A transparent optical medium which only allows electromagnetic waves of one polarization to penetrate. Of the light waves normally moving in all directions, polarization filters only allow to pass those components of an oscillation that come from a certain direction.
These directed light beams hit the measured object and are then reflected partly in a mirror-like way. Unlike diffuse reflection, direct reflection does not alter the oscillation direction of light.
These components can be eliminated with a second polarization filter positioned at 90 degrees.
Densitometers with polarization filters show identical values for wet and dry prints which are, however, slightly higher than on devices without polfilters.