DIC for birefringent objects

[Timor]

Hi,
I read a lot about DIC these days, and I've also read that it offers no real advantage over phase with birefringent objects. I have been struggling with the identification of calcareous nannofossils, due mostly to their size and condition (usually covered with calcite overgrowth, and near the resolution limit of my current setup) and I wonder if there would be any benefit to a DIC setup for this purpose. As far as I know it wouldn't fundamentally increase the resolution, and as I don't need attractive photographs (I'm only trying to make out details of their morphology) perhaps it would be a waste of money.
Any opinions would be welcome.
Cheers, Tim

[TiggerScope]

Not only does DIC offer no advantage over phase with birefringent optics, it offers less. The image is confused and gives little useful information, though it can be quite pretty. It's the reason DIC is rare on inverted scopes where plastic Petri dishes are commonly used. Ideally you'd use Hoffman, though in the absence of Hoffman optics, oblique should yield useful information and is simplicity itself to experiment with. You could just wave a finger under the (wide open) condenser, or a bit of card.

Peter

[Timor]

Peter,
Thanks for your thoughts, I'm sure you're right.
I always use a bit of oblique, with partially crossed polars, and in truth I don't think lack of contrast is my problem. It's just it would be nice if I could squeeze slightly more resolution. I should probably put more work into improving my slide making, and stop looking for easy answers!
I've wondered about Hoffman, but there seem to be few parts available, and not much information. I've never seriously tried COL either, I think maybe that would be worth trying.
Tim

[PeteM]

As said above, Hoffman Modulation Contrast is a good option for birefringent objects. The objectives seem to show up fairly frequently and moderately priced (around $100-$150 for 20x or 40x with correction collars). The condensers are harder to find, but a DIY addition of polarizing sections to clear plates held in a turret phase condenser is possible. As you say, though, you're not gaining resolution - just contrast.

Is it possible that reflected microscopy with suitable specimen prep, possibly darkfied, polarized, or DIC, could help in identifying your fossils?

If you have a colleague with a DIC scope, you might still give it a try. Combined with a wave plate there may be settings that would allow you to visually separate the calcite overgrowth you mention from the underlying fossils.

[TiggerScope]

For more resolution there ain't no substitute for NA. What objective and condenser are you using and how are the specimens mounted?

[Timor]

Hi Pete,
I'll keep my eyes open for a Hoffman objective, although only a 60x or 100x would be of use. I need to read a bit more about that system.
I've never had a chance to try reflected microscopy, but I've never heard of anyone using it for this purpose. I think there's probably a reason for that.
Darkfield transmitted light can be useful, but I find it difficult it to prepare slides that are clean enough for use with a 100x objective.
Most professionals now seem to use EM, and looking at the microtax website the light microscope images shown there, and in the literature, aren't really much different from what I can see with the scope I'm using now. Maybe slightly sharper in some cases? I don't know. I need to try and take some photos, that would make it easier to compare.
It's just that the last few days I've kept noticing the stunning DIC images that seem to be everywhere now - it made me think maybe I was missing out!
Tiggerscope,
My equipment is fairly good although not the newest - I usually use a BHS with a variety of condensers and objectives - my best objective is a Nikon 100x NA1.4 planapo, with an aplanatic/achromatic 1.4 condenser. I also have a phase condenser and 100x objective. It's all a bit old now, but I'm not sure if newer stuff would be very different. I always use polars, slight oblique, and often a 1/4 waveplate. The problem is that the nannofossils are very small! I mount them as a smear in LOCA (the stuff for repairing phone screens - it's very good for keeping everything in a single layer). Norland 61 is usually recommended but I haven't had a chance to try that. I've been checking through some of my slides tonight, and the amount of detail visible varies enormously from slide to slide, so I need to work on that.
Cheers,
Tim

[PeteM]

Hi Tim, You'd probably be disappointed with the resolution of HMC at 100x -- and in any case, those objectives seem to be rare on the used market. It's typically used on inverted scopes, up to 40x objectives. The company did make some at 100x, though.

It sounds like you're on to something with getting the specimen prep as thin as possible.

Perhaps someone near you has BH2 DIC. If it turns out to be helpful, and you are able to find an affordable upper sliding prism, then I may be able to point you to a source for a compatible 100x condenser prism that you could mount in a spare Olympus phase condenser.

[Timor]

Thanks Pete, will do. Although I have a feeling it'll be a long time before I find the BH2 slider, I don't remember ever noticing one for sale.
Tim

[Phill Brown]

I'd wonder about the mountant being an issue.

[Timor]

Yes, the mountant's something else to consider, although I don't think there's anything inherently wrong with LOCA. When I started using it, I compared with one commercial mountant (I forget which) and several home-made recipes, and the LOCA was best out of the ones I tried. For my purpose, at least. I don't know its R.I. but it seems to give about the right amount of optical relief. As it's a UV cured cement I would guess it's composition is fairly similar to the Norland, but I'm going to try some Norland 61, as that is the one that's always suggested.
Tim