Abstract: An apparatus and system for monitoring and treating cataracts, the apparatus comprising: a monitoring light source configured to monitor cataracts by emitting monitoring light in the wavelength range of 350 to 410 nm to excite fluorescence light in the cataracts, a treating light source configured to treat cataracts by emitting treating light in the wavelength range of 400 to 570 nm to irradiate the cataracts, a wavelength selection system configured to monitor cataracts by selecting wavelengths of the excited fluorescence light in the cataracts and a dichroic beam splitter configured to reflect the monitoring light and the treating light towards the cataracts and the excited fluorescence light in the cataracts towards the wavelength selection system, wherein the monitoring light, the treating light and the excited fluorescence light are reflected by the dichroic beam splitter along a common optical axis and wherein the dichroic beam splitter is arranged at 45 degrees to the common optical axis to transmit wa