Abstract: Methods and apparatus for detecting one or more properties of a sub-surface volume of a diffusely scattering sample are disclosed, where the diffusely scattering sample comprises surface enhanced Raman Spectroscopy (SERS) probes, the SERS probes being spectrally responsive to a stimulus. The stimulus is provided in a varying form at the sub-surface volume, and during the varying stimulus, probe light is directed to an entry region on a surface of the sample. A portion of the probe light is collected, including elements of said probe light Raman scattered from said SERS probes, at a collection region on the surface of the sample, the collection region being spaced from the entry region. The collected Raman scattered elements are then measured, and variations in the measured Raman scattered elements which are induced by the variations in the stimulus are detected. One or more properties may then be detected based on the detected variations in the measured Raman scattered elements.