An important feature of Southampton’s Outdoor Lighting Code is its prohibition of blue-rich “white” light. The color of a light source can be described in terms of the amount of energy emitted at different wavelengths, but more commonly is characterized by Correlated Color Temperature (CCT), which is measured in degrees of Kelvin (K). Lower CCTs represent strong red / yellow tones, while high CCTs indicate a strong blue cast.
Most fixtures currently used for street and parking-lot lighting have high-pressure sodium bulbs, which typically have Correlated Color Temperature of 2,100-2,200 Kelvin, with a decided gold / amber tone. However, some newer outdoor light sources - especially LED, but also metal halide and fluorescent - have CCTs of 4,000K and even up to 7,000K or higher. Samples of different CCTs can be seen on a color chart (PDF).
There are a number of problems associated with such blue-rich light, which is often described as “white.” Compared to warmer sources such as high-pressure sodium or incandescent, blue-rich light causes more glare, creates more sky glow, poses greater difficulties for older eyes, has more damaging ecological impacts, and more readily disrupts human circadian rhythms.