1. An exposure control system for a colour printer adapted to produce prints from different types of original colour material comprising: (1) photo-electric means for determining the colour density of an original; (2) a store for storing sets of exposure parameters, each set being associated with a different original material; (3) means for spectral analysis of an original; (4) means responsive to said spectral analysis means to form a coefficient representative of that original; (5) classification means for classifying each coefficient in one of a plurality of ranges of coefficient reference values, each range being characteristic of a different original material; (6) means responsive to said classification means for automatically selecting a set of exposure values from said store that is associated with the respective coefficient reference range of values in which the coefficient for a particular original is classified by said classification means.