Topical Tip: Notes on using the LOMO darkfield condenser for both darkfield and annular brightfield

Compiled by David Walker, with tips by David Walker (UK) and Ted Clarke (USA),
images by David Walker.

Dedicated darkfield condensers by the big name makers can be expensive but for LOMO users, the OI-13 NA 1.2 oil immersion darkfield condenser is quite affordable and often comes up on the secondhand market. Some notes on using this condenser are compiled below, both from the author's experiences and Ted Clarke's, where mentioned.

Note on installation and use

The immersed condenser works very close to the underslide of the slide. Sometimes the condenser upper focus limit stop on the LOMO Biolam stand may have been set conservatively by the makers which although fine with the Abbe condenser, the darkfield condenser may not focus close enough. This applies to the author's stand. The stop pin in theory is adjustable with the captive screw under the stage but the pin may be seized, as in the author's example. A workaround is to unscrew the optical assembly a few mm out of its mount to increase the height.

If adjusting the condenser and or pin height, hold say the edge of a plastic credit card (to avoid scratching condenser) across the stage aperture with condenser focussed fully up to ensure it's not set high enough to hit slide when in use. At the highest mags hitting the slide may risk damaging the slide and / or objective.

In darkfield mode

The maker's manual suggests that for immersion objectives, a funnel stop is required to reduce the objective's NA to 0.85 (if it doesn't have an iris). However, from the author's trials (and Ted Clarke's, personal communication) this NA limit seems a little conservative and it's worth trying darkfield at full NA up to an NA of ca 1.0 as the condenser gives excellent darkfield i.e. an inky black background with, for example, the LOMO 60x NA1.0 apo oil. See example shown below.

Diatom Pleurosigma angulatum, Klaus Kemp eight form test slide in Hyrax available from www.diatoms.co.uk.
LOMO OI-13 oil immersed darkfield condenser, LOMO 60x NA1.0 oil apo with iris set at full NA, green filter, tungsten lamp. LOMO 7x eyepiece.
Sony P200 consumer digicam supported over eyepiece. Out of camera image -0.7EV from auto exposure, resized.

 

Use for annular brightfield ('COL') with high NA objectives

In darkfield, the condenser presents an annular ring of light outside the objective's NA and the familiar image of the bright subject set against a black background is seen. But if the same NA 1.2 darkfield condenser is used with objectives of higher NA, brightfield is created because the annular ring of light now lies just within the objective's back focal plane. This creates a form of annular brightfield or 'circular oblique illumination' (COL) exploiting nearly the full NA of the objective. See footnotes for a selection of resources on the various aspects of annular brightfield.

Using a darkfield condenser for COL is a technique Ted Clarke (USA) encouraged me to try as he has already used it to good effect with a LOMO stand (personal communications). (See my article last month for an example of use with a Nikon Labophot and Nikon NA 1.4 darkfield condenser).

As has been remarked by others on annular brightfield, the technique may or may not work well with a given objective and / or subject, so is very much a 'try it and see' technique. The author and Ted Clarke have found the LOMO 90x NA1.3 apo can work well with certain subjects with annular brightfield with the darkfield condenser.

The diatom Amphipleura pellucida (specimen length 90 µm) using annular brightfield slightly decentered and the LOMO 90x NA 1.30 objective. LOMO K7x eyepiece.
Klaus Kemp eight form test slide in Hyrax available from www.diatoms.co.uk.
Violet 400 nm LED mounted in LOMO OI-19 external lamp, i.e. at the visible light - near UV border. (All work with webcam, see this article for safety notes when using such LEDs.)
Camera 1.3 Mpixel Opticstar PL-130M. Crop of single image from camera, tonal balance adjust and resize.

 

High NA COL on the author's LOMO / camera combination at least, does seem to have a restricted tonal range which although the eye tends to tolerate, digital or film imaging can give a less than striking look, cf the strong contrasty modelling with highlights conventional oblique can give. Tonal balance corrections of the image can improve matters. It can also give a veiled look overall to the imagery possibly because of any out of focus particles being emphasised.

Because the condenser can give extreme oblique, under some circumstances false structure (see top left, image above) e.g. outside the periphery of a diatom can occur; something the microscopist of course needs to be very wary of. This isn't unique to the COL form of oblique but far as I can tell from own imagery, the artefacts when they occur seem to be mainly of out of the plane of focus diatom edges and don't think internal false detail is being created. As with all microscopy techniques, the user is best to be wary of using a single technique in isolation especially on unfamiliar structures without confirming any structures seen are genuine with other lighting techniques.

Comments to the compiler David Walker are welcomed.

Acknowledgements

Thanks to Ted Clarke for personal communications where he has shared his experiences with the darkfield condenser and for encouraging me to try the high NA COL technique. Thanks also for a copy of the darkfield condenser manual.

Thanks to Klaus Kemp for his excellent diatom test plates and type slides (available from his website www.diatoms.co.uk), and for making them so affordable to the hobbyist.

A selection of resources on annular brightfield

Paul James has an excellent suite of Micscape articles on many aspects of COL, in particular using the annular rings of a phase condenser to create the technique.

Frithjof Sterrenburg discusses the technique in Chapter 8 of his 'Microscopy Primer' in the section 'Phase-contrast on the house' hosted on Micscape.

R M Allen, discusses and illustrates an example of its use in 'Photomicrography' with a darkfield condenser, 1958, 2nd edition, p.276.

Willis W. Mathews, 'The Use of Hollow-Cone Illumination for Increasing Image Contrast in Microscopy', Transactions of the American Microscopical Society, Vol. 72, No. 2 (Apr., 1953), pp. 190-195. First page accessible on JStor. The author describes the use of a modified Abbe condenser to create the effect.

W G Hartley in the Quekett Monograph No. 6, 1983, 'The microscope a beginner's guide', p. 32 mentions the technique using a darkfield condenser.

Historical note: An example of a 19th century mention of annular brightfield is by W. Carpenter in his 2nd edition 1857 of 'The microscope and its revelations'. On p. 124 he has a footnote to his discussion on darkfield and references the paper below which is available free on the 'Journal of Cell Science' website archive.

J. C. Hall. 'Original Communications: On an Easy Method of viewing certain of the Diatomaceæ', Quarterly Journal of Microscopical Science. 1856 s1-4: 205-208.

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