Cheap 'n cheerful phase contrast with a LOMO Biolam or similar student microscope

by David Walker, UK

 

For the microscopy enthusiast on a limited budget a dedicated phase contrast microscope can be expensive and a maker's phase upgrade outfit can also be pricey even for student microscopes like the author's LOMO Biolam. However, as other enthusiasts have also shown, it is often straightforward to adapt a student microscope to limited but workable phase at modest expense. It just needs one (or two) cheaper examples of phase objectives that can often be picked up on eBay and the centering annular rings can be homemade for an Abbe condenser filter tray (see footnote below). It won't be as slick to use as a proper phase system but can give competent results and a way of assessing whether full phase is needed.

My occasional need of phase is satisfied by the route described below using a Biolam stand. Similar student stands may also be amenable to such an approach.

Choosing the phase objective: When I owned a dedicated phase microscope with four phase objectives, there was only two, the 20x and 40x that I used regularly, the 10x and 95x I rarely used. (This may differ for other enthusiasts depending on subjects studied). So for my Biolam I looked around for a cheap 40x phase in the first instance. A benefit of choosing a medium power objective is that the annular rings required are usually larger than for low power so are easier to make.

I avoided modern big name objectives and bought an older Zeiss Winkel phase objective off eBay with a very slight but inconsequential mark on the front element which kept the price very reasonable. The 42x NA0.85 objective gave the bonus of a higher NA for brightfield as well as a higher performance phase objective cf the typical achromatic 40x NA0.65 phase objective supplied in the LOMO upgrade kits.

Making the annular ring for the condenser: Various ways of making the rings to sufficient accuracy have been described, the route I prefer is to make a 'ruler' for the condenser filter tray and read off the inner and outer annular ring dimensions seen in the back focal plane of the objective that match the superimposed objective phase ring (see this Micscape article). In the earlier article I used a fine pen to make the ruler, but CAD software can do a much neater and more accurate job as shown below.

The annular ring boundary lines required can be printed onto acetate sheet with the already calibrated CAD drawing package. The black fill-in can be made with a printer, but I haven't found either an inkjet or laser printer to give a sufficiently opaque result. Some enthusiasts report sticking two printed out rings together to improve this. Another way to improve the opacity is to print out with the fill-in then carefully overpaint with a few layers of matt black paint using a fine brush and slide ringing table to give a completely opaque result. Painting almost up to the annular ring borders can be accomplished with a brush trimmed to a fine point and using thinned ink but avoiding the exact boundary to retain the printer's neat phase ring boundaries.

Using the phase ring: The ring is aligned in the usual way by inspection of the objective back focal plane with a phase telescope. The ring is not as accurate as the correct maker's ring and phase condenser but slight changes in the LOMO Abbe condenser focus corrected for these. Slight imperfections in the annular ring's circularity may give less than top notch phase cf a proper phase outfit, but I've been pleased with the results and give that extra satisfaction that can be gained from 'homebrew' cf. using 'off the shelf' solutions!

I prefer a proper phase telescope as it's a one off purchase and such a useful accessory e.g. for polarisation studies, although enthusiasts have reported homemade low power telescope alternatives. Leica and Wild examples recently sold on eBay for ca $40 each, so lesser brands should be cheaper.

Some results with the 42x objective. After using a Cooke phase microscope for many years I believe the results are comparable at this mag. My Sony S75 digicam doesn't cope well with critical focus photomicroscopy so have defaulted back to video stills from a 'C' mount colour security style camera, with Snappy video still capture box.


The diatom Navicula lyra mounted in Hyrax. The visual image gives crisp resolution of detail but the video still can't quite resolve the detail at this mag. 'Test plate 8 forms' made by and with thanks to Klaus Kemp.


Left:
Epithelial cheek cells unstained, brightfield with the 42/0.85 phase objective.
Right: The same subject
but with homemade phase ring and Abbe condenser.

Comments to the author David Walker are welcomed.

 

Footnote: There's downsides of mixing maker's objectives of the same tube length, including potential lack of parfocality with other objectives and incompatibility with eyepieces owned. I'm use to having a mix of maker's objectives on a nosepiece and parfocality is not vital. Another approach is to seek out the 'scope maker's correct phase objective which will be parfocal and make the annular ring as described, thus still avoiding the need for a full phase kit. Even with the latter approach, homebrew phase is not guaranteed to work on all stands. With the Russian Abbe condenser an annular ring placed in the filter tray is nominally in focus in the back focal plane of the objective when the condenser is correctly set for brightfield, thus allowing correct phase adjustment, other condensers may not fulfil this requirement. This sort of approach works best when bargain basement single phase objectives can be found on eBay or club meets so no undue expense is incurred if it doesn't work, but this 'try it and see' method is part of the fun of microscopy for me!

 

 

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