Apologies if this has an obvious answer, my knowledge of this area is very limited.

I am attempting to produce a setup that will allow me to illuminate objects in 940nm wavelength infrared light, and see it via a bandpass filter tuned for that frequency.

My question is whether sunlight will effect the image, I know it produces a lot of IR light, but does it contain all different types of frequency?


The setup will be indoors, however there will be windows around letting sunlight in, so this is not for direct sunlight but ambient.

Any information on this subject would be greatly appreciated, thanks!

  • $\begingroup$ Have you done a search of "solar spectrum" or something similar to that? $\endgroup$ – Kyle Kanos Mar 17 '15 at 12:50
  • $\begingroup$ I have, but it spreads across atmospheric conditions as well (i.e, outside of the atmosphere, to sea level) and I do not know where on the graph "normal" ambient sunlight would fall. if it helps this will be indoors, however there will be windows allowing sunlight in. $\endgroup$ – Aphire Mar 17 '15 at 12:58
  • $\begingroup$ @KyleKanos just out of interest, anything wrong with my answer? just learned that comments are optimum strategy for some questions $\endgroup$ – user74893 Mar 17 '15 at 13:06
  • $\begingroup$ @irish: if you're referring to your meta post about it, I don't believe that is at all what John Rennie said. Your answer is, at best, half an answer: it does not convey anything OP doesn't seem to already know and starts off addressing something irrelevant. $\endgroup$ – Kyle Kanos Mar 17 '15 at 13:12
  • $\begingroup$ @KyleKanos no just dealing with this particular question 2 d/v immediately after I published it ... just learning the ropes no big deal thanks $\endgroup$ – user74893 Mar 17 '15 at 13:24

The solar spectrum looks like the image below. The yellow shaded curve is the observed spectrum at the top of the atmosphere while the red curve is what we get at ground level. The black line is a blackbody model of the sun (a pretty good approximation, just don't worry about the yellow curve peaking above the black line)

enter image description here

940 nm occurs right around the first H$_2$O valley (where some of the sunlight is absorbed by water in the atmosphere). So while this wavelength will be reduced in intensity (as compared to 1000 nm), your device will likely still observe some background IR.


I have looked into this question a bit more than most. I have a 940 nm bandpass filter and it shows that there is quite a bit of 940 nm IR available in mid day sunlight. If the filter is placed over a camera one will observe that cumulus clouds (and most clouds) are black because they are made of water droplets and saturated water vapor. The water vapor absorbs 940 nm light. Therefore cloudy days will show much less IR in this band than clear, sunny days. My goal is to image areas of increased water vapor in the atmosphere by comparing an image at 940 with an image at 850 nm. So far this has not shown anything and I am not sure why. Apparently the difference in absorption between the small differences in water vapor found on a sunny day (maybe 1 to 2 degrees F. of dewpoint) is not enough to show up in pixel subtraction processing.


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