Virtually all lighting fixtures emit a range of colors of light. Some, like incandescent and halogen bulbs,  create a continuous spectrum over a wide range of wavelengths. Others, like fluorescent bulbs, emit a few very specific wavelengths that, when seen by our eyes, mimic white light.

Lighting manufacturers use a characteristic called correlated color temperature (CCT) as shorthand for whether a light looks “cool” or “warm”. As shown here, the higher the CCT value (as measured on the Kelvin temperature scale), the bluer the light looks.

Light that is strongly blue can be environmentally disruptive (and often harmful) to plants, wildlife, and humans. Blue-rich light also scatters readily in the atmosphere, creating lots of skyglow.