,
and produces a larger gamut, incorporating more of the colors in the RGB
color space. (See "Gamuts" for a representation of the Hexachrome gamut.)
CIE L*a*b* (CIELAB)
is the most complete
color model used conventionally to
describe all the colors visible to the human eye. It was developed for
this specific purpose by the
International Commission on Illumination
(Commission Internationale d'Eclairage, hence the CIE
acronym in its name). The * after L, a and b are part of the full
name, since they represent L*, a* and b*, derived from L, a and b.
The three parameters in the model represent the luminance of the color (L,
L=0 yields black and L=100 indicates white), its position between red
and green (a, negative values indicate green while positive
values indicate red) and its position between yellow and blue (b,
negative values indicate blue and positive values indicate yellow). See
Lab Color Space.
The Pantone Matching System (PMS)
is the internationally accepted system for producing spot color by
printing with the specific color ink. Generally, designers should always
use Pantone color specifications to guarantee proper spot color. The
only way to see a color properly is on a printed sample, thus all
designers should buy the Pantone Color Specifiers, which come in coated
and uncoated versions. This may sound expensive, but the specifiers
include color swatches which you use to accompany a print job to
guarantee spot color fidelity. The Pantone web site is
http://www.pantone.com/ . For specific information on color
specifiers, see the "Graphics" section from the home page or click here
for
Pantone
Formula guides for Coated and Uncoated Stock. Pantone also offers
color profiling and calibration tools from the "Color Mangement" link on
their home page. (See "Color Management"). You cannot properly reproduce
the full Pantone gamut (see "Gamuts") in HTML color, but see this
PMS Color Chart for a look at the Pantone
system and select the Pantone color libraries from your desktop
publishing or image editing program.
Duotones: Duotones are used to
increase the tonal range of a grayscale image. Although a grayscale
reproduction can display up to 256 levels of gray, a printing press can
reproduce only about 50 levels of gray per ink. This means that a
grayscale image printed with only black ink can look significantly
coarser than the same image printed with two, three, or four inks, each
individual ink reproducing up to 50 levels of gray.
Sometimes duotones are printed using a
black ink and a gray ink--the black for shadows and the gray for
midtones and highlights. More frequently, duotones are printed using a
colored ink for the highlight color. This technique produces an image
with a slight tint to it and significantly increases the image's dynamic
range. Duotones are ideal for two-color print jobs with a spot color
(such as a PANTONE Color) used for accent.
Because duotones use different color inks
to reproduce different gray levels, they are treated in Photoshop as
single-channel, 8-bit, grayscale images. In Duotone mode, you do not
have direct access to the individual image channels (as in RGB, CMYK,
and Lab modes). Instead, you manipulate the channels through the curves
in the Duotone Options dialog box.
(From Adobe Photoshop CS Help)
(Drawn largely from Wikipedia, the
free encyclopedia�s treatment on Color Spaces at
http://en.wikipedia.org/wiki/Color_model)
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