At least two chemically different substances of interest of an unstained biological specimen (12). For each of a substance image (24, 26) is generated, indicating for every region of the image an amount of the substance. A multicolour image is generated on the basis of the substance images. In a related aspect, a data carrier carries instructions for instructing a computer to control or to perform the method. In another related aspect, a system (10) an unstained biological specimen (12) comprises an optical system for exposing to ultraviolet light (18) of a selected frequency and for measuring for various regions of the specimen the intensity of transmitted ultraviolet light (20) and for repeating the steps of exposing and of measuring for different ultraviolet frequencies, thus generating for each frequency an ultra violet image; a computer for generating substance images on the basis of the ultraviolet images and for generating the multicolour image on the basis of the substance images.