Hinds Instruments PEM-100, Руководство пользователя

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Bessel Function Ratio Methods
PEM 100 User Manual 75
Another method would be to correct for 1) and 2) by blocking the light (near the light
source, not near the detector) and recording the detector output voltage. This
baseline value would then be subtracted from the measured value of DC volts to
give V
DC
.
There is another problem which is more subtle, but just as important. This method
assumes that the responsivity and/or gain (if there is amplification) are the same for
DC (or low frequencies) as for 2f. If this is the case, then the following relationship
should be valid:
( )
( )
( )
( )
0 0
0 2
2
1
2
2
A J
A J
measured V
measured V
DC
f
+
=
(12)
The factor of
2
results from the lock-in amplifier’s measuring V
rms
whereas
theoretical expressions refer to V
peak
. If equation 4 is satisfied then the calibration
using this method may be trusted. This may be tested by comparing the maximum
value of the experimental ratio V
2f
/V
DC
with the maximum value shown in Figure
A.10 (1.481 at about 3.45 radians).
If the condition of equation 12 is not met, the following procedure may be used.
Assume that there is an experimental constant K which relates experimental values
of the ratio V
2f
/V
DC
to the theoretical values of the ratio.
( )
( )
( )
( )
0 0
0 2
2
1
2
A J
A J
measured V
measured V
K
DC
f
+
=
(13)
The term on the right has a maximum value of 1.481. Using the maximum
experimental value of V
2f
/V
DC
, equation 13 may be solved for the experimental
constant and a numerical value obtained. A graph or table of
DC
f
V
V
2
vs A
0
for the
value of K found may be generated and used for determining A
0
.
Reference:
1. Kemp, James C., Polarized Light and its Interaction with Modulating Devices ,
Hinds International, Inc., 1987
2. Oakberg, Theodore C., “Modulated Interference Effects in Photoelastic
Modulators”, PEM Newsletter #8, Summer, 1998