I haven't been able to find much information about this, maybe I'm searching the wrong thing but, how would you diffuse a laser beam from a thing collimated beam, into a "cone" of light? More directional than an LED, but much more spread out than a laser. Thanks in advance.
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$\begingroup$ Wouldn't a diverging lens (like double-concave or plano-concave) of appropriate radii of curvature do the trick? $\endgroup$– user191954Commented Jun 26, 2018 at 4:08
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It is not clear whether you want to simply spread the beam, or want to diffuse the beam. If you spread the beam, you will end up with a relatively smooth distribution of light. If you diffuse the beam, you will end up with a speckle pattern.
To spread a thin collimated beam into a smooth conical distribution, the best way is to send the beam through a low-power (long focal length) lens and place a pinhole aperture at the focal point. The pinhole aperture will "scrape off" the noise in the beam and produce a smooth distribution.
To diffuse the beam, you can send the beam through, e.g., a ground glass diffuser or an opal glass diffuser; or you could bounce the beam off of a white-painted surface. This will cause the beam to scatter in a wide range of directions, and you can place an aperture downstream to limit the beam to a cone of angles.
If you diffuse a very thin beam, you will end up with large widely spaced speckles in the conical beam. If you first spread the beam to, say, 1/4 inch wide using a lens, then diffuse the beam, you will end up with small closely packed speckles.
