PEM200 User Manual
59
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.
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