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For educational purposes, is there a way to make a narrow bright beam of white light without much equipment and without using the sun as the source?

The collimated beam should be about 2 mm wide for 50cm length and be visible when seen on a white screen in a room with very partial darkness.

The usual way is to use a very strong light bulb (very hot), focus it using a large lens, then select only a dot using a hole, then collimating the outgoing light with another lens, and then a slit to select only a small fraction of it.

This requires a lot of equiment and takes a lot of space.

Is there another way ?

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  • $\begingroup$ What's wrong with a big flashlight? $\endgroup$
    – Emilio Pisanty
    Commented Apr 15, 2015 at 14:16
  • $\begingroup$ @Emilio, I now added "narrow" to specify that I want something narrower that the usual flashlight. Plus a flashlight does not create a clean beam. It emits in a wide solid angle. $\endgroup$
    – fffred
    Commented Apr 15, 2015 at 14:21
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    $\begingroup$ A flashlight beam seems pretty narrow to me. (The technical term you might want to use, by the way, is 'collimated'.) Have you got a quantitative measure of the narrowness you want to achieve? $\endgroup$
    – Emilio Pisanty
    Commented Apr 15, 2015 at 15:23
  • $\begingroup$ @fffred, I am not sure whether you realize the power of Emilio's tool. A proper flashlight contains a parabolic mirror and this parabolic mirror guarantees that - in the approximation in which the glowing wolfram is located at one point - all reflected photons are moving exactly in the same direction (the wolfram is in the focal point of the parabola). The photons that get out of the flashlight "directly" have all conceivable directions, but those disappear at a big distance. Only the reflected ones are collimated, and therefore don't fade away at a big distance (in the right direction). $\endgroup$
    – Luboš Motl
    Commented Apr 15, 2015 at 17:06
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    $\begingroup$ @fffred, fair enough. But you're asking people to solve a very technical engineering problem. Physics is really about the individual mechanisms that may make something work. One may discuss the useful components - small holes, strong sources, lenses, mirrors, reduction of size of the source of light etc. It's pretty clear that there is no miraculous way to make ordinary light laser-like without combining the things above. Why don't you buy a laser? They are cheap. You may combine several colors of laser beam, too. $\endgroup$
    – Luboš Motl
    Commented Apr 16, 2015 at 9:26

2 Answers 2

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You might be able to circumvent equipment issues with creative use of Fresnel lenses. Here's the path I would create for the light.

  1. Strong Flashlight
  2. Focusing Fresnel Lens
  3. Collimating Fresnel Lens (inverted lens with image at focus). I'd cut this lens to the correct size for the application.
  4. Small hole to refine size and parallel light.

Depending on your intensity requirements you might be able to do it with just a flashlight. Fresnel lenses are fairly cheap and can also be found on old equipment. You might also find using a parabolic mirror, like those sold at beauty stores, useful. You can insert one between stages 1 & 2 or between stages 3 & 4 depending on your needs/goals.

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This is actually non-trivial. If there was only one wavelength, and a source that was very small, it could be solved with a single lens that has the source at it's focus. For the rather tough requirement of collimation over 500 mm a best form (bi-spherical) lens might not be sufficient and an asphere or multiple lenses be required. Now the white light requirement makes this much harder. To make sure that rays of all wavelengths are bent the same way, you need an achromat (correct at 2 wavelengths in the visible) or better and apochromat (correct at three wavelengths in the visible).

A Fresnel lens is unsuitable, because it is typically of too low quality and is not color corrected. Also the transitions between the 'blazes', i.e. the areas that have the wrong curvature, really mess up the quality.

Luckily people have been building lenses that can do both the low aberration focusing and the achromaticity, because the requirements for visible wavelength photography are very similar. I would look for an existing used fixed focus camera lens. Zoom lenses make too many compromises to be good for this.

Generating a small, bright, evenly illuminated light source is a separate problem, which might require a very bright source, some diffuser or homogenizer arrangement and another achromat or apochromat.

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