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Incandescence is a special case of thermal radiation. Incandescence usually refers specifically to visible light, while thermal radiation refers also to infrared or any other electromagnetic radiation.
For a detailed discussion of the intensity and spectrum (color) of incandescence, see the article: thermal radiation.
In practice, virtually all solid or liquid substances start to glow around 798 K (525 °C), with a very dull red color, when no chemical reactions take place that produce light as a result of an exothermic process. This limit is called the Draper point. The incandescence does not vanish below that temperature, but it is too weak in the visible spectrum to be perceivable.
At higher temperatures, the substance becomes brighter and its color changes from red towards white and finally blue.
Incandescence is exploited in incandescent light bulbs, in which a filament is heated to a temperature at which a fraction of the radiation falls in the visible spectrum. The majority of the radiation however, is emitted in the infrared part of the spectrum, rendering incandescent lights relatively inefficient as a light source. If the filament could be made hotter, efficiency would increase; however, there are currently no materials able to withstand such temperatures which would be appropriate for use in lamps.
The word incandescent is also used figuratively to describe a person who is so angry that they are imagined to glow or burn red hot or white hot.