Geoscience Reference
In-Depth Information
the entrance and exit slit widths (dispersion). The light gathering capacity (LGC) is
defined by:
height
(
mm
)
LGC
=
slit
(5.10)
mm
mm
(/#) *
f
2
dispersion
5.4.4 Polarization Effects
Polarization effects in monochromators and other optical components can present signifi-
cant difficulties in the overall operation and calibration of fluorimeter systems. At the same
time, the introduction of polarizing elements such as polarizing filters or Glan-Thompson
or Glan-Taylor polarizing optics are essential to the measurement of fluorescence polari-
zation or anisotropy.
Reflection diffraction gratings are well described (
www.horiba.com
,
www.newport.
com
) and can exhibit both strong and complex polarization effects. At certain wavelengths
the grating may exhibit diffraction efficiencies in the S- or P-polarization planes. On aver-
age, this has little effect on the overall power transmitted by the monochromator for gen-
erally unpolarized light inputs but the monochromatic beam that exits the monochromator
will, to some extent, be partly polarized, the extent of which can be strongly wavelength
dependent. These effects lead to spectral shifts, signal loss, and a whole range of other
mis-
leading
artefacts in the spectral signal.
Figure 5.14
shows two sets of typical diffraction
grating efficiency curves, from two master gratings, for 1200 g mm
-1
and 500 nm blazed
gratings with polarization angles for 45 degrees (upper boxes) and also S- (perpendicular)
and P- (parallel) planes (lower boxes).
Nondispersive elements such as filters are also widely used in all of the optical spectros-
copy methods, especially fluorescence measurements, and are based on either absorption
or interference. Filters are commercially available for wavelengths above 200 nm and come
in many forms, some of which are bandpass, cutoff, heat-absorbing, heat-reflecting, etc.
The most common types are:
•
Bandpass filters of the interference type are defined by the bandpass wavelength and the
width of the bandpass. It is common to use such filters to select a wavelength; that is,
excitation wavelength for fluorescence and a different filter for detection of an emission
wavelength. Typical values for bandpass are 10-50 nm. Filters can offer excellent out-of-
bandpass rejection, but do not offer the flexibility of a spectrometer.
Cut-on and cutoff filters absorb all radiation at wavelengths either shorter or longer than
•
the transition wavelength. The cutoff wavelength is defined as the spectral position where
50% of the maximum transmission of the filter is observed.
Neutral density (ND) filters have relatively small wavelength dependence in the 180 nm
•
to 2.5 μm spectral range. They transmit a specific percentage of the incident light; the
exact transmission percentage can be very precise to allow strong signals to be measured
