Hinds Instruments PEM-100, Руководство пользователя

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Appendix A: Calibration
74 PEM 100 User Manual
A
0
V
2f
/ V
DC
Figure A.10 Ratio V
2f
/V
DC
vs. retardation (radians) for parallel polarizers
This method gives monotonically increasing values of the ratio V
2f
/V
DC
for low values
of A
0
(3 radians and less). It lacks sensitivity for retardation values near 3.5 radians,
but this is where the flat-topped half-wave waveform occurs (
π
radians). The graph
is used in reverse, for a given value of
DC
f
V
V
2
, A
0
may be determined.
The method is conceived as a systematic calibration of retardation vs. V
Control
for a
sequence of retardations, beginning with low values. Such a study should avoid
ambiguities in A, since the functional relationship is an inverse one. The method,
however, requires considerable attention to detail.
First, care must be taken to ensure that V
DC
is proportional to the average intensity
of the modulated light signal. Other contributions to the experimentally measured
DC signal would include 1) stray light from ambient room illumination and 2)
electrical offset in the detector circuit.
One method to exclude other sources of light from contributing to V
DC
is to use a
mechanical chopper at the light source and a separate lock-in amplifier. This would
eliminate the influence of both 1) and 2) above.
A very effective method of reducing the influence of 1) is to place a narrow-band
interference filter with a pass band at the same wavelength as the light source. The
contribution to V
DC
would be limited to only the light that can get through the filter.
Alternatively, the detector can be shielded from ambient room light. Hinds
Instrument’s detectors have provision for adjusting the electrical offset under dark
conditions.