Re: How does a circular polarisation filter works?

Hello,

You have summmed up circular polarization in your question quite well. However, I would describe circularly polarized light as the *superposition* of two waves with equal amplitude and wavelength, but a phase difference of pi/2. It's a subtle difference, but in the end, only one wave (consisting of two perpendicular components) exists.

The way a circular polarizer is made involves the use of a quarter-wave plate. A quarter-wave plate is a retarder which will introduce a phase shift of pi/2 between the orthogonal components of a plane wave. In mathematical terms, if a plane wave is described as:

E = (x E_{0x} - y E_{0y})*cos(kz - wt)

then a quarter-wave plate will effect either the x or y component by a factor of pi/2. This will make the wave equation read as:

E = E_{0}(x * cos(kz - wt) - y) * sin(kz - wt)

where the amplitudes of the orthogonal components have been made equal. So by combining a plane polarizer with a quarter-wave plate (in that order), we can change randomly polarized light into cirlarly polarized light. For further information, I recommend the text: Hecht, E. 1987. "Optics." Addison-Wesley Publishing Company: Reading, Massachusetts.

Circular polarizers are used in cameras generally when the camera has the autofocus feature. This is because the autofocus feature is sensitive to light levels. Therefore, if a linear polarizer was used, and the camera oriented in such a way that polarized light from the scene of interest was parallel to the polarizer, the light levels would be very high. Turn the camera 90 degrees and the levels would be much lower. The autofocus feature cannot handle this kind of variation, so we compromise by using a circular polarizer. Because the angle of polarization is constantly changing with a circular polarizer, we get the benefits of some depolarization without throwing off the autofocus features. Follow this link for more information about polarizers and cameras.

Hope that helps!
Amber