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The ability to identify the contents of a sealed container, without the needto extract a sample, is desirable in applications ranging from forensics toproduct quality control. One technique suited to this is inverse spatiallyoffset Raman spectroscopy (ISORS) which illuminates a sample of interest withan annular beam of light and collects Raman scattering from the centre of thering, thereby retrieving the chemical signature of the contents whilesuppressing signal from the container. Here we explore in detail the relativebenefits of a recently developed variant of ISORS, called focus-matched ISORS.In this variant, the Fourier relationship between the annular beam and atightly focused Bessel beam is exploited to focus the excitation light insidethe sample and to match the focal point of excitation and collection optics toincrease the signal from the contents without out compromising the suppressionof the container signal. Using a flexible experimental setup which can realiseboth traditional and focus-matched ISORS, and Monte-Carlo simulations, weelucidate the relative advantages of the two techniques for a range of opticalproperties of sample and container.

To focus-match or not to focus-match inverse spatially offset Raman spectroscopy: a question of light penetration