|Metameric Green Tiles|
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.
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