Many lamps generate light by means other than a filament, which creates light with slightly different. In this case, the actual metric used is CCT or correlated color temperature. A common example of CCT is seen when describing fluorescent T5 bulbs, but this has been used to describe the visual spectrum of high pressure sodium lights and LED’s as well. Lamps marketed as 4000K and lower will emit more red light, lamps which will be good for flowering plants and lights rated 7000K and above emit more of a blue spectrum which will be good for seedlings and vegetative growth. Lamps with a Kelvin between 5500-6500 will emit the closest spectrum to natural daylight. It’s important to note that color temperature, CCT and Kelvin only refer to the visual spectrum emitted from a lamp.