3 COLORIMETRY BACKGROUND
This section provides you with useful information about colorimetry and how to get the best results from the system
in real world applications.
3.1 MEASUREMENT GEOMETRY
The CLM-194 colorimeter is designed to measure the samples the way the human eye does in the real world. It
implements a (45°c:0°) geometry, that uses 45° circumferential illumination by means of multiple directional sources
and 0° viewing perpendicular to the sample plane, according to ASTM E 1164. The circumferential illumination is
essential to achieve repeatable measurements on directional and structured surfaces.
The (45°c:0°) instrument is capable of specular excluded (SPEX) readings, that exclude the specular or gloss
component from measurement. The (45°c:0°) geometry simulates the normal condition used for colour evaluation,
where a difference in gloss and texture o a sample with same pigmentation results in a difference in colour. A gloss
sample is indeed visually judged darker by the eye when compared to a matte or structured sample.
Several applications require agreement between measurement and visual assessment, therefore implicitly assuming
the (45°c:0°) measurement geometry:
batch to batch comparison in production and quality acceptance
assembly of multi-component products using different materials
printed materials and the graphic arts industry, where a (45°c:0°) is the conventional measurement geometry
reproduction of the colour of a sample taking into account texture and gloss
and many others.
If only the pigmentation difference is to be detected or different textures of the same material are to be compared, a
specular included geometry (SPIN) that includes the gloss components in readings is recommended. This is usually
achieved with (d:8°) sphere geometries.
Note that only measurements taken under the same conditions can be compared. Measurements taken with (45°c:0°)
and other geometries match only under a very limited set of conditions and often exhibit differences of several E
units.
Relevant conditions affecting measurements are:
measurement geometry
illuminant / observer pair
colour system
sample preparation and orientation.
3.2 SAMPLE PREPARATION
An ideal sample to achieve repeatable and accurate colour measurements with a reflectance colorimeter like the CLM-
194 has the following characteristics (see also ASTM E1164):
a flat surface
completely opaque with no translucency
clean and dry
uniform across the area being measured
large enough to cover the instrument's sample port
its colour is not affected by light (photochromic), moisture (hygrochromic) or heat (thermochromic)
it adequately represents the material or parts under evaluation.
If any of these qualities is not present, the sample must be made as ideal as possible for a correct measurement.
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