Understanding Polarization

This page describes the polarization principle and the main application of it.

Overview

Polarization is an electromagnetic wave like radio, radar, X rays or gamma rays. The difference is a question of wavelength. A wave is something vibrating, in the case of a piano or a guitar, it is a cord. When talking about polarization, it is the orientation of the electromagnetic field of a propagating light wave (an electric and a magnetic field which are vibrating together).

The software only considers the electric field, as the magnetic field can be deduced from it in materials used for light propagation.

A polarization state is the geometrical trajectory followed by the electric field vector while propagating.

As the polarization state is elliptical, the polarization is defined by the azimuth angle of the ellipse, the ellipticity and its rotation sense.

Note: Birefringent materials, polarizer surface or optical polished surfaces (Fresnel) use the polarization. Lambertian reflection is depolarizing. It converts polarized light into unpolarized light by changing randomly its polarization while processing the reflection. However, for Gaussian scattering, the model has been designed not to depolarize light.

Application

The polarization is the main physical property LCDs are working with. LCDs are used together with polarizer.

According to the applied voltage, they rotate or not the polarization axis of the light by 90°. So when this light tries to cross the polarizer, it is stopped (black state) in case of a 90° angle of the polarization axis with the easy axis of the polarizer or transmitted (white state) if this angle is 0°.

We saw that even the simplest surface quality (optical polished) has an effect on polarization. This is the surface quality used each time one deal with a light guide in automotive (dashboards) or in telephony (to lighten the keypad for example).

Since such devices are using multiple reflections inside their light guides, it is important to have an accurate model to describe the light behavior on this surface.

It is possible to build a light guide with a birefringent material to build a special function for the polarization. For example, a backlight for a LCD without any polarizer between the backlight and the LCD reducing the losses due to the polarizer.