A NEWBIE PLAYS WITH CROSS-POLARISED PHOTOGRAPHY
Ever since I was a tyke, I’ve been fascinated by the stars. Age 7 or so, I did a school project about the Solar System – complete with crayon-coloured planets, carefully cut to scale from cornflakes boxes. For my efforts I was presented with a copy of Patrick Moore’s “Observers’ Guide to Astronomy”, which I still have to this day. It was the beginning of a life-long relationship (with the stars – not with Patrick, obviously).
More recently I began collecting meteorites. I’ve seen Mars, Vesta, and the Moon through telescopes – the idea of actually holding a piece of them in my hand still ranks as one of the coolest things I can imagine. I’ve done a bit of astrophotography, and am a keen amateur photographer. Put all these interests together, and you can imagine the ‘Wow!’ I felt when I first saw came across Tom Phillip’s stunning images of meteoritic thin sections. I gotta get me some of those, I thought.
I ordered a section of Dhofar 922, dug out my little toy Bresser microscope, and waited for the post to arrive. I was, to say the least, disappointed – not an awful lot to see, dull browns and greys, none of the detail or beautiful colours I’d seen. A little research explained the problem – the images were taken using ‘cross-polarised light’. I didn’t understand what that was, but it sounded expensive, and would likely need some serious optical hardware. I resigned myself to admiring them from afar.
A few months later I downloaded Tom’s thin-section screensaver, which prompted me to do a bit more reading about XPL (cross-polarised light). It turns out that the principle itself isn’t really all that difficult. The specimen is placed in the optical path of a microscope, as normal; the clever bit is that a layer of polarising material is placed above and below the slide.
A polarising filter blocks all the light passing through it, except that which is aligned in one particular direction. Imagine running a string through a gap in a picket fence, the far end tied to a tree, the other in your hand. If you wiggle the string side to side, or top-left to bottom right, the waves in the cord will get as far as the fence struts and stop. If you move the string up and down, on the other hand, the waves will happily pass through to the other side. In effect, the filter does the same with light.
Normally if you put two polarisers at 90 degrees to each other, they will pretty much block all light – the first cuts out all but the ‘up and down’ waves, and the second cuts out even those. (You can try it with a couple of pairs of sunglasses – just rotate one lens in front of the other). But with a thin section in between the filters, the minerals in the specimen change the polarisation of the light so that most does, indeed, pass through. The composition and orientation of the minerals determine the nature of the transmitted light, which means components of the rock with different makeup are clearly identified at the eyepiece.
Hmm. That doesn’t sound too hard, I thought – if I could get some cheap materials I might have a go at this…
I wasn’t prepared to spend a lot of money – not least, because I had no idea if it was even possible with simple components. Also, I spend too much on meteorites as it is – if I become addicted to thin sections, it’s not going to get any cheaper!
I found a UK supplier of inexpensive polarising film – Middlesex University. They do a lot of science kits and components, and are well worth a look for all home hackers.
They sell 48mm (about 2”) square film for just over £1. It’s cheap, and it’s not remotely optical grade, but for a proof of concept it would be fine. I ordered a few sheets.
As I’d long since sold my Dhofar TS, I also needed to get my hands on a specimen to study. I spotted a nice piece of teschenite on eBay – good Scottish rock! – and it was delivered within 24 hours.
I took out the two most badly scratched pieces of polarising film – they’re quite soft and easily damaged – and put the others away for safe keeping.
The first thing I tried was my QX5 USB digital microscope. It’s cheap and cheerful, but I’ve taken some nice pictures of chondritic material with it, and thought it might be suitable for the TS. Unfortunately, with even a single piece of film in place, the output was essentially black. I dismantled the camera head to remove the filter before the CCD, but this didn’t help – it was probably just an IR blocking filter. It’s possible there’s some polarising going on in the optical train; alternatively, it may be that the illumination just isn’t sufficient to overcome the inherent loss of light through the polarising film.
Although disappointed I couldn’t use the ‘all in one’ microscope with camera, I thought perhaps I could do something with the little Bresser. I placed a slide between two pieces of film at right angles to each other, and mounted it on the stage. Even before I went to the eyepiece, I could see little sparkles of colour on the surface of the slide! Peering my eye down to the scope, I focused up, and was delighted to see something that looked a lot like the pictures I’d been admiring on the web!
I got quite lost panning around the section; long, thin white crystals were set at various angles, like miniature steel girders. Ruby reds and turquoises, yellows, smooth beads and ragged crystals – lovely! The problem with this was as I was moving the slide and polarisers around on the stage, the film was getting quite badly scratched. Worse than that, it was starting to delaminate (I mentioned it wasn’t optical grade, right?) Now that I had the principle, I decided to make a ‘fair’ attempt.
The Bresser has a little filter wheel built in to the underside of the stage, containing red, blue and yellow filters, as well as 3 aperture stop holes. One of these was very close to full aperture.
I carefully cut a piece of undamaged polarising film to size, and glued it over the hole. The piece was aligned so that it would be parallel to the slide when the filter was in position.
Next, I next cut another piece of film, slightly narrower and smaller than a microscope slide, and glued this to a clean slide. I cut it so that in position it would be at right-angles to the film under the specimen – ie, to maximise the relative angle of polarisation.
Professional setups have a rotating plane to allow the polarisation to be continuously varied, but 90 degrees was fine for my purposes. Finally, I placed the filter slide on top of the thin section – film face down against the specimen – mounted the two slides on the stage, and focused in.
I’m pleased to say it worked a treat! The added advantage with this setup is that it’s easy to switch between normal and XPL illumination, by just moving the filter wheel between ‘clear’ and ‘filter’ positions.
The next step was to get some photos – it’s all very nice cooing at your desk, but the idea was always to take pictures.
My first approach was to use a modified webcam. I use this for astrophotography – it’s just a Phillips webcam with the case partially dismantled and the optics removed, attached to a 1.25” adapter for fitting into an eyepiece tube.
Unfortunately I didn’t find this at all satisfactory. I couldn’t find a suitable point to introduce the adapter to the scope tube; I could mount it directly above the eyepiece, but the image was poor, and I struggled with the colour balance of the webcam (a problem I don’t see in astro work). Perhaps with a custom adapter tube it would work, but I didn’t have anything suitable to hand.
I fell back on another technique from astronomical imaging – afocal eyepiece projection. This is a grandiose term for what amounts to pointing a camera into the eyepiece. While not a permanent solution, it did at least provide some images, and a few minutes in Photoshop produced examples of white and XPL images side by side:
Ok – they’re not going to win any prizes. But they do show that the basic technique can be achieved pretty easily, with very inexpensive materials. The scope originally cost about £40 including a set of blank and prepared slides, cover slips, etc. The polarising film cost a few pounds, and the camera was just a standard digital compact. It’s also worth pointing out that even with this crude setup, the visual results were very pleasing – much clearer than the photos above appear.
Next steps? Well, I’m sufficiently encouraged that I’ve ordered a second-hand Olympus microscope and some proper glass filters. I need to find some meteoritic sections to examine, and look forward to seeing what a step up in quality can do.
Oh, and talking of quality… about two days after I took these images, I realised the polarising film wasn’t being scratched, and wasn’t delaminating. I just hadn’t realised that it came with protective plastic film on both sides. Removing this led to somewhat clearer images, and a slightly redder face!
The latest version of this little microscope can be found here: Bresser Junior Microscope Biotar 300x-1200x