A quarter wave plate, which takes place between the laser and the optical cavity, gives to the photon beam a circularly polarization, either right or left.

To check systematic errors on the polarization measurement of the electron beam,
one can reverse this photon polarization.

	Just click on the polarization button, and select Right or Left polarization: The "Elliptique" selection button does nothing. LEFTRIGHTELLIPTIQUE Reversal takes one minute to be executed. It's finished !