The IntelliCEMS 1550 Series UV Fluorescence analyzer measures gas concentration based on the principle that SO2 will fluoresce when exposed to ultra-violet light. The instrument consists of a UV light source and an optical filter, a measuring cell, a second optical filter, and a detector. In addition, there are lenses and baffles to focus the light. The light source emits UV light in the direction of the measurement cell. The light is focused and the wavelength filtered as it enters the measurement cell. The optical filter blocks UV at wavelengths longer than 230nm. The UV is absorbed by the SO2 in the measurement cell. The SO2 absorbs UV at wavelengths between 190nm and 230nm, and then emits the energy as UV wavelengths between 230nm and 420nm. At a right angle to the incoming beam of UV light is the port for the detector. The detector is a photomultiplier tube that is very sensitive. The detector port has a filter that blocks UV shorter than 230nm, as well as a focusing lens. The filters keep the detector from sensing the UV light from the lamp, so only UV light emitted by SO2 is measured. The amount of energy picked up by the sensor is directly proportional to the concentration of the SO2 in the measurement cell.