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Monday, June 2, 2014

How do you use whiteness measurement?

Whiteness is a single number quantity which is based on the entire visible spectrum.  Whiteness is a compromise between colorimetry and brightness measurement.  Colorimetry is a cumbersome but complete three dimensional description of whiteness, whereas brightness is simple to understand and convey but relates only to the blue portion of the visible spectrum. 

The most widely accepted whiteness calculation in the Paper Industry is the following formula which is defined in TAPPI, ISO, ASTM among others.



WI = Y + (WI, x)(xn - x) + (WI, y)(yn - y)

where:
Y, x, y                         = the luminance factor and the
                                       chromaticity coordinates of the
                                       specimen,
xn and yn                     = the chromaticity coordinates
                                       for the CIE standard illuminant
                                       and source used, and
WI, x and WI, y          = numerical coefficients.


Keep in mind that these equations should be used only for specimens with

     40 < WI < (5Y-280) and -4 < T < +2


where, T is the tint value that complements the whiteness calculation.

Some companies use whiteness measurement as a production specification and others use it in marketing. How do you use whiteness measurement?