PEM200 User Manual
48
A
Calibration
by Theodore Oakberg, PhD,
updated with PEM 200 Commands
Proper retardation calibration of photoelastic modulator systems is essential for
optimum performance. This is best accomplished with the modulator in the optical
setup in which it will be used. This application note is intended to assist users with
in
situ
calibration of their PEMs.
Some calibration techniques are very simple. In the vacuum UV with a circular
dichroism CD experiment, the modulator retardation might simply be adjusted to give
maximum signal when a known CD spectrum line is being observed. Even this simple
technique could be improved with the Bessel function methods described later in this
application note.
PEM-200 Calibration
The PEM-200 has been designed to provide the same calibration for a narrow
monochromatic light beam, as compared to the PEM-100
.
For many applications the
calibration built into the PEM-200 is sufficient (retardation uncertainty <5%).
There are still cases where in-situ calibration by the user is advisable or even
mandatory. These cases include, but are not limited to, the following:
1. Where there is a requirement for very high retardation accuracy for the PEM.
2. Where the light beam at the PEM is not narrow but uses a large fraction of the PEM
aperture.
3. Where extreme angles of incidence (>15 to 20º) are involved.
For user-performed calibration, direct control of the modulator drive commands should
be considered (:MOD:DRV #).
Calibration Theory
The optical setup for most calibration procedures is shown in figure A.1.
