Filter-Cap: O2k-Fluo LED2-Module (O2k-Series D-G) sensors (Fluorescence-Sensor Green and Fluorescence-Sensor Blue) and O2k-FluoRespirometer (O2k-Series H-I) sensors (Smart Fluo-Sensor Green and Smart Fluo-Sensor Blue) are equipped with a removable Filter-Cap for exchange of optical filters for the optical pathways from the LED to the sample and from the sample to the photodiode.