Characterisation of Deep Layers of Tissue and Powders: Spatially Offset Raman and Transmission Raman Spectroscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 51

35000

30000

t-stilbene

2.5 mm

25000

2 mm

20000

1.5 mm

15000

1 mm

0.5 mm

10000

0 mm

5000

PMMA

0

750 950 1150 1350 1550 1750

wavenumbers /cm -1

Fig. 3.4. A set of SORS spectra collected from a two-layer system consisting of
1 mm thick layer of PMMA made of 20 μm diameter spheres followed by a 2 mm
layer of trans -stilbene powder measured using 514 nm as the probe wavelength. The
spectra are shown for different spatial offsets. The top and bottom spectra are those
of the individual layers obtained in separate measurements. The spectra are offset
for clarity. The acquisition time was 100 s for each spectrum and the average laser
power 12 mW (reprinted with permission from [18]. Copyright (2005) The Society
for Applied Spectroscopy)

detection system [18, 19, 30-32]. Similar probes have also found use in fluo-
rescence tomography [5-8]. In the case of SORS, the collection fibres can be
arranged into an annular (ring) geometry (see Fig. 3.5) to maximise collection
eciency at a particular offset. With this arrangement the spatial offset is de-
fined as the radius of the ring.

3.3.3 Inverse SORS

The fixed nature of the collection fibres in a conventional SORS experiment
limits the range of spatial offsets available. An alternative approach (inverse
SORS) swaps the laser and collection fibre geometries; Raman light is collected
through a group of fibres (arranged in a disc shape) contained within the
centre of a probed area defined by a ring-shaped laser beam (see Fig. 3.5).