= Spatially offset Raman spectroscopy
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= SORS
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{synonym}
\b[Spatially offset Raman spectroscopy], or <SORS>, collects <Raman scattering> from a surface region laterally displaced from the illumination spot. In a diffusely scattering material, changing the offset changes the mixture of photon paths and the relative contributions of surface and buried layers. It is not simply a tightly focused measurement at a selected depth.
The2005 experiment of <P. Matousek> and colleagues used <trans-Stilbene> powder beneath a $1\,\mathrm{mm}$ layer of <PMMA> powder. Its primary report describes a19-fold improvement in buried-layer contrast before numerical separation.<Subsurface Probing in Diffusely Scattering Media Using Spatially Offset Raman Spectroscopy by P. Matousek and others>[Abstract, two-powder feasibility experiment and lateral collection geometry]{ref} The physical point is that an opaque-looking scattering overlayer need not contribute the same spectral proportion at every collection offset.
An original two-component <linear combination> illustrates <scaled subtraction of two spatially offset Raman spectra>. Let $S(\nu)$ and $B(\nu)$ be fixed surface and buried <Raman spectra>, and suppose dark-corrected measurements give
$$
Y_0(\nu)=8S(\nu)+2B(\nu),\qquad
Y_\rho(\nu)=3S(\nu)+4B(\nu).
$$
If a surface-specific band determines the scale $3/8$, then
$$
Y_\rho-\frac38Y_0=\frac{13}{4}B.
$$
The surface contribution cancels while a scaled buried spectrum remains. These coefficients are chosen examples, not the paper's measured mixing fractions.
A practical arrangement fixes an excitation spot, moves the collection region laterally, rejects the excitation line with an <optical filter>, and sends the collected light to a <spectrograph>. Record comparable exposure, incident <radiant power>, backgrounds and spectral response at each offset. If the offset changes each component's shape, rather than only its scale, the simple subtraction model fails. Strong <absorption>, <fluorescence>, overlapping bands and measurement noise also limit inference. Offset-dependent enrichment provides evidence for depth-dependent contributions; it is not an unqualified depth calibration or unique chemical identification.
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