Metamerism, a color phenomenon, is when a pair
of samples match under a certain light source or set of viewing conditions but
not under another. For example, the sample shown at right* are a close match under
daylight, but do not match when viewed under artificial or fluorescent light. This
is because the spectral curves of these two samples, shown below are not identical.
Any change in conditions such as the light source, observer, or viewing geometry
might result in a perceived mismatch. This problem occurs in commercial color
matching when the colorants used in the production batch are not the same as those
used in the color standard. Controlled lighting conditions and instrumentation
can help solve this problem.
What is color tolerance?
A color tolerance should represent the range of visual difference
that a customer is willing to accept. It is specific to color space,
Illuminant, and Observer. to the right* is a visual representation of
a color tolerance of 1.0 DE CMC 2:1 for Illuminant D65, 10° Observer.
The most effective way to set a color tolerance is to visually evaluate
a wide range of samples against the color standard. Color measurements are
then taken and plotted to generate a numerical description of the maximum
difference allowed for each dimension. Elliptical tolerances, such as those
generated by CMC color difference, are generally accepted to best correlate
with visual assessments. Color instrumentation and software can simplify the
task of setting accurate tolerances for your products.
The samples to the right* are the same white paint at different gloss
levels. When viewing these samples from a perpendicular direction, you see
lightness differences. However, when you tilt them so that the specular
reflection is directed away from your eyes they appear to match. This example
illustrates the effect of gloss differences on perceived color, and verifies
the impact of instrument geometry in solving appearance problems related to
visual color evaluation.
Instrument geometry defines the illumination and viewing conditions. Two
designs of instrument geometry are widely used today: circumferential 45°
illumination with 0° viewing (45°/0°), or diffuse illumination
with near-normal viewing (d/8°). Most instruments using the d/8°
configuration offer two measurement choices, gloss-included (SCI-Specular
Component Included) and gloss-excluded (SCE-Specular Component Excluded).
When samples exhibit differences in gloss and other surface effects, the
appropriate choice of instrument geometry is critical to the correlation
between instrument and visual assessments.
Metamerism: Spectral Reflectance
The spectral curves below show the degree of mismatch resulting from
the use of different pigmentation for the metamers inside. these samples
have nearly identical L*a*b* values for Illuminant D65 10° Observer,
but not for Illuminant A or F2.
L*a*b* Color space
The CIELAB model of color space is the basis for color tolerances specified
in terms of CMC DE or CIELAB DE. The two-dimensional portion of the diagram
below shows the a* (reg-green) direction versus the b* (yellow-blue) direction.
the light-dark dimension (L*) is plotted perpendicular to this plane, as
indicated on the diagram below.
Tips to Manage Color Variation on
Spray Coated Aluminum Extrusions
- Remember: Color panels used as standards for approval are typically
produced in a laboratory setting. Therefore, expect color variation to
occur between the standard and any factory-applied samples. It is
recommended the color be approved using mock-up samples in the environment
in which it will be viewed.
- Expect some color variations when coating different substrates; the
approval of the coated finish should be from the factory-applied
- Color control is more precise on coil coatings than extrusions. Therefore,
it is suggested that the standard be developed using the extrusion first, then
formulating the coil standard to match.
- Remember certain micas and metallics may have different orientations of
individual flakes which will reflect light differently.
- Expect some color variations when matching aluminum extrusions coated
from different paint applicators; it is recommended that production samples
from each applicator be matched against a control color standard.
- In order to avoid batch-to-batch color variation from a paint supplier
and an applicator, it is recommended, if at all possible, to get metal painted
at the same time.
- Aluminum extrusions usually contain intricate details that make consistent
paint coverage across all surfaces difficult. If a profile requires consistent
paint coverage, consider a design with uniform wall thickness.
- Minimize designing profiles with wall thickness variations to ensure best
chances of color consistency during the curing cooling cycle.
- Unless the supplier and the customer both agree to use comparable color
measurement devices, the most common method to measure color consistency is
the Hunter L*a*b* color space. The Hunter L*a*b* color space is a 3-dimensional
rectangular color space based on the opponent-colors theory where "L"
represents the lightness axis (0 is black, 100 is white), "a" represents
the red-to-green axis (positive values are red, negative is green, and 0 is
neutral) and "b" represents the blue-to-yellow axis (positive values
are yellow, negative are blue, and 0 is neutral). Although not completely accurate,
it serves as the most reliable model to measure color differences expressed in
Delta E units and is intended to be a single number metric for PASS/FAIL decisions.
Different color measuring devices can read different L*a*b* values for the same
color due to the inherent differences in technologies. It is best, if at all
possible, to use similar device models when comparing paint color between the
paint manufacturer, applicator, and end user.
For more information, contact your Bonnell Aluminum representative.
*Due to variations in monitors, the color samples represented on
this page may not be accurate.
Please contact your Bonnell Aluminum representative to obtain
a tri-fold brochure on "The Truth About Color".