Energy efficiency of quasi-monochromatic LEDs for road lighting under mesopic vision

The study addresses the energy efficiency of quasi-monochromatic LEDs for road lighting under mesopic vision conditions. It provides the outcome from the initial stage of research into the potential of such lighting at low adaptation levels. Outdoor electric lighting is widespread and essential for ensuring safety after dark, and current developments increasingly emphasize sustainable LED-based solutions. The existing standards define luminance levels for photopic vision, while nighttime conditions on illuminated roads fall within the mesopic range (0.005-5.000 cd/m²). The research aimed to determine potential of quasi-monochromatic LEDs and energy savings resulting from the use of the light sources in road lighting. Laboratory tests included LED chip samples with peak wavelengths between 495 and 605 nm. The analysis considered the spectral power distribution, S/P ratio and mesopic luminous efficacy of the LEDs, normalized power density and annual energy density for road lighting under mesopic vision conditions. The results confirmed that quasi-monochromatic LEDs are an alternative worth studying for mesopic vision conditions and can improve the energy efficiency of road lighting. For LEDs emitting within 495-570 nm range, mesopic luminous efficacy increased as adaptation luminance decreased. The analysis also showed that, compared with HPS lamps, a 570 nm LED and a mixed spectrum combining 2884 K and 500 nm LEDs may offer up to 20% higher energy efficiency at the lowest lighting levels (0.3-0.5 cd/m²). Consequently, annual energy savings in road lighting may reach approximately 20% relative to HPS lamps and even 30% relative to a typical white LED.