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Monday, March 10, 2014

Effects of Metamerism

Metamerism is the phenomenon of two samples matching perfectly under one lighting condition, but not matching under another lighting condition.  A sample and standard (or target) are said to be a good non-metameric match if they are a match under several different lighting conditions.

Metameric Green Tiles
Metamerism can be caused by several different issues: mills that use different dye systems, substantially different raw materials, different fillers, etc. can have metamerism related problems.  A common example of metamerism occurs when two samples are generated to be a match, but are done so utilizing two different dye systems.  What might occur is that Mill A uses a single dye system and Mill B uses a two dye system. Single dye systems will absorb light differently than two dye systems.  The result of this will be the potential of having a good match under one lighting condition and a poor match under a different lighting condition.  This same things could occur when different dye suppliers are being used at the two mills.  Due to the different dye chemistries, absorption characteristics will again vary.

One of the primary challenges with paper is that Optical Brightening Agents (OBA's) almost always lead to a form of metamerism because the different light sources have different amounts of ultraviolet energy.  Differing amounts of OBA's in two samples can result in a good match under one lighting condition and a poor match under a second lighting condition.  Due to "OBA metamerism" or "fluorescent metamerism", all standards should be established for a single, uniform lighting condition in all mills.  Source C or UV level C (which is used in ISO and TAPPI standards) is recommended since it closely simulates many paper viewing conditions.

Classic Metamerism Spectral Curves Cross in Three Places

Metamerism can be detected by using instrumentation or by doing human, visual assessments.  A spectrophotometer can evaluate a sample and associated standard under different lighting conditions and report metamerism.  Human, visual assessments can be performed in a light booth with two standard lighting conditions. However, human assessment is subject to a variety of potential errors including: aging/yellowing eyes, color deficiencies, and psychological effects.

Please contact me, toddp@technidyne.com,if you have questions about metamerism or color measurement.