The forward scatter visibility measurement technology has established its position along transmissometers in safety critical Runway Visual Range (RVR) application at the airports. The ICAO's revised Manual of Runway Visual Range Observing and Reporting Practices, second edition, 2000, has raised wide international interest towards RVR systems based on forward scatter visibility sensors.

The new Vaisala forward scatter visibility sensor design concentrates especially on reliability and safety factors having effect on visibility measurement in airport environment. In addition, the visibility measurement range up to 75 km allows the same sensor to be used in meteorological visibility measurement applications. A new method has been developed to compensate the attenuation effect of window contamination in order to increase the periodic maintenance interval between window cleanings and to ensure that the measurement accuracy is maintained throughout the maintenance interval. This new method is based on monitoring the total reflection properties of the window surface by sending the light beam in a specific angle into the glass material.

The paper will present the instrument design and calibration in detail, as well as measurement results in various weather conditions and window contamination situations.