Category Archives: General

Project Beater – microscope LED light conversion

My journey into the world of microscopy with my little Olympus BHB microscope (Project Beater) has been really fun, and has opened up the possibility of doing some UV microscopy. However, as is inevitable when working with 40 year old equipment there are some challenges. One of which is the in-built lighting system. On my system the electronics were fried, and big scorch marks next to a very complicated looking component made me think that an external power supply would be a good idea. Also the original tungsten bulbs are pretty thin on the ground now, and cost anywhere up to about 50GBP each. Couple that with a lifespan of about 200 hours if they are looked after properly, and it can get pricey, quickly….

As a result of this, converting it to LED lighting looked like a nice option. I wanted to use the base of a broken bulb that came with the microscope as the mount for the LED, just so I could use the original lamp housing. The first attempt with a 3W white light LED was, and I can be brutally honest here, a bit rubbish. It produced a fair bit of light, but the LED was not in the same place as the filament from a bulb would have been, so I lost a lot due to the collimating optics not working that well. Also, 3W for the LED was a little low (or at least that is what I thought). For LED light MkII, I made a couple of changes. Firstly a more powerful LED – 10W white light with a colour temperature of 4000-4500K. This runs at about 10V and 1A, and cost about 2GBP delivered from eBay. Secondly, mount this LED where the filament of the original bulb would have been. This was a little more challenging….. Before we dive into the conversion, a jump to the end result and a pretty image. Here’s a dark-field image of a Maize seed taken on Project Beater using the LED light conversion I’ll discuss below.

Dark-field image taken with 40x Olympus lens

The old bulb was removed easily enough with the use of a mini blow torch to melt the solder holding it in place. The LED needed mounting about 3cm from the top of the existing mount, to enable the LED to be in the same place as the filament of the bulb. I needed a metal tube (to help act as a heat sink for the LED) about 3cm long, and with a nice flat top. This is where work overlaps with a hobby of mine. In my spare time I do some target shooting, and I had some empty brass cases on my workbench. One, from a 338 Lapua was about the right diameter to fit in the lamp housing. Plus it had a nice flat top for mounting the LED on and giving a good thermal contact. A few minutes with a hacksaw, and a bit of JB Weld, and the cut down brass case was mounted.

Original bulb holder modified for the the LED
Re-purposing a spent case

Once this had set fully, the LED could be added (again, thank you JB Weld) and wired in to the original connectors.

10W LED all wired up

Putting this into the microscope produced a little bit of problem. It fit just fine, but was so bright that it was difficult to use in the normal bright-field setup. Even with the current turned down it was still my brighter than the original 30W tungsten bulb. Not good. Or is it? Unlike bright-field imaging, dark-field imaging modifies the light transmission to the sample with the end result of making the areas which are around the sample look dark, and allowing only light that has been scattered be imaged. In doing so it can be used to show features not visible in normal bright-field imaging. But dark-field imaging needs a lot of light. Aha, now we have an LED light source which is too bright for normal use, would it be suitable for dark-field imaging?

A quick test. The sample is Maize seed section, and the images below are taken with a 10x UVFL Olympus objective, and photographed through a 3.3x photoeyepiece. A quick comparison between the original 30W tungsten bulb and the 10W LED for dark-field imaging. Firstly, the 30W tungsten bulb, and a camera exposure time of 1.3s

Original 30W tungsten bulb, 1.3s exposure time

The 30W tungsten bulb produces a usable dark-field image, but there is a bit of a hot spot in the middle of the image. Secondly the 10W LED, same ISO setting on the camera, and the microscope was left the same. All that was changed was the light. 1.3s was too long for this light, and I had to reduce the time by about 15x to 1/13s to get an similarly exposed image.

10W white LED, 1/13s exposure time

With the 10W LED the colour temperature of the image is much cooler than with the tungsten bulb (not surprising as the spectra is different), and the lighting intensity looked to be a bit more even across the sample. But the big difference is the exposure time – about 15x faster with the LED indicating much more light being present. So, yes it does look as though this LED light will be suitable for dark-field imaging, and will be a better option than the original 30W tungsten bulb. It is so bright in fact, that it would probably work well with narrow band pass optical filters, to do multispectral microscopy, but that is for another day.

For now, here are a couple more dark-field images of the Maize see cross section, taken using the LED light (plus of course the one at the top of the post which was done at higher magnification).

Dark-field Maize seed image taken with 10x UVFL Olympus objective
Dark-field Maize seed image taken with 10x UVFL Olympus objective

Not all experiments work out as expected, and we should be really thankful of that – if they did, we wouldn’t really learn anything new. As scientists we shouldn’t be afraid to try new things, and test the limits of the equipment we use. Who knows, it might throw up some interesting new area that is useful for your work.

As for what’s next, well I’ve recently started seeing 10W 365nm LEDs for sale, so perhaps getting one and seeing whether it is useful for UV fluorescence microscopy would be a good idea. But first, time for a coffee. Thanks for reading, and if you are interested in this or any of my other work, you can reach me through my Contact page, here.

UV microscopy of sunscreen formulations

Light microscopy is a powerful tool to help you understand how your product or formulation is structured. As with all imaging techniques though, depending on what you want to image, it is not without its challenges. When the Covid lockdown started, I bought myself a beaten up old Olympus microscope, with the aims of learning how it worked getting it up and running again, and with a long term goal of using what I’d learned to build a UV transmission microscope. Of course, things progressed pretty quickly as we shall see….

First though some of the results of my first attempt at UV microscopy. Taking the standard Olympus BHB microscope, and replacing the halogen light source with a 200W Xe lamp which is filtered to produce light mainly in the UV region from 250nm to 400nm, I took some images of a sunscreen formulation on a glass slide. This product is an oil in water emulsion (oil droplets suspended in a water based continuous phase), and with the UV absorbing component in the oil phase. This would be a nice simple one to try, as the two phases should look very different under UV.

For an objective lens, I used a Leitz 40x UV objective (which is discussed further here), to give me a good chance of getting as much UV to go through it as possible, and this is what a thin film of the sunscreen looked like imaged using a UV modified Nikon d810 camera (overall magnification about 400x).

UV microscope image of sunscreen formulation (approx 400x mag)

So, what are we looking at in the image above? The black areas are the oil phase, where the sunscreen is present (hence they look darker as the UV is being absorbed). In between the dark areas, is the water phase. You can see individual oil droplets, but also some areas where the oil droplets are starting to merge together as the emulsion structure begins to collapse.

While doing the images, I noticed that if the microscope cover slip was touched, the water and oil phases would ‘flow’ as a result. To capture this behavior I took a video clip using the UV modified camera (watch it with the sound off unless you like to noise of cooling fans from the light source).

UV video clip of sunscreen movement under the microscope cover slip

What are we seeing in the image above? Firstly, at the start of the clip, the lighter grey water phase flows from left to right, taking some of the individual oil droplets with it. This is then followed by the oil phase as it collapses and the darker sunscreen containing oil droplets fuse together. The video is darker than the photograph, as there is so little UV getting through to the camera – in order to get anything at all on the video the ISO setting on the camera had to be turned up to 10,000!!!

Overall, a success for a first experiment. The sunscreen containing oil droplets look like they should under UV, and it was even possible to get some video footage of the behavior.

What is going on here though, why is everything so dark in the video clip? The light source was a 200W Xe lamp, focused using a collimator into the back the microscope – there should have been loads of light. This is where the issues with dealing with optical systems comes in to play. This microscope was not designed for UV imaging, and so every lens, every mirror, every beam splitter will be be absorbing some UV. Imaging is done through a photoeyepiece in the top of the microscope which projects a image into the camera. This in itself is a relatively complex piece of optics designed for use in visible light rather than UV. Also the 200W Xe light source was just ‘shone’ into the back of the microscope with the hope that the alignment would be good enough. Add all these together and a lot of the light will be lost before it even reaches the camera. Using a spectrometer it was possible to estimate how much was being lost. Firstly an irradiance scan taken with the cosine corrector of the collection fiber about 3cm in from the collimator on the light source.

Irradiance scan of the light source

This light is producing a lot (and I mean a lot) of UV. I wear gloves, and UV safety goggles while using this light, as the risk of burning and eye damage is high. There is a good range of UV wavelengths though, from about 250nm up to 400nm.

Now a second scan with the spectrometer, with the cosine corrector up against the exit of the photoeyepiece at the top of the microscope (after the light has gone through the microscope, the condenser, the glass slide itself, the Leitz UV objective lens, and the photoeyepiece).

Irradiance after passing through the microscope

The light getting through the photoeyepiece is about 0.1% the intensity coming from the lamp itself. Also the wavelength range of UV has been reduced. Now there is nothing really below 350nm. This is because the microscope is not designed for UV imaging – the glass lens elements present absorb the UV, both in the glass itself and any coatings on them. This is why UV microscopy is so difficult – all the components in the optical train must be optimised for UV rather than visible light, especially if you want to get below 350nm.

There is another issue to consider as well – the filtration of the light before it gets into the camera. On my UV modified Nikon d810 camera, there is an internal filter which lets through light mainly between 320nm and 380nm, while blocking unwanted wavelengths. So as you can see, with the light getting through the microscope, I am only really seeing what is going on between 350nm and 380nm at the moment.

Future direction for this work is for me to build a UV microscope, rather than trying to adapt one to do the job – something capable of being used between about 250nm and 400nm. This will be a bit of along term goal though, as the challenges are many. Still, should be fun though…..

In the mean time if you’d like to know more about this or any of my other work, get in touch – you can reach me through my Contact page.

Leitz UV microscope objectives – historical optics

When I look back at my school days, thinking about some classes still fills me with dread. One of those was History. Memorising endless British Kings and Queens and the dates they reigned. Dull, dull, dull, dull, dull. It’s strange though as I loved archaeology, but History classes just didn’t do it for me. It could also have something to do with the teacher, who had a habit of prowling around the class while barking facts at us, but that is another story. As I have ‘matured’ I’ve gained a bit more of an appreciation for history – without knowing where we’ve come from, how can we make sure we are heading in the right direction, or perhaps more pertinently these days “those who do not learn history are doomed to repeat it”. Anyway, I digress, and back to the science.

As part of my research into imaging science, UV photography has really captured my imagination. Recently I have started looking more at microscopy, and so the idea of doing some UV microscopy seemed like an obvious area to explore. However most objective lenses aren’t ideal for UV transmission, as they block some or all of it. Some of the standard microscope objective lenses, such as the Olympus UV fluorescence (UVFL) lenses have pretty good UV transmission around 360nm, but then it drops off to essentially nothing at 320nm and below. If you want to look at short wavelength UV, say at 320nm and below, then you start needing rather specialised objectives which are no longer based no glass lenses.

These UV lenses tend to be very expensive, especially when bought new, however they do occasionally appear on the second hand market. When they do become available, prices can vary wildly, often depending on whether the seller knows what they are, and whether collectors are getting into a bidding war. One manufacturer of these types of lenses was Leitz, and recently I was fortunate enough to find one of their 16x UV objectives on eBay from a seller in the US, which I quickly snapped up. When it arrived I started trying to track down any information about it, but couldn’t find anything definitive. Even asking a couple of experts on Leitz failed to reveal anything other than ‘they were made in very small numbers’. A few weeks after the 16x arrived, I was given the details of a dealer in Germany – Optik Online, here – who had another couple of the Leitz UV lenses available. Now, for those of you who haven’t looked here, this is an amazing shop, and somewhere an optics geek can spend hours. It turned out that he had two more of the Leitz UV lenses, a 40x one and 100x one (both glycerin immersion lenses), and I quickly put in an order for these as I hadn’t seen them anywhere else. Here they are;

Leitz 40x UV objective
Leitz 100x UV objective

These 2 seemed to be a similar style to my 16x one, but unfortunately the dealer didn’t know anything else about them. I did manage to find a Leitz brochure from 1985 which mentioned three UV objectives, with the same magnifications as mine, and an excerpt from this is shown below;

Excerpt from 1985 Leitz catalogue

In the catalogue to UV lenses are called ‘Quarzobjektive’ but while they magnifications and NA values matched my lenses, the cover slip thickness is different (mine say 0,35mm quartz, while these say 0,17mm). It does seem strange though, and I’m actually wondering whether the catalogue has an error in it regarding the cover slip thickness. More research to be done there, to hopefully get to the bottom of that…..

While looking through the Optik Online site, I also found a Leitz Quartz 300x objective, which looked very interesting, so included that in my order. Here is the lens;

Leitz 300x Quartz objective

Now this one is a bit different in appearance to the other ones, and it is also meant for a different design of microscope (the tube length on this one is 400mm rather than 160mm on the others). After a bit more detective work, I found some information on something similar from the 1960s, and that it is likely to be a reflecting objective rather than a reflex design (although I wont be dismantling it to check that). Here’s a copy of the brochure detail from 1967;

Catalogue excerpt for the 300x objective from 1967

The objective seems to be similar to part number 520 110 in the catalogue, although the actual design looks a little different.

Optik Online also had a 32x Zeiss Ultrafluar lens which is also designed for UV imaging down to 220nm, so that is coming over for testing too and I’ll share more about that once it arrives.

Looking to older scientific equipment for inspiration is fascinating, and this is an area of history that certainly gets my attention (if only we did this at school). The designs that these guys were making 30+ years ago are amazing, and still perfectly usable today for research. If you can find them, and that can be a quest in itself given they were never made in huge numbers, they are often available at a fraction of the cost of new lenses. Digging into their history can also be a challenge – I’ve been able to find some information on these lenses, or at least similar versions to them, but even so I’m not completely sure that what I’ve found is exactly right. If you know any more about these, such as when they were from and what they were designed for, I’d love to hear from you.

Thanks for reading, and enjoy your research.

Zeiss 60mm f4 UV-Objektiv lens and Buttercups in UV

Choice of lens has a huge impact on the nature of your final image. Some lenses are so clinically sharp you can cut yourself on the details in your shot. Others have more ‘character’. Here’s a fun and unusual lens, which falls into the second category, although I suspect under certain circumstances can yield some very usable images. It’s the 60mm f4 Zeiss UV-Objektiv in Exakta mount. Pictures of the lens below, mounted on to my UV converted Canon Eos 7D camera.

60mm f4 Zeiss UV-Objektiv
60mm f4 Zeiss UV-Objektiv

The lens is a simple triplet construction, and while it has pretty good UV transmission, it does not go as deep into the UV as the 105mm UV Nikkor Or 85mm Asahi Ultra Achromatic Takumar. I measured the transmission spectrum between 280nm and 420nm and got the following curve.

Transmission through the Zeiss 60mm f4 UV-Objektiv

This lens is a simple construction – 3 element, 3 groups – and suffers quite horrendously from focus shift as the wavelength changes. To test it out, I got an Eos to Exakta adapter so I could mount it on my UV converted Eos 7D, and took a picture of some Buttercups in the garden, which is shown below.

UV Buttercups with the Zeiss 60mm f4 UV-Objektiv (wide open)

Once white balanced correctly, the Buttercups show strong UV characteristic of the black centers and yellow petals, but the image is definitely a little soft. The UV filter in my modified Eos 7D lets through quite a broad range of UV wavelengths, so I get the feeling using a narrower band pass filter, and on a camera which has live view focusing, would improve the image.

There are some truly fascinating historical lenses, either quirky designs, or amazing engineering (or more often, both) which can be adapted to digital imaging. If you want to know more about this or any of my other work, I can be reached through my Contact page. Thanks for reading.

Project Beater – phase contrast imaging added

For those reading my post you’ll be familiar with my Covid-19 project – Project Beater – my Olympus BHB microscope rebuild, and a chance to learn a new imaging skill. As always when playing with new imaging equipment, it quickly turns into a ‘what can I buy for it?’ situation. Reminds me of when in owned a VW Corrado G60, when I spent most of my time (and money) on improving, tuning and generally modifying it. Ah, that was a fun car, and perhaps was my original Project Beater….

My VW Corrado G60 – the original Project Beater?

Anyway, I digress, and back to the microscope. How you use lighting has a huge impact on the type of imaging you can do with a microscope, and the images you can produce. Bright field imaging, where the sample is illuminated by transmitted light (i.e., illuminated from below and observed from above) white light, and the contrast in the sample is caused by attenuation of the transmitted light in dense areas of the sample. Bright-field microscopy is the simplest techniques used for illumination of samples in optical microscopes. The appearance of a bright-field microscopy image is a dark sample on a bright background. However, this does not always give good contrast between the subject and the background, and there are a host of illumination techniques which can be used for different samples to make them clearer to image.

One such approach is called Phase Contrast imaging. I’m not going to give a detailed review of how this works, as there are plenty of those online. It’s a very useful technique for imaging live cells, as it improves contrast without staining. To demonstrate this, below are two images of cells from my cheek collected with a quick buccal swab, taken with a 10x Phase contrast objective, through the microscope eyepiece (100x overall magnification). Firstly, the normal bright field image.

Bright field image of cheek cells

The cells are just about visible, but with very low contrast against the background. And secondly, a phase contrast image of the same field of view.

Phase contrast image of cheek cells

The cells become much more visible in the phase contrast image. This becomes even more obvious when looking at a 40x phase contrast image (400x overall magnification).

Higher magnification phase contrast image of cheek cells

So, how did I do this on the BHB microscope? For phase contrast imaging you need a different condenser and objectives with the right phase contrast rings inside them. The condenser for the BHB I have seem to be quite rare (and expensive). I managed to find a potential set for sale at Microscope Wizards, here. However it looked like they were for a different type of Olympus microscope to mine (a more recent one), as the condenser mounted using a dovetail mount which mine didn’t have. It looked to me as though the condenser might be the same diameter as mine, and if so, may be possible to mount in the same way. A few emails later, confirmed the size of it was correct and it should work, so I bought it to try it and thankfully it worked. The condenser came with a couple of phase contrast objectives so gave me everything to get me started.

Sometimes when building and rebuilding equipment original parts aren’t available, and you have to modify things or use something originally designed for another piece of equipment. As scientists we have to look at a problem, break it down in to its component parts and then address each one. The skill set is just the same, and a good scientist can turn their hand to any problem.

If you want to know more about this or my other work, you can reach me through my Contact page. Thanks for reading.

Imaging with the Zeiss Luminar 100mm f6.3

A while ago I got a 25mm Zeiss Luminar lens which turned out to be a really useful lens for UV imaging (you can read about that here). As the 100mm is a triplet (even simpler than the 25mm), I had high hopes for its potential as a UV lens. Turns out that hope was misplaced, but the lens is still an interesting one….

I bought my copy as a microscope unit, consisting of the lens and a few other parts. This is what it looked like as the complete unit.

Zeiss Luminar 100mm f6.3 microscope setup

The lens itself has a 35mm screw thread, and removing the extraneous parts of the setup above leaves this.

Zeiss Luminar 100mm f6.3 lens

The lens elements protrude on this lens and mine had been put down on one of them, leaving a tiny mark (<0.5mm) in the middle of one of them. Even so I still wanted to give it a go for photography. The thread size is 35mm, so I bought a 35mm x 0.75 to M42 adapter from eBay. I then mounted this on a range of extension tubes (up to 11cm in total) and on to my Eos 5DSR. I use part of the original unit as a makeshift hood.

Zeiss Luminar 100mm f6.3 on extension tubes

Here’s some shots from the garden, all hand held. First with about 6cm extension, wide open, and full frame image (but reduced in resolution for sharing).

Zeiss Luminar 100mm wide open on about 6cm extension

The short extension gave a focus distance of about 3m, and a really nice swirling bokeh in the background of the image. Then with 11cm extension, and slightly stopped down, again resized for sharing.

Zeiss Luminar 100mm on 11cm extension stopped down slightly
Zeiss Luminar 100mm on 11cm extension stopped down slightly

And finally, this was taken as a crop from the image above before resizing, and is shown as actual pixel resolution.

Actual pixel resolution from the Zeiss Luminar 100mm

Even with a tiny mark on the lens there looks to be plenty of sharpness there. Could make for a really interesting portrait (with a short extension) or macro lens (with longer extension tubes).

Reusing older lenses can give your imaging a unique look, impossible to replicate digitally. If you want to know more about my work, you can reach me through my Contact page.

Project Beater – lights, camera, stacking…

My Olympus BHB microscope refurbishment (affectionately know as Project Beater, given the state it was in when I got it) is coming along during the Covid-19 lockdown.

Initially I thought that changing the lighting to LED would be the way to go. Longer bulb life, and brighter light for much lower power sounded like a win-win. However my initial attempt left me a bit disappointed. C-, more work needed there. So back to the original Tungsten filament bulb. 6V and 30W means a 5A power supply, and my benchtop unit wasn’t up to the job, so a quick ebay purchase of an adjustable 3-12V, 5A supply (for the princely sum of £12), and a couple of 4mm banana plugs, and I has a suitable power supply. This gives a more even light spread than the LED, and is easily adjustable for brightness.

After my initial attempt, it became obvious the focus stacking was going to be needed. I downloaded Zerene stacking software, as it gave a 30 day free trial, and it seems to be the go-to package. Now, to set about trying it out.

My slide prep kit came with a few pre-prepared slides, so these are great for practice, as I don’t have to worry about preparing something. One of these is a Corn seed cross section. Lots of detail, so a simple one to start imaging with. Below is the result of a stack of 22 images, taken with the 4x SPlan objective, through the camera adapter and captured on my monochrome Nikon d850 (final magnification about 80x).

Corn seed cross section, approx 80x magnification

It’s fairly obvious that focus stacking improves things a lot compared to a single image, although it’s not completely fool proof. Certainly a piece of software which requires practice to get the best out of.

My journey into microscopy has been fascinating, and something I will continue to play with moving forward. If you want to know more about this or my other work, you can reach me through my Contact page.

Leitz 16x UV microscope objective imaging

For my UV imaging research I regularly look out for second hand equipment, as new UV imaging stuff, especially UVB capable kit, can be hugely (eye-wateringly) expensive. I recently saw this one advertised. It’s a Leitz 16x microscope objective, marked up as UV on the barrel of the lens.

Leitz 16x UV objective

Interestingly for my work, the markings said it was designed for 160mm tube microscopes (which is what I have) and for 0.35mm Quartz coverslips. Now, quartz coverslips aren’t your normal microscope equipment, and would imply that this is meant for deep UV work (at 300nm and even below). The vendor wasn’t able to tell me much about it, other than it came from a piece of equipment he had stripped for selling a while ago. So I made and offer which was accepted, and it started the long journey of making it here to the UK from the US. I wasn’t able to find anything out about this lens online, so it was a bit of a gamble, although the lack of information often indicates how rare something is.

When it eventually arrived, after the lovely Customs fees had been paid, the first thing I did was measure the transmission through it. And to my excitement this is what I got….

Leitz 16x objective transmission from 280nm to 420nm

The transmission curve was very flat from 280nm to 420nm, indicating that this was indeed a lens designed for use at short wavelengths. Quite a few lenses will transmit light in the UVA region from 400nm down to around 350nm, but below that things get tricky. This Leitz lens doesn’t have any actual glass in it, and the elements are probably either quartz or calcium fluoride (or perhaps both) which is why it transmits so far into the UV.

The next step for me was to mount it on a camera and see what the images were like. I mounted this on my UV converted Nikon d810, and imaged a Buttercup flower using my Hamamatsu LC8 Xenon light source for illumination. Using 6.5cm of extension tubes meant that the final magnification was about 9x, rather than the full 16x. I focused in on the part of the Buttercup petal where it goes from black to yellow in the UV image (the UV world looks very different to the visible world for many things), and this is what I saw.

Buttercup petal in UV with the Leitz 16x UV objective

The first thing to note is that the lens does indeed transmit the UV well, and the region of the petal which absorbs UV strongly (in black) can easily be seen. This is a single image, not stacked, and there is some pretty severe field curvature on it, which is not surprising given there are no other compensating optics being used. Also the magnification would be higher with the correct length extension tubes, which would crop the field of view and flatten the image. Focus stacking would help as well by creating a much sharper image. Amazingly the objective seems to be achromatic between the UV and visible, as almost no focus shift was needed between using the camera viewfinder to focus it and taking the picture in UV. This is very handy for using it and a nice feature to have.

The UV converted d810 camera I have is mainly UVA sensitive, however I also have monochrome converted cameras which are UVB sensitive. So the next thing will be to build an inline optical filter system, so I can start using it in the UVB region. Not a simple task, but science is all about challenges and finding ways to overcome them isn’t it….

I certainly got a bit lucky finding this one. It just goes to show, that sometimes it’s worth taking a gamble to push your work forward. They wont all pay out, but that’s not the point. Without pushing the edges of your research, how can you expect to learn anything new. If you want to know more about my work and the areas I am researching, please contact me here.

Polarisation in the UV to reduce shine

I’ve been playing around with polarisation in my photography for years, originally in visible light imaging of skin to reduce shine and aid in colour assessment, and more recently in the UV when I designed the first cross polarised UV imaging system for looking at sunscreens (published here). For pictorial purposes, full cross polarisation can be quite harsh, making the subject look flat and lifeless. Sometimes you just need to dial down the reflections, rather than completely eliminate them. You can do this by changing the angle between the 2 polarisers.

To show how this works, and how it can be done in UV, I’ve done a sequence of images of a Buttercup flower, using my UV modified Nikon d810. Buttercups in the UV are notorious for showing specular reflection, so are always a test for lighting setups. The lighting I used was a Hamamatsu LC8 with a UV collimating lens. Polarisers are Moxtek UV linear polarisers, although the principle applies to other polarisers too.

Firstly, no polarisers at all – normal image, at 0.6s exposure.

Non polarised UV image

The Buttercup shows its usual shine bands in the UV image, and shine is evident on the glass vase as well, and the edge of the 20% diffuse reflectance standard on the right hand side of the image. In fact there is shine in various parts of the image, anywhere there is direct specular reflection occurring.

Now, adding the polarisers, one on the light source and one on the lens. The exposure has been upped to 4s due to light being absorbed by the polarisers (there is about 1.3 stops loss of light for each polariser with these two).

Parallel polarised
45 degree cross polarised
70 degree cross polarised
80 degree cross polarised
90 degree fully cross polarised

As can be seen from the images above, as the polarisers are moved from parallel to crossed (90 degrees to each other) the degree of specular reflection reduces, reducing the visibility of any shine. While specular reflection is removed by cross polarisation, diffuse reflection isn’t. You can see this as the 20% diffuse reflection standard on the right hand side of the image hardly changes as the polarisers are moved.

Control (and more importantly, understanding) of every aspect of the imaging process is vital when you’re trying to do reproducible scientific photography. When you do understand it, the setup can be designed to image just what you need to, whatever it is you are trying to image.

If you’re interested in this or any other aspect of my work, you can reach me here.

white balancing UV photos with Culture Hustle Black 3.0 paint

I’ve been a big fan of the Black paint that Culture Hustle make for ages (see here), as it has great light absorption from UV to IR, is easy to apply, is easily available and importantly given I’m a Yorkshire man, a good price. I’ve used it to paint the insides of various optical devices I’ve built to cut down on reflections, and I use it for making enclosures for UV imaging.

I got to wondering whether it would make a good white balance standard for UV imaging, and whether it could be mixed with white paints or pigments to make greyer versions with different reflectances. While cheap, normal photo colour charts aren’t great for UV imaging, as they change in reflectance with wavelength. At the other end of the price range is Spectralon, which is available in a range of different reflectances, but is very expensive, and easy to damage if not looked after.

To test out the idea I settled on 5 different white balance targets to try;

  1. Spectralon 10% diffuse reflectance
  2. Culture Hustle Black 3.0 paint
  3. Culture Hustle Black 3.0 paint mixed with their white paint (about 50:50 mix)
  4. Culture Hustle Black 3.0 paint mixed with magnesium oxide powder to make a grey paste
  5. Brown cardboard the paints were applied to

The target subject was a Buttercup in a glass vase on a grass lawn. Images were captured in bright sunlight with a Nikon d810 camera converted to UV by Advanced Camera Services Ltd and a Rayfact 105mm UV lens. A RAW image was taken of the Buttercup, and the white balance targets and then white balanced in Darktable before exporting as reduced size JPEGs. No further processing was done.

Here’s what they all look like.

1. Spectralon 10% diffuse reflectance standard
2. Culture Hustle Black 3.0 paint
3. Culture Hustle Black 3.0 paint mixed with magnesium oxide
4. Culture Hustle black and white paints mixed
5. Cardboard

As per usual in the UV, the Buttercup looks black in the middle as it absorbs the UV very strongly there. Overall white balancing with the Black 3.0 paint, and with the Black 3.0 paint mixed with magnesium oxide powder to make a grey paste, gave almost identical results to the Spectralon 10% diffuse reflectance standard.

Mixing the black and white paint together and white balancing with that gave a different colour balance to the image, making it more yellow and cutting the blue back (very obvious if you look at the grass). Also, the black centre of the flower has taken on a yellow hue. Using the cardboard itself for white balance resulted in a slight yellow shift in the colour balance compared to Spectralon, but not as much as the black/white paint mix.

What’s going on here? The Spectralon has a pretty flat reflectance curve in the UV which is why it makes a good white balance target. The Black 3.0 also has a pretty flat response in the UV, which is something I have tested before, which is why it also is acting as good white balance target. However by itself it is very dark, so might not be suitable for including in images as a white balance target. Mixing it with magnesium oxide powder, made a paste which had about 10% refectance. The magnesium oxide works well, as again it has a fairly flat reflectance in the UV. Mixing the black and white paints, resulted in a grey paint which looked in the UV region to have about 10% reflectance, but resulted in a very different white balance. What is most likely happening here is that the reflectance of the white paint varies in the UV as a function of wavelength. This will lead to an uneven reflectance curve in the UV, and as a result when used for white balancing a different colour balance (a bit like the normal photo calibration charts).

The cardboard surprised me a bit. I thought the final white balance would be way off. It was not the same the Spectralon and was still a little too yellow though.

Overall, I was very impressed with the Culture Hustle Black 3.0 paint for use as a white balance standard for UV photos, and the results compared very well with Spectralon. Adding in a some magnesium oxide powder to make a grey version also worked well, although mixing it with white paint did not.

If you have over 500USD burning a hole in your pocket, by all means go for the Spectralon. However for a small fraction of that the Black 3.0 paint will do the job and do it well, and if mixed with some magnesium oxide powder to make a paste can be made to be grey as well as black, making it more versatile.

If you want to know more about this or any other aspect of my work, you can reach me here.