Specially developed for greenhouses, Lucalox? PSL high pressure sodium lamps offer the benefits of stable lumen and micromole maintenance and a full spectrum content that promotes photosynthesis. Photosynthesis Active Radiation (PAR, measured in micromole/ sec) is essential for plant growth. Simple light or lumen maintenance is not enough to create plant growth. Plants require a certain radiation level to help with photosynthesis. The Lucalox? PSL lamp has been specially developed to provide stable lumen maintenance and increased PAR output.