||Artificial light at night is an irreplaceable technology for our society and its activities at nighttime. But this indispensable tool has detrimental side effects, which have only come to light in the past 10â€“20 years. This chapter reviews ways to implement technology in order to lower the impact of artificial light at night on nature and humans. Further, it provides guidelines for environmental protection and scientific approaches to reduce the increase in light pollution and discusses the urgent need for further research. Measures to prevent obtrusive light and unintentional trespass into homes and natural habitats are
mostly simple solutions like shielding luminaires and predominantly require awareness. Shades are another effective tool to reduce trespass from interior lights. Especially in greenhouses, the use of shades significantly reduces the contribution to skyglow. Artificial light should be switched off whenever it is not needed. Smart, flexible lighting systems can help to use artificial light with precision. The choice of the appropriate illumination has to be balanced by the needs for optimal visibility, human well-being, environmental conservation and protection of the night sky. For visibility, conditions comparable to bright moonlit nights (0.3 lx) are sufficient. Low-level streetlights that produce only 1â€“3 lx at the surface meet the requirement of facial cognition. Although this light level might be too low for road safety, a consideration of maximum illumination levels in street lighting is recommended. The spectral power distribution of illuminants can impact several environmental parameters. For example, illuminants emitting short wavelengths can sup- press melatonin in higher vertebrates (including humans), are attracting many insect species, and contribute in skyglow above average. Recent findings in different measures for energy efficiency of illuminants at scotopic or mesopic vision conditions compared to photopic conditions indicate that the assessment of lighting products needs fundamental revision. Further research is crucially needed to create refuges for light-sensitive species at night, to measure the impact of artificial light on nature, and also to monitor the improvements of light pollution-reducing measures. Decrees in various regions have helped to lower the impact of artificial light at night significantly. Measures to reduce the impact of artificial light at night need to be carefully balanced with the surrounding environment. Thoughtful guidelines are crucial to reducing the rapid increase in sky brightness worldwide. These guidelines need to be made accessible for decision makers especially in areas which require new light installations.