Question: Electronic flash adaptation and Köhler

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enricosavazzi
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Question: Electronic flash adaptation and Köhler

Post by enricosavazzi »

I will eventually modify my Olympus BX50 for flash photography, and of course I want to keep continuous illumination together with flash illumination. So the obvious choice is a beam splitter somewhere in the illumination path. I am aware of several homemade flash modifications on photomacrography.net and other web sources.

I would rather not (substantially) modify the original light source (in my case 100 W halogen), so I would prefer not to use one of the more extensive modifications that place the flash tube at the original position of the bulb filament, move the lamp to the rear of the flash tube and converge the continuous illumination onto the flash tube via a condenser.

I have also seen modifications that place a power LED immediately at the rear of the flash tube, but would prefer not to switch to LED at this time, because of the uncertainties of matching the output of a 100W halogen with a homemade LED modification without overheating the LED or flash tube.

I have seen two basic types of homemade flash modification on the web, that reflect more or less what I intend to do:
Type (1) places a 45 deg. beam splitter (usually a plain large cover glass) directly in front of the lamp. The flash illuminates the front side of the beam splitter from an angle normal to the illuminator optical path.
Type (2) places a similar beam splitter between the field lens of the illuminator and the bottom of the condenser.

Type (1) seems to me the most likely to provide a flash illumination as close as possible in optical quality to the continuous illumination. It is also significantly more difficult to implement in the BX50, because of limited space available within the BX50 illuminator casing and closeness to the unshielded 100W bulb.

Type (2) is much easier to implement, but it would seem to me that one cannot achieve proper Köhler illumination with it. This type also bypasses the field diaphragm when using flash, with potential flare problems. Since my principal goal is to retain maximum illumination quality, how much are these potential problems important in practice?
--ES

viktor j nilsson
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Post by viktor j nilsson »

Can't provide much input I'm afraid, but I am very interested in hearing people's thought on this. I've also been thinking about buildinga flash setup for my microscope (a Wild M20). Because the bulb filament (or LED) is located within the base of my microscope, the only workable solution for me is option (2). In fact, I've had 50:50 beam splitter cubes and -plates in my shopping cart on several occations, but I've never pulled the trigger since I'm not sure if the loss of Köhler would be a big deal or not. I'm sure you read it, but David Walker notes that his flash setup behaves like critical illumination, which doesn't sound all that great to me. I guess it's fine as long as you have diffusion on the front of the flash. http://www.microscopy-uk.org.uk/mag/art ... lash2.html

Beatsy
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Post by Beatsy »

I've been mulling the flash thing for years, but never done much about it. The idea I liked best was a second condenser projecting an image of the filament onto a flash tube, so the Koehler illumination is identical for both sources. But as you say, it's a lot of unwelcome hacking.

My compromise would be the 45 degree coverslip method - if I ever get around to it.

Regarding strict Koehler illumination: providing it's pretty close, I haven't seen *that* much improvement of resolution between a "nearly right" setup and a "spot on" setup. It affects evenness of the illumination more than anything else IMO. The only place this has really mattered to me is when stitching brightfield images of larger specimens (diatom arrangements). There, any appreciable difference between flash and filament distance *would* make a significant difference (gradients on the background).

This makes me wonder if the 45 degree glass would be better if it were frosted - and focus that at the correct plane for Koelher, rather than an image of the filament and/or flash bulb. Essentially, the frosted glass would become the light source. It could also be square on to the light path (avoiding path length differences) if you could arrange for both flash and filament to be illuminating it from behind. Just a thought...

Edit: You could probably use the flash to illuminate the frosted glass from the front too, off axis from the light path. My Zeiss illuminators have just such a piece of glass that can be swung in front of the filament. All I'd need to do is cut some of the case away to get flash onto the glass. I feel itchy fingers coming on... :)

JohnyM
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Post by JohnyM »

Does Oly lamphouses for your scope have adjustable collectors? Are you willing to buy an extra lamphouse and completely modify it?

enricosavazzi
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Post by enricosavazzi »

JohnyM wrote:Does Oly lamphouses for your scope have adjustable collectors? Are you willing to buy an extra lamphouse and completely modify it?
I already have an extra BX 100 W halogen illuminator that I can modify :D .

However, the BX halogen illuminators are designed on the principle that they require (and offer) no adjustments. All the centering is done only with the condenser. Nothing appears to be adjustable or centerable in the illuminator housing, not even the lamp socket.

The optics of the illuminator and the associated optical path in the scope base are quite wide (50-60 mm diameter), at least twice as wide as those in the finite Zeiss scopes I am familiar with. This is probably not to clip or vignette even if there is a moderate misalignment of the lamp.

The lamp is very close to the first collimator (which seems to be aspheric). I don't have the housing at hand right now, but if I remember correctly the distance between lamp and collimator is only about 10-20 mm.

The illuminator housing is quite large and has a black metal lining on its inside. There is no reflector at the back of the lamp. Opening the housing removes the lamp from the optical path and disconnects it electrically. This further makes it a little more difficult to modify the housing.

PS - One possibility might be to use a twin optical guide and to put its shared end in place of the illuminator lamp. One of the light guides can then go to a stand-alone fiber halogen illuminator, the other light guide to the flash. For this to work the two bundles of optical fibers should be well mixed together at their shared end, however. If they are not, the surface of the shared end will have bright and dark areas, which may cause problems with uniformity of illumination.
--ES

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Post by Pau »

I think that most microscopes don't use true Köhler illumination because it is pretty tricky to perfectly set up. They put diffusers in the light path that, despite eating light, improve illumination evenness over imperfect Köhler. The exception can be epifluorescence illuminators (BTW including confocals).

My old Zeiss Standards have a removable frosted glass. According with the instructions it must be removed for centering but they recommend to put it in after. In fact I prefer true Köhler (no diffusers) despite needing re-centering more often.
Pau

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Post by Pau »

Hi Enrico,

I'm sure you have taken a look at the linked setups at http://www.photomacrography.net/forum/v ... hp?t=26185

If your collimating optics were focusable (I doubt it) and you can focus (at Köhler) the lamp placed 20 mm recessed from the normal lamp position the "1" option could be implemented designing the adequate tube to hold the beamsplitter.
If not, I can't figure how to implement the "1" option without lenses and maintaining the Köhler position for both lamp and flash tube.

Better than a cover, you can buy plate beamsplitters: more efficient and they are made in different R/T ratios, 80/20 would be ideal in your case

Option "2" in practice also works very well, did you see Anne's setup?
http://www.photomacrography.net/forum/v ... 160#237160

Of course you can also make a LED/flash illuminator without destroying your beloved halogen lamp housing and switch them when required

(from a former PM I sent to Enrico)
Pau

JohnyM
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Post by JohnyM »

enricosavazzi wrote: PS - One possibility might be to use a twin optical guide and to put its shared end in place of the illuminator lamp.
That's something i've been thinking of, but my attempts were proven unfruitful.

I have 2 of theese. One is for epifluorescence (HBO/Halogen/Flashgun) with manually rotatable mirror (100%/0%/0%).
Other is for DIA and this one im gonna discuss.

My current setup on FrankenMicrophot (Mk.IV), that im pretty content with looks like that:
Image
Image

Im in the process of manufacturing adaptors for TTL flashguns, as what you see currently is old manual flashgun.

Setup is completly reversible (with exception of one broken lamphouse) and consist of:
- Unmodified HMX lamphouse 100W halogen.
- Custom manufactured box that houses beamsplitter and serves as attaching point for flash / lamphouse
- 3 position beamsplitter (100/0 , 75/25 , 0/100 )
- Broken lamphouse that i've used to obtain:
*collector lens for flashgun
*parts to make "box" attachable to microscope - male/female locks

Thanks to movable collectors im perfectly able to maitain koehler illumination on both halogen / flash while filling the aperture.

Im not sure how well this solution could adapt to Y shaped olympus scopes.

zzffnn
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Post by zzffnn »

Enrico,

I have tried your type (1) and type (2) designs and prefer type (2). My design is similar to Anne's, except that I simply place a big 50/50 be splitter cube under condenser.

Type (1) design waste too much light for my application (high magnification oblique / darkfield light, fast flash speed, live fast-moving protists). But my illumination methods are not even to start with and do not require Kohler. They also waste a bit more light than DIC. So your mileage will vary.

I did not try Y fiber optic design. For that to work in my rig, I would need a wide and good quality FO cable that is very short and efficient. That is not easy or cheap to achieve.

I did try a cover slip as mirror, but my rig needs wider and more efficient mirror or beam splitter. A good beam splitter mirror worked well for me, but I prefer cube BS for easier handling and modularity (my DIY Lego mirror enclosure is not as good as Anne's). My BS mirror provided slightly better light efficiency though.

I am sure you already know this, but want best possible light quality. You can easily move flash head back and forth and diffuse it. You may just have to try and see, how good is good enough. A 40mm beam splitter cube cost less than $60 USD in US, from Surplus Shed. Easy/cheap to try.

I did not expect the type (2) design to work so well for DIC. But Anne has shown that it can be more than good enough. For me, at least.

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Post by Pau »

My experience differ from zzffnn's: I find illumination with option 1 at Kölher a bit more intense and more controllable, but I must confess that I only did some test time ago with 2.
Pau

pbraub
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Post by pbraub »

Dear all,

I have been passively enjoying the forum since a while but now I feel that I can provide some input to that question – especially because I am interested in the flash-to-Olympus question myself. In my spare parts drawer I have a BX50 stand which I plan to restore given some time – I has been not treated kindly in its previous life, but for the purposes of testing I have set up a small test rig. (BX50 with UPlanFl60xOI NA1.25, SWTR and 1x c-mount adapter with a GS3-u3-41c6c 1" c-mount camera).

The BX50 indeed has a diffuser approximately 6 cm from the collimation lens of the 100W lamphouse (3 cm from the back of the lamphouse mount). When set up as intended I can see a diffused but clearly differentiable image of the filament when observing the back focal plane.

When I detach the lamphouse and move it back by 10 cm, which is as much as the standard cable allows, the image of the filament becomes blurred. During this, the available light reduces significantly and there is some drop in contrast.

Image
BX50 with 100W Lamphouse moved back as far as the built in cable allows.

Image
Central 100% crop of an H&E stained sample with a small blood vessel in adipose tissue. Left:Lamp house in indented position, middle: moved half way back (~ 5cm) and left moved as far back as the cable allowed (~10cm). Brightness adjusted on all images simultaneously.

Image
The same picture as above with brightness adjusted per picture.

In conclusion I think the impact on the image quality is minimal, the loss of light however significant. I will also try to test evenness of illumination, but I have some issues there that probably come from inadequate equipment/setup rather than the modification of the microscope.

I really like the Y-shaped design of the BX stands, especially of the BX1 (40,50...) series with its very clear cubistic lines. However this makes modification of illumination in the back of the microscope very difficult.
Mounting a BS prism directly under the condenser is out of the question for me because the setup should be also compatible with my other scope and this has already to much stuff in that position.

Kind regards
Peter

discomorphella
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Post by discomorphella »

On my BX-60, which has a very similar if not identical lower illumination path to your BX50, I just milled out a slot in the back of the LH100-3 lamphouse for a Nikon SB-80 or similar flash and shot it straight through the lamp. A real kohler system won't be too perturbed by the small distance change between the filament and the flash. I just am careful to run the lamp at low power so I don't overheat the flash; if I am going to use the lamp I just remove the flash. So far its worked quite well with my D810 in remote command mode for the flash.
Best,

David

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Post by enricosavazzi »

discomorphella wrote:On my BX-60, which has a very similar if not identical lower illumination path to your BX50, I just milled out a slot in the back of the LH100-3 lamphouse for a Nikon SB-80 or similar flash and shot it straight through the lamp. A real kohler system won't be too perturbed by the small distance change between the filament and the flash. I just am careful to run the lamp at low power so I don't overheat the flash; if I am going to use the lamp I just remove the flash. So far its worked quite well with my D810 in remote command mode for the flash.
Best,

David
That sounds interesting. One reason why I did not consider the flash-through-filament solution is that the folded filament of the 100W halogen bulb looks quite compact (as seen by naked eye and through the glass envelope of the bulb). But on the other hand the filament thread itself must not touch its adjacent whorls and may actually block less than 50% of the total "footprint" area of the folded filament. Since you are using this solution, obviously it must work well enough.

If so, just adding a baffle (e.g. a thin blackened aluminium sheet with a roughly 5 to 8 mm hole) about 10 mm at the rear of the lamp could block most of the light that the lamp sends rearward and help to prevent overheating the flash.

Multiple stacked baffles spaced from each other by a mm or two would be even more efficient at dissipating IR caused by bulb illumination, as well as enhancing air flow. Stacked baffles can be easily built by folding an aluminium sheet a few times like an accordion, with additional pieces of aluminium placed in each fold to keep the right spacing, then drilling the baffle hole in the stack and removing the extra sheets.
--ES

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Post by Pau »

Enrico, David, to prevent (up to some point) IR heating the flash tube a IR blocking filter would be convenient.

Schott KG1 and BG38 are usual in Zeiss microscopes. Slide projectors also are a good source of them, and of course they can be found at filter makers sites
Pau

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Post by enricosavazzi »

Pau wrote:Enrico, David, to prevent (up to some point) IR heating the flash tube a IR blocking filter would be convenient.

Schott KG1 and BG38 are usual in Zeiss microscopes. Slide projectors also are a good source of them, and of course they can be found at filter makers sites
I have been thinking about this too. The best placement would be between baffle and flash window (so that as little as possible of the light/NIR from the lamp hits it).

There is of course plenty of NIR in the emission from halogens, but not as much as from the incandescent bulbs of old (only1/3 to 2/3 for the same bulb wattage). Still enough that it might be troublesome even after most of the emission has been blocked by the baffle. Considering that most of the "classic" NIR blockers work by absorption and get hot (thereby emitting longer IR wavelengths), a dielectric NIR mirror might have the advantage of getting less hot.
--ES

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