Use of polarization filters in print color measurement
Photographers use polarization filters to cut out glare. On color measurement devices, the use of these filters minimizes differences due to gloss. Differences in appearance from specular reflection can come from paper coating and from ink finish. Also drying of wet ink changes the appearance. Whether a surface is glossy or matte can impact color measurement.
There has been debate about the use of polarization filters and there are different camps. As always, it is important to understand both sides of the issue and see what fits your situation.
When and why should I use a polarization filter?
A polarization filter minimizes color measurement variability from gloss effects, making the color control process more reliable. You are reacting to changes in the base color, not changes in gloss level.
For ink formulation, process control with effect pigments and for wide gamut printing the use of polarizing filters can be particularly useful. When comparisons need to be made between online measurement of wet ink and offline measurement of dry ink, the use polarization filters is also considered useful.
The M3 measurement standard incorporates the use of polarization filters.
Why should I NOT use a polarization filter?
Not all measurement instruments offer small aperture measurement with polarization filter. This is because the use of small aperture with polarization filter can dramatically cut down the amount of light reaching the color sensor. This increases measurement noise and reduces the precision of measurement. If your instrument does not support the combination of small aperture and pol. filter this can be a limitation.
Most standards and guidelines published by SWOP, GRACoL etc. and ANSI Status T are based on unpolarized density readings. So the use of these filters puts you in a place where you cannot compare with standards. The difference in readings from different instruments can be higher with the use of polarized filter. Also, for ICC profiling, the use of polarization filter is not recommended.
It is also argued that non-polarized measurements show a difference that aligns better with the what the eye sees, even though it does not let you get to base color minus the effect of gloss.
The density measurements with and without filter are linearly related for a particular type of paper. Unfortunately this relationship is not universal. It changes with paper type as shown by the data posted here.
It is important to get a device that can automatically move the pol. filters in and out. It is also important to get a device that enables the use of BOTH polarization and small aperture at the same time while still providing acceptable precision. SpectroDENS is a modern device that enables you to use pol. filter in a 1.5 mm view are without sacrificing precision.