Sometimes I see something for sale, and despite not knowing quite why, know that it’ll be interesting or useful to have. So it was the other day when I saw a microscope slide for sale. This was a diatom slide made in 1950 by a chap called Horace Dall, but what caught my eye was that the slide said that the diatoms were mounted in titanium dioxide (TiO₂), and then it gave a refractive index measurement – ‘µ = 2.90’. Before I show you the slide though, I want to share an image taken of it, as it turns out it makes for some really spectacular photos.

I’ll speak more about the images from it in a minute. Back to the slide itself. Here’s a photo of the slide;

I’d not seen anything like this before, and guessed that it had been done to improve contrast of the diatoms for imaging (given the high refractive index of the TiO₂). The sample under the coverslip had a sheen to it a bit like an oil film on water. After a bit of a bidding war I ended up with it and the imaging began. As a simple set of initial images I did some brightfield and darkfield ones with a 20x objective (Olympus Splan 20x) and a 63x objective (Zeiss Neofluar Pol 63x) as I could use both of these without needing to mess around with immersion fluids. Images were captured using a white LED light, and with a normal unmodified colour camera. The colours you see below have not been modified by me, these are as captured by the camera. Anyway, less chat, more images;

The darkfield images especially were very pretty, showing a range of colours from blue through to red for different diatoms. Even the brightfield image showed the range of colours. The effects reminded me a bit of looking at mica particles which are put in cosmetics, or an oil film on water, and I presume this is due to the thin layer of TiO₂ deposited on the diatoms causing interference colours. This led me to wondering whether anything had been published on this process or these samples. I spend my days tracking down information on stuff, so set about looking for details on this, and, ……..nothing. So I approached the Facebook group of the Quekett Microscopical Club that I am a member of, and within an hour loads of useful information came back, including reference to a paper in the Quekett Journal from 1985 written by Horace Dall about making these types of slides. So, I need to learn more about searching for information….

Horace made these slides by exposing them to fuming titanium tetrachloride, which was then transformed into TiO₂. Back when I was a student in Durham I won the Tioxide Scholarship for the year during my Chemistry degree. This came with a trip around the Tioxide plant on Teesside, where I was given horror stories about titanium tetrachloride and how dangerous it is. In Horace’s paper, he mentions “These experiments were carried out only when suitable winds would quickly evacuate the fumes.”. Legend. After this he went on to develop other approaches including the evaporation of aluminium (or aluminum for some readers) in a vacuum and using that instead. This brings me to another interesting point. I searched for information on this and found a number of articles which discussed coating diatoms with metal and metal oxide films for different purposes. None of them that I have found so far have referenced Horace Dall’s work though, despite being done many years afterwards. Horace was many years ahead of of his time with some of his work.

As this process was done to improve resolution, it got me wondering whether I could try imaging with UV as my previous work has shown how this improves resolution (see here). However I assumed that the slide was made of glass, so it likely wouldn’t be feasible to image all the way down at 313nm. It did make think that 365nm may be possible. Out came the Ocean Insight spectrometer, and I measured the transmission spectra through the slide itself and then through the slide/diatoms/coverslip, and got the following;

The slide itself is glass, and the transmission drops below 350nm, down to about 290nm where it becomes opaque. The region with the diatoms and coverslip though absorbed quite a lot of light at all wavelengths, but in the UV, transmission fell down to about 320nm. As I expected imaging at 313nm wouldn’t be possible, but 365nm would be doable. The transmission curves are interesting. The coverslip is likely the same type of glass as the slide, and is much thinner than the slide. Therefore it should block less of the UV. The shape of curve is likely driven by the TiO₂ layer, as it does block short wavelength UV.

I went back to the slide and managed to find the same region I imaged with one of the darkfield images (the 63x one). This time though I imaged it in brightfield at 365nm with my Leitz 100x NA 1.2 glycerine immersion objective, and got the following image.

Just for comparison here’s a crop of the 63x darkfield image showing the same diatom.

The 365nm brightfield image has slightly better resolution than the darkfield image, which is not a huge surprise – better objective with a higher NA (1.2 vs 0.9), and used a shorter wavelength of light, but some of the structures visible in the 365nm are still visible in the darkfield visible light image.

Just for giggles, here’s a crop of the 365nm darkfield image, looking at the pore structure of the diatom.

Even with 365nm it looks as though I am getting down to well below a micron in terms of the features being resolved. Keep in mind that the slide and coverslip were glass (not quartz) and the coverslip thickness was also wrong for the objective, so although this may not be as sharp as it could have been, it is still a respectable result.

It’s funny how a single historical sample can be such a fascinating thing to image. It is all too easy to forget, or just not realise, that amazing scientists existed before the days of the internet, and it can be hard to track down their work if you don’t know exactly what you are looking for. I can feel a new paper coming on looking at some of Horace Dall’s work as he was such a pioneer, and bring it to a whole new generation of scientists. If you’ve made it this far, thanks for reading, and if you’d like to know more about this or any other aspect of my work, you can reach me here.

“Few objects are more beautiful than the minute siliceous cases of the diatomaceae: were these created that they might be examined and admired under the higher powers of the microscope? The beauty in this latter case, and in many others, is apparently wholly due to symmetry of growth…”, Charles Darwin, On the Origin of Species, 1866.

While I spend a lot of my time doing research for other people, I am also a simple scientist at heart. Like many people I appreciate beauty in art, but for me, I tend to seek patterns and structure in what I am looking at. Something that gives me both beauty and structure is looking at diatoms under the microscope. These are the solid structures left behind when certain species of algae die off and they are beautiful structures. I’ve not graduated to making my own microscope slides just yet, so tend to keep an eye out for second hand ones on eBay. Some can be had for a few GBP (although for big arrangements you can pay thousands of GBP, especially from certain makers as there is a big collectors market) and they make for fascinating viewing under the microscope at the end of a long week – microscope therapy as it were. Today I want to share with you some images from a couple of slides which arrived this week.

Two slides arrived this week – one is an antique slide made by John Barnett which has an arrangement of 4 examples Eupodiscus radiatus diatoms, the other is a 7 form test slide made by Klaus Kemp. Low magnification images (using an 20x Olympus Splan objective) of the samples on the two slides are shown below.

The images above where made using 405nm light on my UV microscope. How on earth anyone is able to move an arrange these things on such a small scale amazes me. But it is the structures in the diatoms that become more clearly visible at high magnification which are really fabulous. On the first slide, moving to a 60x objective (Olympus 60x Splan Apo), gives the following.

Using the 60x brings me in a little too close for this sample, but it was an objective I wanted to have a play with. Cropping the original image further, allows the structure to be more easily observed.

While the majority of the features are hexagonal, there are pentagons and heptagons spread throughout it, which allow the structure to take on the beautiful curves that it has. Not too bad for an algae.

Moving into the Klaus Kemp slide, I decided to look at the one which was third from the right (Pleurosigma angulatum). This one has one nice small features and makes of a good one to look at. Here it is, again with the 60x Olympus Splan Apo objective.

As with the Eupodiscus radiatus, the use of the 60x was slightly too big a magnification for the Pleurosigma angulatum (note to self use a smaller magnification photoeyepiece next time). However this one has some really small features, as can be seen in a cropped image below.

The repeating dot like features in the image above have been reported in the literature to have about 16 per 10 microns, and that is indeed what I see here.

I used 405nm light here, as this provides some improvement in potential resolution vs normal white light. Using 405nm gives a nice boost to resolution without all the hassle of going to full UV imaging, and it means being able to use normal slides, coverslips, objectives and condensers. However this time I didn’t quite get the setup right as there is a hotspot in the middle of the 60x images and a bit of fall off in one of the 20x ones. I actually cheated a bit as well and used glycerine as the immersion fluid for the 60x objective, rather than oil. This was due to laziness – its easier to clean off the glycerine than oil – and perhaps I sacrificed a bit of image quality as a result. I also didn’t oil the condenser to the underside of the slide – due to not wanting to make a mess.

Before I leave todays post, I wanted to share images of the slides themselves. First the antique John Barnett one.

The John Barnett slide is actually paper wrapped, and the diatoms are in the middle of the glass in the centre of the slide. They are of course not visible at this scale. Next we have the Klaus Kemp slide.

The Klaus Kemp slide is a simpler affair, and the diatom are inside the small ring in the middle of the slide.

The world of the small can be a beautiful place and for a relatively small outlay is within the reach of anyone with a simple microscope. I’m no expert on diatom imaging (check out the Photomacrography forum for some amazing images) but I just find it endless fascinating to look at them. Thanks for reading and if you’d like to know any more about this or my other work, I can be reached here.

Some photos are just pretty, some are challenging, others are shocking. In the world of research an image, if captured in a controlled way, can provide as much information as any other form of measurement. In some recent work, I presented imaging of a diatom at 313nm on my custom made UV microscope (see here), and one question which came from the work was how could I quantify the resolution of my objectives at different wavelengths of light, including well down into the UV. This brings me to the topic of today’s post – resolution testing of my microscope using the Newport Highres-2 high resolution microscope test slide.

To test my microscope I knew I was going to need a high end test slide – something with very small features. UV microscopy offers improved resolution compared to using visible light, and looking at my previous diatom image taken with the Leitz 100x NA1.2 UV objective at 313nm it seemed like I was resolving features down to about 200nm if not below. I also needed the slide to be made from quartz of fused silica rather than glass, so it wouldn’t block UV. As a final requirement, it needed to be able to be used with immersion oil or glycerin and a coverslip without damaging the test target. Taking all this into account I decided on a Newport Highres-2 positive slide (opaque lines on a clear background), and having spoken with them it looked like this would indeed survive being using with immersion fluid and a coverslip, and would be UV transparent. So I went ahead and ordered one.

The test slide is a USAF pattern, which has lines of different widths arranged on it. An image of the full area of the test slide taken in visible light using a 4x objective on my microscope is shown below.

The pattern is chrome deposited onto a quartz substrate, which is mounted into the middle of a 3″ by 1″ aluminium slide as shown below. You can just about see the test pattern in the middle of the diamond shaped piece of quartz in the middle of the slide.

Overall it looks to be well made and with a good container which will hopefully keep it safe.

Back to the test pattern and resolution testing. The pattern is presented as a series of Groups from 4 to 11, with Group 4 having the largest lines, and Group 11 the smallest. Each group is further subdivided into 6 Elements, which again get smaller going from Element 1 to Element 6. The widths of the different lines are given on the Newport site – here.

With using the Leitz 100x NA1.2 UV objective for imaging diatoms, it looked like I was seeing features down to about 200nm if not a little smaller, so the key parts of the test slide which were of interest to me were Groups 10 and 11 as these have the smallest features. Looking with 313nm gave the following image (cropped slightly just to show Groups 10 and 11). Images were taken using a monochrome converted Nikon d800 from MaxMax, which I can use down to 300nm. I also took the image as a Raw file and processed it in Monochrome2DNG to try and get a little bit more from it as the camera sensor has had the Bayer filter removed.

All Elements in Group 10 were clearer resolved using 313nm light. These lines go from 488nm down to 274nm wide. In Group 11, Elements 1, 2, 3 4 and 5 are clear to me, but Element 6 I’m just starting to struggle to break out the individual lines, although they can just be seen. In the image Group 11, Elements 4, 5 and 6 are identified by the dots instead of numbers. Keep in mind that Group 11, Element 6 has lines which are only 137nm across (Group 11 Element 5 is 154nm and Group 11 Element 4 is 173nm). So it does look as though that objective will certainly resolve features down to below 200nm and perhaps around 150nm. Put this into perspective – a single Covid-19 virus is about 100nm across. Not too shabby for a home built microscope.

Just for comparison here are the images taken using 365nm and visible light (546nm) captured at the same time.

As expected, as the wavelength gets longer the resolution gets worse. Physics in action….. The quest for resolution was reason why microscopists started looking at UV transmission microscopy back in the early 1900’s. They used even shorter wavelengths of light – 254nm and 276nm – which makes me wonder how much more resolution I could get out of my microscope if I used these more extreme UV sources.

Imaging to produce nice photos is one thing, but there are times when we need to be able to put numbers against the things we image, to be able to quantify the things we are seeing. With the Newport Highres-2 resolution test slide, I have been able to do just that with my UV microscope and now have a way of being able to determine the resolution which can be achieved with different objectives and at different wavelengths.

Thanks for reading and if you’d like to know more about this or any other aspect of my work, I can be reached here.

Having the right tool for the job. It is important when doing measurements that we have access to the right techniques (and of course know how to use them – a fool with a tool is still a fool). When characterizing optical filters, I’ve normally just done transmission measurements. However with some filters they have dichroic coatings present on them which can reflect unwanted wavelengths of light even before the light passes through the filter. While I had tried doing some basic measurements of reflectance from these types of filters before, the results were never optimal. I recently decided to bite the bullet and order a proper reflectance probe so I could do this type of work in a more controlled manner. Today I’ll be sharing some initial data from it.

The probe I went for was one by Ocean Insight. They worked with me on the design which is always good when a company does that, and it arrived about 3 weeks after ordering it. I needed one which would work from the UV (250nm) to the IR (1100nm) to match the spectrometers and light source I have, and because those are the wavelengths which are most relevant for photography with normal cameras. I also needed it to be the right diameter for me to use on my Ocean Insight RTL-T stage. The incoming light illuminates the sample through 6 fibers arranged around a 7th one in the middle which is the one which goes to the spectrometer. I have an Ocean Insight FX spectrometer and an STS-NIR one as well, so can measure between 250nm and 1100nm by using the two devices.

For a test filter I use a Baader U, which is a UV pass filter commonly used for UV photography. It has good blocking of unwanted wavelengths, and importantly for me, dichroic coatings on both surfaces which look different to each other. On mine one side is red/pink, and the other green/gold and I have remounted it in a 49mm filter thread for using on my camera lenses (see below).

What do the filter coatings look like in terms of reflectance? Combining the measurements from both sides using both spectrometers gives the following rather complicated graph;

It’ll be clearer to show each side individually. First the red/pink side;

And the same from the green/gold side;

There are some big differences in terms of the reflectance from the two surfaces of the filter, especially in the red/IR region. The red side has the better blocking from 600nm to 1000nm, while the green/gold side has better blocking above 1000nm. It is common practise when using these filters for UV photography to reverse the filter compared to how it is normally supplied. This is because the mounting on a camera lens would naturally reverse it compared to mounting it on a telescope (one uses a female thread the other a male). In UV photography when using it in a camera and not a telescope it is recommended (by fellow UV photographers) to mount it with the red/pink side facing the subject. This makes sense as it presents the side which reflects the most red/IR before it can even enter the bulk interior of the filter.

The filter itself is used for UV photography, and has good transmission between about 330 and 400nm. The coatings on both sides are designed for this, as they both have very low reflectance in that region;

I have deviated slightly from the normal approach for using this probe, and will explain why now. Normally when using this probe, Ocean Insight would recommend using a high reflectance specular reflection standard for calibrating the spectrometer before the measurement. However this does vary in reflectance across the wavelength range, and I could not justify the cost. Instead I bought two mirrors from Thorlabs – a UV enhanced aluminium one, and a protected silver one. I use the UV enhanced one for UV measurements and the protected silver one for visible and IR measurements. They offer very similar properties to the Ocean Insight specular reflectance standard and were about 20% of the cost. If I needed a NIST traceable standard I’d invest in a proper one, but for my work, these mirrors will be sufficient. It should be remembered though that the % reflection scores are vs the mirror, and that does vary slightly in reflectance at different wavelengths. This is one reason why full experimental details are needed whenever reporting data.

As always, thanks for reading, and if you’d like to know more about this or any other aspect of my work, you can reach me here.

When it comes to testing equipment, I like to push it and see what it is capable of, trying to find the limits of what it can do. So I come to the latest diatom image taken on my UV transmission microscope. This one was imaged using 313nm light, and using a Leitz 100x NA 1.20 glycerine immersion objective (mentioned previously here).

The image above was of the central part of a diatom and has been reduced in resolution for sharing as the original (taken with my monochrome converted Nikon d800 from MaxMax) was 7360×4912 pixels. Going closer into part of the original image gave the following.

The ’round’ features are about 500nm across and the gaps between them are about 200nm wide. For a home built microscope, I’m very happy with the resolution, and it seems to be approaching the limit predicted by Abbe (using 313nm light and a NA 1.20 objective, the theoretical resolution should be about 130nm). Keep in mind here that every aspect of the imaging setup needs to be optimized to get the best image. The condenser I used was only NA 0.85, while the objective was NA 1.20. This will reduce the maximum obtainable resolution. At these types of extreme magnifications, every little thing matters. Is there more resolution to be had from this setup? Maybe, but I think not, at least not without going to even shorter wavelengths. The microscope should be usable down to around 250nm, but the camera, light source and filters would all need to be changed for that so at the moment it is not a viable option.

Here is a lower magnification image showing the whole diatom (taken at 313nm with a Zeiss 32x Ultrafluar).

Thanks for reading, and if you’d like to know more about this or any other aspect of my work, you can reach me here.

Testing, testing, testing. As always with new equipment and methods it is helpful to test them to see what they are capable of and where their limitations are. With my UV microscope, I recently had a test slide made using quartz components and which had a variety of diatoms mounted on it. These diatoms have very small features (often sub micron in size) and are great for testing optical setups. Also, being made from quartz it is usable in the UV where glass would block the light.

Todays images were done with 3 different objectives – but before we get into the specifics of the objectives, I’m going to share one of the images produced, because, it is pretty special. This was made with the Leitz 40x NA 0.65 objective which will be discussed below and is a 313nm brightfield transmission image.

Back to the objectives. Three were used for this – a Zeiss 32x NA 0.4 Ultrafluar, a Leitz 40x NA 0.65 UV objective, and a 36x NA 0.5 Beck reflecting objective, all of which are shown below.

I normally use the Ultrafluar when I need an objective in this sort of magnification range, but wanted to check the other two out. The Zeiss and Leitz were both used with glycerine immersion fluid, and the Beck was used dry. The Beck is very different to the other two being a mirror lens, and I checked the alignment of it before using it. The diatom was imaged at 313nm, and each image is a stack of about 13 images made in Zerene stacker. Some processing of the final images was done in Photoshop, but all images were treated similarly. They have been reduced in size for sharing online.

First the 32x Ultrafluar.

Next with the 36x Beck reflecting objective.

And finally with the 40x NA 0.65 Leitz UV objective.

Obviously the field of view is different with the three objectives as their magnifications are not the same. The 32x Ultrafluar image is certainly very good, especially for a NA 0.4 objective. The 36x Beck reflecting objective image is lower contrast and not as sharp, but still shows a lot of detail. However the star of the show here was the Leitz 40x NA 0.65. The final image it produced was extremely sharp and full of detail.

Not much is known about the Leitz 40x UV objective, certainly nowhere near as much as about the Ultrafluar objectives, however I have written a bit about it here. To try and get a bit more detail on the resolution it was producing I did a crop of the original image in ImageJ and took some measurements. The area assessed is shown below.

The yellow line in the image above is 449nm long, so it looks like this objective is easily resolving features below 0.5micron in size. Abbe’s resolution equation says that this objective should have a resolution of 241nm when used with 313nm light, and from the looks of this image it probably isn’t far off that. Makes me wonder what my Leitz 100x NA 1.20 objective is capable of, but that will have to wait for another day….

I’m certainly no diatom expert, but they are beautiful structures and make for great photos, so I will be doing more in the future. It’s been great to finally be able to test some of my UV objectives with samples like this, and to be able to produce some real life images with these historic optical pieces. Thanks for reading, and if you’d like to know more about this or any other aspect of my work, I can be reached here.

Time for a bit of historical microscopy today. I’ve previously written about the Zeiss Monochromat objectives – quartz objectives originally produced in the early 1900s and designed to be used at 257nm or 275nm (see here). Now the microscope is up and running for UV work, I thought I would try out one of these historical pieces and see what type of image it can produce.

The objective of choice was a 6mm NA 0.35 Zeiss Monochromat designed for 160mm tube length and for imaging using 275nm light from a Cadmium arc source. The objective is shown below.

This objective is made from quartz lens elements and has an RMS thread for mounting it. It is also designed for 160mm tube length microscopes which means it is ideal for use on my modified Olympus BHB. Here it is mounted up and in use for imaging a diatom slide.

What about the image quality? To test this I did a single picture of a diatom at 313nm with it and compared it with an image produced by a 32x NA 0.4 Zeiss Ultrafluar which is what I use for my UV imaging research. Firstly with with the 6mm NA 0.35 Zeiss Monochromat.

And now a comparison shot with the 32x NA 0.4 Zeiss Ultrafluar at 313nm.

The 6mm Monochromat had almost exactly the same field of view as the 32x objective. It produced a usable image at 313nm, although was not as sharp as the Ultrafluar. This is not a huge surprise – the Monochromat has only quartz elements (the Ultrafluar is quartz and calcium fluoride), it has a slightly lower NA, and is also being used at a wavelength that it wasn’t designed for. It is also a much older design and construction. The Monochromat also had a huge focus shift compare to visible light which the Ultrafluar does not. Again, this is not unexpected, but must have made working with it in the days of film challenging to say the least.

Overall, it was amazing to use such a historical piece in the manner in which it was originally designed for (bearing in mind the wavelength used wasn’t what it is specified for). Over 100 years later and it is still capable of producing images. Thanks for reading, and if you’d like to know more about this or any other aspects of my work, I can be reached here.

Over the past couple of years I have been building a UV transmission microscope, with the main idea being to use this in my sunscreen research (a brief summary of the key points of the build are given here). I’m now using the microscope to look at sunscreen emulsions in the visible, UVA and UVB regions, and I’m now starting to publish the work as well.

However going back over a 100 years, UV transmission microscopy was done in the quest for resolution – as the wavelength is reduced then for a given numerical aperture (NA) the resolution increases. Therefore moving to the UV offered the chance for more resolution.

I thought about using diatoms as a way of testing the resolution of my system. But the issue I had was the normal slides are made from glass and have mountants, both of which block the short wavelength UVB. After a look around I found someone who was willing to make a diatom slide for me, but using a quartz slide and coverslip (so that it wont block the UV) and the diatoms were mounted with gelatin. As it turned out the chap making it thought it could be better (as the mount is too visible) but I still wanted to try it out.

After a lot of experimenting on his behalf (for which I will be forever grateful) the first test side arrived recently, and I wanted to share some very preliminary images. This was of one of the diatoms on the slide.

These were done with with my UV transmission microscope, and with a 32x NA0.4 Zeiss Ultrafluar objective. Images captured at 546nm (visible), 365nm and 313nm using a monochrome converted Nikon d800 camera. Images processed similarly, and collected as stacks in Zerene. They have been reduced in resolution for sharing here.

First, at 313nm in the UVB region.

Next at 365nm in the UVA region.

The finally in normal visible light at 546nm.

The increased resolution which comes from using 313nm is quite dramatic. Remember, the same objective (a 32x NA0.4 Zeiss Ultrafluar) was used for all these images. It would be interesting to replicate some of the early work using 276nm and 256nm, but that would require different light sources and filters to the ones I have now. However as they are even shorter wavelengths, there should be more resolution to be had.

The objective used for the images above (a 32x Zeiss Ultrafluar) had a relatively low NA 0.4. In theory, there would be more resolution to be had with a larger NA objective. I do have a 100x NA 0.85 Zeiss Ultrafluar, so took some images of a different diatom on the slide. Firstly at 313nm using the 32x objective (single shot, no stacking, hence not everything is in focus).

Then at 313nm with the 100x NA 0.85 Zeiss Ultrafluar.

And then finally at 546nm with the 100x NA 0.85 Zeiss Ultrafluar.

As with the first test, the difference between the 313nm image and the 546nm one is striking. Looking at Abbe’s calculation for resolution, with the 100x NA 0.85 objective, with 546nm light the theoretical resolution is 321nm while with 313nm light it is 184nm. The larger ‘circular’ features in the middle of the images above are of the order of 2 microns across. The smaller ‘dots’ around the outside of the central features are about 500nm across. Theoretical limits for resolution are one thing, but in practice the other components of the optical setup will also influence it, as such I am not surprised that it looks like my limit to resolution is above the theoretical limit. In a perfect world I would get some form of calibrated test chart for ultra high resolution microscopes, but that will have to wait for now as they typically cost about 1000GBP.

I hope you have enjoyed this little journey into the micro world, and an evaluation of how the light we use to look at things can influence what it is that we see. Thanks for reading, and if you’d like to know more about this or other aspects of my work, I can be reached here.

‘Jon, you should always be open to new experiences’, is something I get all the time, and to be honest new experiences often make me nervous. New science though, now that is a different matter, especially when it leads to experiments and perhaps new toys to work with. For a while I’ve been wondering about getting a camera for my microscope which wasn’t a SLR or mirrorless one, but it needed to be something I could make work in UV down to 300nm (and ideally below). After consideration of various options, I approached Altair Astro in the UK about their 26M monochrome APS-C camera.

This camera has a BSI Sony IMX571 monochrome sensor, and has the option of cooling as well. Importantly for me, they are UK based, and they were willing to work with me on some adaptations. The window on the camera which is there because of the cooling setup would block the UV so I ordered one without the window and am having a fused silica replacement made instead by one of my regular optics suppliers – UQG Optics. Also was pretty certain that the sensor on the coverglass will be UV blocking (like most modern sensor coverglasses) and be no good down at 300nm, so the plan will be to send it away to my friendly camera modifier MaxMax and get the coverglass replaced with fused silica. I will however be keeping the microlenses on (bit of an experiment, hopefully they will still work in the UV – my experiments say they should, but not everyone is convinced).

I went with the APS-C one for a couple of reasons. Cost – the full frame sensor version was over twice the price. Risk – given it will have the coverglass removed this is a dangerous process and doesn’t always work first time. Damaging this one will be a lot cheaper to replace than the full frame one. I would have loved the 61Mp fullframe one they do (Sony IMX455 sensor), but it was overkill for my microscopy work, and I just couldn’t justify it at this stage. I’ll also be using it on the Questar UV telescope I’ll be getting later this year (from their UK dealer, The Widescreen Centre) and APS-C will be better for that as well.

The camera arrived the other day, along with a nice adapter for Nikon lenses and I managed to get it up and running (steep learning curve as I have never used a camera like this before). Some shots of the brief setup and test with the Rayfact 105mm lens (the picture of the garden was taken with the Baader U UV filter on the lens, through the glass of the kitchen window).

The next stage was to understand more about its spectral response in its unmodified form. For this I used the approach I developed for looking at cameras response in the UV (see here for background). As a reference camera I used my monochrome converted Nikon d850 which also has a BSI sensor. Normalizing and plotting the curves from the two cameras gave the following.

The two cameras have very similar response curves between 400 and 700nm as would be expected. However below 400nm the 26M response drops rapidly, being essentially non-responsive below 340nm. This can be seen more easily if I plot the response of the 26M vs the monochrome d850 and express it as a percentage.

Below 340nm I have not bothered plotting anything as it means dealing with two very small numbers and it all gets a bit noisy.

What’s different between the two cameras (apart from the sensor size)? The monochrome d850 has a fused silica coverglass, and has had the Bayer filter and microlenses removed. The 26M has the stock coverglass, and (I am assuming) has microlenses and something between the microlenses and the sensor. I suspect the drop in sensitivity below 400nm for the 26M is being driven by the coverglass, as I’ve seen a few coverglasses now which are starting to block UV around 400nm before becoming opaque by about 320nm. Not sure as to why the slight drop about 700nm – perhaps AR coatings on the stock coverglass which are optimised to visible?

Now I’ve measured this response curve, the next stage will be for me to send the camera to Dan at MaxMax and have the coverglass removed and replaced with fused silica.

Overall the build quality of the 26M seems impressive, so I am looking forward to having a bit more of a play with this and seeing what it can do once it is fully modified.

Thanks for reading, and if you’d like to know more about this or any other aspects of my work, you can reach me here.

This post follows on from my previous work making an autofocus UV lens using a Nikon 50mm f1.8 lens body (which you can read about here). Having a basic autofocus lens body like this makes it possible to swap other lenses in and out and test how they perform. Camera lenses themselves vary from incredibly simple (a single element, or even a pinhole) through to extremely complicated (up to 20 or more individual glass elements). For UV imaging, simpler is usually better – fewer elements means less glass, and fewer air/glass interfaces. When it comes to simple lenses, there are a few much more simple than a single mensicus lens. This leads us to the one used for the conversion discussed here today – the lens from a Boyer Serie VIII camera. Here is the lens mounted into the Nikon 50mm f1.8 autofocus lens body and on the camera (note the use of white-tac to keep it in place – yes, sometimes simpler is better).

What did this lens come from? Originally it was in a Boyer Serie VIII roll film camera as shown below.

The camera was bought from eBay for about 10GBP and was non functional (the shutter was all gummed up). The fact that it wasn’t working wasn’t an issue for me as I only wanted the lens. After a bit of time with a Dremel and a hand file (and a mask as the dust from cutting the Bakelite was horrendous) the lens was removed.

This was cut to the right diameter to fit into a Thorlabs SM1 mount which I’d put on the lens body. The Boyer lens itself seems to have a focal length of around 60mm and is a single meniscus element. I started this little project thinking that it might be handy as a cheap lens for UV imaging, and the transmission spectrum of it looks promising, as it is letting through light down to about 300nm which for a normal glass lens is pretty good (quartz, fused silica or calcium fluoride would obviously go lower, but would not cost 10GBP).

With a 11mm extension tube between the lens and camera body, this will focus from infinity down to about 1m in normal visible light which isn’t too bad for usability. I’d expect some shift of that focus range in UV, and I might need to get rid of the extension tube when using it there.

I took a few test shots in the garden today (nice, dull, grey December day, the joys of English winters). These were taken on my monochrome Nikon d850, with no filters (this will be visible and a bit of IR and a little UV) and no lens hood. All at ISO 640 and varying the f stop setting from f1.8 to f22. It should be kept in mind though that the focal length of this lens is not 50mm so the absolute f stop values are not relevant here.

Here’s it wide open at f1.8;

At f1.8 the image is very soft and glowy, which is not a huge surprise.

Now at f2.8;

At f2.8 it is slightly less soft, and contrast has improved slightly.

Next f4;

At f4 there is a big increase in contrast.

Now, f5.6;

F5.6 is similar to f4, and this behaviour continues through to f22 along with an increase in depth of field;

The Boyer lens does cover a full frame sensor, and there isn’t any noticeable vignetting at the corners. It certainly isn’t going to win any sharpness prizes, and I think is best described as ‘artistic’. However keep in mind that this is a single element, and in the images above it is dealing with visible, IR and whatever UV is present, so there will be a degree of chromatic aberration contributing to the image softness.

What has this achieved? I put an old camera lens into a modern autofocus body and managed to make a soft, autofocus lens. Great. Fascinating. But that is not the point here. This lens will transmit light down to around 300nm, so this is an autofocus lens with UV capability well down into the UVB region. With a cost of about 10GBP (Ok, granted that does not include the cost of making the Nikon autofocus body which was about 80GBP including the Thorlabs ring) it is at the cheap end of lenses when it comes to UVB capability. Using narrow band pass filters is likely to sharpen things up as well by reducing chromatic aberration. This could actually end up being quite a useful little lens. It was a fun build, and perhaps that is as important as anything else – keep work fun. If you’d like to know more about this or any other aspect of my work, I can be reached here. Thanks for reading and Happy Holidays.