|A Case For Streamlining Lighting
A Personal View
By Paul James
It's been quite awhile since I made my own self discovery of what the Victorians called 'Annular Illumination'. It was the image's subtle contrast enhancement that I noticed when incorrectly setting my substage phase condenser with the x50 phase annulus when using the x40 BF objective, instead of leaving it in brightfield mode where there would be no annular stop involved. It was of course pure luck that the slide I was scrutinising at that time, leant itself to contrast enhancement by the ring annulus on the phase disc. Many years on, it has become a more widely employed illumination technique owing to the global web's popularity. Prior to the existence of the Internet, this illumination technique was likely practiced by only a relatively small group of microscopists, though the actual numbers of enthusiasts cannot be known with any accuracy. However, what is from my perspective a curious mystery, is that despite having collected a dozen or more books on the subject of microscopy since around 1957, I find that "annular lighting" is not mentioned in any........?? Yet I have been informed that the technique was well known over a century ago? Why I wonder would such an anomaly exist? The truth might be that annular lighting works more effectively with high quality objectives, with inherently high contrast raising anti-reflection coatings, with highly corrected glassware in apochromatic and fluorite combinations. I'd imagine that the amateurs of those times would not have exotic objectives of the sort commonly available today. Thus the contrast enhancement, inherently raised by annular illumination, would have been rather more subtlely rendered than is the case today? Thereby a sort of evolutionary process would have entered into the equation, in which those microscopists who had better optics naturally would tend to carry on with COL usage. Given the paucity of information about annular illumination, especially in more modern times would suggest that the latter must have been a minority, who quietly pursued their observations with annuli outside the limelight of brightfield where the core of enthusiasm and tradition reigned.
Though Köhler's clever employment of substage optics yielded a more uniformly generated field for photographic purposes over a century ago, it did not bring about any significant advances in visual microscopy that Critical illumination was incapable of replicating. Yet I think Köhler illumination bolstered brightfield considerably in the ensuing years insofar as many of the professional stands that are now found in amateur hands, were rigged for Köhler fields,and were perfectly capable of satisfying the most pernickety observer. Therefore there seemed little point in mass producing stands that did not employ Köhler's photographic capabilities. Critical illumination didn't fail, not at all: it just didn't make sense to pursue it principally because it couldn't raise a homogenous field to professional photographic standards. Having said that I'm fairly sure that there are many individuals still practicing critical illumination, and justifiably so.
Phase contrast followed some years after Köhler's lighting insights. Its easily seen contrast enhancing properties guaranteed a welcoming clutch of professionals, and an ever growing crowd of amateurs in latter years. By then, the inherent virtues of Brightfield, Phase contrast, and of course Darkfield, were truly cemented into the microscopist's arsenal by the early 1930's. Anything else would for one reason or another be sideline techniques, in terms of numbers of adherents, though the emergence of DIC later on, scooped a following, but its intrinsically high cost kept its universal adoption by the amateur ranks to a minimum. There have been other illumination variants too, but again mostly too costly, and therefore not common place in amateur hands.
There is however another anomaly regarding "annular illumination" besides its scant profile in microscopy literature, and that is its titling. I have thought that the phrase "annular lighting", is as misleading as it is accurate. It is a simple but incontrovertible fact that annuli can raise both darkfield and phase contrast illumination too. All accounted for from varying diameters of annuli. Interestingly too is the fact that darkfield and phase contrast's titling are accurately descriptive, insofar as the reader understands in an instant what type of lighting it raises. The Victorian 'annular lighting' title does not indicate what the effects are, given that DF & PC are variants of annular lighting too.
Though I'd self discovered a lighting technique with an annulus, which I innocently coined COL.....Circular Oblique Lighting to represent it, I think its time that the microscopy community should decide to rename "Annular Lighting" to bring it in line with its cousins : DF& PC lighting techniques. I'm not advocating the use of the acronymn COL in place of Annular Lighting, but that the microscopy community, amateur or otherwise should face up to the fact that there should be a reshuffling of illumination nomenclature in order to simplify the perception of all illumination techniques , so that the novice can understand it all on a more easily assimilated footing.
EG.......................Brightfield techniques :-
1) No Substage Annulus generated lighting technique :
Pure BRIGHTFIELD (image quality modified by subtle closure of substage iris diaphragm, up to the full NA of the objective.)
2) Substage Annulli generated lighting techniques :
a) DARKFIELD. ( Image quality and appearance modified by varying annulus diameters all beyond NA of objective )
b) PHASE CONTRAST ( Contrast enhancement generated by interacting phase plates in substage condenser and objective, altogether well below objective's NA)
c) CIRCULAR OBLIQUE LIGHTING ( Contrast enhancement by natural phase reversal processes in the objective, generally increasing as the diameter of the annulus rises up to the objective's NA )
It's not perfect I know, BUT I wish I had this information the day I started to peep down the eyepiece, or a little later perhaps. It would have been a major advancement in my ability to understand what I could do and therefore generate the best imagery at that time. It seems that the natural, even logical desire to unclutter matters for the Botanist and Zoologist to classify species etc., doesn't seem to extend to the lighting techniques of one of the most important tools of their trade. ........the 'scope ??
|All comments welcome by the author Paul James|
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Published in the January 2013 edition of Micscape.
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