PEM200 User Manual
50
(
)
{
}
t
A
I
Ω
−
=
cos
cos
1
2
1
0
(3)
and
( )
{
( ) (
)
t
A
J
A
J
I
Ω
+
−
=
2
cos
2
2
1
2
1
0
0
0
(4)
higher
+
}
..
terms
These four equations are basis for the calibration procedures described in the
remainder of this application note.
Light Sources for Modulator Calibration
The procedures described below for PEM calibration each require a monochromatic
light source. A brief discussion of light sources suitable for this purpose is in order.
Lasers are excellent sources of monochromatic light. At Hinds Instruments, we use
HeNe lasers to calibrate most PEMs which we manufacture. Laser beams are intense
and well collimated, and do not normally require any focusing optics in the calibration
optical setup.
Interference effects may be present when using a laser. In those cases, care must be
used to eliminate the interference effects before performing the calibration. A
discussion of modulated interference and methods for reducing them may be found in
PEM Newsletter #8, Summer 1998
(2)
.
Spectral light sources, such as mercury lamps, are also excellent light sources when
used with a monochromator or selected interference filters which match the spectral
lines. These allow calibration of the PEM over a range of wavelengths. Interference
effects should be negligible. Systems based on interference filters may offer
significantly better light throughput than monochromator systems.
White light sources may also be used with monochromators, but here some caution is
advised. The error in accuracy of the wavelength of the light should be recognized as
approximately the optical bandwidth of the monochromator. Within the bandwidth of a
spectrum line, the system sensitivity will be strongly affected by the slope of the spectral
radiance of the light and by the spectral sensitivity of the detector. The situation also
applies to using interference filters, but the “effective modulation wavelength” may be
even further from the nominal center bandwidth of the filter.
For the near IR, visible and near UV, the hottest light source available should be used.
If an incandescent lamp is used, it should be of the halogen variety. High pressure
Xenon lamps, carbon arc lamps and the like would be even better.
As a general rule, the light source being used in a particular experiment may be the best
one to use for the calibration.
