The sensitivity of commercial lidar ceilometers for detecting aerosols within the boundary layer is sufficient to give reliable information on parameters like the mixing layer height. The traditional optical concepts of these lidars are either biaxial two-lens systems or one-lens systems with separate compensation techniques for the internal optical cross talk. The single lens concept is superior to the biaxial system when reliable data from the near range up to 300 m elevation is required. This paper introduces an enhanced single lens system removing the internal optical cross-talk between the laser transmitter and the receiver. The new Vaisala Ceilometer CL31 uses this optical layout to improve cloud and vertical visibility detection performance in precipitation, fog and haze situations. On the other hand it also provides advanced boundary layer scans during clear sky weather. Such scans are performed around the clock in the Vaisala test field by ceilometers using the three optical systems discussed. A comparison of cases covering a variety of different meteorological situations illustrates the advantages of the new optical concept. Mixing layer height values are determined with a gradient method and compared to radio sounding data.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.