Correlated Color Temperature

550px-Planckian-locusHaving discussed color temperature elsewhere, this article talks a little more about how it applies to LED lighting and other technologies.

In incandescent bulbs, including halogen, light color is remarkably consistent across all products and brands. One 2700K bulb looks very much like another. This, you might think, should be obvious, but the same is not necessarily true of LED and fluorescent lighting.

The color consistency of an incandescent bulb is dictated by science. In an incandescent bulb, the usual 2700K color temperature is also the physical temperature of the filament. The filament is incredibly hot at around 2426°C (i.e. 2700K).

Because an incandescent bulb behaves like a “blackbody radiator” and produces light through the use of heat, its light output follows a precise, defined path known as the “Planckian locus” in physics. The bulb’s color temperature sits on an exact point along this path.

LED and fluorescent bulbs do not have a “true” color temperature for the simple reason that they do not use heat to produce light. Though an LED bulb certainly generates heat, the heat is not carried in its light. That’s why you can freely touch the front of an LED bulb when in use without burning your fingers.

Instead of having an actual color temperature, like an incandescent bulb has, the color of an LED bulb is dictated by the precise properties of its manufacture (e.g. thickness of yellow phosphor and wavelength of blue LED). Before being sold, the manufacturer assigns a color temperature to the product, which is known as a “correlated color temperature”.

A correlated color temperature, as its name suggests, is one that correlates roughly with an actual color temperature. The main difference is that a correlated color temperature (CCT) is allowed some leeway along a magenta-green axis rather than following the precise color path of a filament bulb. Because of this leeway, an identical color temperature does not necessarily mean identical color when applied to LED products from different manufacturers.

The takeaway is this: when buying multiple LED bulbs for a large lighting scheme, choose one reputable brand.