An optical device for determining the presence and/or concentration of analytes in a sample is presented. The optical device comprises a detector and a detection unit comprising optical path components. The detection unit has wavelength-dependent responsivity. The optical device further comprises a light source for emitting light of different respective usable wavelength ranges. The light is guidable through the optical path to the detector to generate baseline signals and response signals relative to the baseline signal indicative of the presence and/or concentration of analytes in the optical path. The intensity of the light reaching the detector is adjusted inverse to the wavelength-dependent responsivity with respect to at least two respective usable wavelength ranges so that a reduction of the ratio between the maximum baseline signal at one of the selected usable wavelength ranges and the minimum baseline signal at another of the selected usable wavelength ranges is obtained.