See and measure things in the right light
Color consistency from batch to batch is of course a “must” requirement for an industrial coating. The “correct” color has to be ensured across different material types and gloss levels. Additionally, color consistency needs to be checked under multiple light sources, because multi-component products are utilized under different lighting conditions. Otherwise, parts painted with different batches have the potential risk to appear the same under daylight, but show an apparent mismatch under indoor room lighting. This phenomenon is known as metamerism.
Visual test of metamerism
In a light booth standard and sample are viewed at the reference light source – most of the time D65. Then the light source is changed to at least one test light source, which is significantly different from the reference light source. A common practice is to visually evaluate the sample pair under illuminant A and a fluorescent light source representing TL84 or CWF. This can be easily done using the byko-spectra pro lighting cabinet - the new reference for visual color evaluation. CIE Publication 51.2 describes a method to rate the quality of daylight sources by means of metameric sample pairs with a metamerism index (MI) of zero for standard illuminant D65. The higher the MI value the poorer the daylight simulation. For critical color evaluation, ASTM D1729 recommends in the visible range Class B or better. The new byko-spectra pro uses an innovative combination of filtered tungsten-halogen lamps and LEDs achieving a never seen before quality with CIE Class A rating: MIvis ≤ 0.25.
|Picture 1: Daylight simulation with byko-spectra pro|| Picture 2: byko-spectra pro|
Light booth for standardized visual color appraisal
Instrumental test of metamerism
The reason for metameric paint batches is that the pigments or colorants used in the formulation are different. This can occur when e.g. raw materials are no longer available because of environmental issues or more cost efficient solutions require raw material changes. In any case, the spectral curves of the metameric pair are different. Typically, the curves cross each other at least three times.
Picture 3: Principle of metamerism
However L*a*b* values calculated for one illuminant are the same for both specimen, but are different for a second and third illuminant. The graph below shows measurements taken with the spectro2guide. The ΔL*, Δa* and Δb* values are significantly different for illuminant F11 (TL84) and even out-side of the tolerance ellipse. This is a typical sign for metamerism. The part being measured matches well for illuminant D65 and A, but is visually rejected for illuminant F11.
|Picture 4: Color differences for 3 illuminants: D65/A/F11|| Picture 5: spectro2guide|
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