Geoscience Reference
In-Depth Information
is also the possibility of simultaneously measuring a full spectrum of wavelengths when
employing a photo diode array (PDA) or CCD detector. So the popularity of photodiodes
rests in the tradeoff between gaining the benefits stated above and sacrificing the high sen-
sitivity of PMT detection.
6.3.3 Optical Filters
A variety of spectral filters are used to isolate the desired wavelengths of light along the
excitation (incident) and the emitted light path of the resulting fluorescence. There are
also neutral density filters that are used to reduce intensity without spectral discrimination.
Spectral filters are of three general categories: bandpass, interference, and optical edge and
are used to discriminate wavelengths. The former comprises absorptive and interference
(dichroic) types. Absorptive filters are made of glass with the addition of inorganic and
organic compounds to selectively absorb and transmit wavelengths of light. They are inex-
pensive and exhibit low peak transmittance and broad peak shape (
Figure 6.8
). Interference
filters, however, consist of sequential layers of reflective cavities that resonate with desired
wavelengths and destructively cancel or reflect nondesired wavelengths. Controlling the
thickness and sequence of the layered coatings means that exact color ranges are possible.
Interference filters have high peak transmittance and narrow peak shapes. Because of these
properties and improvements in coating technology, interference filters are an ideal choice
for precise scientific work requiring high spectral resolution (Macleod,
2001
). Bandpass
optical filters are described by critical features including peak transmittance, center wave-
length (CWL), or nominal wavelength, and full width at half maximum (FWHM), or
effective bandwidth (
Figure 6.8
). CWL is the average of two half-height wavelengths in a
spectrum and FWHM is the wavelength range at half the max transmittance. Optical edge
filters are referred to as cutoff or blocking filters and can be used to transmit much of the
UV-visible regions of the spectrum, but reflect far UV and near infrared energy. UV pro-
tection filters also reduce photochemical degradation without damage to the components
when the light output is intensified. Two or more cutoff filters can also be used to produce
a peak similar to a bandpass filter (
Figure 6.8
).
6.3.4 Optical Configurations
Most submersible fluorometers are configured to utilize one or two LEDs, interference and
cutoff filters, and photodiodes. Combinations of filters for effective fluorescence measure-
ments can include the use of UV-violet blocking filters and an interference filter (centered
on desired excitation wavelength(s)) to discriminate against the scattered UV incident light
and reject small amounts of red light emitted by the blue LEDs. For emission detection
a Schott (blue glass) glass cutoff filter (with optimal stopband or passband limits) and
a bandpass filter (centered on desired emission wavelength(s)) can be used to select red
fluorescence to be detected by a photodiode (WetLabs, Inc. fluorometers). The physical
