When is a filter not a filter – dichroic dilemas

As a photographer who spends a lot of time working outside of the visible spectrum I regularly use filters on my lenses or inside my camera to isolate the wavelengths of light I’m interested in seeing.

Buttercup in UV, taken with UV converted Canon EOS 7D camera and Zeiss 105mm UV Sonnar on extension tubes

However filter choice can be far from straightforward, especially when imaging in the UV region. Today, I’ll give a quick example of why this can be a problem. First though a question – when is a filter not a filter? Take a red filter, which is often used in black and white photography to darker skies and lighten foliage. What is it doing to the light? What a dumb question, it’s letting red light through. Well, yes, it is, but then a simple clear glass filter would let red light through. With filters it’s not so much about what they are letting through (although that is obviously important), it’s about what they don’t let through. A red filter is red because it blocks light that isn’t red – it lets red through while blocking other wavelengths. When imaging in the UV region, this blocking becomes very important, because cameras are relatively insensitive in the UV region compared to the visible and IR regions. If the blocking on the lens isn’t up to scratch hen wavelengths you’re not interested in can make it through to the sensor and contaminate the image.

There are two main types of filters that photographers use – ionic filters such as Schott UG11 or Hoya U-340 which filter the light using the bulk properties of the glass itself, and dichroic filters which have a thin coating on one or both surfaces the glass which provides the filtering. Dichroic filters can just be on plain, colourless glass or they can be applied to an ionic filter, so the filtration is provided by the coating in addition to the bulk of the glass. An example of a dichroic filter in widespread use for UV photography is the Baader Venus U (commonly known as the Baader U).

Dichroic filters can be tailored to allow for very sharp cutoffs on the filters, and also high transmission, while ionic filters tend to have much smoother transitions. On the face of it, dichroic filter would seem to be perfect, for the UV photographer, offering high transmission and in theory excellent blocking of out of band wavelengths.

But there is a bit of an issue with dichroic filters which can be a problem if not considered. That is the effect that the angle between the incoming light and the filter has on its transmission properties.

Let me explain. I shall start with an example of an ionic glass UV filter – the LaLa U made by UVIRoptics. The LaLa U is an ionic filter stack which lets UV through while blocking visible and IR. Here’s what the transmission through the filter looks like at two angles – light incoming at 90 degrees to the filter, and at approximately 45 degrees.

LaLa U filter stack transmission with the light at 90 degrees and 45 degrees to the surface of the filter

Transmission was measured using an Ocean Insight FX spectrometer and light source. Tilting the filter to 45 degrees drops the transmission (as the light is going through a lot more glass), but the shape of the transmission peak doesn’t change. If the graph is replotted to look for leaks, I get the following.

LaLa U graphs replotted to look for leaks

As expected, the LaLa U has good blocking in the visible and IR regions, and tilting it does not make a difference.

Now, what happens when we look at the dichroic Baader U filter in the same way? First graph, full scale, second graph magnification to look for leaks.

Baader U transmission with the light at 90 degrees and 45 degrees to the surface of the filter
Baader U transmission graphs replotted to look for leaks

The Baader U behaves very differently as it is tilted. Unlike the ionic LaLa U filter, the Baader U transmission spectra shifts to a shorter wavelength when the light is at 45 degrees to the surface of the filter. In addition the UV transmission profile is much more jagged in shape. Also, and even more concerning it is now letting in light above 680nm, and while 0.2% transmission may not seem like a lot, this can be an issue for UV imaging.

Some dichroic filters have even tighter transmission regions than the Baader U. Examples of these include Edmund Optics and Thorlabs 10nm bandpass filters. These both claim to have high blocking of out of band wavelengths, while at the same time giving high throughput in the region of interest. I use these for filtering light sources and also for imaging in front of camera lenses, although they are normally used more for filtering light sources than imaging.

How do these behave? Firstly, the Edmund Optics 330nm, 10nm bandpass filter which claims OD4 blocking (<0.01% in the out of band regions) and has a mirror finish on both surfaces of the filter as a result of coatings.

Edmund Optics 330nm 10nm bandpass filter transmission with the light at 90 degrees and 45 degrees to the surface of the filter
Edmund Optics 330nm 10nm bandpass filter transmission graphs replotted to look for leaks

And next, the Thorlabs 340nm 10nm bandpass filter.

Thorlabs 340nm 10nm bandpass filter transmission with the light at 90 degrees and 45 degrees to the surface of the filter
Thorlabs 340nm 10nm bandpass filter transmission graphs replotted to look for leaks

First, I should mention that the Edmund and Thorlabs graphs highlight some of the issues when using a solid state spectrometer like the one I have for looking at filters with really sharp peaks, and that is the artefacts that can occur at either side of the main peak in the profiles. The slight dip in transmission below 420nm and above 700nm for them both is not the spectrometers fault. That was all mine. I didn’t recalibrate the baseline while running these 4 filters, so the last two (Edmund Optics and Thorlabs) are showing a slight drop below 0% transmission below about 420nm and above 700nm. It was a sunday morning, I hadn’t had my coffee yet. Excuses over, and despite those effects, how does tilting impact the spectra? With the Edmund Optics filter, peak transmission drops from 330nm to 310nm, and reduces in intensity. Very worryingly though, it starts to let significant amounts of light through across the visible spectra and into the IR. This would be expected to seriously contaminate a UV image if not dealt with properly.

The Thorlabs filter peak transmission drops from 340nm to about 315nm, along with a huge drop in intensity. However unlike the Edmund Optics filter, it doesn’t develop obvious leaks in the visible or IR region (at least up to 800nm).

I should emphasize at this stage that these Edmund Optics and Thorlabs filters have not been developed for use as camera filters, and are designed to work with the light hitting them at 90 degrees to their surface, so it is not unexpected to see their performance being degraded when the light hits them at 45 degrees. Also, I’m not clear on whether other filters in their ranges behave in exactly the same way. Dichroic filters are tailored for the wavelength they are to be used at, as such they could well vary in the transmission they show at different angles.

If you’ve made it this far and are suffering from graphical overload, you can now take a breather – no more graphs. Why are these changes important and more importantly, what can be done about them if using these filters for photography?

As for the ‘why’ consider how light goes through a lens and reaches a camera sensor. It doesn’t just come from directly in front of the camera, but as a cone of light. The wider the focal length of the lens, the wider this cone. 45 degree either side of normal to the lens is pretty wide, equating to a focal length of about 22mm on a camera with a sensor the size of a 35mm SLR. As a result of this the spectral distribution of the light reaching the edges of the image could be very different to the light in the middle of the image, leading to strange colour shifts across the final UV image.

The 45 degrees used for the test here is a pretty tough harsh based on this, and the effects would be expected to become less severe as the angle gets closer to 90 degrees to the surface of the filter. For example a 105mm focal length lens has angle of about 23 degrees on a full frame (35mm) sensor camera. On longer focal length lenses, like a 105mm one, the effects described here would be expected to be much less pronounced.

What can be done about this? Longer focal length lenses will be less of an issue, so be aware that dichroic filters can have an issue when using wide angle lenses. Use a good and appropriately sized filter hood to block as much light coming in from the side as possible. If the mechanics allows for it, try mounting the filter behind the lens (between the lens and body of the camera). Although be aware that mounting a mirror finish filter just in front of the sensor can present its own problems with reflections. Perhaps the most obvious thing to do is consider using an ionic glass filter stack instead of a dichroic one. If the slightly reduced transmission is something you can live with, and there is no need for ultra sharp cutoffs to the light, then this can be the most flexible approach and can be used on a wide range of focal length lenses without changing the spectral distribution of the wavelengths getting through to the sensor.

This aim of todays discussion is not to have a downer on dichroic filters – they are extremely useful, offering high transmission and well defined cutoffs – the aim is to point out that depending on how they are being used they can have issues which need to be dealt with to get the best out of them.

There’s been a lot of graphs today, so well done if you’ve made it this far. Thanks for reading, and if you want to know any more about this or any other aspect of my work, you can reach me here.

digital Collodion – A modern take on a historic photographic process

The Collodion imaging process harkens back to to early days of photography. A photosensitive layer comprising a nitrocellulose carrier and silver halide on a glass plate forms the image capture media, and the result is a negative image of the subject – essentially a precursor to the more recent use of film photography.

Unlike modern films and digital cameras, Collodion plates are not sensitive to the full visible spectrum, being essentially unable to capture the longer wavelengths, while at the same time being sensitive to some UV. As such the images captured with them look very different to modern images, with very washed out skies being a common feature, along with darkened foliage. Skin tends to be captured as darker than it appears in normal visible light images (due to the increased absorption of short wavelength light be melanin) along with an emphasis of surface texture (again due to the short wavelengths being imaged).

Something I’ve wondered about since starting with digital photography is how to go about replicating the effects of Collodion in a digital camera. The first step was to try and understand the spectral response for Collodion plates. Some work has been done on this by LundPhotographics, who showed that it was mainly sensitive between 320nm and 520nm. This correlates well with the spectral response of an un-sensitized photographic plate as shown in “The Theory of the Photographic Process” 4th Ed by TH James (Figure 17.42 on page 512) which showed sensitivity between about 360nm and 510nm. With the work discussed by TH James they acknowledge that drop in sensitivity towards the blue end was a factor of the use of tungsten light which has limited blue and UV. Therefore the LundPhotographics range of 320nm to 520nm is a good start point, especially when imaging with sunlight.

The next stage was filter choice. I had a feeling Schott BG25 would be a good filter material to use for this as it has good transmission in the UV and blue regions. However it has a problem in that it transmits IR as well, and as digital cameras are very sensitive to IR this would need to be blocked. There are a range of filters which can be used for this such as Schott s8612 and BG39. During an eBay trip one day I came across a seller who had some 2mm thick BG25 along with 2mm thick BG18 (which is also a good IR blocker). Even better these were large – 75mm diameter – and cheap. So a quick purchase later, and some 77mm filter rings from Kood International, and I had some filters which could be used on lenses with a 77mm filter thread making them very versatile.

This first thing to do was measure the transmission spectra of the filters, and I did this using my Ocean Insight FX (UV and visible) and STS-NIR spectrometers which I use for a lot of my work. This gave me transmission spectra for the filters between 250nm and 1100nm as shown below.

BG25 and BG18 transmission spectra, individually and combined

By combining the two filters, the result was transmission between about 330nm and 510nm, with good blocking at shorter and longer wavelengths (especially the IR) – the green line in the graph above. This correlates well with the range for Collodion sensitivity derived by LundPhotographics. One slightly odd thing with the spectra – the BG25 was letting through less IR than I would have expected for a 2mm thick one based on the Schott online filter calculator. Measurement of the thickness showed it to be 2.1-2.15mm thick, but even that could not account for the transmission in the IR. Perhaps it’s just an odd batch (the filters were old ones) or perhaps the specs have changed slightly over the years. I’ll never know for sure on that. However the BG18 would do a good job of blocking even if the IR transmission of the BG25 was little more than it is.

To capture an image, I need a camera which was sensitive to the 300-550nm region, and for this I used my Canon EOS 5DSR multipectral conversion. The lens was a Canon 17-40 f4 (at 19mm). Image captured in my garden in direct sunlight, and as a comparison shot, a visible light image taken with a camera phone at the same time.

Firstly the digital Collodion image.

Digital Collodion image

And the normal visible light image.

Visible light camera phone image

As expected the digital Collodion image has washed out sky and darker green foliage when compared to the visible light image. In hindsight I could have used a monochrome converted camera as this would have boosted the UV sensitivity compared to the camera used here, although this would have needed me to use a different lens as well, as this one wouldn’t have let that much UV through. So my Collodion image here isn’t as ‘UV heavy’ as it could be and is mainly comprised of long wavelength UV from about 380nm and blue light. Lens and camera choice is certainly something to consider for future work with the filters.

Revisiting old photographic processes forces us to think about the image capture process as a whole and the factors that are important to it. In doing so we learn more about the methods we are using, their limitations and how they can be improved. This type of thought process is vital to the world of supporting claims for products, as without knowing how the methods we use work, how can we understand their limitations – what they can and can’t be used for?

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

Working with Universities – Forensics

I try and publish as much as I can about the work I do – it gets the science out there so it can be built on by others which is hugely important for pushing research forward. Sometimes I may not have enough experience or data in a specific field myself to be able to pull together an entire publication, however I do get asked to help with work in a wide range of research fields especially with regards to imaging.

One area which has strong links with skin and imaging is that of forensics, and UV photography has applications in that field. Due to my work on the measurement of lens transmission in the UV, Dr Kevin Farrugia, at De Montfort University, approached me in 2020 with some questions about UV imaging, which led to some interesting and thought provoking discussions about what was actually important when doing UV imaging.

Dr Farrugia recently published an article on UV imaging of fingerprints, and looking at how the choice of equipment can influence the results. The article “A pseudo-operational trial: An investigation into the use of longwave reflected UV imaging of cyanoacrylate developed fingermarks” was recently published in Forensic Science International.

Always nice to get a mention in the acknowledgements on a paper (you’d be shocked to find out how often this doesn’t happen after these types of discussions), so thanks Kevin. Part of having expertise in an area is the responsibility to teach others and pass on that knowledge, especially to Universities, and it is something I will continue to do in the future.

Getting customer service right – Thorlabs

Some purchasing experiences can be a bit of nightmare. Unfriendly or unhelpful staff, packages that go ‘missing’ during shipment, and courier companies that feel as though leaving your parcel on someone else’s doorstep and providing you with some tantalizing clues as to where it is, is as much fun as going on an organized treasure hunt.

Other companies though, just get it right. Enter Thorlabs. Thorlabs produce and sell a huge range of optical components, and I often use their parts and adapters when building lenses such as a UV lens I made based on their 79mm UV Aspheric lens.

Lens build based on the Thorlabs 79mm UV Aspheric lens
Thorlabs 79mm UV aspheric lens in place

This has proved to be used for UV imaging and can potentially even be useful well down into the short wavelength UVC region.

UV image of a Dandelion taken using the Thorlabs UV aspheric lens

But this where they get a big ‘tick’ from me – customer service. When you ring them up and ask for advice, they genuinely sound as though they want to help. Thank you, thank you, thank you. Add that to the range of kit they sell and the good prices and you have a winning combination. And then we come to the icing on the cake. In with the deliveries you often get Lab Snacks – nibbles and treats to help make the day go more smoothly (by the way Thorlabs, my wife says thank you so much for these).

Thorlabs Lab Snacks with the delivery

In a world of mediocre customer service, it is great when you get something like this. Thank you Thorlabs, I’m a very satisfied customer and shall continue to buy many more optical components from you in the future.

My UV imaging journey

As part of life’s journey sometimes it is useful to look back to see where you’ve come from, so today, please excuse my self indulgence as that is just what I’m going to do with regards to my experiences with ultraviolet (UV) photography. I’ll put a few links to pieces I’ve written along the way, to provide more background where needed. I’ll also mention a few of the UV equipment related suppliers and contacts I’ve made along the way, although I’d like to make it clear that I am in no way sponsored or funded by them.

UV photo of a Buttercup taken using a Zeiss UV Sonnar 105mm lens and UV modified Canon EOS 7D camera (modified by Advanced Camera Services Ltd, UK)

Photography has been a part of my life for a long time – even when a boy I would often have a camera in my hand, and I suppose I have one of my brothers to thank for that. He was a film cameraman so was (and still is) a great inspiration to me. My other brother (also an inspiration) while not a photographer was an artist so pictures and picture taking was always around while I was growing up. In 2000 I bought my first proper film camera under supervision of a good friend from University and of course being a scientist looked for things to do with it beyond taking ‘snaps’.  This led me to infra-red (IR) photography and the even more mysterious UV photography. UV imaging was one of those mythical areas which used mysterious lenses made of exotic materials such as quartz and calcium fluoride, and needed strange filters which blocked all the visible light, and it was all very intriguing. I even tried doing some with a normal camera lens and some UV sensitive film, and got some ‘less than optimal’ results. So as a technique I filed it away as one of those ‘to be done later’ approaches.

Fast forward a few years and I was working for P&G in their Skin Care group, and again UV imaging appeared as an area of interest from my mentor. Like me he found it a fascinating area and even had some of these mythical UV lenses from his research. However it was still well beyond my means at the time to go into this in any serious way, and I settled for IR photography as a way of getting my geeky fix for non-visible photography.

We now jump a few years forward again to the winter of 2017 and someone I was working with asked me if I could come up with a way of visualising sunscreens and how they spread to help with development of new topical sun protection products. Given how sunscreens absorb UV light, it immediately got me wondering whether UV photography could be used for this. But there was a problem – when you hit a film of sunscreen with a bright light, you get a lot of reflection from its surface which hides information on the film morphology. Easy I thought, let’s just cross polarize the light source and camera like I do for visual light photography. Oh, the naivety of youth. As with many things UV related this proved to be a little less straightforward than I originally thought, but it started me down the route of seriously looking at UV photography. This was where I jumped down the rabbit hole…..

After working with a UK supplier (Advanced Camera Services Ltd) to get a UV converted camera, a lens and UV flash, I then designed a cross polarisation setup for it, the result of which was a system which could do cross polarised images of sunscreen films showing the film morphology. This was written up and published in the International Journal of Cosmetic Science, as being able to combine UV absorption of the film with information about the film morphology looked to be giving a more accurate correlation with in-vivo measured SPF values that did not considering the morphology.

Cross polarised image of a) a poorly spreading sunscreen product, and b) a sunscreen product which spread well. The poorly spreading product shows streaks corresponding to variations in the final film thickness

It was about this time that I also came across the Ultraviolet Photography forum. This collection of individuals from around the world had a tremendous background knowledge of all things UV photography related, and time spent on the forum was time well spent. I also found Dr Klaus Schmitt who is a UV imaging expert and dealer in unusual lenses. This led me to my first personal UV purchase – an Asahi Ultra Achromatic Takumar 85mm f4.5.

Asahi Ultra Achromatic Takumar 85mm f4.5 lens

There’s something very special about looking at a camera lens and then realising that those lovely lens elements aren’t actually glass (or plastic as is sometimes the case these days). Of course I was bitten by the bug after getting the Asahi, and I’ve been fortunate enough to get hold of a few other unusual UV lenses since then, including a 105mm f4.3 Zeiss UV Sonnar (designed for medium format 6×6 cameras), a prototype Leitz Elcan 52mm f5.6 which was designed as a UV lens, a Katoptaron LDM-1/s 800mm long distance microscope, a Zeiss 60mm f4 UV Objektiv, a Nye Optical 150mm f1.4 mirror lens and an Astro Berlin Quarz 120mm f2.1 large format lens.

Zeiss 105mm f4.5 UV Sonnar
Carl Zeiss 60mm f4 UV Objektiv
Astro Berlin 120mm f2.1 Quarz Objektiv

I suppose I’ve become a bit of a collector, but I see it more as the role of a curator. Being made in small numbers most of these are of historical interest, so I look after them and will eventually pass them on to the next generation of photographers. All of these lenses have good (and some bad qualities) when it comes to use in UV but some of them have enough reach to get down well into the UV region below 300nm. The plan is to eventually write up research done on or with all of these lenses at some point. Somehow I get the feeling this will end up being a retirement project…. I also make my own lenses from the range of parts that Thorlabs sells (big boys Lego).

Of course lenses are only part of the story and you need a camera to capture the image. For my initial sunscreen work I bought a UV converted camera from Advanced Camera Services Ltd in the UK, which I still use for day to day UV imaging. Having a built in UV filter over the sensor is great for an SLR camera as you can still use the eyepiece (although focus point will likely be different in the UV depending on the lens). One of the big issues with modern camera sensors with regards to UV imaging is the presence of the Bayer filter – the transparent coloured layer which creates the red, green and blue image on most sensors. While this is great for the visible region, the dyes used absorb loads of UV, especially at the shorter wavelengths, so has a big impact on camera sensitivity. Fairly early on I came across a company in the US – MaxMax (Llewellyn Data Processing LLC) – who were offering monochrome conversions of cameras, where they were removing this Bayer filter layer in a very controlled way, so I got one of my cameras converted. In fact I was so impressed with his work, that I’ve ended up getting various cameras worked on by him, including two Canon EOS 5DS R’s (one to monochrome and one just for multispectral imaging with the Bayer filter still intact). It was these Canon cameras which were the basis for my publication in the Royal Photographic Society Imaging Science Journal in which I used a device I’d built for measuring camera sensor sensitivity to look at how thee conversions impacted UV imaging ability. I actually have another paper looking in more depth at monochrome camera conversions and spectral sensitivity in the UV, visible and IR which will hopefully be published later this year.

Next we come to filters. Blimey, filters for UV imaging. This can be a bit of tricky area as of course a filter for UV imaging needs to be able to effectively block anything which isn’t UV. For a typical camera sensor this mean blocking from 400nm up to around 1200nm and because camera sensors tend to be more sensitive to visible and IR than to UV, the blocking of these wavelengths needs to be good. Very good. A lot of people use the Baader U filter from Baader Planetarium for their work. This is a filter designed for telescopes, so typically needs a bit of modification for using on a camera lens, and is dichroic which can lead to some odd colour fringing on wide angle lenses. I’ve also found the filters made by UVIRoptics to be very good, and he offers non-dichroic ones which don’t have the colour fringing issue with wide angle lenses. Also Invisible Vision here in the UK offer a good 308nm UVB filter with a mount suitable for normal camera lenses. I must admit, I’m a bit of a filter junkie and have far too many of them, but the set of 10nm band pass filters which go from 310nm to 390nm from Edmund Optics and Thorlabs have been very very useful, although they are only 1″ diameter, they are fine with the 105mm UV Rayfact macro lens (and other small element lenses).

Actually, while working in the UV I’ve found that a lot of stuff needs to be built as it cannot be bought (or if it can be bought, you need very, very deep pockets). This has led me to building my own setups for measuring camera sensor sensitivity and lens transmission, both of which have led to publications in peer reviewed journals. The paper containing the lens transmission work was awarded the Paper of the Year for 2020 in the International Journal of Cosmetic Science which was a great honour to receive.

Building my camera sensor sensitivity measurement setup
IJCS 2020 Publication prize winner announcement for my paper “UV reflectance photography of the skin: what are you imaging?”.

The last year has of course been hugely difficult for many people. When the first lock down came around I found myself in need of a new project to keep busy. As I had relatively little experience with microscopy, and given its very visual nature, I thought it would be a good skill to learn to compliment my other imaging work. I decided to buy an older microscope in need of fixing up, so settled on an Olympus BHB from the 1980’s. This poor thing looked like it has been stored in a field for a while, but could be taken apart and cleaned with relatively simple tools. Like many projects it became somewhat obsessional for me, and I soon got to wondering whether it could be transformed into a UV transmission microscope. This would be no simple task as there is a lot of glass in a microscope. So I set about learning about UV microscopy and kept an eye out for second hand equipment such as objectives and condensers. I had a couple of lucky purchases relatively early on and managed to find a Leitz 16x UV objective with good UV transmission down to and below 300nm, and an vintage Zeiss quartz condenser (although I did also make my own condenser at one point using a UV fused silica half ball lens).

Leitz 16x UV microscope objective lens
Antique Zeiss quartz microscope condenser

I also came across the Quekett Microscopical Club which was founded in 1865 and even has its own peer reviewed journal. What better place to learn about old microscopy equipment I thought and applied to join. As with the photography I’ve found myself no collecting anything related to UV that I can find and use so have a wide range of UV lenses, both refractive (such as the Zeiss Ultrafluars), and reflecting (mirror lenses). All of these have been bought second hand as new they would have had eye-wateringly high prices.

Zeiss reflecting microscope objectives and condenser

The UV microscope is now complete, although as a tinkerer it will be improved upon, and I am starting to do research into sunscreens using it.

Scientifically, my UV journey has been challenging, fascinating, frustrating at times, but ultimately very rewarding and is an area I shall be continuing to work with in the future. In addition to the obvious areas of interest such as sunscreens and skin imaging, my plan is to explore different areas such as forensics, geology and even anthropology to see what looking at things in a different way can bring to research in those fields. Microscopy has proved to be a fascinating research area for me and I’ve already written a couple of UV related articles on it with more to come in the future.

Thank you for reading and if you’d like to know more about my work you can reach me here. I’ve included a small publication list at the end of the article which includes some of the key articles I’ve written on this area along with the talks I’ve given to date.

UV related publications and talks

“Yooperlite – Imaging the fire within using UV”, JM Crowther, Quekett Bulletin, 2021, 80, 53-56.

“Chapter 28 – Dermatological imaging – A survey of techniques“, A Davies, JM Crowther, in Photography in Clinical Medicine, Ed. P Pasquali, 2020. ISBN 978-3-030-24543-6. https://doi.org/10.1007/978-3-030-24544-3_28

“Chapter 29 – Beyond the visible: UV, IR and fluorescence imaging of the skin“, JM Crowther, A Davies, in Photography in Clinical Medicine, Ed. P Pasquali, 2020. ISBN 978-3-030-24543-6. https://doi.org/10.1007/978-3-030-24544-3_29

“UV reflectance photography – what are you imaging?“, JM Crowther, International Journal of Cosmetic Science, 2020, 42(2), 136-145. https://doi.org/10.1111/ics.12591

“Visualising Sunscreens Using UV Photography”, presented at the Sun Protection Conference, London, UK, 2019.

“Imaging of the skin – UV, visible, IR”, presented at the Royal Photographic Society Imaging Science Group meeting, London, 2019.

“The big reveal: UV imaging uncovers sun protection, skin dryness and microbiome“, JM Crowther, Cosmetics and Toiletries, Sept 2019, p.32-45.

“Understanding colour reproduction in multispectral imaging: measuring camera sensor response in the ultraviolet, visible and infrared“, JM Crowther, The Imaging Science Journal, 2019, 67(5), 268-276. https://doi.org/10.1080/13682199.2019.1638664

“Calibrating UVA reflectance photographs – standardisation using a low-cost method“, JM Crowther, Journal of Visual Communication in Medicine, 2018, 41(3), 109-117. doi: 10.1080/17453054.2018.1476819.

“UV Photographic Imaging – Sunscreens and Skin“, The Cosmetic Chemist, 2018, 15th October.

“Understanding sunscreen SPF performance using cross polarised UVA reflectance photography”, JM Crowther, International Journal of Cosmetic Science, 2018, 40(2), 127-133.

Microscopy – cross section of an objective

Have you ever wondered what the inside of a microscope objective lens looks like? Why am I even asking that, of course you have, haven’t we all. All the tiny components, the little lenses. Very cool. Sometimes we get to see schematics of what they look like in cross section in manufacturers brochures or in other publications, but usually that is as close as we get to actually seeing what is going on in there.

I recently saw an objective for sale which had been sectioned from end to end by Spectrographic Ltd in the UK, so put in a bid and was lucky enough to buy it. The objective was an Olympus HI M100 1.30 which was unfortunately broken (no working lenses were sacrificed in the making of this section). Looking from the side it looks ok, a bit beaten up perhaps, but ok;

However turn it around and it looks very different.

Keep in mind that this objective lens is only about 25mm long. By sectioning it the amazing engineering that it takes to make one of these is revealed. The lens elements themselves are remarkable, as can be seen when we zoom in on them.

Going from left to right it looks like there are two singlets, and then two cemented doublets. Keep in mind that these are only a few mm across.

The images were captured using a Hoya Super EL 60mm enlarger lens as a macro lens as discussed here.

I’m very impressed by the quality of the section from Spectrographic Ltd, especially considering it is such a small item. This is something that is handy to have when explaining about microscopy – being able to show someone the inside of an objective is a step up from just looking at a sketch on a screen. Now then, where is that broken Beck reflecting objective I got a while back…..

Thanks for looking and if you’d like to know more about this or my other work you can reach me here. And please remember that the images in my site are copyrighted to JMC Scientific Consulting Ltd. If you’d like to share them, please ask first.

Photography – Hoya Super EL 60mm f4 enlarger lens

Sometimes in the world of photography we fall into the trap of ‘must buy that expensive lens everyone has been talking about’. However it is possible to use rather humble lenses to take good photos and they can often produce very high quality images for what is a very modest outlay.

Enter the Hoya Super EL 60mm f4 enlarger lens. This one cost me the princely sum of £30 on ebay. Here’s the lens;

Hoya Super EL 60mm f4 enlarger lens

As mentioned above this was a lens for an enlarger. It has a rather unusual 8 element 4 group lens construction and was designed to cover 6x6cm. The thread is M39 (common for enlargers) and is easy enough to adapt to M42 which is mountable to most cameras with an adapter. I used a helicoid on mine to, to allow me to vary the focus more easily.

What are the images from it like? Here’s a few from the garden, taken at either f8 or f11 on my Canon Eos 5DS R and hand held. I’ve reduced the resolution for easier sharing here.

Overall I like the images it produces, and the colours are nice. But how about resolution? The image below was pretty much the full frame of the original shot.

And now a crop from the middle of the flower image, kept at the original pixel resolution (i.e. not reduced for sharing).

I think there is plenty of sharpness there.

Enlarger lenses like this tend to make very good macro lenses and can often be had for very little money. Normally they are M39 threaded as well, so once you have the right adapters for your camera it is easy enough to swap between them if you want to try different ones.

We often want the latest and greatest in terms of equipment, but always keep in mind that new equipment isn’t everything. Also, check out the abundant web resources such as the MFlenses forum, and Photomacrography.net for inspiration and get snapping.

Thanks for looking and if you’d like to know more about this or my other work you can reach me here. And please remember that the images in my site are copyrighted to JMC Scientific Consulting Ltd. If you’d like to share them, please ask first.

Lens transmission – from UV to IR

Measurement of lens transmission in the UV has interested me since I started working with UV photography, and a I’ve ended up building my own system for determining it (see here). However with this setup I was limited to UV and only just getting into the visible, as it could measure from 280nm to 420nm. With recently being in a position to evaluate an Ocean Insight STS-NIR microspectrometer (initial work discussed here) it got me wondering whether I could now measure transmission from 280nm all the way up to 1100nm, as this covers most of the range of camera sensor sensitivity.

To measure lens transmission over such a wide wavelength range meant juggling light source and spectrometer. I ended up using the following combinations;

280nm to 420nm – Hamamatsu LC8 200W xenon light source, Ocean Insight FX spectrometer.

420nm to 650nm – Moritex MHAA-100W halogen light, Ocean Insight FX spectrometer.

650nm to 1100nm – Moritex MHAA-100W halogen light, Ocean Insight STS-NIR spectrometer.

In theory these combinations should have allowed me to see all the way from 280nm to 1100nm. For an initial test I looked at 2 very different camera lenses – Canon 40mm f2.8 STM pancake lens, and a Rayfact 105mm f4.5 UV lens (the modern version of the UV Nikkor 105mm). Here’s how the two lenses looked.

Canon 40mm f2.8 STM and Rayfact 105mm f4.5 UV lens transmission

The graph above contains 6 lines – 3 each for the 2 lenses, with each line covering a different spectral range. You’ll notice that the wavelength range only goes to 1000nm, not to 1100nm where the STS-NIR can measure to. This is for a couple of reasons – my Moritex light source light intensity is dropping quickly above 1000nm, and the STS-NIR sensitivity also drops as you get closer to 1100nm. Add these factors together, and then throw in the integrating sphere needed for doing lens transmission measurements and it means that the data gets very noisy above about 1000nm. So I decided that it was only worth plotting it to 1000nm. While I had measurements from 280nm, I started it at 300nm on the graph to make it look prettier – starting at 280nm would make the x-axis scale look odd.

What does this tell us about the 2 lenses? The 2 lenses behave very differently to each other. The Rayfact 105mm f4.5 UV lens has a relatively flat transmission spectrum from 300nm all the way to 1000nm, varying between about 75% at 300nm to about 70% at 1000nm. This is actually pretty close to the data Rayfact share for the lens, which starts at about 78% at 300nm and drops to about 70% at 900nm. The Canon 40mm f2.8 STM pancake lens is very different. It very effectively blocks the light below 350nm, but in the visible region has very good transmission – up above 90% for most of the visible spectrum. Its transmission then drops quickly in the IR as the wavelength increases. This is not surprising, as understandably the Canon lens is optimized for imaging in the visible spectrum. The visible spectrum data is a bit noisier than I expected, especially towards 420nm. Longer acquisition times would help there.

What have I learned here? With the light sources and spectrometers I have I can measure lens transmission from deep in the UV all the way up to the IR. Sensitivity at the top end is lacking a bit which limits me to about 1000nm, although I have a couple of ideas about how to address that in the future. Lens alignment is critical for the measurements here. My setup is horizontal and to get the most accurate results I need to make a stand for each lens to be tested to hold it in exactly the right position for all the measurements. With a vertical setup this would be easier, but at the moment I have no way of safely positioning my lights in a vertical orientation, so horizontal it is for now.

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.

Spectroscopy – Ocean Insight STS-NIR microspectrometer

As small business owner working in the research and development area, it is vital for me to keep expanding my capabilities and refining the portfolio of what I can offer to clients. Although I already have a wide range of UV, visible light and IR imaging equipment, along with UV and visible spectroscopy capability (Ocean Insight FX spectrometer), which I use for looking at lens and filter transmission, IR transmission spectroscopy is an area which I am currently lacking. Given the financial upheavals of the last 12 months, the decision to invest in a new piece of kit is not made lightly and new kit must fill a gap in my measurement ability.

My spectroscopy interest is driven mainly by my photography. Camera sensors are mainly sensitive between about 300nm and 1200nm, so this is my area of interest. My Ocean Insight FX spectrometer covers my needs from 250nm to 800nm, and I’ve been very happy with that over the years. Going above 800nm is something I’ve been thinking about for a while, as understanding the behavior of filters for UV photography in the IR is very important – even small leaks in the IR region can be a huge problem for UV imaging. In an ideal world I’d like a spectrometer that would be good for up to and just above 1200nm, however that is a bit of an issue as standard solid state spectrometers tend to only be good up to about 1100nm. Above that you tend to get into more exotic sensors such as InGaAs (indium gallium arsenide) and the costs go up very quickly. As a compromise then, I decided to look at the IR ones with conventional sensors and able to measure up to 1100nm. Between 1100nm and 1200nm the camera sensitivity is dropping rapidly, so this is a compromise I am willing to accept for now.

I do a lot of my research from home and my workshop space is limited. As a result of that I tend to look for compact equipment. Combining sensitivity from around 700nm to 1100nm with small size of equipment led me to the Ocean Insight STS-NIR spectrometer and I’ve been fortunate enough to get one of these to evaluate. Initially I was a little skeptical as to whether such a small spectrometer would give good results. And when I say small, I mean small. Here it is in the flesh, next to a 2p coin.

Ocean Insight STS-NIR microspectrometer next to a UK 2p coin

This spectrometer is tiny – 40x42x24mm – with a fiber optic connector at the front, and a USB socket at the rear. Could this really give good data from 650nm to 1100nm?

As a first test I decided to look at a range of IR photographic filters – Heliopan 715, 780, 830 and 1000 – which I use for IR imaging. Light source wise I used my Ocean Insight DH-2000-BAL and set everything up for transmission measurement. Here’s how the filters look.

Heliopan 715, 780, 830 and 1000 IR filter transmission

Well, that was pretty impressive for an initial test. The filter transmission curves were as expected and very clean. Up in the 1000 to 1100nm region, the data get a little more noisy, but the sensor is losing a bit of sensitivity up there and the light source is dropping in intensity as well.

As a next test I took some of my Hoya U-340 filters (from UVIRoptics), and started stacking them up on top of each other. Hoya U-340 is a bandpass filter which has good UV transmission, but does have an IR leak at around 725nm. Using a combination of 2mm and 4mm thick ones I measured the transmission of 4mm, 6mm (4mm+2mm), and 8mm (4mm+4mm) between 650 and 1100nm.

Hoya U-340 transmission at different thicknesses

Each of the lines above was the average of 10 individual runs, and the standard deviations of the scans are shown as paler coloured error bars on either side of the lines. As expected the 4mm Hoya U-340 showed an IR leak of about 0.25% at around 725nm. This leak was nice and clean with the STS-NIR. Above about 1020nm the data for all three filters gets a bit noisy – as mentioned above the sensitivity of the sensor is low up there at the extreme top end, and the light source intensity drops too, so I’m not surprised to see that.

How about if we zoom in to the the 725nm and look in a bit more detail?

Hoya U-340 filters of different thicknesses – transmission in the 725nm region

The standard deviation error bars can be seen more cleanly like this and it shows very good reproducibility. The leak in the 6mm stack looks to still be visible and different to the 8mm stack. This can be seen more clearly by zooming in yet again.

Hoya U-340 filters of different thicknesses – transmission in the 725nm region, zoomed in

At this scale, the leak in the 6mm stack at 725nm can still be seen, as about 0.01% (this equates to Optical Density OD4 blocking of the IR). By the time you get to 8mm thick, the leak can no longer be seen due to the extra thickness of the filter stack. So, again, very impressive result from the little STS-NIR microspectrometer.

What have a learned so far? I’ve been very surprised and impressed by the Ocean Insight STS-NIR. For such a small spectrometer it gives very good results. It’s proved that it is capable for assessing filters, and I hope to try it for looking at lens transmission too, although this will be a bigger challenge for it (and my light sources). In theory with the lights I have I should be able to measure lens transmission between 280nm and 1100nm using the FX and STS-NIR spectrometers, which will cover most of the area that normal camera sensors are sensitive too.

Thanks for reading and I hope you enjoyed my latest foray in the measurement world. If you’d like to know more about this or any other aspect of my work, you can reach me here.

Winner of the International Journal of Cosmetic Science publication prize 2020

A very personal update today. I’m very proud to have been awarded the International Journal of Cosmetic Science’s publication prize for 2020 for my paper “UV reflectance photography of skin: what are you imaging?“.

The award has been sponsored by the Society of Cosmetic Scientists and the paper was aimed at demystifying the aspects of the UV imaging process to enable researchers to understand what it is they are actually seeing. It brings together various aspects of my research, and many of the methods I’ve developed and built myself to characterise cameras and lenses in the UV region.

I’ve always been a firm believer in the peer review process for the critiquing and publication of research, and will continue to actively publish my work in this area in the future.