Blu-ray^TM micro structures resolved - in an ordinary light microscope!

In our previous issue we had discussed the size of the Corona virus, whether it might be visible in a light microscope - well, agreed, as a dot, not as a full-blown image. As DIY Corona virus microscopy was deemed to be slightly unadvisable we were looking out for an object of similar size - accessible for everybody. And in fact we found one: the beloved Wikipedia names Coronaviridae with typical sizes in the range between 120 and 160 nano meters. This can be perfectly mirrored by the fine structure of a Blu-ray^TM disc (as a single pit of a Blu-ray has a similar size as a Corona virus):

Fig. 1: The fine structure of the CD, DVD and Blu-ray, as pictured by the Wikipedia.
Image source: Wikimedia Commons (Cmglee - Own work, CC BY-SA 3.0, Link)

When looking into the respective textbooks and articles one might end up not too optimistic. Let's have a look at a (translated) statement from the German microscocopy magazine MIKROKOSMOS:

"Even with an 1.4 apochromatic objective one will achieve thoroughly true object representations only when the structure under investigation is at least 1 µm in size. Under less favourable conditions this limit can be even 4 µm ..."

Source: [Krammer 1979], sloppy translation by ourselves, sorry

With respect to fig. 1 (a Blu-ray pit ranging at about 130 nm in diameter) one might conclude from the citation that the light microscope would be far off the necessary requirement. But we should not underestimate our classical light microscopes and hope that they might crack the alleged resolution limit of the ordinary bright field light microscope! In any case the Blu-ray is readily available for everybody and its structures are clearly on the fascinating borderline of light microscopic resolution. As a consequence the Blu-ray disk can be considered as the ultimate test specimen for high-res light microscopy. In most cases there will be no need to buy an expensive dedicated test specimen with similar specification. Instead, the Blu-ray disk will make up a bargain test specimen in case you would like to compare your own microscopic capabilities, equipment and results with anybody else on earth!

There is no need for further discussion if you should have some personal expertise in practical microscopy and the necessary equipment at hand. In order to keep our own experimental investment as low as possible we looked out for a low priced Blu-ray disk, ending up with a "King Maker" disk, at a cost of about 3 US $, (i.e. 50 Cent for each lethal weapon illustrated on its cover).
There was definitely no need to check the DVD content. Instead we used a knife to cut out a rectangle with the approximate size of a microscope slide. There is a thin (0.1 mm) protective foil above the pits and lands which has to be stripped off so that our microscope can actually come close enough to grasp the fine details. We were using an ordinary bright field light microscope, no tricks and no frills, no STED or similar refinements - but instead oil immersion and raking light. Just have a look at the result:

Fig. 2: Light microscopic representation of the lines (with pits and lands) on a Blu-ray disk. The red window on the left was magnified by a factor of 2 for increased clarity.
The smallest structures on this image have the size of a typical Coronavirus!
Technical annotations: we measured 31 lines per 10 µm which is in perfect concordance with the dimensions shown in fig. 1. The equipment used was vintage, definitely on the modest side: an old Meopta DN 816 microscope stand, fitted with a LOMO HT-11 trinocular head; a 90x/N.A. 1.30 APO oil immersion objective; an Abbe type condenser (three lens system, aperture 1.4, with an iris diaphragm that can be decentered); ordinary white light was provided by a low voltage "Köhler" bulb system (LOMO); Leitz Orthoplan 10x eye-epiece as a projection eye-piece, a few M42 distance rings and on top a SONY NEX-5N digital camera. As requested for highest resolution tasks, both the objective lens and the condenser top side had to be rinsed with immersion oil. The condenser iris was decentered and optimized in orientation until the best resolution was obtained. Please note that we did neither use an expensive high class N.A. 1.4 objective nor UV light nor dark field. The image shown is unretouched, including some dust on the CCD ...

We tried to find similar images by other microscopists in the internet but were not successful on first try. But in the end we came across a further, earlier light microscopic Blu-ray image by an other amateur (Jean-Marc Babalian, photography.net, in 2017). So there can be no doubt that our photomicrograph image is not a Fata Morgana and that the Blue-ray can be actually conquered by the light microscope! We will discuss this in appropropriate depth, with further technical explanations and source citatations in the upcoming magazine.

Stay tuned!

Practical hints for those among you who would like to give it a try

It is advisable to begin with a CD, to continue with a DVD and finally try to master the Blu-ray disk. The following advice is referring to the use of ready-made commercial media media only (not for those burnt by yourself).

The CD (easygoing!) has its metal layer with the pits and lands on the side with the label inscription, directly beneath the cover print (cf. fig. 1). As the thickness of the CD is about 1.1 mm it is still possible to study the pits and lands from below, through 1 mm of transparent polycarbonate. But of course this is far from ideal. Alternatively one can try to use much light in order to look through the label layer, coming from the top side. With a bit of luck the pits can become visible already under a 10x objective. Of course you will get dramatically better results with higher resolution objectives. The following image (fig. 3) was made by means of a high-res 90x N.A. 1.3 oil immersion objective:

Fig. 3: For comparison. The light microscopic image of a Computer CD - easy!
Did you see it under your own microscope? Excellent!

The DVD (medium difficulty) has its pits and lands positioned between two 0.6 mm polycarbonate layers. In order to come close one has to break the disk and look out for some laid open metallic layer in the fragments. The DVD pits can become visible under a 40x/0.65 objective. But, as with the CD one will see more by means of the oil immersion objective.

Fig. 4: For comparison. The pits of a computer DVD as seen under the light microscope. Image taken by means of a high-res oil immersion objective.
Did you find this structure under your own microscope? Bravo - you are making progress!

The Blu-ray (difficult!) is easier to access but much more difficult to resolve: the pits are positioned on the underside, covered by a 0.1 mm protective layer which an can be torn off. Okay, also this procedure is definitely destructive. So you won't be able to watch all the brutalities contained in the Blu-ray video afterwards (which might be more prudent anyway). Good luck!

Literature

Kurt Krammer: Zur Deutung der Diatomeen-Feinstrukturen im Lichtmikroskop.
Mikrokosmos 68 (1979) p. 66-71.

© 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.

Main Page