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Is there a difference between thermal radiation and infrared radiation?
Category: Physics
Published: December 13, 2023
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Yes, there is a meaningful distinction between the terms "thermal radiation" and "infrared radiation". With that said, there can be significant overlap between these two terms, which is probably why many people use these terms interchangeably in everyday life. "Thermal radiation" is electromagnetic radiation with any frequency that is created by the thermal emission process (not just infrared frequencies), while "infrared radiation" is electromagnetic radiation with a frequency in the specific range of 0.3 THz to 400 THz that is created by any process (not just the thermal radiation process). Infrared radiation can also be called infrared light, infrared rays, or infrared waves. There are many ways to make infrared radiation besides thermal radiation. Also, there are many other types of radiation that the thermal radiation process can create besides infrared radiation. This is summarized in the Venn diagram below. Note that this Venn diagram is in no way complete or comprehensive.
Thermal radiation includes all forms of electromagnetic radiation that are emitted by an object through the thermal emission process. This is the process in which the atoms and molecules that make up an object bump around randomly, according to the object's temperature, and emit electromagnetic radiation as a result. Thermal radiation can also be called radiant heat.
All objects made of atoms are always emitting thermal radiation. In order to emit zero thermal radiation, an object would have to be at exactly zero absolute temperature. However, exactly zero absolute temperature is impossible because quantum uncertainty does not allow it. The hotter that an object gets, the more thermal radiation it emits. Also, the hotter that an object gets, the more that the spectrum of the thermal radiation that it emits expands to include higher frequencies. Depending on the temperature of the object, it can emit various combinations of radio wave thermal radiation, infrared thermal radiation, visible light thermal radiation, ultraviolet thermal radiation, x-ray thermal radiation, and gamma ray thermal radiation. (Note that microwave radiation is a type of radio wave radiation and therefore does not need to be mentioned separately.)
There is always a minute amount of radio wave radiation emitted as part of the thermal radiation. However, for all temperatures except cryogenic temperatures, the part of the thermal radiation that is radio waves is miniscule, so that we can ignore it and still get the same answers to four significant figures. More specifically, for all temperatures except cryogenic temperatures, radio waves make up less than a ten-thousandth of the total thermally radiated energy.
All objects that are at a temperature below about 1000 K (but above cryogenic temperatures) continuously emit thermal radiation that is 100% infrared radiation. This includes objects at room temperature, such as chairs and tables; objects at cold winter temperatures, such as snow and ice; objects at normal biological temperatures, such as birds and humans; and objects cooked in household ovens, such as pizzas and cookies right out of the oven. That is why none of these objects visibly glow.
The plot below shows the thermal radiation spectrum emitted by an object at room temperature (294 K) such as a chair or a table (assuming that the object is an ideal blackbody emitter, which is usually close to the truth). Do not be confused by the word "blackbody" as it is just the name for the idealized explanation and does not refer to colors in the spectrum. As you can see in the plot below, 100% of the thermal radiation emitted by an object at room temperature is infrared radiation. In fact, an object at room temperature is nowhere near to thermally emitting visible light or higher frequencies. This is why objects at room temperature do not visibly glow through thermal effects. In the two plots below, the entire curve is not shown because I wanted all wavelength plots in this article to be on the same horizontal scale so that you can visually compare them.
The plot below shows the thermal radiation spectrum emitted by an object that is at the temperature of a living human (310 K), again assuming that the object is an ideal blackbody emitter, which is usually close to the truth. As you can see, the thermal radiation still consists of 100% infrared radiation. An object at this temperature is nowhere near to emitting visible light or higher frequencies. This is why humans at normal temperatures do not visibly glow, i.e. they are only glowing at non-visible infrared frequencies. (A human at abnormal temperatures can certainly visibly glow, such as when on fire.)
All objects that are at a temperature between 1000 K and 100,000 K continuously emit thermal radiation that consists almost entirely of infrared thermal radiation, visible light thermal radiation, and ultraviolet thermal radiation. This includes objects such as candle flames, campfires, incandescent light bulbs turned on, hot electric cooktop elements, hot toaster elements, hot coals, and hot lava. The visible light that you see coming from an object that is so hot that it glows is literally visible light thermal radiation. Such objects are called incandescent.
The plot below shows the thermal radiation spectrum emitted by an object that is at the temperature of an incandescent light bulb filament (2820 K) when the light bulb is turned on (assuming an ideal blackbody emitter). As you can see, the thermal radiation still consists mostly of infrared radiation. However, it also consists of a significant amount of visible light and a small amount of ultraviolet radiation. In other words, an incandescent light bulb that is turned on emits far more non-visible infrared radiation than it does visible light. This is why incandescent light bulbs are so inefficient.
The plot below shows the thermal radiation spectrum emitted by an object that is at the temperature of the sun's surface (5778 K), again assuming that the object is an ideal blackbody emitter. As you can see, the thermal radiation now consists of huge amounts of infrared radiation, visible light, and ultraviolet radiation. This is why sunlight does a great job of warming us up, illuminating the world around us, and giving us sunburns.
Some people show the plot below and claim that because the curve peaks in the visible-light range, that means that the sun emits mostly visible light and that's why human vision evolved to be tuned to these frequencies. This is incorrect. The same data in the plot below plotted as a function of frequency does not peak in the visible-light range, as you will see later. This is because the peak of a broad spectral distribution does not have much meaning. Also, to be clear, the plot below does not exactly show the spectrum of the light emitted by the sun, although it is very close. The plot below shows what the spectrum of the light emitted by the sun's surface would look like if the sun was a perfect blackbody emitter.
Note that most of the plots above have different vertical scales. The vertical scales were adjusted in each case to make the plotted curve fill the graph. All of the plots above show the thermal radiation spectrum as a function of the wavelength.
There are other meaningful ways to plot a spectrum. We can replot the four situations shown above, but now plotting as a function of frequency. The results are shown in the four plots below. The curves look a little different when plotted as a function of frequency, because frequency means something different from wavelength, but the overall concepts that I have been explaining are still the same. Room temperature objects and normal human-temperature objects are still found to emit thermal radiation that consists of 100% infrared radiation. Incandescent light bulbs are still found to emit thermal radiation that consists mostly of infrared radiation but also contains a significant amount of visible light. Objects at the temperature of the sun's surface are still found to emit thermal radiation that consists of huge amounts of infrared radiation, visible light, and ultraviolet radiation.
All objects that are at a temperature above 100,000 K continuously emit thermal radiation that consists of infrared thermal radiation, visible light thermal radiation, ultraviolet thermal radiation, and x-ray thermal radiation. This includes objects such as hot stars and supernovas. Note that the sun's surface is not hot enough to thermally emit much x-ray radiation, but the sun's corona is. Interestingly, objects in the universe that are hot enough to thermally emit significant amounts of gamma-ray radiation are exceedingly rare. Processes that can create temperatures this high are so powerful that they generate far more gamma rays through other mechanisms than through thermal radiation.
The plot below shows the percent of the emitted thermal radiation that is infrared radiation at each object temperature (assuming an ideal blackbody emitter). More specifically, it shows the percent of the thermally radiated energy that is in the range of 0.3 THz to 400 THz, as a function of the radiating object's temperature. This plot summarizes much of the information that I have presented above. Note that the complications that arise at cryogenic temperatures (less than 120 K) are not visible in this plot because the plot's temperature scale is so large.
As you can see from this plot, the emitted thermal radiation consists of 100% infrared radiation for (non-cryogenic) temperature less than 1000 K, which includes cold winter outdoor temperatures, hot summer outdoor temperatures, room temperature, the temperature of a living human, and every temperature at which a human can touch an object and not get burned. Furthermore, most of the ambient infrared radiation in everyday life is created by the thermal radiation process. This is why the terms "thermal radiation" and "infrared radiation" are often used interchangeably in everyday life, even though they mean different things. This means that in everyday life (ignoring objects that are so hot that they glow), if you use the terms "thermal radiation" and "infrared radiation" interchangeably to describe objects in the room, then you won't be very wrong. In contrast, if you use these terms interchangeably to describe hot stars, then you will be very wrong. For instance, less than half of the sun's thermally radiated energy is infrared radiation. Only 2% of the thermally radiated energy emitted by the star Sirius B is infrared radiation. The table below presents the values of some of the data points in the plot above, where the percents are in terms of radiated energy.
| Temperature | Percent of the thermal radiation that is infrared |
|---|---|
| Room temperature (294 K) | 100% |
| Human body temperature (310 K) | 100% |
| Temperature at which water boils (373 K) | 100% |
| Hottest household oven temperature (533 K) | 100% |
| Temperature at which aluminum melts (933 K) | 100% |
| Temperature of incandescent light bulb (2820 K) | 91% |
| Temperature of the sun's surface (5778 K) | 46% |
| Temperature of the surface of Sirius A (9940 K) | 17% |
| Temperature of the surface of Sirius B (25,200 K) | 2% |
Infrared radiation includes all electromagnetic waves that are between radio waves and visible light on the electromagnetic spectrum, regardless of what created them. More specifically, infrared radiation includes all electromagnetic radiation with a wavelength between 1 millimeter and 650 nanometers, which corresponds to a frequency in the range of 0.3 THz to 400 THz. Infrared radiation is most commonly generated by the thermal radiation process, but it can also be produced by lasers, LEDs, spectral line emitters, cyclotron emitters, and so forth.
In summary, the term "infrared radiation" refers to electromagnetic radiation of particular wavelengths regardless of the source, while the term "thermal radiation" refers to electromagnetic radiation created by the thermal emission process regardless of the wavelengths. Therefore, the term "infrared thermal radiation" is not redundant, but literally means "infrared-wavelength radiation created by the thermal emission process". The visible glow that you see from a campfire is not infrared thermal radiation. It is visible light thermal radiation. The infrared signal from a remote control is infrared radiation but is not thermal radiation. The visible light from your LED flashlight is neither infrared radiation nor thermal radiation, but is visible LED light. In contrast, the radiant heat from a hot rock near the campfire that warms you after the campfire has been put out is 100% infrared thermal radiation. These concepts are summarized below.
| Example of radiation | Infrared radiation? | Thermal radiation? |
|---|---|---|
| Visible glow of a campfire | No | Yes |
| Visible sunlight | No | Yes |
| Signals from a remote control | Yes | No |
| Signals in fiber optic cables | Yes | No |
| Visible illumination from an LED flashlight | No | No |
| Ultraviolet radiation from a tanning bed | No | No |
| Radiant heat from a dark hot rock | Yes | Yes |
| Radiant heat from any object that is not visibly glowing (and not cryogenic) | Yes | Yes |