Geoscience Reference
In-Depth Information
1.0
Detection
volume
0.8
“Low concentration”
0.6
0.4
0.2
“High concentration”
0
0
2
4
6
8
10
Distance inside cuvette / mm
Figure 5.9. How the inner filter effect impacts on the illumination of the sensing volume.
5.4.2 Continuum Light Sources
Most photoluminescence instruments employ the use of high-intensity continuum sources
with UV and visible output. Continuum light sources have wavelength-dependent intensity
profiles and may also change in both intensity and spectral output as a function of time. For
this reason many fluorimeter systems are now equipped with a reference silicon photodiode
detector that monitors a portion of the excitation light that is incident upon the sample. The
most common light source that is used in bench-top fluorimeter systems is the xenon short
arc lamp. This has an intense, almost continuous spectral output from the UV (220 nm) to
the near-IR (800 nm) with some fine structure superimposed. Line spectra are prominent in
the 800- to 1000-nm range, and optical output can be observed well into the IR (2.6 μm).
Xenon lamps have a small arc length (0.5-2.7 mm for 75- to 450-W lamps) and a high flux
density. They are available in a variety of envelope materials allowing output from 185 nm
and have relatively long operating lifetimes of up to 2000 hours.
Typically, xenon lamps are operated with DC power supplies, but pulsed versions are
available with 1- to 10-μs pulse widths for use in time-resolved measurements such as
phosphorescence studies. Xenon lamps offer the highest spectral flux density short of a
laser. Their spectral output is close to that of the sun at 6000°C, and they make ideal solar
simulators in many experiments. Special considerations when using an arc lamp include
envelope materials (materials that allow the transmission of light within a given or defined
spectral range); ozone dissipation; and convection current effects on noise, cooling, arc
gap, and lamp life. Although they are the lamp of choice for most fluorescence instruments,
these lamps require a complex power supply with an ignition circuit to strike the arc and a
well-designed lamp housing to provide proper lamp cooling. In addition lamps that exhibit
