Circular polarizer filter wavelength dependency? I don't fully understand the circular polarizer filter (CPL) for my camera. It contains a linear polarizer and a quarter wavelength plate (QWP). The QWP causes a time delay between the ordinary and the extraordinary component of the light wave. This delay should correspond to a phase shift of 90° to create circularly polarised light.
I do understand that for monochromatic light, but not for white light. If the time delay is the same for all wavelengths, the phase shift cannot be 90° for all wavelengths, can it? It would range from about 60° to 120°.
 A: You are right. A QWP is designed for a particular wavelength. The phase shift changes linearly with frequency. See Waveplate in Wikipedia.
For photography, this doesn't matter much. Two polarizers are typically used. A linear polarizer can be adjusted to darken the sky and manage reflections, because a component of light from these sources is linearly polarized. The linear polarizer matters for artistic reasons.
This is followed by a quarter wave plate in modern cameras because linearly polarized light can mess up autofocus and light metering. See Polarizing filter (photography)

Normal light is a mix of all wavelengths. The QWP was probably designed for wavelengths in the middle, green. So green will be circularly polarized, red will be elliptically polarized, and blue will be elliptically polarized perpendicular to red. They will be close enough to circular to not mess up the camera's autofocus and light metering sensors. This does not change the appearance of the photo, other than preventing a messed up photo from a camera malfunction.
The linear polarizer affects appearance. It prevents light that is linearly polarized in a direction you choose from reaching the QWP and camera. You can choose a direction that darkens the sky because light from the sky is partially polarized. You can choose a direction that reduces or eliminates reflections because reflections are more strongly polarized.
With a linear polarizer and no QWP, all the light from all wavelengths would be linearly polarized. The camera might not autofocus properly. It might be overexposed if the light meter did not register light from the wrong polarization.
