The function of the polarization conversion system or PCS is to convert all of the light from the lamp assembly from unpolarized to linearly polarized light. Several of the current spatial light modulators like the LCD and LCoS modulators must have linearly polarized light to work properly. Mercury and Xenon light sources output unpolarized light and if only the light which was polarized in one direction was used, too much of the light would be wasted.

The PCS was invented by engineers at Epson for use with LCD panels and it has also found use elsewhere.