Hinds Instruments PEM-100, User Manual

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PEM 100 User Manual 63
A
Calibration
by Theodore Oakberg, PhD
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 100 Calibration
The PEM 100 has been designed to provide significantly improved calibration for a
narrow monochromatic light beam, as compared to the PEM-90. Specifically, the
accuracy of retardation vs. wavelength has been improved. For many applications
the calibration built into the PEM 100 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 parameter Vcontrol should be
considered.