The truth about color: color spectrum

Metamerism

The truth about color: metamerism example

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?

The truth about color: color tolerance example

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.

Surface Characteristics

The truth about color: surface gloss effects

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.
The truth about color: spectral reflectance graph
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.
The truth about color: CIELAB model of color space

Tips to Manage Color Variation on
Spray Coated Aluminum Extrusions

  1. 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.
  2. Expect some color variations when coating different substrates; the approval of the coated finish should be from the factory-applied extrusion sample.
  3. 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.
  4. Remember certain micas and metallics may have different orientations of individual flakes which will reflect light differently.
  5. 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.
  6. 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.
  7. 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.
  8. Minimize designing profiles with wall thickness variations to ensure best chances of color consistency during the curing cooling cycle.
  9. 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".