A real time monitor, specifically designed to measure hydrogen fluoride (HF) concentration at the exit of the air purification system of aluminum smelters, has been tested and evaluated over a full year. The system has been designed to be rugged, with a low operating cost and easy to install and maintain in operation. These objectives have been achieved using a cheap halogen lamp as a light source, a simple micromotor as a light modulator, a pair of interference filters as optical analyzing elements and optical fibers as light guides between the central unit and the remote measurement points. In this application, the use of optical fiber provides two great advantages. Firstly, measurements in very demanding conditions become possible since the central unit, which has the task to make the entire optical electronic and digital processing, can be left in a control room where the conditions are much less difficult. Secondly, the capability of the central unit to process the optical information coming from two probe heads significantly reduces the overall costs by measurement points. The detection sensitivity limit achieved is 0.1 mg/m3 with normal stack diameter. The accuracy is around 5% depending on the care devoted to the calibration process. The response time can be adjusted over a large range but is typically set at 10 seconds.