Beware, kelvin temperature is another mind bender! Kelvin temperature (abbreviated as K), is the scientific unit for temperature, and is often used to measure the color temperature of light, or more accurately, the measurement of the temperature of an object emitting black body radiation, also known as thermal radiation, or radiant heat as visible light. All objects at a temperature greater than absolute zero emit some form of thermal radiation, although at room temperature the wavelength of this radiation is too large for the human eye to perceive. Still confused?
Think of it this way: at room temperature, your stove burner doesn’t emit any visible light. However, as it increases in energy (temperature), the energy associated with the burner increases, and the blackbody radiation the burner is emitting changes frequency, and emits that energy in the frequency of visible light, and that’s when you know it’s hot!
There is a great deal to know about kelvin temperature and thermal radiation (most of it mathematical) that exceeds the scope of this article, but kelvin temperature is a very useful way to measure the color spectrum of a bulb.
Kelvin temperature, in the hobby, is used virtually interchangeably with spectrum. Spectral range is more useful for determining PAR, but kelvin temperature has become the norm among bulbs produced for the hobby. Kelvin temperature is often associated with PAR: a light source with a given intensity from an object with a color temperature (kelvin) of 6500K has more PAR (but not necessarily more PUR)than light emitted from a source of the same intensity but having a color temperature of 14,000K. A light source with a color temperature of 6500k places more of the energy between the 400-700nm wavelengths, thus there is more energy available for photosynthesis to occur.
As the hobby progresses, especially towards LED lighting, kelvin temperature is becoming obsolete, and should be (and is being) phased out as a measure of 'color'.