Is the infrared light from the sun (above 700nm) collimated like the visible spectrum or does it become diffuse due to the atmosphere? I mean during direct sunlight, so a clear sky. Basically, I want to know if it is possible to use infrared light only to power a heat engine or not.

  • $\begingroup$ What do you mean "use infrared to power a heat engine"? $\endgroup$ Apr 8, 2020 at 8:20
  • $\begingroup$ Like a normal concentrated solar power, but instead I want to use only the infrared part and send the visible part to plants. $\endgroup$ Apr 8, 2020 at 9:23
  • $\begingroup$ When you are asking if the infrared light is collimated, are you just worried about the practicality of using mirrors to focus it to a single point? (as opposed to some fundamental thermodynamics question about heat engines?) It's worth pointing out that visible light is scattered by the atmosphere (which is why the sky isn't black during the day), but the overwhelming majority of the visible light intensity is still the direct parallel rays coming from the sun. $\endgroup$ Apr 9, 2020 at 11:44
  • $\begingroup$ I am asking about the practicality of reflecting the infrared radiation using mirrors :) $\endgroup$ Apr 11, 2020 at 22:45
  • $\begingroup$ You need to explain what you mean by collimated. $\endgroup$
    – ProfRob
    Apr 14, 2020 at 7:55

2 Answers 2


This picture

suninfra**strong text**

says yes, as the image of the sun is seen in infrared. Go to the link to see the effect of changing infrared wavelengths.

  • $\begingroup$ What link do you mean? The picture only goes to imgur and I can't find any other link in your post :S $\endgroup$ Apr 8, 2020 at 9:26
  • $\begingroup$ Edited, Sorry, the first line was erased when I put in the image, and when I wrote it again I did not check for the link. $\endgroup$
    – anna v
    Apr 8, 2020 at 9:35

You need to specify the exact wavelength. In general the atmosphere is transparent to near-infrared radiation from about 700-2400 nm, with some notable absorption bands due to water and carbon dioxide. Beyond that, the mid- and far infrared can be heavily absorbed, particularly by water vapour. There are transmission windows at around 3.5, 5 and from about 8-14 microns. Beyond that, the atmosphere is nearly opaque until mm wavelengths.

A transmission curve for the atmosphere (at zenith) is shown below (from https://en.m.wikipedia.org/wiki/Infrared_window ). I think this is for a "dry" site with low amounts of precipitable water vapour.

IR transmission of the atmosphere

This tells you how much direct radiation from the Sun, reaches the ground and arrives as a parallel beam (if the Sun is at zenith). This is what most people understand to be meant by collimated. Note that the amount of infrared radiation scattered in the atmosphere and hence the diffuse illumination from the sky will be small.

  • $\begingroup$ I'm not asking about which wavelengths are being transmitted, but if the ones which are transmitted stay collimated. $\endgroup$ Apr 14, 2020 at 7:44
  • $\begingroup$ @BeaconofWierd that is the same thing. The infrared flux from the rest of the sky can be neglected . $\endgroup$
    – ProfRob
    Apr 14, 2020 at 7:51
  • $\begingroup$ Great :D So the infrared radiation which passes through the atmosphere is collimated then :) $\endgroup$ Apr 14, 2020 at 8:39

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