The present application relates to an optical system (1) comprising a plurality of optical components, a pulse generating arrangement (3) configured to generate a treatment pulse along a treatment pulse optical path through said optical components, said pulse generating arrangement also being configured to generate a probe pulse along a probe pulse optical path extending through said optical components, a sensor (8) configured to generate information indicative of an optical characteristic of said probe pulse that has passed along said probe pulse optical path through said optical components, and a controller (5) configured to control said pulse generating apparatus (3) to selectively emit said treatment pulse along said treatment pulse optical path (2), in dependence on the information generated by the sensor (8).
