A Seamless WideField Substage Condenser (part 2)

Optimising Light Levels and Image Quality

By Paul James


Shortly afterposting last month's article I managed to increase the numerical aperture of the condenser by further tweaking of the distance between the top lens and the main optic body. The maximum aperture of the condenser rose to just over 0.5na without incurring loss of field for the x3 objective, which was approaching the traditional 0.65na of the standard x40 achromat objective.

I'd cobbled together a DIY condenser that raised satisfyingly full fields from all 5 objectives, so the next stage in my personalisation of substage lighting was to explore the possibilities of moderating the field intensities from those objectives to a similar level ( I had good reasons to moderate this natural illumination discrepancy which I'll explain later). This would involve reducing the output from the x3 and x6 objectives so they would equate to the field intensities of the x10/x20/x40 objectives. Theoretically this could be achieved in a number of ways:-

1) Photo sensor control of the source's current. 2) Crossed polar control for each objective. 3) A single axial stop of complex form sited at the anterior focal plane of the condenser would compensate for increasing apertures of all objectives......... Entirely feasible but would take a while to determine the optimum silhouette by trial and error. Added to this would be the significant increase in diffraction in the aerial image generated by excessive length of edging from such a complex stop. 4) A circular axial stop of a diameter which can be varied for each objective, which would generate negligible diffraction in the aerial image.

The Challenge

As I am an advocate of the principle of simplicity, the last option appealed considerably. I'd realised from past experience that blocking the central portion of light entering an objective reduces image intensity whilst improving contrast without incurring a reduction in resolution. This simple mechanism would control light intensity and raise COL imagery simultaneously, and was therefore very appealing to me. But the principle problem here was that ideally each objective would require a different stop tailored to output image of similar quality levels. If a given stop was sited in the plane of the anterior focal length of the condenser (the ideal placement for stops, irises and phase rings) it would work to some extent because its illumination diminishing effects would be greatest with the x3 objective and progressively less with the rest. In fact it works to a limited extent as well as raising COL illumination in the lower powers, which diminishes in effectiveness with each incremental increase of na from higher power objectives.

But there was a more interesting and useful technique which I'd chanced on before, by adopting an unconventional and somewhat imperfect approach to substage illumination.

The Solution

I reasoned that if the substage condenser images the field source from the lamphouse onto the specimen plane, a suitably sized stop would remove its central component if placed below the condenser's anterior focal plane and physically detached from the condenser so its distance from it could be varied.

The field lens on the base of the Wild M20, (roughly illustrated above), was a convenient site for the stop, which would block the central light from the source. The stop's proximity to the anterior focal plane of the condenser ( A few millimetres below its lowest lens ) was fortunately not too critical, so long as the height of the tip of the tapering shadow cast by the stop could, by adjustment of the condenser, be set a little above and below the specimen plane to allow for variation of illumination form and its quality. The phase telescope's view of the back of the objective confirmed this principle. So placing the stop on the centre of the field lens would achieve this goal because the existing condenser's focussing compass was able to cope with the necessary variations of elevation.

The Effects

I found that this arrangement was capable of removing a visually noticeable proportion of the central light from the lowest power objectives and progressively less with the higher powers. It worked quite well despite its simplicity. Altering the condenser's elevation directly varied the stop's apparent diameter as witnessed through the phase telescope. I'd thus contrived a very simple lighting technique that: 1) Evened light levels across the objective battery; 2) Raised in succession a seamless continuum of BF/COL/DF illumination. It wasn't perfect, but it was a promising start and certainly a workable technique that I thought was deserving of some refinement.

It was also curiously enough, a method of altering COL's effectiveness in a stepless manner.......somewhat akin to the Leitz Heine condenser. The only constant that had to be figured out was the stop's diameter which was gauged by trial and error with a keen eye through the phase 'scope. It was ultimately a compromise that satisfied both extremities of the objective battery, so I eventually settled for a circular stop 4.5mm wide.

I'd thus managed to control the light intensity variation reasonably well between the x3 to the x40 objective AND coincidentally raised imaging quality in one fell swoop. For a given condenser elevation the stop's apparent diameter was at its greatest in the x3 objective and least in the x40, but of course this could be now be instantly altered to favour a given objective by altering the condenser's elevation. Throwing tradition to the wind, further controlling of the condenser's elevation induced DF, though this useful facility's success with a given objective varied. The DF component is not available in the higher x20 and x40 objectives since the DIY condenser's limited na cannot generate the necessarily oblique cone of light to fulfil this role. However at this juncture my appetite for an all embracing substage optic increased significantly, but so did the frustration with problems attending to its realisation !

A Watershed

I'd now come to a point in this venture not unlike that I'd found in the midst of the photomacroscope project. Despite the basic success of the unadulterated diy condenser as a thoroughly capable BF condenser, its field of excellence remained in the observation of prepared slides. In this role its uniquely individual lighting of static specimens was without peers. This feature of its lighting qualities hadn't been 'engineered' by me but was inherently part of the main optic I'd chosen for the role. What I was presently doing was spoiling its unique optical quality by trying to stretch its capabilities which didn't suit it at all. I therefore had to concede that another optic would be required for aquatic observations that possessed a fraction more numerical aperture than the diy version, which would raise DF more universally. I also realised that this 'aquatic condenser' I was searching for wouldn't illuminate the full fields of the low power objectives I'd originally hoped to achieve. But before conceding defeat I tried several classic condensers in the same role, each without its top lens, but still illuminated with central stop in situ on the field lens.

Condenser Mark 11

Out of a handful of old condensers I found an example of a Vickers achromat oil immersion 1.3na which seemed a likely candidate. With its top lens removed, it furnished a suitably wide cone of light in excess of 0.8 na.. As I'd expected it didn't provide the field coverage for the x3 objective, but as things turned out that didn't matter at all. With the DF component now well established into the fold of illumination I'd unwittingly contrived a situation which was more useful in aquatic observation than I had originally realised. The fact is that whilst this 'new' condenser did the business by raising BF/COL/DF as per diy condenser MK 1, its rendition of full fields in DF from the lower power objectives x3 & x6 was a double bonus. It was perfectly acceptable simply because the DF image is generally much dimmer than the equivalent BF, plus the fact that DF imparts low power imagery with distinctly more eye friendly acuity............ideal for its intended role of searching etc..

Concluding Thoughts

The initial sentiment I'd felt of failing to achieve my desired goal of tweeking a single condenser's parameters into an all embracing substage optic soon faded. I now realise that 2 condensers, MK 1 tailored to cope with the mounted slide, and the other MK 11 tailored to suit aquatic conditions made much more sense. I most certainly didn't want the Swiss knife equivalent below decks, though changing a condenser for an hour's pursuit into the aquatic world or that of the static prepared slide would be well worth the effort to realise a little more perfection from each discipline.

Now I have the basis of a really smooth and stepless imaging technique which incurs no disruption save that caused by the unavoidable changing of objectives, and all achieved without diverting attention away from the eyepieces........the principal incentive to change substage traditions. To be able to raise stepless variations of Brightfield/COL/Darkfield by merely adjusting the substage condenser's elevation soon becomes an intuitive operation, which is virtually instant in effect, and provides more visual information and pleasure during a 'live' observing session, than I have ever formerly experienced.


I am presently experimenting with a refinement which optimises imagery further, and need a little more time to assess its virtues etc.. Next month's article continues this journey of exploration toward more satisfying substage setups.


All comments welcome by the author Paul James


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