Schneider 138mm Tru-Pol Circular Polariser Filter (p/n 68-013138). Circular True-Pol Polarising filter for cameras which use polarising elements in the beam-splitters of the viewing system, such as the ARRI 435 and the Movie-Cam Compact. This filter may also be required on certain HD video cameras with complex optical blocks such as the Sony PMW-EX1, PMW-EX3, and PDW-700 which polarise the red and blue channels more than other cameras and therefore require a circular polariser to prevent cross polarisation and associated colour shift in the image.
Beam splitters send light in two directions: to an eyepiece and to a video tap. Light in a camera’s viewing system can become polarised, and when a standard (linear) polarising filter is attached to the lens, the light becomes cross polarised. This hinders visibility and may create total black-out of the viewing image and/or the video tap.