In the visual test the yellowness is determined by pouring the sample into a tube and comparing it to a known standard. The standard that the sample falls closest to then becomes the value for the liquid. This procedure is highly subjective due to variationsof observers, illumination and to some extent the standards themselves.
Most products are not strictly yellow and therefore require a three dimensional description of color: red/green, yellow/ blue and light/dark differences. Modern instruments read this information by the use of standardized color scales like CIE L*a*b* or L* C* h°.
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