1. A method for characterising a sequence of images represented by a plurality of pixels whose intensity and/or colour can change with time, each pixel having a pixel value indicative of the intensity and/or colour of the pixel, the method comprising the steps of: (i) determining the temporal rate of change of the pixel value for each pixel of a group of pixels; (ii) combining the determined rates of change for each of the pixels so as to provide a combined rate of change value for the sequence of images, the combined rate of change having a plurality of temporal frequency components associated therewith; (iii) determining the size of at least some of the temporal frequency components associated with the combined rate of change; and, (iv) characterising the sequence of images in dependence upon the sizes of the frequency components.