Extreme laser speckle

[rjlittlefield]

I was just playing around with a laser pointer tonight, using it to axially illuminate a small black beetle by bouncing the laser beam off a 45° inclined cover slip whilst watching through a 10X NA 0.30 objective.

The result was some interesting abstracts.

full frame


crop


These are HDR images, constructed using Photoshop CS5, File > "Merge to HDR Pro", from four images of exposure times ranging over a 75:1 range.

What's particularly interesting to me about these images is that due to the height of the inclined cover slip, I couldn't get the objective down low enough to focus so the subject is just a complete blur in normal light. I would show you the view, but basically there's nothing to see -- it's like looking at clouds through fog. What we're seeing here is entirely a speckle pattern, formed by the laser beam interfering with itself before and after reflection from the beetle. Sort of like a hologram, I guess, though I'm far from clear that it would be possible to reconstruct anything from a capture like this.

--Rik

Edited to add, 06 Nov 2022: Lou Jost at viewtopic.php?p=224498#p224498 has a nice illustration of how to use dilute milk for de-speckling.

[yanyading]

Sir, do you think this laser improved the resolution or not, thanks.

[rjlittlefield]

do you think this laser improved the resolution or not

Not.

When the lens is focused on the specimen, coherent laser illumination is a disaster compared with conventional diffused incoherent illumination. At best I can still see structure of the specimen, but it is overlaid with a strong speckle pattern that makes it impossible to tell what's real texture and what is just speckle.

With proper capture and processing, I suspect it's possible to recover the specimen structure. In fact I once read an abstract talking about a low cost system using coherent illumination to build a high-DOF, high-resolution imaging system. I ordered a copy from the local campus library, but somehow it never came in and I forgot about it. I never did find out what they meant by any of those terms, or anything else about their system.

--Rik

Edit, 06 Nov 2022: add word "coherent" to distinguish from de-speckled laser illumination.

[yanyading]

Thanks sir!