HP IndiChrome - Basic Concepts; Color Space

96 Chapter A Special options, basic concepts, and spare parts (all presses) ENWW
Basic concepts
Color space
Every color can be represented by the CIELab coordinates: L*, a*, and b*.
The lightness, L*, indicates whether a color is light, such as Yellow, or dark, such as
Dark Blue. An L* value close to 100 indicates a very light color, whereas a value close to
zero indicates a very dark color.
The coordinate a* indicates the green or red content of a color. A negative value of a*
indicates a green color, whereas a positive value indicates a red color. Most colors used in
printing have values of a* between -80 and 80.
The coordinate b* indicates the blue or yellow content of a color. A negative value of b*
indicates a blue color, whereas a positive value indicates a yellow color. Most colors used
in printing have values of b* between -80 and 120.
The three coordinates L*, a*, and b* form a color space, which is a three-dimensional
space where every color is represented by a point in the space.
The chroma C* is a measure of the brightness of a color. Colors with large C* values are
referred to as bright (saturated) colors, whereas colors with small C* values are referred to
as dull colors.
C* is defined as:
∆E*
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As a default, the HP IndiChrome ink mixing system uses ∆E*
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to quantify differences
between colors. If the value of ∆E*
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is small, the two colors being compared are similar. If
the value of ∆E*
94
is large, the two colors being compared are not similar.
The CIE 1994 Color Difference Model defines ∆E*
94
as a replacement of the more familiar
CIELab ∆E*. The ∆E*
94
color difference improves upon the CIELab ∆E* for color matching
of saturated colors. Furthermore, ∆E*
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is more uniform than ∆E* in the sense that the
perception of the difference between two colors is a direct function of ∆E*
94
and not
dependant upon L*a*b*.
In the IMS software, ∆E*
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and ∆E* refer to the color difference between the actual mix or
best mix and the target color. The ∆E*
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color difference is defined by:
The ∆E* color difference is defined by:
where:
L*, a*, and b* are the color coordinates of either best mix or actual mix.
C
∗
a
∗
()
2
b
∗
()
2
+=
2
*
*
2
*
*
2**
015.01045.01
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
∆
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
∆
+∆≡∆
targettarget
94
C
H
C
C
LE
2*2*2**
baLE ∆+∆+∆≡∆
***
target
LLL −≡∆
***
target
aaa −≡∆
***
target
bbb −≡∆
***
target
CCC −≡∆
2*2*2**
CbaH ∆−∆+∆≡∆