Method and apparatus for calibrating a spectrometer

A calibration moans suitable for, and intended for use in a spectrometer comprising a source 4 of electromagnetic radiation, a detector 16 suitable for detecting electromagnetic radiation from the source, recording means for recording the output of the detector and at least one optical element 8, 10 in the optical path between the source and the detector, comprises a variation means 18 to enable the input of electromagnetic radiation to the detector to be varied and an adjustment means 20 to adjust the variation means adapted so that the input of the detector substantially reproduces the input of electromagnetic radiation that the detector would receive in its normal operation from a sample which the spectrometer is intended to analyse. The variation means comprises an electrodiachromatic (liquid crystal) mask which during calibration using a ceramic standard 22 is controlled by a computer so that in each wavelength band the mask optical density is adjusted by reference to stored values. The radiation reflected by the standard 22 is thereby adjusted to correspond to that which would be reflected by a sample. The invention obviates the need to re-analyse a sample for which spectra have already been obtained on a reference instrument.