Simple Darkfield Stop for BH2

[Craig Gerard]

I want to take some transmitted darkfield images with my BHT.

Why is my dark field not dark?

The CH2 was easy, just pop the darkfield stop in the filter tray.

The condenser on the BHT is a flip top (swing-out) Achromat (BH2-BC 0.8-0.16).

How do I implement a simple darkfield stop using this condenser?

I'm using SPlan objectives 2, 4, 10, 20, 40.

Do I need a different size stop for each objective?

Ideally, do I need a separate condenser for darkfield? If so, what is the appropriate part number?

In most cases, for transmitted light observation, you will only need a darkfield stop in the condenser. At higher magnifications, however, you will need an objective with an iris as well as a special darkfield condenser.

Reference: http://www.olympusamerica.com/seg_section/seg_faq.asp

Found an article by Wim:
http://www.microscopy-uk.org.uk/mag/art ... field.html

And another link here:
http://micro.magnet.fsu.edu/primer/tech ... field.html

I would still appreciate specific answers to the questions above......


Craig

[Craig Gerard]

Anybody?

[Tom Jones]

Craig,

When I was playing with Rheinberg, I machined a thin wall, hollow plug, with a recess for a tray at the top, to fit up inside of the condenser to hold the Rheinberg filters as close to the iris as possible. It's easy to do with a lathe and a piece of ABS rod. I was trying (in vain, actually) to use 40x. It took me awhile to realize I needed an iris objective to get the NA down low enough at 40x since I was using a UPlanFl with an NA of 0.75. The home made tray worked well with 10x and 20x, and even with a darkfield stop at the same magnifications. As you noted, it would probably work just fine with the filter tray directly under the condenser, but I had a BX-40 without a tray. Several months ago I finally managed to obtain oil/iris 40x, 60x and 100x objectives but my schedule hasn't allowed my any time yet to play with them!

Pretty much anything you can use to hold the darkfield stop just below the condenser should work. Years ago we cobbled up darkfield to look for syphilis spirochetes using a dime placed on the flat top of the swing-out lower condenser lens on an AO (110 I believe) in a clinical lab. It worked surprisingly well.

As for the size of the stop, I suspect their are optimal sizes (see the articles you linked to) for each NA, but the universal condenser Olympus sells for BX series uses a single stop size so I doubt it makes much difference in practice. Olympus has both dry and wet dedicated darkfield condensers (there's a seriously overpriced dry one on eBay for $500, and a wet one for $400). A better choice is probably the universal since it has both the dark field stop and stops for phase contrast. The top lens is interchangeable for wet or dry on the BX one, I don't know about the BH. One for the BH series is up, (along with phase objectives) now: http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT It's still a bit overpriced as a set went for a bit over $600 just a few days ago. Neither listing for the BH condensers mentioned a darkfield (DF) stop, some I'm not certain that series has one. I'm sure someone here has one and can let you know.

You don't need to get carried away with a special condenser. All you really need to do is find a way to place a centered stop somewhere below the iris on your condenser and you should be good to go at 2x (with the swing-out lens out), 4x, 10x and 20x (with the swing-out lens in). 40x may or may not work at the 0.70 NA of your SPlan. You may need to find an iris objective (or a cheaper (!) Dplan 0.65NA, which *might* be too high an NA too). It's just that without a tray of some kind you'll have to find a way to support the stop. Play with the sizes until you get what you like.

Since my experience with darkfield is at best limited hopefully someone here with a bit more experience will correct my potential foolishness!

All I know for sure is that darkfield is really pretty when it works!

Good luck!

Tom

[rjlittlefield]

I would still appreciate specific answers to the questions above......

I offer the following...

Why is my dark field not dark?

Is it somewhat dark, but not black? If so, that indicates that the stop is allowing some light to reach the objective directly. This may be due to incorrect diameter of the stop or incorrect placement in the optics chain.

Do I need a different size stop for each objective?

The best stop will be a different size for each objective, because each objective has a different aperture. However, you may be able to find some size stop that works for all objectives, and just ends up blocking more light than it really needs to for some of them.

In addition to adjusting the diameter of the stop, you may have to adjust placement of the stop with respect to the condenser.

To facilitate seeing what needs to be done, try removing the objective or swinging it aside, then placing a piece of tissue paper above the stage at a height equal to the front of the objective that you're trying to use. At that height, the condenser with stop added should be projecting a ring of light whose center hole is big enough to fit around the front glass of the objective. The edges of that hole will be more or less blurred, depending on placement of the stop. If it is drastically too blurred, then to get a black field you might need such a large diameter stop that it would cause other problems.

Ideally, the stop should be sized and positioned so the light field remains uniform at the level of the subject, but crisply separated into a light ring with a dark center at the level of the objective. Adjust accordingly...

--Rik

[Charles Krebs]

The condenser on the BHT is a flip top (swing-out) Achromat (BH2-BC 0.8-0.16)

BH2-SC 0.9-0.16 ?

How do I implement a simple darkfield stop using this condenser?
I'm using SPlan objectives 2, 4, 10, 20, 40.
Do I need a different size stop for each objective?
Ideally, do I need a separate condenser for darkfield? If so, what is the appropriate part number?

The BH2 condensers (and condensers on the majority of "newer" microscopes) mount via a dovetail at the base. This generally makes it much harder to place darkfield stops and Rheinberg filters near the condenser aperture. (Quite simple and straightforward on the older "collar-mount" condensers that had a filter tray at the bottom). I've never seen a BH2-SC condenser. But Olympus made an "insert" that fit up inside the BH2-AAC with single phase annulus at the top (It may well fit other Olympus condensers from that series). Such an arrangement could be used or made (As Tom has mentioned he did) to get a darkfield stop up near the condenser diaphragm. See this article by Ted Clarke:
http://www.modernmicroscopy.com/main.asp?article=9

You should be able to use the same size stop for the 4, 10 and 20 objectives. (As Rik said, a single stop will need to accommodate the largest NA, and in this case it will be larger than needed for the 4 and 10). You could try to make the stop large enough for the 40/0.70 but that getting a little "iffy" and will cut down quite a bit on the light intensity when used with the lower powers. Also you'll likely need a different (smaller) stop for the 2X when the top condenser lens is flipped out.

While it is not impossible to use a "brightfield" condenser with a "stop" with objectives of NA 0.70 and slightly higher it is really preferable to go to a "dedicated" darkfield condenser at that point. The Olympus BH2-DCD is marked as 0.92-0.80. This means that the "ring" of light it produces has an "inner" NA of 0.80 and an "outer" NA of 0.92. So you could use this with an objective with a NA smaller than 0.80. The BH2-DCW is rated 1.40-1.20, so it is useful with objectives with an NA smaller than 1.20. (This condenser must be oiled to the bottom of the slide, and it will not illuminate an overall area large enough for the field of view of a lower power objective. It is really meant to be used with about 40X and up). Dedicated darkfield condensers will generally provide a "blacker" dark field.

[Charles Krebs]

Craig,

After posting earlier I remembered a file I had prepared some time ago to help find proper darkfield "stop" sizes with my condensers. (Could be used for Rheinberg filters as well).

These are colored concentric circles, from 10mm diameter to 30mm diameter. If you download the jpg (link at end of post) and print it (no resizing for printer) you will have a bunch of targets that look like this:


Print on thin paper or, if you happen to have some use printable clear plastic sheets.

Once you have figured out where and how you will be placing the stop in the condenser, temporarily place one of these targets in that location instead. Put a slide on the stage and focus on a subject. Don't refocus, but move the slide so that you are looking a blank section on the slide. Next, pull out one of the eyepieces and peer down the eyepiece tube. You should see part of the target at the back of the objective. It should be centered. You want to find out how large the circle must be to completely "fill" the back of the objective. This is the required diameter for a "stop" for that objective.

You may want choose a size the next ring larger. This should take care on any slight misalignment of the stop when placed in the condenser. As seen down the eyepiece tube, with the eyepiece removed, you don't want any light at all "creeping" into the objective from the edge of the stop if darkfield is the goal.

Here's the "target" file:

http://www.krebsmicro.com/forumpix/10to30.jpg

[Craig Gerard]

Thankyou for the detailed responses Appreciated.

I have read through the posts and will do so again....and again while I attempt to put the advice into practice.

I understand the concept; but it takes my brain time to absorb, arrange content and recognise pattern (I can see the pattern); so, allow me some time and I will respond to the questions in your posts.

All have provided valuable information, practical approaches and some answers in the form of questions. I will respond.

Once again, I am now better prepared and informed as I embark in search of the, so far, elusive darkfield


Craig

[g4lab]

Dark Field condensors appear regularly on fleabay.

Some of them are in outfits which may include objectives that are matched to the condensors or funnel stops which fit (into the backs of) objectives made by the same manufacturers. The funnel stops also block the direct light from participating in the image formation.

I have only played with the usual spider stops but I think the best dark field pictures are done with the higher NA reflecting DF condensors like Cardioids
and with objectives with funnel stops.

http://cgi.ebay.com/Swift-Bi-Reflecting ... 1211wt_911


http://cgi.ebay.com/OLYMPUS-MICROSCOPE- ... 500wt_1154


Most of the dedicated dark field condensors are intended to be oiled to the slide which allows the light to come in from a more oblique angle. The objective may or may not be oil immersion.

Also the condensors used to come with fused quartz slides which since they were a bit harder than glass would take a higher polish and also were selected for transparency so that they did not scatter the light for you. The cover slips were similarly quartz and selected and both were expensive. There were draconian methods of cleaning them usually starting with nitric acid. Every particle of dust scatters light.
I think you can still buy such things if you look.

http://www.mccronemicroscopes.com/store ... gory_id=28

Finally because you are blocking a great deal of the light flux you need a pretty powerful source in the first place. This is where they used to use the carbon arcs (which were usually about 1200 watts) and various short arc lamps.

[Craig Gerard]

Still working through this; but have not applied all the suggestions - yet.

Charlie, the printout has been very handy in determining diameter of individual stops.
I am using a BH2-SC condenser. There was an error in my initial post (as you spotted). (an image of the BH2-SC is hosted on http://earth2geologists.net)
http://earth2geologists.net/Microscopes ... denser.JPG

Until I get a handle on this process, I've placed various sized stops on the element above the condenser iris; probably not the ideal place in the "optics chain", but for now, it is the easiest place regards access.

What I am seeing is not true darkfield and imperfections in slides are just as visible as the subject; but this has been the case no matter where I place the stops in the "optics chain". (I will take an image an upload). There maybe other contributing factors, in addition to the level of my understanding (or misunderstanding).

I'm still absorbing the information provided and waiting for the penny-to-drop. I can hear it rattling $$$

Gene,

I've just noticed your post and will read through after lunch.


Craig

[Craig Gerard]

Gene,

Good point regarding the quality of the actual blank slides and coverslips; surely that must play a role, especially with transmitted darkfield?

What brands, specs do members recommend? You've mentioned quartz?

Every particle of dust scatters light

That is exactly what I'm seeing, even when using new slides. The slides I have also show some anomalies even in brightfield; but in darkfield, it resembles a meteor storm:
http://www.nightskynation.com/pics/meteor-showers.jpg

I don't think my condenser is designed for transmitted darkfield; even if I were to use NASA grade slides, I still feel this condenser would be unable to deliver acceptable darkfield. It can't seem to deliver the light above the slide horizon; but, as I've mentioned in my initial post the less complex CH2 system did deliver acceptable, basic transmitted darkfield. (using the same blank slides I have now); so I am thoroughly confused.

I'm very reluctant to upload any images because they are seriously, seriously bad; so be prepared to be shocked and amused.....some may even cry

You have been warned

Craig

[Craig Gerard]

Just a dusting of scales on a slide; but you can see what I mean by a meteor storm.

Raising or lowering the condenser has no impact. Flip-top is swung out of the way.

Microscope slides used: Lomb Scientific Precleaned Microscope Slides 26 x 76mm, 1-1.2 THICK (these are the details on the box) freshly opened.

SPlan 4x.





The dust and most other imperfections are on or in the glass slide, not on the sensor.

................................................

The image below is one captured previously using the CH2 with a darkfield stop in the filter tray. DPlan 10x.



Craig

[Charles Krebs]

Flip-top is swung out of the way.

This will likely vary with condensers, but the couple times I've tried I have had poor success doing darkfield with the top condenser piece "flipped out". Try it with the top in place. (You'll need a different size stop, and be sure to have the condenser diaphragm fully open).

Darkfield is brutal when it comes to dust and unwanted debris. Also (regardless of brand) you really need to clean the slides and covers scrupulously. And even then, you'll probably need to do much digital "spotting".

[g4lab]

Take one of your precleaned slides and examine it with a laser pointer.
I bet they just aren't clean enough.

The methods I have read involve soaking the slides in nitric acid and washing them in double distilled water. Methods that are post WWII might recommend the ultra sonic cleaner if you have one of those available.

Your background is black enough. You just have lots of dust. Like the whole world.

[Craig Gerard]

Still no joy

Gene,

The third image in my previous post was not taken using my current setup. It was taken using a CH2; same slides and similar subject (but no meteor storm or starfield).

I'm beginning to suspect, that in addition to my ineptitude, there may be something physically out-of-wack with the condenser. All the instruction I have received and read seems to indicate I should be seeing better results; but I am yet to achieve true darkfield.

The information below was provided by a supplier of darkfield stops (and echoes one of Charlie's suggestions):
"This item is placed in the filter holder located underneath the condenser.
The condenser iris should be fully opened and the flip-top lens in the top position."

hmmm... I've done that too.

Might need to look at a dedicated Darkfield condenser, oil or dry, whichever comes first, or a general purpose alternative to the flip-top condenser (I went with the flip-top (swing-out) condenser initially, because it plays well with the lower power objectives; but apparently not in darkfield. The BH2 is more complex than the CH2 and obviously the complexity requires more finely tuned or dedicated components for specific applications.

I have tried different 'slides' to see if slide thickness and glass properties may be having an undesirable effect; short answer = yes.

I need to solve this riddle, for my own satisfaction and for future projects. It may sound like I am a bit scattered (I am at times); but that is usually the case when learning something new that doesn't quite turn out as expected. I scramble for answers.

Tom, thanks for sharing your experiences. I have taken onboard all the information you provided and also Rik's nifty tissue paper calculator and placement of the stop in the "optics chain".

I've just located a couple of alternate Olympus condensers for the BH series, not dedicated darkfield (that may take another 5 days or so), but they should enable me to compare results.

Thanks for all the assistance. Loads of good advice in this thread.

Also (regardless of brand) you really need to clean the slides and covers scrupulously.

Can anyone suggest a practical approach to the 'cleaning' process? My coverslips are exceptionally brittle due to the nature (and purpose) of their construction.


Craig

[Craig Gerard]

I managed to get hold of a couple of BH2 condensers: an Aplanat Achromat 1.4 (BH2-AAC) and a 1.25 Abbe (BH2-CD).

I have been able to achieve a darkfield by placing the 'stop' on the top element and positioning the condenser appropriately. It is not the ideal position for the 'stop'; but it works. The Aplanat Achromat 1.4 (BH2-AAC) seems to provide the best result, mostly due to the wider aperture, I presume.

I'm not certain if this is pseudo-darkfield?

I need to open the condenser aperture completely and turn the BHT illumination to maximum.

I'm beginning to appreciate the extra illumination power a BHS stand and lamphouse would provide; but there must be other ways to get more light into a BHT?
I'll see what happens when a fibre optic illuminator is connected to the backend of the BHT (any precautions?).

Craig