Let's say that I have a source of parallel light rays (e.g. from a laser and a beam expander). I pass that light through a volume that contains some particles. Particles at different locations and with different sizes and shapes will essentially block out the incoming parallel rays, and cast shadows that have the size, shape (cross section) and location of the particles. I would like to image the shadows of these particles with a CCD. I've tried to sketch the setup here, and to put a scale on things the plano-convex lens shown would have a diameter of 5 - 10 cm, and I thought a focal length probably around 20 cm.

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I know that I can use a plano-convex lens to focus parallel rays to a single point, but that is probably not what I need, or at least not all I need. Will I be able to get a good image with just a plano-convex and a plano-concave lens, like a reverse beam expander?

Fig. 1 in the paper linked below shows a system like the one I describe, but in that case they use a regular telephoto lens in front of the CCD (specifically a Nikkor 85 mm, according to the description). I was wondering if I could get similar results with simpler lenses.



1 Answer 1


You will get a kind of picture only if the particles farther away than the focal length , sharp pictures only at the exact locations where you also get pictures of lit objects. So no good shadows or very unsharp ones for particles in different distances from the lens, To be a little better you have to use a smaller opening in front of the lens.

  • $\begingroup$ Could you be a bit more specific about why this is the case? And when you say "a smaller opening in front of the lens", which lens do you refer to? Referring to the paper I linked again, the authors pass the parallel light rays through a volume of water of length more than 20 cm, and are able to get sharp images of anything in that volume. Granted, their 85 mm Nikon lens probably has an aperture that can be set to a small opening, and I couldn't find any information about that setting in the paper. $\endgroup$
    – Tor
    Sep 28, 2023 at 20:47
  • $\begingroup$ I could read just the introduction of the cited paper, but it seems they use a lot of computer power and digital cameras. Even with a modern smart phone you can take pictures in different distances and then use software to get an overall picture. It seems they are very proud of this progress, so I doubt you can do something similar wir just on lens. $\endgroup$
    – trula
    Sep 29, 2023 at 17:27

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