colour & masterbatch | Understanding colour
and metamerism. The latter is a phenomenon in which two objects have a given relationship under one set of viewing conditions and a different relationship under another. This happens because the two objects are spectrally different. Preventing metamerism is dependent on making sure the two examples have the same spectral curve. Figures 5 and 6 illustrate and explain metamerism.
Measuring colour Figure 5: An example of metamerism
There are two ways to measure colour. One is by a visual examination, relying on the human eye. The other is via instruments and equipment.
Samples must be pre-qualifi ed to: represent the batch; be free from contamination; be in the same form as the target; and be reproducible within acceptable limits of repeatability. Then, a single sample is chosen as the ‘standard’ by which all others will be judged. Visual examination: Because visual colour measure- ment depends on consistent judgment, the observer would need to provide repeatable judgments.
Figure 6: Why metamerism happens
individually. It is the brain’s ability to detect edges that makes visual examinations of product designs so valuable. Our optical system can discern specs, streaks and blotches that no spectrophotometer could ever detect. The spectrophotometer would simply average the specs into the overall colour. It also allows us to evaluate surface textures and recognizes specular refl ections.
With all of these factors considered – light, object, the human eye and the human brain – do all people see colour the same? We most defi nitely do not. We all have different spectral response curves even with normal colour vision. Then there are also other factors, such as individual colour defi ciencies and colour blindness that make our perceptions of colour vast and unique person-to-person. What we see is not all about colour. It’s all about
appearance. Colour, gloss, scattering, light sources, surroundings and temperature all impact appearance. For example, Figure 4 shows the impact of surrounding colours. For one to understand how the human observer sees
and evaluates colour, one must have a clear under- standing of the light source, how the object modifi es light, and how the human observer responds to light
36 INJECTION WORLD | April 2014
Examination by instrument: First and foremost, you must understand the instrument, how it works and its specifi cations. Choose an instrument that gives you the best mix of functions to meet your design needs: Spectral data, including absolute coordinates and metamerism; Geometry;
Precision and accuracy;
Software, including the ability to complete the necessary calculations, as well as the user interface; It is also important to remember that no instrument
can replace the importance of the human eye. An instrument may indicate a fractional difference in colour that may not be discernible to the observer, so allow for some margin of difference to avoid unnecessarily scrapping an entire lot for a difference no one would ever notice.
Possibilities are limitless The colour match is only one component of product design. The fi nal product’s end use creates a list of attributes that must also be met to have a successful product design. Depending on where the product will be stored, factors that might need to be considered include: bleach resistance, light stability, weathering and chemical resistance, among others. After all, what good is the right colour that provides
just the right feeling if the product itself fails to stand up to its environment? With assistance from experts in colour and product design, there’s virtually no limit to what designers can accomplish.
www.americhem.com
www.injectionworld.com
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