In a Raman spectroscopy apparatus, exciting light is focussed on a sample (26) as a line focus 38. Spectra from points in the line focus are dispersed in rows 46 on a CCD detector 34, having a two-dimensional array of pixels. The line focus moves longitudinally in a direction Y (arrow 48) relative to the sample. Simultaneously and synchronously, charge is shifted in a parallel direction Y′ (arrow 50) within the CCD, so that data from a given point in the sample continues to accumulate. This ensures that the data from each sample point arises from illumination which is integrated along the line focus, and makes it easier to stitch the data together subsequently to form an image of the sample. In order to provide averaging in the X direction during fast, low resolution scanning, the line focus is swept across the sample in a zig-zag fashion, between boundary lines 60.