A wavelength tracking system comprising: a wavelength tracking unit with reflection surfaces at stable positions providing a first reflection path with a first path length and a second reflection path with a second path length, wherein the first path length is substantially larger than the second path length, and an interferometer system , wherein the interferometer system comprises: a beam splitter to split a light beam in a first measurement beam and a second measurement beam, at least one optic element to guide the first measurement beam, at least partially, along the first reflection path and the second measurement beam, at least partially, along the second reflection path , a first light sensor arranged at an end of the first reflection path to receive the first measurement beam and to provide a first sensor signal on the basis of the first measurement beam, a second light sensor arranged at an end of the second reflection path to receive the second measurement beam and to provide a second sensor signal on the basis of the second measurement beam, and a processing unit to determine a wavelength or change in wavelength on the basis of the first sensor signal and the second sensor signal. a wavelength tracking unit with reflection surfaces at stable positions providing a first reflection path with a first path length and a second reflection path with a second path length, wherein the first path length is substantially larger than the second path length, and an interferometer system , wherein the interferometer system comprises: a beam splitter to split a light beam in a first measurement beam and a second measurement beam, at least one optic element to guide the first measurement beam, at least partially, along the first reflection path and the second measurement beam, at least partially, along the second reflection path , a first light sensor arranged at an end of the first reflection path to receive the first measurement beam and to provide a first sensor signal on the basis of the first measurement beam, a second light sensor arranged at an end of the second reflection path to receive the second measurement beam and to provide a second sensor signal on the basis of the second measurement beam, and a processing unit to determine a wavelength or change in wavelength on the basis of the first sensor signal and the second sensor signal. a beam splitter to split a light beam in a first measurement beam and a second measurement beam, at least one optic element to guide the first measurement beam, at least partially, along the first reflection path and the second measurement beam, at least partially, along the second reflection path , a first light sensor arranged at an end of the first reflection path to receive the first measurement beam and to provide a first sensor signal on the basis of the first measurement beam, a second light sensor arranged at an end of the second reflection path to receive the second measurement beam and to provide a second sensor signal on the basis of the second measurement beam, and a processing unit to determine a wavelength or change in wavelength on the basis of the first sensor signal and the second sensor signal. <EOS>