Biomedical Engineering Reference
In-Depth Information
Longpass
or
notch filter
Light collection
(a)
Light delivery
Bandpass filter
Light collection
(b)
Light delivery
Bandpass filter
tablet
Fig. 10.4.
Raman probing principles based on backscatter (
a
) and transmission
(
b
)geometry
of the background response from fibre-optic Raman measurements has been
described by Myrick and Angel [9]. Most fibre probes are designed around the
principles described in this chapter. Figure 10.4a shows a typical probe design
that filters background responses. A bandpass filter is positioned at the front
of the delivery fibre to remove unwanted Raman from the fibre. A longpass or
notch filter is placed in front of the collection fibre(s) to remove the intense
light at the laser wavelength propagating through the collection fibre to the
analyser.
A typical fibre probe will illuminate the sample with a laser spot whose
dimensions are dependent on the launch optics and the position of the sample
relative to the probe end. For a static sample, the volume interrogated will
depend on the Raman scattering properties, working distance and the accep-
tance angle of the fibre collection optics. For a sample moving in the laser
beam, the sampled volume is also dependent on the time taken for signal
averaging and the velocity of the sample.
Non-contact measurements with the probe head sited remotely from the
process are well established. Tumuluri et al. [10] have described a probe-based
process measurement for monitoring the drug content in an extruded film. A
commercially available probe head with a working distance of around 5 cm
gave excellent results for quantitative monitoring of the process in real time.
Appropriate working distance can be up to tens of centimetres depending
on the application. However, for many pharmaceutical processes that involve
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