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BluRay scanning microscope

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This is a suggestion for a possible hack.

BluRay players are astonishing pieces of engineering - they focus a laser beam down to less than 500nm, and have the transport mechanism to align with the spiral of data on the disc at a pitch of only about 300nm. All in all, it's quite fantastically precise, yet BluRay players are dirt cheap because of mass production.

So, here's the proposed hack to convert a BluRay player into a high-resolution scanning microscope.

The hard part will be producing a sample-carrying dummy disc. It will need to be a CD-sized disc of glass, only 0.1mm thick. Fortunately, such glass exists - it is widely used for microscope cover-slips. I am pretty sure that the glassmakers would be able to supply this is CD-sized discs.

Next, take a regular CD and punch a bunch of holes in it, maybe 1cm in diameter. Then bond the glass disc to the holed CD. The CD provides structural support to the glass disc. Samples are placed in the holes, in direct contact with the glass.

Now comes the electronic part. You need to be able to access the read-head drive mechanism (can't be that hard to hack), and to collect the raw signal coming from the read head.

By accessing the raw signal from the read head, what you have is an optical scanner with a resolution of better than 500nm, which should be able to image the samples placed in the holes in the CD (through the 0.1mm glass layer). Add some fancy software and - gadulka! - a high-resolution scanning optical microscope.

I haven't yet figured out if the optics in a BluRay player are confocal or not, but I believe they are (since BluRay discs have more than one data layer). If so, and if the Z-axis control can also be hacked, then you actually have a scanning confocal microscope, capable of generating 3D images of samples.

One drawback is that you'd only be working at a single wavelength (unless you did a lot more hacking); but even so, a high-resolution scanning confocal microscope built from a cheap BluRay player would be pretty amazing.

MaxwellBuchanan, Oct 14 2016

Someone has already... https://hackaday.io...g-laser-microscope/
...been there, done that. [MaxwellBuchanan, Oct 14 2016]

Metalens works in the visible spectrum, sees smaller than a wavelength of light High efficiency ultra-thin planar lens could replace heavy, bulky lenses in smart phones, cameras and telescopes https://www.seas.ha...wavelength-of-light
[beanangel, Oct 19 2016]

Blu Ray Lens Control http://www.nt.ntnu..../ecc03/pdfs/456.pdf
[bs0u0155, Oct 19 2016]


       perhaps you could get the right depth of field with a microtome slice of something, then polish it down to the right focal plane for the CD   

       I think it would work.
beanangel, Oct 14 2016

       Do you mean put a whole slice in the machine, instead of the glass-windowed disc? Possible, I guess - but the windowed disc would be more versatile.   

       One limit might be centrifugal forces at normal disc- playing speeds; however, there'd be no real need to spin the disc that fast, and in any case fixed samples wouldn't be a problem.
MaxwellBuchanan, Oct 14 2016

       Oh arsely arses. This has been done <link>.
MaxwellBuchanan, Oct 14 2016

       Yeah, but you get credit for having thought of it second.
normzone, Oct 14 2016

       Story of my life, [norm], story of my life.
MaxwellBuchanan, Oct 14 2016

       Hmm, so I disappeared down the rabbit hole of Blu Ray optics. So could this work, could you make a microscope by just having a special disc and getting access to the output of the photodiode?   

       Well, maybe sort of a bit. The drive has rudimentary focus control for the disc, it only cares about pit Vs no pit representing 0&1 so working out which is 0 and 1 is relatively easy and focusing on the plane of the disc is also easy. You could retain this system so that it auto focuses on the plane of the disc. Problem here is that you would have to get your sample EXACTLY in that plane. At this point the holder becomes impossible to make practically. You'd need a much better microscope to make sure it lined up OK.   

       This is where you start chucking things away and replacing them with real microscope components, like the guys on Hackaday did. The first thing they chucked was the whole disc spinning and head movement system. Replaced with a "high precision x-y-stage" I don't know which one they have but they're all in the thousands of $. This means you can move the sample in a lovely X Y sweep whereas the disc system would have created a series of arcs.   

       Once they have control of the X Y movement, mechanism of checking focus then they're off to the races. Except they're not. What they've made is a polarized reflective sort-of-confocal microscope. They can scan across their sample and assign values to pixels based on the reflected light received by the photodiode. To separate the excitation light from the reflected light they have a polarizing beam splitter. So they shine light of one polarity and look for another.   

       This creates problems, edges and angles really mess with linearly polarized light. So is there no reflection? Or has the shape of your sample messed with your polarization? They thought of that though, that's what the 1/4 wave plate is there for, changes the linearly polarized light to circularly polarized light. The sample reflects the circularly polarized light but in the opposite handedness the plate changes that back to linear and you are good.   

       The problem with the sample creating artifacts remains however. What if your sample does not uniformly reflect 405nm light? You will never know the difference between something that doesn't reflect back along the incident light path and something that isn't there. That and reflection is great for mirrored discs, and stuff made of metal. Not much else reflects considerable light at those scales. Cells for example are essentially transparent.   

       Next problem is with moving the sample about. I assume they are not stepping the sample while it scans past the lens, but instead just binning the output by units of time. Commercial confocals instead scan the excitation laser accross the back of the objective using unbelievably precise mirrors. The stage isn't going to be able to match that precision. The advantage though, is that you can get away with a really dreadful lens. Which is what they have. A commercial microscope essentially has the whole of the sample in focus, and the sample remains fixed relative to the objective during the scanning. This means you need an incredible bit of engineering to maintain an even field of view despite the fact that the light path to the center of the sample and the edges is different. That's where a lot of money goes. The Blu Ray lens just has to focus one pixel at a time.   

       It is a viable strategy, as long as you have a very, very good stage. You could use this to make a fluorescence microscope too, simply removing the polarizing elements and replacing with dichroics.
bs0u0155, Oct 19 2016

       metalens material should be mentioned [link] this is like a velvet of waveguides rather than a refractive optical element. much thinner, with higher resolution at the visible, and presumably UV
beanangel, Oct 19 2016

       //Problem here is that you would have to get your sample EXACTLY in that plane.//   

       But... the BluRay head has a voice-coil that constantly adjusts the head height to keep the correct layer of the disc exactly in focus. For example, the first layer is directly underneath the 100µm (0.1mm) coating of the disc face. I don't see why the same system would not keep the focus 100µm behind the face of the coverglass we're using - i.e. focussed perfectly on the specimen.   

       In other words, the player is already designed to image (by scanning) sub-micron pits with sub- micron spacing, in a plane which is a precise distance from the surface of the disc - and to do so even though the disc itself may be slightly warped and non-flat.   

       Points such as polarization artefacts and sample reflectivity are well made. But I do like your idea of making it a fluorescence 'scope instead - much more versatile. A good many dyes are excited around 400nm, as are my favourite things - Q-dots. you'd want swappable emission filters, though.
MaxwellBuchanan, Oct 19 2016

       //But... the BluRay head has a voice-coil that constantly adjusts the head height to keep the correct layer of the disc exactly in focus//   

       I suspect, and read, that this took a while to get right. I expect the reflectance values of the surface are fairly tightly controlled. As you move the lens toward the disc, you should get increasing values to a peak, then decreasing. So you can get a ball park range before the disc starts spinning. Then it's simply moving the lens if the value starts to fall off. Move it down a tiny bit and it falls further, then you need the opposite direction. If the disc is standary blu-ray surface then BOOM! A neonatal cardiomyocyte, there might be issues with that algorithm.   

       // keep the focus 100µm behind the face of the coverglass we're using//   

       The working distances actually work out OK, you need #0 or #1 thickness, but that's easy. I suspect there will be a problem with the glass though. Your light is then travelling an awkward path, Air-plastic-air-glass- sample. Bit of a Zig-zaggy mess of refractive indicies like the spoon in the half-full glass of water. It gets worse if you have an aqueous sample... this is why many confocals use water immersion lenses, they're not as good as oil in terms of sheer NA values, but the aberrations are kept to a minimum. (I'll bet the lens plastic and the disc plastic on the blu ray match) Interestingly, I don't think this matters too much, because you are viewing such a restricted volume, any aberrations wont have the same effect. I'll have to have a think about that.   

       //dyes are excited around 400nm, as are my favourite things - Q-dots. you'd want swappable emission filters//   

       Well, it starts to get tougher here. Individual fluorophores aren't as bright as say, a mirror. This is one of the reasons the laser diodes are so conspicuously powerful for a domestic device. You need a bit of brute force if you're going to fire your light through such a crappy lens and lots of other optical elements. Especially when you're dealing with a low efficiency photodiode. Your 10mW of laser is losing say 30% in the collimators and other organizing elements, so 7mW. say another 30% in the lens and getting to the sample, say 5mW, then your fluorophore will yield 50% at best, 2.5mW, Your light will be emitted in a full sphere, but you're 0.8 NA lens will only cover say 20% of that. Then Back through your optics, and filters, say 50% gone, to slam into your detector which is about 25% efficient. About 15uW, which is actually workable. With QDots it is OK, with anything else, you lose efficiency which is amplified down the chain, worse, almost everything else will bleach with that power.
bs0u0155, Oct 19 2016

       // Your 10mW of laser //   

       Milliwatt schmilliwatt. A BluRay player has about half a watt of laser. And bleaching scmeaching too. Qdots all the way.   

       Next week: how to fashion a fully-functional heart- lung machine from a rotary-dial telephone, a disposable camera and a Corby trouser press.
MaxwellBuchanan, Oct 19 2016


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