An optical apparatus comprises an ultraviolet light source (100) configured to transmit the ultraviolet light to a sample (102), one or more wavelength dependent beam splitters (104) and at least two separate detectors (106, 108). Each beam splitter (104) receives, from the sample (102), a band of excitating ultraviolet light and at least one band of fluorescence associated with an interaction between the excitating ultraviolet light and the sample (102) in the optical path through the sample (102), directs the band of excitating ultraviolet light passed through the sample (102) towards a first detector (106), and directs the at least one band of the fluorescence towards at least one separate detector (108). The first detector (106) and the at least one separate detector (108) are simultaneously configured to form electrical signals carrying information on powers of the bands of the ultraviolet light and the fluorescence, respectively. The signal processing unit (110) configured to measure the at least one property of the sample (102) on the basis of a signal from the first detector (106) and each signal from the at least one separate detector (108).