Coaxial illumination on the cheap...

[Chris S.]

A 50-50 mirror/splitter would attenuate much of the light returning to the tube lens from the subject. In a dual mode, (side and coax lighting) I would need to crank up the side lighting to compensate for the reduced transmission through a 50-50 mirror.

Keith, I'm pretty certain you are aware of this, but just to make sure: Beamsplitters are available in configurations far from 50-50. Widely variant specifications for transmission and reflectance are available.

Cheers,

--Chris

[BugEZ]

Chris S wrote:

Beamsplitters are available in configurations far from 50-50. Widely variant specifications for transmission and reflectance are available.

Thanks Chris for pointing this out. Beam splitters are mature technology and much more sophisticated than I first thought.

I think the split ratio that is present in the cover slip gizmo described above is not too bad. Certainly I expect to be able to work around whatever it has turned out to be.

After reflecting on "method 2" for a week or so, I think a commercially available beam splitter is the way to go if possible when starting from scratch. I ended up spending ~ $35 for cover slips and elf invested both time and materials in machining the housings. A purpose built splitter no doubt incorporates "lessons learned" that I have yet to discover. I guess it depends on how easily the prefabricated device can be adapted to your rig.

I hope to try method 3 (with the light source directly in front of the objective) next week if I can fit it into my schedule. It may be very cost effective ($.10 for LED, $ .89 for flat black paint, $.03 for construction paper sleeve to position in in front of the objective). But it is rather odd to place something between the lens and the subject.

K

[BugEZ]

I had to put this project on hold while I addressed a computer crash and a short vacation. I am back now, my computer is back up to speed and so I was able to resume this interesting project.

Here is a crop from one of the images that I gathered to create a stack. This was made with composite illumination. Both side lilght (from the bucket light) and the coaxial illuminator constructed with elf's help. I have the coax LED hooked up to the circuit that controls the side light so it switches on and off to minimize vibration effects. The results are interesting.





For comparison an image with side illumination only...



I think that the comments of others regarding spreading the light out a bit that enters from the coaxial path will probably fix the white dot surrounded by a black circle issue. Still a work in progress, but promising!

Keith

[rjlittlefield]

spreading the light out a bit that enters from the coaxial path will probably fix the white dot surrounded by a black circle issue.

When thinking about these lighting setups, remember to imagine the world the way it looks from the subject's standpoint. Regardless of how much you spread out the light, it's only going to be coming through the glass and from around the barrel of the objective. That black circle represents the metal ring that surrounds the glass. You can't get any light coming through that area, so to get rid of that ring I think you'll have to do something like a Lieberkühn reflector that incorporates or sits in front of the metal so as to bounce light off it.

--Rik

[BugEZ]

I did some playing around with diffusers internal to my light arrangement to soften the coaxial illumination. First I tried a simple paper diffuser snipped from a sheet of computer printer paper. The snippet was placed directly on top of the LED to diffuse the light before it strikes the cover slip reflector. It reduced the light intensity too much. Then I pulled out the typing paper snippet and inserted a layer of facial tissue (kleenex) and it produced a much more satisfactory result. Below I show a comparison of my final result with the coax illuminator and side light (left) and side light only (right). The color of the corneal filters on each ommatidium are clearly shown with the coax light.



Regarding Rik's last post:

Regardless of how much you spread out the light, it's only going to be coming through the glass and from around the barrel of the objective. That black circle represents the metal ring that surrounds the glass. You can't get any light coming through that area, so to get rid of that ring I think you'll have to do something like a Lieberkühn reflector that incorporates or sits in front of the metal so as to bounce light off it.

This is quite insightful and explains what I see in my results. The bug's eye still sees a ring of dark sky... More to noodle on after our first frost.

After googling Lieberkuhn reflector this is what I found:

http://books.google.com/books?id=KafFAQ ... &q&f=false


[Admin edit 9/29/2014 RJL to fix url tag error causing post to print as blank]

[GemBro]

Lovin' this thread .... reminds me of my Uni days playing with lasers and prisms ... totally different I know but I just got thrown back there for a while ...

Keepin' an eye on this progression ...

Gem

[Troels]

I am in doubt how relevant this is, but I noticed that your beamsplitter consists of a thin sheet of glass.

Some professional manufactured beamsplitters are built by connecting two glass prisms resulting in a cube shaped splitter with a 45 degrees boundary crossing the cube corner to corner.

Se this Carl Zeiss beamsplitter . Notice Picture no. 7 where you se directly through the cube and no. 8 where you se "round the corner".

I could imagine that the light rays from object to camera is less disturbed (diffused/reflected/refracted) by passing two 45 degr. glass surfaces extremely close to each other (in the cube) than two 45 degr. surfaces separated by a relatively large fraction of a mm (glass sheet).

That consideration is of course only valid if we assume that the light rays are much less affected by passing a surface at right angel (90 degr.) to the rays than passing a surface at 45 degr. angel. My intuition supports this.

I am not sure if Carl Zeiss' opticians were motivated by this chain of arguments. But it could be worth considering. Especially when the price is so low.

Troels

[Troels]

It just occured to me, that manufacurers in the camera world must have come up with solutions to the problem of image degradation created by a 45 degr. glass sheet operating as a beamsplitter.

Sony has developed a series of cameras, the Sony SRT models, where SRT stands for Single Lens Translucent (mirror) in contrast to the conventional SLR principle (Single Lens Reflex).

In stead of the flipping reflectiong mirror in the camera house they use a fixed semi-translucent mirror. This mirror catches only 30% of the incoming light, directs it to a separate censor creating a live image for the viewfinder and focus electronics. The rest of the light goes to the image censor.

I suppose they have solved the problems with image degradation by means of advanced coatings.

If you could get such a mirror from a destroyed camera it would definitely be a better beamsplitter than any sheet of untreated glass.

Troels

[jnh]

Since cube beam splitters were mentioned earlier here, I wanted to point out this thread:
http://www.photomacrography.net/forum/v ... highlight=
Used without polarizers and quarter wave plates, cubes give rise to unwanted reflections and hence plate beam splitters seem to be better suited for coaxial illumination.