A sample is illuminated by laser light and the resulting Raman spectrum is dispersed at a high spectral resolution along one or more rows or columns of detector elements of a CCD. The resulting charge is shifted in a direction Y′ and binned in an output register of the CCD. The dispersed spectrum is moved along the rows or columns in a direction X′, synchronously with the shifting of charge in the output register. Thus, data from a given wavenumber in the spectrum continues to accumulate in the output register during the movement. This enables data from a wide spectrum to be collected at high resolution, without the need to subsequently stitch blocks of data together in a computer, even where the CCD is arranged such that row-by-row transfer of charge towards the output register is orthogonal to the direction of dispersion.