Linear Polarization filters are for range finder- and reflex cameras, however not the reflex cameras with dome light by the use of mirrors (TTL) and automated lenses. In other words, use Circular filters with you Digital cameras.
Polarization filters reduce the direction of oscillation of the sunlight waves. The filters are neutral in color and can be therefore used in color photography
The physics of sunlight: the suns light is not polarized but scattered evenly. Since there are particles of dust and so on in the air, polarization of the suns light happens when reflected partially from these particles.
This type of scattering of the sunlight reduces the transparency of the air known as haze but on the other hand makes up for very blues skies.
Additionally polarization of light happens also when reflected from non metallic surfaces. With the polarization, filtered colors become more saturated.
However polarization of light is also produced by normal reflection on surfaces (the non-metallic). Such Reflexion lead to a higher saturation of the color when a polar filter is used.
The polarization filter basically does one thing: It makes the air transparent! This is particularly desirable when doing remote shooting or trekking in the mountains. Additionally blue tinting in landscapes is eliminated but in the same time enhances the blue of the sky.
Filters used in image processing can not replace polarizing filters or any other filter. The outcome is different.
Make sure to use Circular polarizing filters with all digital cameras because linear ones affect the result due to distorted information on the digital cameras chip.