Abstract: The present invention relates to a method and apparatus for forming a filtered image filtered with an at least partially negative filtering spectrum. In the method a plurality of exposure combinations of exposure times and wavelength channels are determined so that at least some of the wavelength channels are on the positive area of the filtering spectrum, on which area the exposure is essentially determined to correspond with the filtering spectrum, while at least some of the wavelength channels are on the negative area of the filtering spectrum, on which area the exposure is determined to essentially correspond with the inverted filtering spectrum. Partial images are formed of the target using the said combinations of exposure times and wavelength channels by illuminating the target with the light source, by filtering the light emitted by the source of light and by detecting the light reflected or transmitted.