Putting Together a Darkfield Microscope

Some of you are considering purchase of a microscope. I thought I would offer some suggestions that cover a fairly wide range because I know you all have different budgets.

First, let me explain in a simple way how darkfield imaging is created. In a normal light or brightfield microscope, the light comes straight up through a condenser and the objects on the slide are generally seen as dark against a more or less clear field, one that is so bright that some objects are invisible because the light washes them out.

In darkfield, the light path is obstructed so that the light appears to hit from the edges. This causes a sort of silhouetting of the objects against a darkfield, usually almost black. This type of microscopy requires a special condenser and better imaging is possible with oil immersion objectives with irises than with ordinary objectives. Because the exact path of light is critical to what is illuminated, the condenser has to be perfectly centered in order to get good results. If severely off center, nothing at all can be seen.

There are people providing DF scopes that can only facilitate adequate DF at 40x magnification. At this level, no oil is required so less skill is needed than with oil objectives, but I take exception to the notion that 40x is sufficient to "see everything of importance." You do, in fact, see much more at higher magnifications.

The oil immersion objectives have irises or diaphragms that allow for very tiny adjustments to compensate for differences in thickness of cover slips. Variations within the same box of cover slips can be significant so this ring is very helpful, but one can sometimes see quite a bit with a much cheaper objective that has a stop instead of an iris ring. For hobbyists, this makes entry level work affordable, but for more serious work, the oil and iris are both essential.

The four big microscope companies are Zeiss, Leica, Nikon, and Olympus, but fierce competition is arising out of China and to a lesser extent Russia and India. Mergers and acquisitions combined with outsourcing of production has resulted in a situation in which many components of microscopes are made in third world countries, including a lot of low-end objectives for the big four. In other words, it is possible that an inexpensive Zeiss scope is not really made in Germany and thus cannot be expected to have the same production standards as a higher-end Zeiss scope.

This said, brightfield and phase contrast scopes vastly outsell darkfield scopes so very few manufacturers have a model that is ready for darkfield straight out of the box. The exception might be Hund, a German company that produces a decent DF scope for around $4000. The Lomo that I sell is modified for DF by the importer. In short, it is not built for DF but rather converted in the U.S. It is a professional scope with surprisingly good optics and even more bang for the buck than some Chinese scopes.

For instance, one can get a Chinese scope with camera and monitor for about the same price as the Lomo but the allocation of funds in the budget is totally different. I think the Chinese have catered to a captive market where someone thinks of a microscope purchase in somewhat the same way as a computer and they want all the pieces in one box, but each is a little scaled down so as to capture the entry-level market. The Lomo is klutsy and anything but modern looking, but it is hugely functional, especially with the addition of the fiber optic illumination, an optional extra. Once one has the right condenser and objectives and enough light, this scope can really do the job, albeit a little awkwardly but professionally. It still prices out under the Hund but a little above the cheapest of the Chinese scopes but less than some of the better quality Chinese scopes. This is the market the Russians have staked out and want to hold. Lomo beats all the big four on pricing and basically has all the features required plus a built-in magnifier, something I have not seen on other scopes. What this means is that whereas most scopes ship with 10x eye pieces and four objectives, with 100x being the highest magnification, Lomo ships with two sets of eye pieces, 10x and 15x, plus a magnfier so that the maximum magnification is 3750x rather than 1000x. This makes for a significant difference in what one can see. Perhaps more important even that this is the fact that the oil immersion objectives are really good, not perhaps of the same quality as Zeiss but still very, very good, meaning clinical work is possible on a budget of $3000 as opposed to double or triple that with a name brand. In short, you get the optics without the design aesthetics, but the scope is rugged and has a lifetime warranty on everything.

I posted a few pictures of similar objects shot on four different scopes with four different cameras to show that although the last, my Nikon with Sony camera, is clearly the purest in terms of what is sought in classical DF, the other three pictures are totally adequate.

http://darkfieldstudies.com/comparison_images/phagocytes.html

Many blessings,

Ingrid

 

Copyright by Ingrid Naiman 2007