Properties

There are four main properties that characterize thermal radiation:

• Thermal radiation, even at a single temperature, occurs at a wide range of frequencies. How much of each frequency is given by Planck’s law of radiation (for idealized materials). This is shown by the curves in the diagram at right.
• The main frequency (or color) range of the emitted radiation includes higher and higher frequencies as the temperature increases. For example, a red hot object radiates enough in the long wavelengths (red and orange) of the visible band to see, which is why it appears red. If it heats up further, it also begins to emit discernible amounts of green and blue light, and the spread of frequencies mentioned in the first point make it appear white. We then say the object is white hot. However, even at a "white-hot" temperature of 2000 K, 99% of the energy of the radiation is still in the infrared. This is related to Wien's displacement law. In the diagram the peak value for each curve moves to the left as the temperature increases.
• The total amount of radiation, of all frequencies, goes up very fast as the temperature rises (it grows as T⁴, where T is the absolute temperature of the body). An object at the temperature of a kitchen oven (about twice room temperature in absolute terms: 600 K vs. 300 K) radiates 16 times as much power per unit area. An object at the temperature of the filament in an incandescent bulb (roughly 3000 K, or 10 times room temperature) radiates 10,000 times as much per unit area. The total radiative intensity in a cavity that contains a black body in thermodynamic equilibrium rises as the fourth power of the absolute temperature, the Stefan–Boltzmann law. In the plot, the area under each curve rises rapidly as the temperature increases.
• The rate of thermal radiation of a particular kind of electromagnetic wave is proportional to the amount of absorption that the same type of wave experiences. Thus, a surface that absorbs more red light thermally radiates more red light. This principle applies to all properties of the wave, including wavelength (color), direction, polarization, and even coherence, so that it is quite possible to have thermal radiation which is polarized, coherent, and directional, though polarized and coherent forms are fairly rare in nature.

These properties apply if the distances considered are much larger than the wavelengths contributing to the spectrum (most significant from 8-25 micrometres at 300 K). Indeed, thermal radiation here takes only travelling waves into account. A more sophisticated framework involving electromagnetics has to be used for lower distances (near-field thermal radiation).

°C	Subjective colour [2]
480	faint red glow
580	dark red
730	bright red, slightly orange
930	bright orange
1100	pale yellowish orange
1300	yellowish white
> 1400	white (yellowish if seen from a distance through atmosphere)