The invention relates to a system and a method for optical measurement of a target, wherein the target is illuminated, either actively illuminated, reflecting ambient light, or self illuminating, and a measurement light beam received from the target or through it is detected. The prior art optical measurement systems generally include mechanical filter wheels and photomultiplier tubes, which cause the equipment to be expensive, large-sized and often not sufficiently accurate and stable. The objective of the invention is achieved with a solution, in which the illuminating light beam and/or measurement light beam is led through a Fabry-Perot interferometer or a set of two or more Fabry-Perot Interferometers, and the Fabry-Perot interferometer or a set of two or more Fabry-Perot Interferometers is controlled into different modes during the measurement of a single target. The invention can be applied in optical measurements where, for example, reflectance, absorption of fluorescence of the target is measured.