System for optical measurement of a target (350, 450, 452, 675), the system comprising - a detector (339, 380, 381, 439, 481, 539, 582, 598, 682, 698) for receiving measurement light beam from or through the target (350, 450, 452, 675), and accordingly, - a light source (310, 410, 510), for illuminating the target (350, 450, 452, 675) with an illuminating light beam, wherein the system comprises at least one Fabry-Perot interferometer within the path of the illuminating light beam and/or the path of the measurement light beam, wherein the system has first means to control at least one light transmission characteristic of a first Fabry-Perot interferometer that is controllable into at least two modes with different values of said characteristic during a measurement of a single target, wherein the first Fabry-Perot interferometer include one Fabry-Perot interferometer or a set of two (341, 371) or more interferometers, characterized in that said first Fabry-Perot interferometer has one mode in which during a measurement of a target (350, 450, 452, 675) said first Fabry-Perot interferometer is controllable to a shutter mode, to block received light of all wavelengths within the overall measurement range. - a detector (339, 380, 381, 439, 481, 539, 582, 598, 682, 698) for receiving measurement light beam from or through the target (350, 450, 452, 675), and accordingly, - a light source (310, 410, 510), for illuminating the target (350, 450, 452, 675) with an illuminating light beam, wherein the system comprises at least one Fabry-Perot interferometer within the path of the illuminating light beam and/or the path of the measurement light beam, wherein the system has first means to control at least one light transmission characteristic of a first Fabry-Perot interferometer that is controllable into at least two modes with different values of said characteristic during a measurement of a single target, wherein the first Fabry-Perot interferometer include one Fabry-Perot interferometer or a set of two (341, 371) or more interferometers, characterized in that said first Fabry-Perot interferometer has one mode in which during a measurement of a target (350, 450, 452, 675) said first Fabry-Perot interferometer is controllable to a shutter mode, to block received light of all wavelengths within the overall measurement range. wherein the system comprises at least one Fabry-Perot interferometer within the path of the illuminating light beam and/or the path of the measurement light beam, wherein the system has first means to control at least one light transmission characteristic of a first Fabry-Perot interferometer that is controllable into at least two modes with different values of said characteristic during a measurement of a single target, wherein the first Fabry-Perot interferometer include one Fabry-Perot interferometer or a set of two (341, 371) or more interferometers, characterized in that said first Fabry-Perot interferometer has one mode in which during a measurement of a target (350, 450, 452, 675) said first Fabry-Perot interferometer is controllable to a shutter mode, to block received light of all wavelengths within the overall measurement range.