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Blu-rayTM micro structures resolved - in an ordinary light microscope! (III)

Once more - why are we making so much fuss about the Blu-ray microscopy? It is quite simple:

No matter what kind of leisure time activity or professional work you are performing with your microscope - you should know about its performance and its physical limits. CD, DVD and Blu-ray have a big advantage in common: they are standardized all over the world. As a consequence everybody in possession of one of these can participate in a universal microscope performance check. This is a big difference when compared e.g. to diatom test specimens. As some among you will know anyway, there is a certain variability among diatom test specimens. This variability can be explained by different embedding media with different diffraction indices and by tiny, but not neglegible differences in the sizes of the actual diatom individuals. In contrast the industrial media can serve as true sub micrometer rulers.

Last but not least each Blu-ray is a wonderful demonstration object in order to illustrate the physico-chemical borderline of light microscopy. For this sake please simply look once more at our light microscopic photomicrograph of a Blu-ray (fig. 1, green color center area):

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Fig 1: Combined photomicrographs of a CD, a DVD and a Blu-ray in uniform magnification.
Please note that the Blu-ray part of the image is slightly retouched with a few dust particles removed. Cf. additional technical informations in our previous magazine issues [ English keywords: Blu-ray disk, microscope, microscopy, diffraction limit ]

When looking around in the internet you will read on and on that the light microscopic resolution is diffraction limited. This has to be accepted, of course. But, on the other hand, nobody can actually precisely say what this means in practice. It sounds reasonable to state that the resolution limit should be approximately half of the light wavelength of "average" blue light thus defining the resolution limit as circa 200 nm.
When looking back into the original primary literature it becomes clear that even Ernst Abbe, a mathematical genius, finally resorted to a non-numerical, verbal definition of the light microscopic resolution limit [very free translation by us - sorry]: "As it is by no means possible, even by immersion, to reach a viewing angle above 180° (with respect to air) we have to accept the consequence that the resolution limit with central illumination will never exceed the value of a full wavelength, with extreme oblique illumination the value of a half wavelength of blue light" [Abbe, 1873, S. 456]

For those among you who are able to read German, please consider to use the better, original text by Ernst Abbe: "Da nun auch beim Immersionssystem der Oeffnungswinkel durch kein Mittel erheblich über diejenige Grösse, die 180° in Luft entsprechen würde, hinausgeführt werden kann, so folgt, dass […] die Unterscheidungsgrenze für centrale Beleuchtung doch niemals über den Betrag der ganzen, und für äusserste schiefe Beleuchtung niemals über den der halben Wellenlänge des blauen Lichts um ein Nennenswerthes hinausgehen wird."

Kurt Michel, not an intellectually light weight personality either, being the chief of the ZEISS optical engineering at his time, also tended to a soft verbal definition, namely [once more in free translation by us]:
"There is no exact borderline for microscopic resolution. Instead, the visibility of detail resolution is continuously decreasing within a certain range which we should call a 'critical range of the resolution'"[Michel 1950].
In German: "Es besteht keine scharfe Grenze für das Auflösungsvermögen. Die Deutlichkeit der Auflösung nimmt vielmehr innerhalb eines gewissen Bereichs, den wir den 'kritischen Bereich der Auflösung' nennen wollen, allmählich ab."

Gerhard Göke, a more modern author and writer of a prolific microscopy textbook is supporting this line of argument as well, by using "upper" and "lower" resolution pairs in his monograph [Göke 1988].

When looking at fig. 1, 2 and 3 in their relationship it becomes obvious that all three distances marked in the schematic cross-section of fig. 3 can be more or less "seen" (in fig. 1). As a consequence it does make sense to stick to a conservative microscopic resolution limit of "circa 0.2 µm" but it doesn't make sense to fix it at a precise numerical value of "200 nm". Instead it might be somewhere in the range between ca. 130 and 190 mµ.

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Fig. 2: The well-defined fine structure of three computer storage media: CD, DVD and Blu-Ray. From this image it becomes clear that all three of them can serve as high precision "nano rulers" and light microscopic test specimens at the limit of light microscopic resolution.
Image source: Wikimedia Commons (Cmglee - Own work, CC BY-SA 3.0, Link)

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Fig. 3: Schematic Blu-ray cross section on the basis of fig. 2. Now, what do you think - which of the three distances marked in red can can be actually perceived and resolved in fig. 1? We think, all three of them, more or less. This would imply a microscopice resolution limit well beyond (below) 200 nm.

Now we might argue in the direction of either of those three lines. But still there can be no doubt that the Blu-ray fine structure is well within the reach of the classic light microscope and that all publications stating the opposite are (slightly but definitely) wrong.


It might help to have a closer look at the original German literature:

Ernst Abbe: Beiträge zur Theorie des Mikroskops und der mikroskopischen Wahrnehmung. In: Archiv für mikroskopische Anatomie. Volume 9, Nr. 1, December 1873, p. 413–468 [quoted on the basis of the German Wikipedia chapter "Auflösung (Mikroskopie)"]

Kurt Michel: Die Grundlagen der Theorie des Mikroskops. p. 211. Stuttgart 1950.

Gerhard Göke: Moderne Methoden der Lichtmikroskopie. p 53. Stuttgart 1988.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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