1. A method of image formation wherein a sample is scanned by use of a scanning electron microscope to cause an electron microscope to cause an electron beam to be incident on the sample, and three image signals are generated by respectively detecting the intensity of secondary electron emission in a first energy band resulting from said incidence, the intensity of secondary electron emission in a second energy band resulting from said incidence, and the intensity of back-scattered electrons resulting from said incidence, said image signals being used to form a colour image of said sample in which the relative intensities of three image colours at each location in the image are repesentative of the prevailing relationship between said intensity of secondary electron emission in a first energy band, said intensity of secondary electron emission in a second energy band and said intensity of backscattered electron, at a corresponding location on the said sample.