A novel, working, fiber optic sensor is described which samples scattered light from a flowing white powder stream and monitors the yellowness with very high sensitivity and precision. The monitor does not make a primary measurement of color but is correlated to spectroscopic reflectance measurements of pressed pellets of the powder. It was specificaly developed for, and is used with, purified terephthalic acid, a monomer of polyester. It is, however, clearly applicable to any light scattering power or slurry.
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