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I know that concentrated photovoltaic systems use Fresnel Lenses to concentrate incident sunlight to a point. I also know that this point isn’t infinitely small and has an actual size since the lenses don’t concentrate to infinity.

My question is how do I calculate the true spot focus size of the concentrated sunlight? Is there an equation that relates distance, focal length and other properties perhaps?

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The sun is not a point source, so you can't get a point image. You get an image of the Sun. You can use basic lens maker's formulas with the angular size of the Sun as seen from the Earth (same as the angular size of the Moon) and the focal length of your lens.

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  • $\begingroup$ Does this still apply in the context of non-imaging optics? Since the Fresnel has the sole purpose of concentrating the sun’s irradiance to a point, I would like to know how to calculate the size of that point. $\endgroup$
    – J0KerJ4y
    Feb 1, 2018 at 10:21
  • $\begingroup$ A Fresnel lens is a bunch of slices of conventional lenses. Even a tiny bit of flat mirror will form an image - it is the same as a pinhole $\endgroup$
    – C. Towne Springer
    Feb 3, 2018 at 6:08

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