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Light + Object + Observer |
The Colorimetry is
defined as the measurement of colour. The measurement of colour allows
coloured objects to be described, ordered, and compared. These operations
must be accomplished in a logical and repeatable manner, in order to
allow successful colour communications. And, successful colour
communications are essential if satisfactory industrial colour control is
to be accomplished.
Colour is an aspect of visual perception that is not easy to define,
and certainly not easy to measure. It is a sensation whereby a human
observer can distinguish differences between two fields of view, where
such differences are caused by spectral composition differences in the
observed radiant energies. From this it can be concluded that colour is:
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A sensation,
dependent upon the observer.
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Only of interest if the observer can
distinguish differences in sensations.
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Caused by spectral
(wavelength-by-wavelength) energy compositions (distributions).
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The spectral energy compositions that
are sensed by the eye/brain system of the human observer, result
from both: Sources of light, and objects that modify light.
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Although various systems have been developed for the measurement and
ordering of colour, the most important system, by far, is the CIE system.
First published in 1931, this colourimetric system is based on the
principle that the colour of an object is a combination of light, object,
and observer properties. The CIE (Commission Internationale de
l'Eclairage) is an international organization concerned with light and
colour, that continues to further methods and standards concerning these
subjects.
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Elements
Which Cause the
Colour Stimulus
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The CIE system is based on the premise that the stimulus for
colour
is provided by the proper combination of a source of light, an object,
and an observer. The sensation of an object's colour is produced by the
combination of: |
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A light source - illuminating an object.
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An object - reflecting or transmitting
light to an observer.
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An observer - sensing the reflected light.
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The combination of these three is
considered on a spectral (wavelength-by-wavelength) basis.
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Light Source
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Electromagnetic energy exists as waves, which
can be described by their wavelengths or frequencies. The wavelengths of
these waves are distances, with 1 nanometer (nm) equal to 1e-9 meters.
Humans can "see" electromagnetic energy over a range of wavelengths from
about 400nm to 700nm. This part of the electromagnetic spectrum is
called the visible (or colour) spectrum.
Light sources can be described by their
relative energy outputs, wavelength-by-wavelength. These outputs are
called relative spectral energy (or power) distributions. The colour
producing effects of light sources result from the relative amount of
energy available, not the absolute amount of the energy.
Light sources are also sometimes described by
their correlated colour temperatures. The correlated colour temperature of
a source is the temperature of a black body radiator that is most
similar to the source. A blackbody radiator is an ideal surface that
absorbs all energy incident upon it, and re-emits all this energy. The
spectral output distribution of an incandescent (tungsten) lamp
approximates a blackbody at the same temperature. Correlated colour
temperature is typically presented using the absolute centigrade scale,
degrees Kelvin (°K).
The CIE has published spectral output data for
various illuminants, in order to facilitate and standardize colourimetric
computations. These illuminants include:
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D65 - daylight, colour temperature
6500° K.
A - tungsten, colour temperature
2856° K.
F2 - fluorescent, cool white.
F11 - fluorescent, narrow band
cool white.
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CIE Illuminant spectral output data is
used in the process of calculating the colour of illuminated objects.
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Optical Characteristics of
Coloured Objects
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The spectral distribution of light
reflected from an object depends upon: |
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The light illuminating the object; and
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How the object modifies the incident
light.
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The spectral energy compositions that
are sensed by the eye/brain system of the human observer, result
from both: Sources of light, and objects that modify light.
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For opaque objects, reflectance is
determined by the following optical characteristics:
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Surface reflection - diffuse (rough
surface), or directional (smooth surface) reflection
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Absorption - light enters the object and
does not emerge (on a wavelength-by wavelength basis), as it is
converted to heat.
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Scattering - light enters the object and
is deflected (on a wavelength-by-wavelength basis); and is then
eventually absorbed, or exits the object.
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The reflectance of an object is
determined by a spectrophotometric measurement, with calibration
relative to an ideal white, and perfect black. Spectral reflectance
curves, graphical plots of the reflectance data, are often a useful way
of presenting this information.
Reflectance data (of the object) is used
in the process of calculating the colour of the object.
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Observer
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The human eye/brain system senses colour
through three types of sensors (cones), located in the eye's retina.
These cones are sensitive to light in three different wavelength bands,
referred to as the L, M, and S bands. Processing of the cone signals, by
the brain, eventually yields output sensations interpreted as red,
green, and blue (and/or combinations and differences of these primary
colours).
There are two CIE standard observers that
can be used when computing CIE tristimulus values. They are as follows:
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2° Observer (CIE 1931) - for small
objects.
10° Observer (CIE 1964) - for large objects
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The colour matching functions of these observers, with tabulated data
wavelength-by-wavelength, are utilized in the tristimulus calculations.
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Tristimulus Value
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CIE tristimulus values X, Y, and Z, are coordinates of
colour sensation,
and form the foundation of the CIE color space. |
