An optical system comprising - a light source for emission of a first light beam, - a first beamsplitter being adapted to transmit/reflect a secondary output light beam in response to said first light beam being incident upon said beamsplitter, and further being adapted to reflect/transmit a primary output light beam in response to said first light beam being incident upon said beamsplitter, the power of the secondary output light beam being a substantially fixed percentage of the power of the primary output light beam, the substantially fixed percentage being less than 5 %, the substantially fixed percentage being substantially invariant to wavelength variations of the first light beam within a predetermined wavelength range, and - a detector being adapted to measure the power of the secondary output light beam, and providing on the basis of the measured power a control signal to the light source, so that parameters of the light source are adjusted in such a way that the output power of the primary output light beam is kept substantially constant, characterized in that - the first beamsplitter has a dielectric coating, - the first beamsplitter, for an incident light beam having a wavelength within the predetermined wavelength range, induces a variation in the power of the transmitted/reflected secondary light beam being within +/-10 % of the power of the transmitted/reflected secondary light beam at a given wavelength within the predetermined wavelength range so as to provide a variation in the substantially fixed percentage of the primary output light beam being within +/-10% of the substantially fixed percentage at the given wavelength, and - the predetermined wavelength range is chosen from a group consisting of the wavelength ranges: from approximately 520 nm to approximately 585 nm, from approximately 620 nm to approximately 650 nm, from approximately 780 nm to approximately 830 nm, from approximately 910 nm to approximately 1100 nm, from approximately 1450 nm to approximately 1550 nm, and from approximately 1600 nm to approximately 1900 nm. - a light source for emission of a first light beam, - a first beamsplitter being adapted to transmit/reflect a secondary output light beam in response to said first light beam being incident upon said beamsplitter, and further being adapted to reflect/transmit a primary output light beam in response to said first light beam being incident upon said beamsplitter, the power of the secondary output light beam being a substantially fixed percentage of the power of the primary output light beam, the substantially fixed percentage being less than 5 %, the substantially fixed percentage being substantially invariant to wavelength variations of the first light beam within a predetermined wavelength range, and - a detector being adapted to measure the power of the secondary output light beam, and providing on the basis of the measured power a control signal to the light source, so that parameters of the light source are adjusted in such a way that the output power of the primary output light beam is kept substantially constant, - the first beamsplitter has a dielectric coating, - the first beamsplitter, for an incident light beam having a wavelength within the predetermined wavelength range, induces a variation in the power of the transmitted/reflected secondary light beam being within +/-10 % of the power of the transmitted/reflected secondary light beam at a given wavelength within the predetermined wavelength range so as to provide a variation in the substantially fixed percentage of the primary output light beam being within +/-10% of the substantially fixed percentage at the given wavelength, and - the predetermined wavelength range is chosen from a group consisting of the wavelength ranges: from approximately 520 nm to approximately 585 nm, from approximately 620 nm to approximately 650 nm, from approximately 780 nm to approximately 830 nm, from approximately 910 nm to approximately 1100 nm, from approximately 1450 nm to approximately 1550 nm, and from approximately 1600 nm to approximately 1900 nm.