Test result in last post - Beam splitter cube vs plate

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ray_parkhurst
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Post by ray_parkhurst »

Lou Jost wrote:Enrico, that design doesn't solve the aberration issue. The point of my design is that no image rays pass through any glass.

mjkzz, my design might solve the problems you encountered. Front-surface mirrors are available at lambda/8 flatness, which might work at least for the magnifications that Ray uses. Ray, do any of your beam-splitters work in the way that I proposed here? I would think someone makes these somewhere.
Lou...had to go back and re-read your bump post. The concept seems feasible, though it bends the optical axis 90-deg so would require some revamping of the setup to be hybrid horizontal/vertical. Seems worth a try given the elimination of glass from the image path, though contrast may still be an issue.

mjkzz
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Post by mjkzz »

mjkzz, my design might solve the problems you encountered. Front-surface mirrors are available at lambda/8 flatness, which might work at least for the magnifications that Ray uses. Ray, do any of your beam-splitters work in the way that I proposed here? I would think someone makes these somewhere.
My failure building a coaxial light source is like what Ray described. However, what I was saying is this: swap the positions of light source and camera, ie, put camera at light source position, so it will be getting reflected lights, this could solve the contrast issue and even with 50/50, the ghost image is about 5.3 stops weaker (40:1 if it conforms to Rik's assumption, 5% is reflected at second surface). Commercial coaxial products might have better quality mirror and it is probably easier to modify an existing product.

So sure, an 1/8 lambda first surface mirror might be able to improve my coaxial design, but I think it is the very idea of yours (Lou's, letting camera to receive reflected lights) actually solves it.

mjkzz
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Post by mjkzz »

enricosavazzi wrote:
Lou Jost wrote:Could we glue a flat half-silvered mirror to an ordinary prism using lens cement?
That is not a standard component, but a beamsplitter made by cementing the hypotenuse of two right-angle prisms (one of them typically 50% transmitting, 50% reflecting) is called "beamsplitter cube" and is a commonly used optical component.
I tried that, there are commercially available cubes like that, but I somehow got some color issues, maybe my cube is not lab grade (it was for classroom demonstration). But I think it does solve aberration issue as joint surface acts like mirror optically (vs physical thin aluminum deposits).

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Post by Lou Jost »

But I think it does solve aberration issue as joint surface acts like mirror optically (vs physical thin aluminum deposits).
As long as the image rays go through glass, there will be problems with aberrations, I think.[/quote]

ray_parkhurst
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Post by ray_parkhurst »

mjkzz wrote:
I tried that, there are commercially available cubes like that, but I somehow got some color issues, maybe my cube is not lab grade (it was for classroom demonstration). But I think it does solve aberration issue as joint surface acts like mirror optically (vs physical thin aluminum deposits).
I have a few cubes to try as well as partial mirrors. Will be interesting to see what works best and how it compares with normal lighting.

mjkzz
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Post by mjkzz »

ray_parkhurst wrote:
mjkzz wrote:
I tried that, there are commercially available cubes like that, but I somehow got some color issues, maybe my cube is not lab grade (it was for classroom demonstration). But I think it does solve aberration issue as joint surface acts like mirror optically (vs physical thin aluminum deposits).
I have a few cubes to try as well as partial mirrors. Will be interesting to see what works best and how it compares with normal lighting.
I was replying another post, but this will definitely be a good test. I have thrown away all of those stuff.

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Post by enricosavazzi »

If the point is to correct aberrations, one way to correct some of the aberrations (but not ghost reflections) is to use two thin 45 deg beamsplitter plates, arranged at 90 deg to each other. The first surface (to be hit by incoming light) is silvered, all the others AR coated. The second beam splitter cancels out the dispersion and transversal chromatic aberration (and perhaps spherical aberration, but this should be verified) of the first one.

The two beam splitters are preferably placed in the infinity space between objective and tube lens.

When beamsplitter cubes are designed into an optical system, they are considered part of the optical scheme, and their aberrations are corrected by other parts of the optical system. This is what microscope designers always do. Adding them to the system as an afterthought/DIY modification is of course problematic.

The Mitutoyo FS-60 and FS-70 microscopes (which are designed to use Mitutoyo M Plan Apo objectives) use a single, very thin 45 deg beam splitter for coaxial illumination in the infinity space, with no correction of the aberrations elsewhere in the system. This type of beam splitter is sometimes so thin that it cannot be cleaned, so it is usually placed well inside the optical device where dust is unlikely to arrive.
--ES

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Post by Lou Jost »

The infinity space is the best place for them, but finite set-ups require something different. And it also has to be very short because of generally small working distances. Putting extra glass between the lens and subject will not cancel out aberrations in this case.

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Post by rjlittlefield »

In the infinity section life is easy because glass plates of any thickness do not add spherical aberration. In diverging/converging sections, extra glass does add spherical aberration, and that should not be canceled by adding a second plate of complementary tilt. (Simple argument: in the axis orthogonal to the tilt, the plates are not tilted. Path lengths along that axis will be perturbed the same as a somewhat thicker plate that is not tilted. Whatever amount of spherical aberration is added by the first plate in that direction, the same amount more is added by the second plate.)

--Rik

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