Microscopy – Leitz Ks Objectives

When looking out for interesting imaging equipment, I normally have a bit of an idea what I’m after – usually my focus is on UV imaging, and that then has a bearing on the types of things that I am looking for. Since getting into microscopy though my horizons have been broadened as there is some really quirky and interesting stuff out there. This leads me to today post – the Leitz Ks series microscope objectives. These were part of a set of lenses that Leitz produced for a very specific job – looking at the grain in photographic emulsions. Having spent a fair bit of time inhaling fumes in a dark room when I was younger, I thought this was a cool idea and set about trying to find some of these objectives….

Finding these was not too hard, but finding them for a reasonable price was a bit more difficult. In the end, one came from Germany, one from a dealer in Sweden, and two from a lab equipment reclamation company here in the UK. Here they are.

The 4 Leitz Ks objectives

In the range there were 4 objectives. From smallest magnification to largest, there was the 22x Oel (oil immersion) A 0.65.

Leitz Ks 22x objective

Then we have the Ks 45x A 0.65 dry objective.

Leitz Ks 45x objective

Next is the Ks 53x A 0.95 fluorite oil immersion objective.

Leitz Ks 53x objective

And finally the Ks 100x A 1.32 apochromatic oil immersion objective.

Leitz Ks 100x objective

All of these objectives were designed for Leitz 170mm finite tube length microscopes. Normally, on an objective after the tube length is the thickness of the coverslip that they are designed for, however these are different. With these there are different numbers and then a ‘mu’ symbol.

So what’s going on with these objectives? At this point it is a good point to dig back through the archives for some information. There was a Leitz brochure produced for these objectives, and I downloaded a copy from online. Here’s the front page.

The first page of the brochure give a bit more of a background to the objectives in the range.

As can be seen, the full range had 4 different Ks objectives, and two others. The numbers after the tube length, refer to the maximum layer thickness of the photographic film that they were designed for. Oddly on the 22x objective it says 0-2300, instead of the 2200 it has in the table above, while on the other objectives, the numbers correspond to the ‘max layer thickness’ in the table.

The next page in the brochure gives more details on the objectives.

After getting these I thought I’d see what type of images I could get from them, so dug out an old slide and the immersion oil (for the 3 objectives that needed it). My Olympus microscope has a 160mm tube length (as opposed to the 170mm tube length that these were designed for), but I have used other 170mm tube length objectives with success so thought it was still worth a try. At this point I should be sharing some lovely images showing the grain structure in the slide, however I have nothing for you, sorry. The images were actually not that good, from any of them, and I got sharper ones with my normal objectives. Not sure why this is the case to be honest. I struggle to believe that they are all damaged. Online, I was only able to find one thread talking about using them, and they were struggling too, so perhaps there is something odd about their design which makes settling them up and using them difficult. So for now they remain an interesting piece of history.

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

UV Microscopy – Zeiss 50W HBO light source modification

Some of the experiments I do fall into the ‘Please feel free to try this at home’ category. Others however are of the ‘Do not attempt to do this yourself without full understanding the risks involved. No really, I mean it, stop what you are doing now and step away from the work bench’. Today’s post is firmly from the second category. Please do not attempt to do anything like this unless you understand the risks of working with high intensity UV light. With that out of the way, let’s begin….

For my UV microscope build, almost everything has needed modification in one way or another, in order to be usable down in the short wavelength range of UV below 300nm. My main UV light source – a 200W Hamamatsu LC8 mercury xenon lamp – gives out plenty of UV down to just below 300nm, but is really awkward to use with the microscope, due to the position of the output from the lamp. I have a couple of mercury xenon lamps for microscopes – a 100W Olympus one, and a 50W Zeiss HBO – but when I initially checked the output from both of these there was a bit of a problem. While they both had a good output at 365nm, down below 320nm there was essentially nothing. If I’m wanting to look down at 300nm and below, this is a bit of an issue as I need light down there. Initially it struck me as a bit odd, and had me wondering what was going on. Both microscope lights were using mercury xenon lamps like the Hamamatsu LC8, so why was output so restricted? Perhaps the bulbs were made of different materials which didn’t have as good a UV transmission? This could be possible, but it would be very odd. So was it something else?

For now I have settled on trying to use the Zeiss 50W HBO setup rather than the Olympus one. This is mainly driven by space constraints – the Olympus light source is huge, and I just don’t have space to have it out on the bench all the time. The Zeiss HBO is dinky, so is much less of an issue to leave out. Here it is on the bench.

Zeiss 50W HBO light on the bench

If you look down the output port of the light (with it switched off obviously), there is a metal tube which can be moved in and out by turning a knob on the side of the light housing.

Movement of the metal tube

At the far of the tube near the bulb itself is a lens, and the movement of this tube and lens actually focuses the light. But the lens is a problem. It is highly unlikely that this lens is a UV fused silica or quartz one, so it’s going to be glass, and glass absorbs short wavelength UV. This lens will be blocking the light that I want to see. To determine if this was indeed what was happening, the first thing to do was remove the tube and lens. This was actually pretty simple, by loosening the small screw near the focus knob. Once removed, the tube looked like this.

Focusing tube assembly

The lens is held in with a simple spring clip, but this was down at the bottom of the tube and was really, really tricky to get out. Once out, the lens itself could be removed, and this is what it looked like.

Lens from focusing tube

It’s a odd looking, asymmetric design, with the highly curved side facing the bulb. Focal length was about 20mm, which make sense given where it sits in relation to the bulb. Once removed I could measure the irradiance spectra of the light, with and without the lens to see what effect it had on the output. This aim here was not to determine absolute irradiance, but to understand the relative output with and without the lens to understand what wavelengths it was absorbing. For this I used my Ocean Optics FX spectrometer, and got the following.

Spectral output of the Zeiss HBO 50W with and without the lens

The really obvious difference between the two plots is in the UV region – with no lens in place there are a lot more lines there than when the lens is in the way, especially below 350nm. To try and show this better I normalised the lines to the 578nm peak (which should be pretty similar whether or not the lens is there), and replotted the graphs with an emphasis on the UV and near-visible region.

Irradiance with and without the lens, normalised to 578nm peak

Plotting the data like this really emphasizes the effect of the lens. With the lens in place the 365nm line is starting to be attenuated. By the time you get to 330nm, most of the light is absorbed, and then below 310nm there is nothing, all the UV has been blocked. With the lens removed, there are still well defined UV peaks at 313nm, and even below 300nm, down to where the spectrometer reaches the edge of its calibration at 250nm. This is not a huge surprise – that lens is thick, and there’s no wonder it is absorbing loads of the UV. Success then, removing the lens brings back the peaks I’d expected from a mercury xenon lamp. Yay.

Simply removing the lens means that this light source can be used for UVB imaging at around 300nm and even below. What is the downside here though? Removing the lens means losing some light intensity, as the light is no longer focused. But in theory I could replace the lens with a planoconvex lens of around 20mm focal length, so give me some rudimentary focusing capability. I could even use a custom UV fused silica condenser lens, but this will be an expensive option. While probably not as good as the complex shaped lens originally used, a simple planoconvex lens should offer improvement over no len, and is something I may consider in the future if light intensity isn’t sufficient.

What have I learned here? Well I certainly know a lot more about the inside of mercury xenon lamps now. Not everything works the first time (and no, this is not the first time I have realised that). However think logically and approach a problem systematically and a solution can usually be found. While I have gone quickly through the description of what I did here, I must emphasize that working with these types of UV lights is not without risk. Exposure to the skin can quickly result in burns, and to the eyes can result in blindness. I made sure I was well protected when doing this work.

Another part of my UV microscope build can now be ticked off, and I am one step closer to being able to image in the 300nm region and below for my sunscreen work. 2021 is starting to look better already……. 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.

UV Microscopy – Vintage Zeiss quartz condenser

While learning about microscopy during the last year, I’ve also been on the look out for information on historical items which were designed for use in the ultraviolet (UV) region. Although the vast majority of lenses are based on glass optical lenses, manufacturers have occasionally made UV lenses using materials such as quartz and calcium fluoride instead of glass, in order to let short wavelength UV light through. Leitz made a number of objectives and condensers for UV, which I’ve previously written about here, here and here. Lomo made a wide range of UV objectives, photoeyepieces and condensers, containing quartz lenses and their 10x objective is discussed here and photoeyepieces here. Zeiss have made probably the widest range of lenses and objectives for UV, marketed under the Ultrafluar label. These were available for the 160mm finite tube length microscopes, and continue to be made for the more modern design infinite tube microscopes. I have a few of these Zeiss objectives, and will write further on them after a bit more work with them. Before the Ultrafluars though they also made a quartz condenser and objectives, however it is harder to track down information on them given their age.

A few months ago I came across someone selling what was advertised as a Zeiss quartz condenser, and after a bit of discussion I bought it. For nearly 6 months it remained sitting on a shelf in the US, and I finally got it a few weeks ago. It consisted of 3 parts – the main body of the condenser with an adjustable iris, 2 additional lenses, all contained in a wooden box as shown below.

Zeiss quartz condenser

A search online revealed nothing about this, so my first check was a transmission measurement, which showed the following.

Transmission of the Zeiss quartz condenser components

It was a huge relief to see it confirmed that these were definitely not glass but quartz lens elements (glass transmission would drop in the UV while these remained constant). Phew.

The history of the equipment I have is important to me, as I like to learn about what things were used for before and when they were made. To try and find out about this condenser set I approached the Zeiss Archives, with the hope that they could cast some light on it and when it was made. Unfortunately they couldn’t find record of this one, but that wasn’t a huge surprise as these would have been rare items with very few being made. To try and find something out about it, the next step was to get back in touch with the vendor. They pointed me towards a Zeiss Microscopes catalogue from 1934 which listed it. This was a period of time when catalogues were real hard back books, and I managed to track down a copy here in the UK. On examination it was indeed in the catalogue as shown below (with the part number given as 11 42 50).

Quartz condenser from 1934 Zeiss catalogue

Interestingly it also appears in a 1927 Zeiss book with the same part number.

Quartz condenser listing from 1927

Something with the same part number (this time shown as 11.4250) but slightly different numerical apertures for the lenses even appears as far back as the 1913 catalogue.

Excerpt from the 1913 Zeiss catalogue showing the quartz condenser

Looking back through some pre-1913 ones I couldn’t find any mention of it. It looks therefore that mine is probably from somewhere between 1913 and 1934, although I’ve not been able to verify how long it was made for after 1934. They really did make these well – the iris moves and adjusts as if it were made yesterday.

In the research world it is very easy to become fixated on the future, the goal of the work. We should not forget the great science and engineering that has already been done in order to get us to where we are. As with all historical research, it was fascinating to dig into the archives to try and find out about this condenser. It introduced me to the Zeiss Archives, a vast online database of equipment (although only in German, which makes things a little more challenging), and along the way I’ve been able to learn about other areas which will help with my work in the future.

If you’ve made it this far, thanks for reading, and if you’d like to more about this or any other area of my research, you can reach me here.

AquiSense Technologies UV-C LED light sources

In my work on UV imaging, I’m always on the lookout for suppliers who can offer solutions for UV lighting.  Normally, I’d use xenon flash systems, xenon and mercury xenon continuous light and fluorescent sources.  Problem with all these is that you get a lot of unwanted wavelengths, and when using a camera for UV photography, what you don’t want is lots of visible and IR light that needs to be filtered out.  As such light emitting diode (LED) technology would seem to be an ideal solution as it can offer relatively narrow bandwidths in a small package without the need for additional filtration. 

UV LED technology is evolving rapidly.  A few years ago 365nm was pretty much all that was available, but the range of wavelengths available is now extending further and further into the short wavelength UV regions.  This is being driven in no small part by the use of UV-C light for cleaning and sterilising materials, an area which has become even more important given what is going on globally at the moment.

I was recently searching for a collimated UV source that would be able to provide light in the 250nm to 300nm range for my UV microscope build, and this lead me to AquiSense Technolgies, and their PearlLab BeamTM system. This is a compact, collimated beam device which employs UV LEDs and emits radiation in the germicidal range. Its small size makes it ideal for use as a table top system, and it is available in a range of options from 1 to 3 different wavelengths.   Normally used for imaging Petri dishes, I can see this type of technology becoming more and more widespread for the UV imaging and microscopy world, for both reflectance and fluorescence work, especially as LED powers and efficiencies continue to rise.

AquiSense Technologies is the global leader in UV-C LED systems design and manufacture. They work with leading LED manufacturers to evaluate their devices and then design efficient disinfection products. Using a combination of patented technology and in-depth know-how, AquiSense integrates LED devices into products that solve real world problems in water, air, and surface applications. You can find out more about their range of LED UV lighting solutions here.


UV Imaging – Astro Berlin 120mm f2.1 Quarz Objektiv

2021 is well under way now, and of course that means that here in the UK we are in another lockdown. On the plus point with the nights being long and the weather cold, I have time to share something with you on one of the recent lenses to arrive here – the Astro Berlin 120mm f2.1 Quarz Objektiv. For those of you who haven’t heard of Astro Berlin they were optical company based Berlin, and were founded in the early 1920’s (I’ve seen 1921 and 1922 mentioned for this). While originally they produced astronomical optics, they soon changed to camera and cinecamera lenses. Remaining active in East Berlin after WWII the company finally went into liquidation in 1991. They have gained a bit of a legendary reputation amongst photographers and cinematographers, due in part to their combination of long focal lengths and fast apertures with the optical characteristics of older designs of lenses. As such they tend to demand high prices from those wanting to use them as well as the usual array of historical optics collectors.

During my historical research, I’ve come across a couple of Astro Berlin lenses which were designed for UV imaging – the 44mm f2.5 Quarz-Tachar, and 135mm f4.5 Quarz-Anastigmat – and as you can imagine, if the normal glass Astro Berlin lenses are rare, these are almost mythical. However a few months ago I saw an advert for an Astro Berlin 120mm f2.1 Quarz Objektiv, which was a new one to me. This looked to have been designed for large format photography and the seller mentioned it fitted a Compur II shutter, and that they believed it was a quartz lens as they had used it to capture images in UV light (I’ll come back to these points later). After a bit of discussion, I made my offer for it which was accepted, and it arrived just before Christmas. Here’s a picture of the lens (along with a nice reflection of my kitchen in the front element).

Astro Berlin 120mm f2.1 Quarz Objektiv lens

The specifications of this lens were quite impressive – 120mm focal length and f2.1 aperture. Compare this with the UV Nikkor (105mm f4.5), UV Sonnar (105mm f4.3), Asahi Ultra Achromatic Takumar (85mm f4.5), and even the Zeiss UV Objektiv (60mm f4), and as you can see this Astro Berlin lens has an extremely large aperture. Image size was mentioned as 6x6cm or even 6x9cm for macro work. As you can imagine, I was rather excited when this arrived, and the first thing to do was test it for UV transmission. The lens itself easily dismantles to produce a single front element, a central section for Waterhouse stops, and then a rear section which is actually a cemented doublet (which started to get me a bit nervous as cemented elements for a UV lens, especially an old one, would be unusual). What did the transmission measurement show? Let’s take a look.

Astro Berlin 120mm f2.1 lens transmission

Hmmm, what is going on here? The front element is not glass and it’s safe to assume this is quartz based on the transmission spectrum which is still looking good down at 280nm. However the rear group does not look to be quartz as it is showing a drop in transmission from 400nm down to being essentially opaque by 320nm. Don’t get me wrong, this is pretty impressive transmission for a glass lens, but obviously when the 2 parts of the lens are used together, this limits the overall transmission to wavelengths longer than 320nm.

What about this doublet at the rear of the lens? This rear element can be seen in the image below.

Cemented doublet and retaining ring

The image above shows the side view of the rear cemented doublet on the left, and the lens retaining ring on the right. You can see it is a doublet, due to the depression in the black coating about a third of the way in from the left hand side. Whether one (or both, but that would be odd) of the two parts of this doublet are quartz will have to remain a mystery for now, as I don’t want to scratch off the black coating from around the edge to check. It could also be that the adhesive holding the lens elements together (most likely Canada Balsam) is blocking some of the UV, so that is something I’ll be testing in the future as I haven’t been able to find anything definitive on that.

The lens does not have an aperture at the moment, and is built for Waterhouse stops, so that is next on the list of things to make, which will enable me to stop down the lens.

How to capture images with this lens? The lens mount was apparently that of a Compur II shutter. Problem with that is that the Compur II shutter design parameters (thread diameter and pitch) were never actually standardised. Measuring thread diameter is fairly easy, and this was 50mm. Thread pitch is a little harder to measure (well it is for me anyway), and as far as I could tell this was a 1mm, or close to 1mm. To adapt this to a camera, I took the following approach – a M50x1 to M65 adapter from RafCamera, a M65 helicoid, a M65 to Hasselblad bayonet adapter (also from RafCamera), and then a Noveflex adapter to go from Hasselblad to Canon or Nikon. Most of these parts I had already from other projects, but the M50x1 to M65 needed to be ordered. Putting all these together and mounting the lens on a camera looks as shown below.

Astro Berlin 120mm f2.1 mounted on the camera

The length of the helicoid I have limits this to macro work only at the moment (need to get a shorter M65 helicoid at some point). Also the M50x1 adapter seems to have a very slightly different thread pitch to the lens. I mentioned above the thread pitch is hard to measure accurately and I now suspect the lens is threaded with something slightly different to M50x1. It is close enough to mount it well, but just doesn’t screw all the way on. Despite that this setup at least allows me to capture some photos with it. The sun came out today, so I got a few shots in the garden, just using visible light with a normal colour camera, which are shown below.

Image with the Astro Berlin 120mm f2.1 lens

As can be seen above the lens is quite ‘glowy’, which is not surprising as I am using this wide open without any stops, and my finger covering the hole in the barrel of the lens, where the stops would go. Below is a shot showing the entire 35mm frame (reduced in resolution from the whole 50Mp for sharing).

Image with the Astro Berlin 120mm f2.1 lens

Cropping this and showing the central part of the image at the original resolution it was captured at gives the following.

Crop of the larger image at actual pixel resolution

There certainly is plenty of glow there, but that thin thread is a strand of spider silk, showing the lens is certainly capable of capturing plenty of detail.

The nice thing about this lens is that the rear doublet can be removed, and the lens still fits the same M50x1 adapter. This allows imaging using just the quartz singlet at the front of the lens, and an example image is given below.

Image using just the quartz singlet from the front of the lens

Funnily enough the image with just the quartz singlet gives quite a similar image to the one taken with the complete lens shown earlier in the post, at least on my 35mm full frame digital camera. The good thing about this is that being quartz it can be used for imaging in the UVB region below 320nm.

The lens does not have an adjustable aperture, but a slot for taking Waterhouse stops. While these are normally made from brass, I thought I would make a set from 1mm thick black plastic sheet.

Waterhouse stops for the Astro Berlin 120mm lens

I made a set which should in theory go from wide open to about f11, although this is just approximate, and the hole sizes were determined by what size drills I had as much as any maths. How do they work? To try this out I got a set of shots of a pine tree in the garden, firstly without any stops in place and then from largest to smallest.

Going from ‘no stop’ to the largest stop, produced only a very slight increase in depth of field, and I suspect this due to the metalwork on the inside of the lens. After that, as the hole of the Waterhouse stop decreases in size the depth of field increases, just as it would when you make the aperture smaller on a more modern lens. Also the images become much less ‘glowy’. Success. As they are at the moment, they are a bit loose in lens when slotted in the lens, so I may add a layer of black tape to one side to just increase the thickness slightly and make them more snug in the slot.

After all this, where have we got to with the Astro Berlin lens? While seemingly not an all quartz lens, this certainly has good UV transmission, and has a quartz element at the front. Using some off the shelf adapters, it can be mounted to a range of different cameras including 35mm SLR, and medium format – it’d be lovely to try this on a UV sensitive Hasselblad at some point. It also can be dismantled and even when using just the front quartz singlet, can still produce a usable image which would enable it to be used for UVB imaging. When there is a bit more UV around I shall get some images with it and see how it performs.

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.

UV microscopy – modifying the binocular/trinocular head, part II

Happy New Year everyone. 2021 brings promise of a new start and I hope to bring you plenty of cool science over the next 12 months. I have some pretty special lenses and microscope kit to review, so watch this space. Today though, we start with an update on the UV microscope build. This has been a project I’ve been working on for about the last 6 months, and has required some major modification of my Olympus BHB to make happen (see here for the previous summary of the work to date). Today an update on the modification of the binocular/trinocular head, and the results of some major mechanical surgery.

When I took the Olympus head (I’ll just refer to it as a ‘head’ from now on, rather than repeating myself every time) apart, I realised that the construction was going to be quite difficult to adapt for UV. There was a lot of thick glass in there, along with a prism which acted as beam splitter – see here for the initial assessment. Instead of trying to replicate the prism exactly, I made the executive decision to cut the prism in half, and replace half of it with a simple block of UV fused silica. This would mean that when the prism was in the position for taking a photo, the light would go straight through to the camera, rather than being split between the camera and the eyepieces. This would result in more light for the camera (a good thing for UV imaging), and with most cameras having some form of live view, I don’t need to look down the eyepiece for the final focus. The window in the bottom part of the head also needed replacing, but this would just need a simple disk of UV fused silica. However both parts needed to be custom made and the prism would need cutting in half to make this all work. To supply the fused silica components and cut the prism down, I went to a company I’ve worked with quite a bit over the last year – UQG Optics – who were able to supply the parts I needed and do the work on the prism. When anything needs to be custom made the costs go up dramatically, and given the tolerances involved were tight, it was a case of ‘measure many times’ before the dimensions went to UQG for cutting.

When the two UV fused silica parts arrived, along with the cut down prism beam splitter, I assembled everything and glued the circular window into its metal housing, as you can see below.

New UV fused silica parts for the Olympus BHB head

Transmission of the new parts, compared with the old ones is given below.

Transmission of the new UV fused silica parts compared with the original ones

Note the absolute transmission here is probably slightly higher than in real life (by a couple of percentage points). Even after collimation of the light beam for the transmission test, it will still be slightly divergent. Including such thick pieces of glass / fused silica in the path of the beam means that some more light will reach the spectrometer detector than would happen without the glass / fused silica in place. I have tried to minimise this with the setup of the device, but can’t guarantee that it has corrected it fully. The important thing to note is that the new fused silica components have a flat transmission curve as a function of wavelength, unlike the original glass ones, which block varying parts of the UV. The new UV fused silica parts will allow UVB (and even UVC) imaging down to and below 280nm.

Once the glue had set on these, the head needed re-assembling. Not a simple task, and akin to putting a quart into a pint pot. The picture below give you and idea of the parts that needed to go back into it.

Parts from the Olympus BHB head

Once back together (after a lot of loosening and tightening screws to get everything aligned, and cleaning of any glass after putting fingerprints on it) it looked like this.

Olympus BHB head re-assembled (without the top trinocular tube attached)

The next step was to put the yellow filters in the base of the tubes for the eyepieces – the idea here being to eliminate any UV that could otherwise reach the eyes. This is part of my ‘belt and braces’ approach to safety when using this device. Not only would any UV have to pass through the glass part of the beam splitter, and the glass eyepieces, but now has to go through yellow filters which have a transmission of <0.01% of anything below 400nm. As mentioned before, I’d still use UV protective glasses when using this, the yellow filters just another layer of protection. Once it is up and running, I’ll be measuring the spectral throughput to the eyepieces to see how low it is. If in any doubt, I’ll just use live view on the camera for all the focusing work. Here’s a view down the tube of one of the eyepieces to the yellow filter.

Yellow filter in place in the eyepiece tube

One thing I haven’t got right was the diameter of the filter needed. The eyepiece tubes are 23.2mm, and I assumed 23mm filters would be fine. However at the bottom of the tube the diameter opens up and the filters rattle around a bit unless the eyepiece tubes are screwed all the way down. This can be rectified by using larger diameter filters, or by gluing the ones I have in place, and I’ll see if I can get access to where I need to to be able to do this. For now the tubes will be screwed fully down, locking the filters in place.

There was a lot of glass in the head, all aligned very precisely, and I had real concerns that this would simply not work well enough, leaving me with a very big bill for a new paperweight. Once assembled, for a quick test I did a visible light image of one of the Diatom Lab strewn diatom slides using an Olympus 20x SPLan objective, and the normal tungsten light source on the microscope. Stacked in Zerene Stacker, here is how it looks (I used a Schott S-8612 filter as this was taken with my multispectral camera and I wanted to remove IR from the image).

Diatom image using the modified Olympus head

The image above has been reduced in resolution for sharing on the web, and is a crop from the full area imaged, but demonstrates that this modified head can at least give a usable image in visible light. Note, the fuzzy areas in the largest diatom are more down to my stacking technique than anything else.

In theory this is the final part of the UV microscope build – all the glass parts have now been converted to fused silica, and the next stage is to try imaging in the UV. To start with I’ll use 365nm as this is simpler and safer, and I have a couple of light sources (LED and mercury xenon lamp) to try out. I also have a Diatom 2.0 test slide on order from Diatom Lab, so I can look to see how the resolution of the imaging changes when I go from visible to UV. See, I told you 2021 would be exciting.

The original plan was to use this for sunscreen formulation imaging – to be able to image UVA and UVB absorbing components separately to see where they are in the formula – and I’ll certainly be using it for that first. After that we shall see, as there are plenty of other applications for UV imaging, in biology, forensics and other fields.

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.

UV Microscopy – Tiyoda UV 1.2-1.4 darkfield condenser

As I write this it is the the last day of what for most has been a ‘character building’ year, and I shall start today’s post with a little question – As scientists, what is it that drives us to do the things we do? See, nothing to serious, and with that slight philosophical diversion, let’s dive into the subject of this post.

With UV imaging, things aren’t always what they seem. Equipment designed for UV work could be only for certain wavelengths for instance – normal glasses can still be used for UVA imaging work, while materials such as quartz and calcium fluoride are needed for shorter wavelength UVB work, as normal glass would just absorb the light. Some microscope objectives are described as ‘UV objectives’ but are not meant for imaging in the UV, rather they have low fluorescence and are used for visible light fluorescence imaging after illuminating the sample with UV. With older equipment, it becomes hard to try and find original source information about it as it often doesn’t exist digitally, so there is little alternative to hands-on testing to try and determine if something is suitable for a specific application.

In my searches for UV imaging kit, I came across a microscope condenser called a ‘Tiyoda UV 1.2-1.4 darkfield condenser’. This piqued my interest as it sounded as though it was a dedicated UV darkfield condenser, and got me wondering whether it was made of quartz or other exotic materials. Problem was, there wasn’t any info out there on the transmission properties of it. I saw a couple for sale, for rather large amounts of money (hundreds of GBP), which was too much to invest just to answer a question. Eventually I found one for sale here in the UK which cost a lot less, but was best described as a ‘fixer-upper’. So I bought it and here it is.

Tiyoda UV 1.2-1.4 condenser

The condenser came in a mount for a Reichert microscope (which I don’t have) but my main interest was to assess its UV transmission properties. Main problem when it arrived, was that the coating around the central part of the condenser was damaged, as shown below.

Damaged coating

The black paint should be present in a ring around the central part of the condenser, and on this copy it was missing on about half of it. The construction of these things is quite amazing and goes back a long way. They are known as ‘cardioid condensers’ and use a specially shaped mirror to project a cone of light onto the subject. Shining a torch through this one from below produces the following.

Light transmission through damaged condenser

In the image above, the black ring around the central disk should be blocking all the light from below, and it’s pretty obviously damaged. The central disk is actually transparent, and this can be seen more easily if viewed from the side with the torch still on. This is what would sit just below the sample on the microscope stage.

Damaged condenser viewed from the side

The unit consists of two main parts – the main top piece, in and above the Reichert mount, and then a screw-on lens underneath which is marked with the name ‘Super, Wide PAT. PEND.’, as shown below.

Super Wide lens below condenser

The view through this lens is pretty trippy, and it has a dark spot in the centre. It also looks very, very delicate.

View through the Super Wide lens

This lens actually creates a circular ring of light from a point source as can be seen below, as it projects the image of my ceiling lights onto the desk.

Circular ring of light from the Super Wide lens

In use, this ring of light is then projected into the main body of the condenser and reflects off the mirror towards the front of the condenser. A very clever design minimizing the amount of light which would otherwise be lost.

The damage to the top of the condenser was actually a blessing as well as a problem, as it allowed me to measure the transmission straight through, so I can see whether the components were attenuating the UV. Measuring the transmission of the individual parts and complete condenser gave me the following.

Transmission of the Tiyoda UV 1.2-1.4 condenser

Now there is a lot going on in the graph above, so I shall explain. The red line is the transmission through the Super Wide Pat Pend lens by itself. This actually shows good transmission down to about 300nm, which suggests it is a thin layer of conventional glass, rather than something unusual like quartz. The green line is the transmission through the top part of the condenser with the damaged black paint. Measured transmission is very low here, as even with the damage the light still has to bounce off mirrors and through a small hole to get to the detector, which attenuates the total signal. When the two parts of the condenser are recombined, and transmission measured, we get the blue line. Now the incoming light is focused by the Super Wide Pat Pend lens, onto the mirrors and out of the top of the condenser. As can be seen the overall transmission drops from 400nm down to about 320nm, below which it is essentially blocking the light. Note that absolute transmission of the condenser would be higher than this as the diameter of the beam of light I use for my tests is relatively large, and some light is lost by not being able to pass through the small optics of the condenser. The key thing to note is the drop in transmission at the short wavelength end. The Tiyoda UV 1.2-1.4 would therefore be good for the longer wavelength UV down to around 320-330nm, but not good below that as it would prevent the light from transmitting.

OK, so I have a damaged condenser which does not have short wavelength UV capability. What next? First step was to repair the damaged paint, and I did this using Semple Black 3.0 paint which as I have shown in other applications has great light blocking properties. After painting, the condenser looked like this.

Condenser after repainting

Shining the torch through it now, and the light is only coming from the central disk.

Torch through the repainted condenser

If I turn down the exposure on the camera, and capture the torch through the condenser again, I get the following.

Torch through the repainted condenser

Basically the only light now getting through the condenser is coming through the central circular opening, as it should be. However it is pretty obvious now that there are some major scratches in the glass of the circular opening especially close to the edges. This condenser has had a hard life….

Where does this work leave me? I have a condenser in a mount which does not fit any of my microscopes. Even if the mount was modified, the diameter of the condenser means that my microscope stage would need modifying to make it fit. The condenser does not have the great UV transmission I was hoping for (although it will still be good for UVA work). The one I have was damaged, and while I have given it a new coat of paint to cosmetically repair it, I have no doubt that a few uses with immersion oil and that paint would come straight off.

So after all this, what was the point of this exercise? Looking around online I was unable to find anything definitive about the transmission of light through a Tiyoda UV 1.2-1.4 condenser. By buying one myself I was able to test it, and fill a gap in the knowledge base about these old pieces of equipment. It answered a question that was relevant to my research, and has allowed me to definitively say how it works, and where it can and cannot be used. And now I write up and share my experience, so that anyone else who may be wondering this, perhaps as part of their work, has something to refer to.

At the beginning of this post I asked a simple question – As scientists, what is it that drives us to do the things we do? Motivations for the work we can do take many forms. Money – sure, we have to make a living and some questions need answering to push projects forward. To push our own knowledge forward – absolutely. To fill a gap in the knowledge that wasn’t there and share what you know, enabling the work of others in your field to move forward as well as your own. Now there’s a nice thought as we move from what for many has been a difficult year, into one which hopefully will be a better one.

If you’ve made it this far, and would like to know more about this or any other aspect of my work, I can be reached here. Happy New Year everyone.

Leitz UV microscope objectives – historical optics, part II

Working in the area of UV imaging and microscopy gives me 2 options. Firstly, win the lottery (or sell a kidney or two) so I can afford to buy new equipment. Secondly, try and hunt down older equipment which can be re-used. Given that I’m quite attached to my kidneys (literally) and that unfortunately that lottery win still evades me, that leaves the second option as being the preferred route. Even this route presents significant challenges though. Very often these piece of equipment were made in very small numbers, most dealers have never even seen or heard of them, and some are well over 50 years old now so tracking down information on them is extremely difficult. Today, I have a little bit of an update on a post I shared earlier in 2020, and also a new historical Leitz objective that I’ve tracked down.

Firstly, the update. Earlier this year I shared a post which talked about a 300x reflecting objective lens – here. This lens is based on a mirror system rather than refractive optics, and is suitable for UV, visible and IR imaging. I bought it as a bit of a collectors items really, and while I’d love to use it some day, it’s going to take some pretty complex work to make that happen, as the tube length it has is much longer than that microscope I have at the moment. While I did find a mention of what I thought was it, in a catalogue from 1967, I didn’t know any more about it.

Fast forward about 6 months, and I was talking with a dealer in Sweden and he mentioned that he had a couple of interesting items. One was a Leitz microscope condenser which had the markings “Glyc Q” on it, and the other a lens which had “Quarzglas” and “Glyz” on it. Now these make my ears prick up a bit as the use of glycerine (marked as “Glyc” or “Glyz”) often implies they were designed for use with ultraviolet light, as does “Quarzglas” (as quartz is UV transparent). So after a quick discussion they were on their way over to me.

I’ll come back to the objective lens with “Quarzglas” in a minute, and cover the condenser first. The 300x reflective objective I already have was pictured in the catalogue alongside a couple of small condensers (see below).

Leitz reflecting objectives and condensers (1967 catalogue)

Being quartz glass the reflecting condensers these should also be suitable for UV work. The one I have is one of the 0.6 NA ones, mounted in a Leitz holder.

0.6 NA Leitz reflecting objective

While the wording on the condenser is not exactly like the one in the brochure, I think this is the same one as advertised. The difference is probably down to being different batch or production run, as I’ve seen small differences like this before.

How does the transmission of this condenser look. I measured transmission between 280nm and 420nm and got the following.

Leitz 0.6 Q Glyc condenser

At first glance the transmission of the condenser doesn’t look great. However it should be kept in mind that the light gathering part of the condenser is quite small, smaller than the diameter of the light beam in the my transmission measurement setup, therefore some of the incoming light is not hitting the lens. This of course limits the max transmission possible. The key thing here is that there is no significant drop between 420nm and 280nm, indicating that it has good transmission in the short wavelength UV region, and as such would be suitable for UVA and UVB imaging. Hurrah.

As a slight aside I also found out a little more about the 300x objective I have. In a paper from from 1964 (Klaus Weber, “New developments in microscopes at Leitz”, Applied Optics, 1964, 3(9), 1045-1048), it describes the mirror objectives as follows; ” The mirror objectives expand the application of microscopes into an additional spectral range; the objective Q 170/0.50 and the glycerin immersion Q Glyc 300/0.85 are suitable for a spectral range of from 2200A [220nm] to 7000A [700nm]; and they are fully achromatic. Suitable condensers are also provided. Mirror objectives consist of spherical mirrors on quartz bodies. The main characteristics of the Leitz mirror objectives are the very small central zone (only 5% of the pupil size), thus guaranteeing very good contrast even for extremely fine object structures. The special type Q Glyc 300/0.85 s, which has a somewhat reduced light transmitting capacity, produces maximum contrast even for finest structure elements over the entire spectral range. It is particularly suitable for microphotometry.”. My 300x one is marked with the ‘S’ on the end, indicating is is one of he slightly higher contrast ones.

Now for the second item that arrived. This is an objective lens with a 50x magnification and is shown below.

Leitz Pv 50x Quarzgl objective
Leitz Pv 50x Quarzgl objective

There’s a lot going on on the label, so what does it all mean. ‘170/0.18’ means it is designed for a 170mm tube length, and a 0.18mm thick coverslip. So far, this is fairly standard. ‘Pv Glyz. 50:1’ means it is for phase contrast (Pv), meant to be used with glycerine as an immersion fluid, is 50x magnification and a NA of 1.00 (the ‘A 1,00’ also shows that). The phase ring can be seen in the photo below.

Phase ring in the Leitz Pv 50x Quarzgl objective

The next line on the lens is really interesting – ‘Quarzgl.’ as this indicates it is quartz so should in theory have good UV transmission. After that, the ‘A 1,00’ has already been discussed. Next we have ‘Muster-Fasserei’ in red. This is an odd one, and not something I have seen before. Talking with a couple of optics expert the overall consensus is that this was a prototype or manufacturing pattern for this lens design, making it a rather interesting piece. While Leitz did make a few refractive optics for UV work (which I have), the ones I have seen so far were not phase contrast ones, again potentially adding to the uniqueness of this one.

What about the history of this 50x lens, what was it designed for? This proved to be a little harder to figure out. I eventually found reference to it in a Leitz document “Image-forming and illuminating systems of the microscope”, where it showed the following (in the line for Pv Glyc 50);

Leitz 50x Pv Glyz objective document

This seems to suggest that it actually wasn’t for use on a normal microscope, but rather a microspectrograph. So far I have found a couple of references to Leitz microspectrographs and microspectrometers from approximately the era of this lens, but nothing that specifically mentions this lens yet, so for now it remains a bit of an enigma. In the document above, the coverglass is given as being Quartz, which might explain the ‘Quarzgl’ on the barrel of the objective.

As with the 0.6 condenser above, I measured the transmission spectra through this objective and got the following.

Leitz Pv 50x Quarzgl objective transmission

Hhhmmm, this is where things stopped going to plan. As with the 0.6 condenser, this objective is relatively small diameter, so the light source is probably being clipped slightly by the objective, which limits the maximum signal I am measuring. However unlike the 0.6 condenser, the objective is blocking the light below 320nm. Not good, and means it wont be any use for UVB imaging. This is a bit odd, as it has ‘Quarzgl’ on the objective lens barrel, and it should have better UV transmission based on that. Does it have a glass element in there in addition to some quartz ones? Does it have coatings on the lenses or adhesives between them? If it is a mock up, pre-production lens, have they just used glass instead of quartz? I shall probably never know the answer to the last question, unless I ever manage to find another one to compare it with. I shall get some images using it once my glycerine arrives, so for now it is just a technical assessment.

Tracking down old optics has been a fascinating part of my UV imaging research. Often there is very little information on these items, as when they were made they were done so in very small numbers. As with all research the road to knowledge is not always a smooth one, as is the case with the objective lens described above. Sometimes what we thought was the case proves not to be so. This is the very nature of research though, and we must take the rough with the smooth. We should not see it just as a negative – if everything we did was right, how can we learn anything? Very often I have seen data which didn’t fit the hypothesis discounted, with every effort made to discredit it, rather than have someone ask the awkward question “Was the original hypothesis right?”. As scientists we should never be afraid of that question, and always be ready to ask it of our own work as well as that of others. Thank you for reading, and if you’d like to know more about this or any other aspect of my work, you can reach me here.

UV Microscopy – Summary and new UV fused silica optics

For a few months now I have been working on converting an Olympus BHB microscope to create something that I can use for UV microscopy down to below 300nm. This will be of use with my work on skin and sunscreen development as well as other areas. As can be seen here, the existing optics of the BHB very effectively block the UV below about 340nm which means the microscope needs modifying for it to be able to do this. To make it UV capable below 340nm means getting rid of all the glass optics and replacing them with UV transparent items such as UV fused silica, quartz and calcium fluoride. I’ve been providing updates as I’ve been doing the work on different aspects of it, but these are spread across a number of posts. I think it is time now to give a bit of summary about where I have got to, and also discuss the latest part of the work – replacing some of the optics with UV fused silica lenses.

Summary first, and I’ll give links to other pages which go over the topics in more details. In the Olympus BH series brochure (from Alan Wood’s excellent Olympus site) there is a nice cross section of a BHB microscope, and this is shown below.

Olympus BHB cross section diagram

With the cross section above I’ve added some labels to help explain the areas I’ve been working on with the UV conversion. Colours are purely there to help identify each section.

Starting at the light source (lower right). The conventional light source is Tungsten filament bulb which gives no UV. The aim will be to use either xenon or mercury xenon light sources for the UV imaging, as shown here. I may also use LED’s especially for the 365nm imaging, but not sure yet on that.

After the light, there is a set of lenses used for high magnification work. These can be moved in and out the beam as needed, depending on the objectives being used. These are glass, and I wont be replacing them, basically because I don’t need to. Some of the mercury xenon lamps I have are focusable, which means I can focus the light without needing these additional lenses, so I can just leave them out of the beam when they are not needed. This cuts down on cost and complexity for me.

After the high magnification lenses there is a mirror which turns the light through 90 degrees. That mirror is actually a good UV reflector so I am leaving that as it is. Then we come to the field iris lens, the auxiliary lens and the condenser, as discussed here and here. All of those are being replaced with UV Fused silica versions, and this will covered in more detail in the second half of this post. The condenser I am building is a simple one with just 2 lenses (simpler than the one shown in the image above).

Above the condenser, but not shown on the diagram is where the slide sites. Even that and its coverslip need to be made of UV fused silica or quartz, as discussed here.

Above the slide is the objective. I’ve written a few pages on these, as I have a range of different ones which can be used for imaging below 300nm. These include the Leitz 16x UV, Leitz 40x UV and 100x UV, Lomo 10x UV, and some others such as Beck reflecting objectives and a 32x Zeiss Ultrafluar which I have not written about yet on here.

Above the objective sits the binocular/trinocular head. I’m going to be swapping out the window and part of the prism for UV fused silica as mentioned here and parts have been ordered for that from UQG Optics. In the top part of the head is the photo eyepiece. Even this needs to be changed from the normal Olympus ones, and I’ve got a range of Lomo ones which are made from quartz as shown here.

After that comes the filters for selecting the UV wavelengths to be imaged and the camera, but that is a whole different issue and will be discussed another day. Phew, we got there in the end, but I hope that gives you some idea of the complexity of converting a microscope to be able to image UV down below 300nm.

Now the the latest update. As mentioned before, part of the conversion requires replacing the field iris and auxiliary lenses, and also the condenser assembly (which in its simplest form on the BHB has 2 lenses which I refer to as ‘Abbe top’ and ‘ Abbe lower’). UV fused silica parts for these came from a couple of optical suppliers in the UK – Laser2000 and Knight Optical – based on what was available from the ranges they had without things needing to be custom made (to help keep costs down). Let’s start by looking at the transmission spectra of the 4 lenses, as measured using my Ocean FX spectrometer and the lens transmission setup I designed.

Transmission of original Olympus lenses (individually and combined)

As can be seen above the 4 lenses cut the UV between about 280nm and 310nm, and when looked at together are effectively blocking everything below about 320nm. Obviously no good for UV imaging below 300nm. This transmission is similar to the modeled data shown here, and given that there were a lot of assumptions in that model, it is not surprising that it isn’t a perfect match. So what do the UV fused silica replacement lenses look like?

Transmission of UV fused silica lenses (individually and combined)

The UV fused silica lenses have essentially flat transmission spectra down to 280nm which is far as I can currently measure the lens transmission. However based on the specs for the fused silica, they would be good for use down to 250nm and even below.

Success. I now have 4 replacement lenses which will transmit the UV below 300nm. But the big question, do they work – can I get an image using them if I mount them in place of the existing Olympus lenses? For this test I just did a brightfield visible light image (this is purely a test of whether they can control light in a manner by which I can generate images of a subject, so at this stage of testing UV is not needed). For this I used a strewn slide of Simbirsk, Russia diatoms from Diatom Shop (very high quality mounted slides) and an Olympus 20x Splan objective. Images were stacked in Zerene (Pmax method). No additional sharpening or image modification has been done to enable comparisons to be made more easily.

Firstly, with all the original lenses still in place (these are low res versions of the original images).

Diatom image using the original Olympus lenses

Now, the same slide imaged with all 4 lenses replaced with UV fused silica ones.

Diatom image using the 4 replacement UV fused silica lenses

And then finally an image taken with the UV fused silica field iris lens, and the other lenses kept as original Olympus ones (as I want to be able to use this microscope for normal visible light work too and didn’t want to compromise its functionality).

Diatom image using the UV fused silica field iris lens but normal Olympus Auxiliary lens and condenser

The good news is that the replacement UV fused silica lenses still allow me to capture images. On the low resolution images above it is difficult to draw comparisons between the different setups, although the UV fused silica lens image looks to be a little lower in contrast. However I also created a set of cropped images from each of the original ones, and they are shown below at actual pixel resolution. Firstly, all 4 original Olympus lenses.

Cropped diatom image using the original Olympus lenses

Next, with all 4 UV fused silica lenses.

Diatom image using the 4 replacement UV fused silica lenses

And finally, with the UV fused silica field iris lens, and the auxiliary lens and condenser as standard Olympus components.

Cropped diatom image using the UV fused silica field iris lens but normal Olympus Auxiliary lens and condenser

What is going on here? Firstly replacing the field iris lens with a UV fused silica one (but keeping the Auxiliary lens and Condenser standard), I get a very similar image to when the field iris was left standard. Good, I can leave the UV fused silica field iris lens in all the time and still use the microscope for normal visible light work. Hurrah. Next, with all 4 UV fused silica lenses, I get a good image, although perhaps not as high contrast as with the original Olympus components. That’s ok, I can live with that as after all I am modifying a device designed and manufactured by a major microscope builder and I wasn’t expecting final image quality to be just as good. Interestingly, with the original Olympus Abbe condenser, there is evidence of strong chromatic aberration (red and green fringes) on the diatoms, but I do not see that with my UV fused silica lenses, and I’m not sure why at the moment. I somehow doubt I have created an APO lens setup, so more work needed to look into that.

It’s been a bit of a large update today, but I thought it was necessary to try and bring everything together as there is so much work that has gone in to getting this far. The UV fused silica lens replacements have enabled me to create an image, and of course their transmission will allow me to work down below 300nm once the final few components for modifying the binocular/trinocular head arrive towards the end of the year.

I’ve been told I am a bit mad (in a light hearted way I hope – anyway, I prefer the term ‘eccentric’) for trying to modify a microscope to image down below 300nm, and that starting with an Olympus BHB was not the right way to go about it. However by using this microscope it has been possible to modify and dismantle and reassemble it easily, something which wouldn’t have been possible with a more recent ‘plastic’ microscope. It certainly hasn’t been straightforward so far but if I look back over the last 8 or so months, it’s made me learn loads about a whole new area of research. It’s that drive to learn new things that keeps us as scientists going, and while yes I’ll use this for research in future, the learning of new things is in itself a worthy goal.

Thanks for reading, and if you want to know more about this or any other aspect my work, please contact me here.

UV Microscopy – modifying the binocular/trinocular head

Earlier this year I started my journey into the world of microscopy, and after a few weeks in thought ‘wouldn’t it be such fun to build a UV microscope’. This idea came along with all the optimism and naivety of someone who really didn’t know what he was letting himself in for. After all, I’ve been researching UV photography for a while and everything can just be reapplied, right. It’ll be easy, right. Well as Arnie said in Commando, that cinematic classic from the 80s, “Wrong”. Even though it has been challenging, it has been great fun and scientifically rewarding. You don’t really know how something works until it is in bits on your work bench.

A few weeks ago in an earlier post, I discussed about the glass elements in the binocular head which contains the eyepieces (covered here). The issue here was that the 2 elements – a circular window and a prism – which were present, blocked the short wavelength UV and would be effectively opaque below 300nm. Now this was a bit of a tricky situation. While I could potentially just get rid of the binocular head and have a tube which the camera would connect to, I wanted to keep the eyepieces and make it look like a normal microscope. To do this, there was only one thing to do, and that would be the replace the window and the prism with UV fused silica or quartz (or calcium fluoride) versions. The problem with this is that both parts would need to be custom made, and anything custom made out of these types of materials is expensive, especially something as complex as a prism made of multiple components.

For help with this I approached UQG Optics, who I have bought filters from before and who have always been good to work with. After drawing up a set of plans they are able to make components in the sizes that I need. Firstly the window. Being a circular this was relatively easy, however the size was unusual so is a custom build. The prism presented more of a challenge. As shown in the previous post, the prism is held within a metal slider. A few screws loosened, and a bit of gentle persuasion with a thin bladed screwdriver, and the prism was released from its cradle to reveal the following.

Binocular head prism

The prism is split into 2 halves – left and right in the image above. When the right hand side is slid into place, the light coming from underneath is bounced into the eyepieces, and none of it can pass directly upwards towards where the camera would be (when the vertical tube is added to the binocular head to make it a trinocular). When the prism is slid across so the left side of it is in the optical path, the light is split with some of it going to the eyepieces, and the remainder going vertically upwards to the camera. To replicate all of this in UV fused silica would be a stretch. It’s a big block of glass, and with some complex angles and shapes. Rather than trying to replicate the whole thing, I decided on a different approach – to cut the prism down the middle, and keep one half while replacing the other half with a simple block of UV fused silica. By keeping the right hand side of the prism, I can keep the side which directs light to the eyepieces and allows focusing. By replacing the left side with a block of UV fused silica, the light would go straight up to the camera. In this configuration, all the light would go to the camera, helping to improve optical efficiency and final focusing for the camera can be done using the live view function. The block of UV fused silica would simply be glued to the remaining part of the prism, and the whole thing mounted back into the the slider. Being a block with parallel sides that is smaller than the overall prism it is also much easier (and cheaper) to make.

Both the UV fused silica window and block have been ordered, and the prism will be sent off to be cut down and prepared. These should all be back with me in December, and are the last UV fused silica components required to convert the microscope.

In all of this the safety of the user (i.e. me) needs to be considered. UV light sources for microscopy tend to be high power, and when focused to tiny pinpoints are extremely dangerous. Couple this with the light then being passed through eyepieces, and the risk of eye damage becomes a very real threat. Retaining parts of the original glass prism for the light going to the eyepiece is actually an advantage here, as the glass itself will block some of the UV, especially the UVB. However there would still be a lot of UVA passing through. How to address this? I wear glasses anyway which have UV blocking layers on them, and I would also wear UV safety glasses when using the device, but it would also be good to have some UV blocking filters in the eyepieces themselves to block anything below 400nm. The eyepieces are 23.2mm diameter so rather than having some long pass filters custom made, I wondered whether there were any 23mm yellow camera filters out there. A quick on eBay search led me to Vintage Cameras Ltd, who had some yellow and orange filters in 23mm diameter for substantially less than they would have been to have custom made. There were two light yellow, two dark yellow and two orange filters as shown below.

23mm filters

Measuring the transmission spectra of the filters using my Ocean FX spectrometer gave the following.

Transmission spectra for the yellow and orange filters

In the scan above, Yellow 1 and 2 are the light yellow filters, Yellow 3 and 4 the dark yellow ones and Orange 1 and 2 the orange ones (if that wasn’t obvious). Yellow 3 and 4 were almost identical, hence the lines overlap. All showed well defined long pass behaviour, although the light yellow and orange filters had a bump in the transmission below their cutoff. Zooming in to the short wavelength end gives the following.

Short wavelength filter transmission

The light yellow filters (Yellow 1 and 2) start letting some UV in at around 300nm and then transmission gradually rises from there. However the darker yellow and orange filters are effectively blocking all the UV light from 400nm to 250nm (less than 0.05% transmission). As the goal is to block the UV while still retaining usability the darker yellow filters are the best trade off between UV blocking and visible transmission, and these will be the ones I use in the binocular head for the UV microscope. It should be emphasised though, that these filters do not obviate the need for UV protective eyewear, they just reduce the risk of damage when using the device while enabling it still be to be used like a normal microscope. I shall still determine the light coming through the eyepieces spectroscopically in the final build.

This UV microscope build has been a really interesting scientific challenge, teaching me more about optics (and microscopes) than I thought there was to know. All the replacement UV fused silica components are now ordered so hopefully in early 2021 I’ll be able to give an update on how it is working. Thanks for reading, I hope you enjoyed it and if you want to know more about this or my other work, you can reach me here.