An optical system and a method of exciting and measuring fluorescence on or in samples treated using fluorescent pigments using such an optical system are disclosed. The optical system comprises at least one first laser (1); a deflection element (7); a mirror (4); an optic (8); and a unit (10), which includes mirror (4) and optic (8) mounted fixed in relation to one another. The unit (10) is positioned so it is linearly movable back and forth along the optical axis (5) and is mechanically connected to an oscillating linear drive (11). The optic (8) additionally acts as a collimator and the mirror (4) additionally acts to deflect the collimated light (12) diametrically opposite to the direction of incidence of the bundled light of the laser (1). The optical system additionally includes a table (13) for receiving sample holders (14) for samples (6); an optical arrangement (16) for imaging a second focal point (17); an aperture plate (18) positioned in the second focal point (17); a first spectral filter (19); and a first detector (20). The optical system is distinguished in that a focusing beam (25) of the light (2), which is emitted by the laser (1) and is deflected by the deflection element (7) into the direction of the mirror (4), is arranged to extend off-center and in a distance (A) to an optical axis (5) of this optical system. A preferred embodiment of the optical system is distinguished in that the oscillating linear drive (11) comprises a connecting rod for linearly moving back and forth the unit (10).