Fig. 1.

Schematic comparing variants of sample excitation (solid line) and signal collection [dashed line(s)] used in Raman spectroscopy in measuring turbid media. (a) Backscattered Raman is a commonly used geometry that uses a single site of excitation with collection of signal close (<1 mm) to the excitation. As applied to Raman microscopy, this approach is called epi-illumination as a single microscope objective is used to excite the sample and collect Raman signal. The sampling volume in backscattered Raman is generally small, both in the lateral and axial dimensions. Thus, backscattered Raman is a good approach for measuring a surface. (b) Overlapping a defocused or wide laser beam with multiple collection fibers in a backscattering-like geometry is called wide area Raman (also known as large volumetric or global illumination Raman). (c) Separating collection fibers from the sample excitation by a small distance (Δd = 1–6 mm) enables collection of diffusely scattered Raman photons, known as a spatially-offset Raman spectroscopy (SORS). (d) Transmission Raman collects Raman photons diffusely scattered through a sample. Wide area, spatially-offset Raman spectroscopy (SORS) and transmission Raman provides representative sampling in turbid media and enables collection of Raman signal from buried layers in a layered sample