Appendix A: Calibration
72
PEM 100 User Manual
A sample calculation is given below for quarter-wave retardation. The required
Bessel functions are:
For half-wave:
( )
485
.
0
2
=
π
J
For quarter wave:
( )
250
.
0
2
2
=
π
J
The required setting for quarter-wave retardation is:
( )
( )
( )
=
2
2
4
2
2
2
2
λ
π
π
λ
f
f
V
J
J
V
(7)
( )
515
.
2
2
=
λ
f
V
( )
2
2
λ
f
V
(8)
( )
2
2
λ
f
V
is the lock-in reading for half-wave retardation, as set by the oscilloscope.
Parallel Polarizer Calibration at a Single Wavelength
The primary calibration technique discussed in “Calibration Using an Oscilloscope” is
observation of the waveform at half-wave retardation. This gives calibration only at
half-wave retardation or integral multiples of half-wave retardation. Other methods
such as the Bessel function zero method give retardation only at specific values of
retardation (e.g. 3.877 radians or 5.136 radians). Another method of accurately
measuring retardation at arbitrary values of V
Control
is much desired.
Calculating the ratio of V
2f
/V
DC
is in principle a good approach. However, if
calibration is being done with the PEM between crossed polarizers at 45º there are
problems, especially at low retardation levels. From theoretical analysis of the
resulting waveforms we learn that:
DC
V
α
( )
0
0
1
A
J
−
(9) and
f
V
2
α
( )
0
2
2
A
J
(10)
If one plots the ratio V
2f
/V
DC
the graph shown in Figure A.9 is obtained.
Summary of Contents for PEM-100
Page 1: ...PEM 100 PHOTOELASTIC MODULATOR USER MANUAL Hinds Instruments Inc P N 010 0000 021 UM Rev H ...
Page 4: ......
Page 8: ...Table Of Contents viii PEM 100 User Manual ...
Page 12: ...Table of Tables xii PEM 100 User Manual ...
Page 22: ...Unpacking the PEM 100 10 PEM 100 User Manual ...
Page 46: ...Chapter 5 PEM 100 Controller Display 34 PEM 100 User Manual ...
Page 52: ...Chapter 6 Operating the PEM 100 Controller 40 PEM 100 User Manual ...
Page 68: ...Chapter 9 Troubleshooting 56 PEM 100 User Manual ...
Page 74: ...Chapter 10 Maintenance 62 PEM 100 User Manual ...
Page 88: ...Appendix A Calibration 76 PEM 100 User Manual ...
Page 92: ...Appendix C Optical HeadSpecifications 80 PEM 100 User Manual ...
Page 100: ......