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I'm trying to project a very thin line at a low angle over some surface defects (peaks and troughs). The scale is ~20mm square, but the defects can be sub-mm (0.2 - 1.0mm).

I am currently using a line generated by a laser pointer. The $4 laser diodes work well enough, but the line spreads out due to the low angle and poor focus(?).

Are there any better / safer alternatives, such as some LEDs + lens that could also focus the light into a line?

I'm new to engineering and optics so any advice would be much appreciated.

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    $\begingroup$ You might go to the hardware store and buy a laser line/level tool. They're cheap and can produce either a line or a swept line in a plane. $\endgroup$
    – Carl Witthoft
    Jun 21, 2021 at 12:47
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    $\begingroup$ What about an interference plate and monochromatic light source? $\endgroup$
    – DKNguyen
    Jun 22, 2021 at 22:33
  • $\begingroup$ This sounds very interesting - how does it project a line? I've had a look on Google, but I don't understand. Thanks! $\endgroup$
    – John
    Jun 23, 2021 at 9:17

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A few points that don't fit into a comment but could help you:

  • Get a better laser source than a 4 \$ laser diode. "Better" means a laser with higher beam quality. When you look at a laser's data sheet, it should specify a value $M^2$ or "BPP" (beam parameter product). Both these numbers are related, the smaller they are, the better you can focus the laser beam.
  • Use a cylindrical lens (if you don't already), they are meant exactly for your application, focusing a laser onto a line instead of onto a spot.
  • From how laser diodes are built, compared to other laser sources, their beam quality differs in the two lateral directions. That means, a diode laser beam diverges for example faster in its height than in its width. It's only an assumption derived from this, but rotating the diode by 90 degree around the beam axis could possibly improve the focus of the laser line in this particular application.

I think that with a laser you are already on a better way than with an LED, it's just about a few tweaks to improve your laser line.

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  • $\begingroup$ Thanks for your answer! Editing... one sec $\endgroup$
    – John
    Jun 21, 2021 at 11:43
  • $\begingroup$ I tried a better laser source (it didn't list a BPP or M2) that lists its Beam Divergence (mRad) as <0.5. Is that one any better or should I be shooting for a completely different class of device? global-lasertech.co.uk/wp-content/uploads/2020/09/… Do I understand correctly that I may just need to literally turn the laser diode 90 deg around the beam axis in order to get better focus? I use a camera to focus at the moment as the laser glasses prevent me from seeing the beam. Is it possible to turn down the line brightness with a filter? $\endgroup$
    – John
    Jun 21, 2021 at 11:57
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    $\begingroup$ @John if the answer helped solve your problem then remember to click on the grey tick mark to accept the answer. $\endgroup$
    – KingLogic
    Jun 22, 2021 at 4:19
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    $\begingroup$ @John Yes indeed, I meant to rotate the diode around the beam axis. However, the influence might be small as the difference between the two axes is not that big. To reduce the brightness on the camera, you can use ND (neutral density) filters. $\endgroup$
    – OpticalResonator
    Jun 22, 2021 at 7:32
  • $\begingroup$ Thanks, much appreciated! $\endgroup$
    – John
    Jun 23, 2021 at 9:10

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