PART I - CHAPTER III
BASICS OF THE COLORIMETRIE
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| Two red balls. How to
describe the difference in color?
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| Attributes
of the color
The
color of an object is given according to three criteria
or attributes which are hue, clearness or luminosity
(value) and saturation (chroma). Thus we can define
very precisely the properties of a color, which will
be useful within the framework of a colorimetric measurement. |
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HUE
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The
Hue is related to the real wavelength on the visible
spectrum, and thus the name defines color of the object.
In colorimetry, we use this hue concept to express the
name of the color which is employed daily by each one
among us. All these colors can be represented on a wheel,
called wheel of the colors.
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THE LUMINOSITY |
The
luminosity, for each one of these colors, can vary the
dark one with light. It is determined by a percentage
of light reflected by the colored object. We will see
that certain colors are darker than others (for example,
the red is darker than the green, color for which the
eye is most sensitive). The measurement of this parameter
can be taken independently of the hue of the coloured
object. |
SATURATION
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The
last attribute of the color is saturation. It utilizes
the concept of purity of a color.
A color having a hue specifies will be dessaturée
if this one is associated with gray having an identical
luminosity. This concept of saturation is independent
of the two preceding ones.
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Colorimetric Systems C.I.E. "Device-independent"
Colorimetric space CIE Yxy
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Diagram 4: Representation of colorimetric space CIE
Yxy 1931
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| We
can represent these three attributes of the color in
a colorimetric space called TSL (Teinte, Saturation,
Luminosity). It is also entitled HSL (Hue, Saturation,
Lightness), HCL (Hue, Chroma, Lightness), HSB (Hue,
Saturation, Brightness) or HSV (Hue, Saturation, Value).
The
measurement of the colors knew during its history of
many means of colorimetric representation. A.H.Munsel
developed a method of visual comparison of coloured
samples, classified according to the three attributes
of the color. This representation shows how the colors
could be classified, what makes it |
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to carry out a catalogue of the colors called Atlas
of Munsel. Later, Commission Internationale de l’Eclairage
(CIE.) adopted a color space called CIE Yxy (1931).
This space was declined in 1976 and another space called
CIE L*a*b* was introduced, we will see the characteristics
and the evolutions. |
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Diagram 5: Spectral curve of the standard observer
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| Colorimetric
space CIELAB
In
1931, C.I.E. developed the space representing colors,
Space Yxy. This colorimetric system is based on the functions
of mixtures described by Wright and Guild. Its development
is a linear transformation of these functions. Trichromatic
components X, Y and Z are virtual primary educations on
which rest the development of the functions of mixtures.
Trichromatic co-ordinates x and y are resulting from these
components X, Y and Z, are used |
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Diagram 6: Colorimetric representation of colour space
CIELAB
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| with an aim of
being able to graphically represent the colorimetric results
in the diagram of chromaticity. With its origin, the tristimulus
values XYZ corresponds to the three components red, green
and blue are deduced and calculated starting from the
functions of mixtures obtained for an average observer.
This concept is called standard observant, and was represented
in 1931 on a graph determining the spectral sensitivity
of the average human eye ( shown in diagram 5 ). The colors
are thus calculated according to these tristimulus values
XYZ, and represented in color space Yxy in two dimensions,
for each value of Y. The primary Y was given in such way
that the function of mixture is close to the function
of visibility of the observer of reference. |
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Colorimetric space CIELUV
In
this precise case, the three parameters which are components
X, Y and Z can be divided into two groups representing
chromaticity and the luminosity. It is for that colorimetric
space Yxy can be represented in two dimensions. Calculations
to obtain co-ordinates X, Y and Z starting from the
tristimulus values XYZ are as follows: |
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Diagram 7: Representation of colorimetric space CIELUV
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Measurements
for the standard observer depend on another factor which
is the angle of observation or the field of vision. Two
values were retained by C.I.E. (2° and 10°). This
color space Yxy is also known as Device independent since
it is based on the human vision and not on machines values.
In
1976, C.I.E. sets up a space color called Co L*a*b*, currently
it is very much used for the measurement of the colors.
One of the problems of color space Yxy was that the observable
differences in color on the graph did not correspond to
the variations of color perceived by the eye. Space Co
L*a*b* tries to solve this defect, by integrating the
three attributes of the color in these formulations. The
co-ordinates of chromaticity has A* and B* represent axis
of colors. The co-ordinate has a* defines the red / green
axis (+a * and -a *), and B * represents the yellow /
blue axis (+b * and -b *). Saturation increases as one
moves away from the center of the diagram. This color
space is thus three-dimensional and uniform (known as
pseudo-uniform). The L*a*b* values are calculated starting
from their tristimulus XYZ.
L* = 116(Y/Yn)1/3 -16
a* = 500((X/Xn)1/3-(Y/Yn)1/3)
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b* = 200((Y/Yn)1/3-(Z/Zn)1/3)
X, Y, Z are the tristimulus values of the sample.
Xn, Yn, Zn are the tristimulus values of a perfect diffuser
by reflexion.
If values X/Xn, Y/Yn or Z/Zn are values lower than 0,008856,
the equations are different
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Diagram 8: Representation of colorimetric space RGB
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(they are consultable in standard AFNOR X08-14 - "Colorimetry. Colorimetric
spaces pseudo-uniforms: CIELUV and CIELAB - Formulas
of variation of color associated ". 1998). Space
Co L*u*v * was developed the same year as the Co L*a*b
*, and corresponds to an improvement of space U*V*W
* defined in 1964. It is about uniform and has properties
different from those of space CIELAB. It can be represented
in a diagram called Yu' v'.
Calculations and
formulas of differences in colors
Calculations
and formulas of differences in colors call upon a certain
number of mathematical functions which it is not useful
to detail here. It is however interesting to note that
these variations measured between the colors are extremely
useful as it acts to compare colors in an extremely
precise way. Thus, in space CIELAB, the variation of
color represents the degree of variation of color without
indicating the direction in which it is. The mathematic
formula is as follows |
ƒ¢E*ab=ã((ƒ¢L*)2+(ƒ¢a*)2+(ƒ¢b*)2)
The other formulas for colors definies in default spaces
are as follows :
space L*C*h* : ƒ¢H* =ã((ƒ¢E*ab
)2 - (ĢL*)2 - (ĢC*)2)
space Hunter Lab : ƒ¢Eh=ã((ƒ¢L
)2 - (Ģa)2 - (Ģb)2)
space L*u*v* : ƒ¢E*uv=ã((ƒ¢L*
)2 - (Ģu*)2 - (Ģv*)2)
space CIE 1994 :
ĢE*94=((ĢL*/SLKL
)2 + (ĢC*ab/SCKC )2 + (ĢH*ab/SHKH)2)1/2
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ĢE*ab
greater than or equal to 1 is visible. In same reality
a difference in color is more important and is not
visible in certain area of color, like the yellow.
In impression ĢE* between 4 and 8 is located
in general interior of the tolerance of the standards. |
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Colorimetric Systems C.I.E. Device
- dependent
Space RGB
The
monitors and scanners use a colorimetric space of representation
of the colors in three dimensions called space RGB (RGB
for Red, Green, Blue). It is a space which rests on
the known system of the additive synthesis. It is known
as Device –
dependent because it is in close relation with
the peripheral which uses it. It can be represented
graphically by a cube on which hue of the primary colors
(RGB) and secondaries (CMY) are present at each top.
The last two tops represent black and white. We will
see that this space RGB can be declined in several forms
(sRGB, AdobeRGB, ColorMatch RGB etc.). Space RGB is
purely an artificial space.
Space CMYK
This
colorimetric space is also dependent on the peripherals
on which they are associated, each one of them differs
in a more or less proportion.
The nuanciers
colors
These
systems are called systems of referencing. Range of
the Pantone colors, for example, show the whole colors
according to a well defined classification of the colors.
The pallete of colors is especially used by the artistic
directors and the creators in the field of publicity
and the communication, for which the instantaneous visual
location is essential. The advantage is that they do
not have the other system representation of the colors.
It is necessary to distinguish the systems nuanciers
from direct tone (Pantone, Focoltone, Toyo, Trumatch,
etc.), nuanciers in quadrichromy. For the latter, the
indications for each color represented are made in values
CMYK. It is however necessary to take with much precaution
this system of referencing since it is directly depend
on the type of ink as well as type of paper and system
of impression used. |
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| The
nuanciers Pantone
major yellow can be assimilated to Pantone 123,
a clear green in Pantone 358C etc. It is possible
to choose the type of support on which these samples
are represented. Thus, Pantone proposes its nuanciers
on art papers and nonart papers (coated &
uncoated). The screen is also integrated in these
nuanciers since each sample has a precise tramage
(175 lines in the last versions), element essential
to control any graphic application and to which
the user can refer himself here. Specific nuanciers
for the Web, graphics, textile or the plastic
can be chosen by the users according to their
needs. The nuanciers can be associated with a
software of visualization on screen, with the
possibility in certain cases, to examine the whole
of the coloured samples while varying the woven
bottom.
Software can indicate possible closest CMYK values
to a Pantone color, in the colorimetric space
of the selected profile:
For example CMYK values, to reproduce Pantone
180 CVC on a machine offset, art paper are advised
by the software as follows: C: 17.2%, M:100% Y:
99.2% K: 13.3% for an impression "Newspaper
280", the software advises C: 0% M: 87.5%
Y: 100% K: 0% |
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| - PART
I - CHAPTER III -BASICS OF THE COLORIMETRIE- |
