Hi Say I have a magnetic field point in some well-known direction, and I have linearly polarized light incident in that direction. I want to convert my π-polarization to σ-polarization, so I find my lambda-quarter plate. However, how do I know whether it produce σ+ or σ-? Do they come with a manual, or do people calibrate them by themselves? I'm not sure how I would do this. Best, Niles.