Human perception of an object's color originates from color sensors in the eye responding to electromagnetic radiation reflected off the object. Under mesopic vision, the human eye is more sensitive to light of short wavelength. Hence, phosphor-coated white LEDs that have higher blue light component would have higher luminous efficacy under mesopic vision than under photopic vision. This trait has been in the past described as an advantage of white LEDs. Nevertheless, higher CCT is associated to lower transmission in fog or haze. Since there is Rayleigh scattering and Mie scattering, light fails to fully transmit through fog or haze. Fog penetration ability is an indispensable consideration for the reason that street lights are utilized to illuminate the road under foggy or hazy weather. Generally speaking yellow light has better fog penetration capabilities than white light. The fog penetration ability of most white LEDs is poor. Low fog penetration ability also results in higher urban skyglow pollution from white LEDs owing to molecular (Rayleigh) and aerosol (Mie) scattering. Such scattering not only reduces the illumination of the ground and make ground objects hard to observe (and thus make the roadway even less safe), the skyglow from scattering also holds an undesirable effect on astronomical research, and adversely affect the general aesthetics of the sky at night.
Inefficient dark adaption is one important factor that causes traffic accidents during the night. In an attempt to make dark adaption faster and improve driver's driving safety and efficiency, low color temperature illumination typically used for illumination. LED lighting with a cool white or daylight CCT has relatively high amount of blue light components and the longest dark adaption time. On the contrary, warm white LEDs have relatively low CCT and a high amount of long wavelength components, and therefore they have short dark adaption times. Dark adaption should be an important design consideration for roadway lighting and is critical in tunnel lighting.
