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Paper: Understanding sunscreen SPF performance using cross polarised UVA reflectance photography

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#1 JMC

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Posted 24 December 2017 - 11:44

I've just had this paper accepted for publication in the International Journal of Cosmetic Science;

Attached File  Cross polarised photography of sunscreens DRAFT Dec 2017.pdf   737.22K   276 downloads

This is in draft, not final form as I haven't been sent the proofs to check yet, so is only in Early View at the moment (hence formatting looks a bit weird). Sharing in case it's of interest, as it uses a cross polarised UV reflectance photography system I designed to image the UV absorbance of reflective sunscreen films, and try and better correlate physical parameters with their ability to protect vs sun damage.

#2 Andy Perrin

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Posted 24 December 2017 - 16:35

This is interesting. I cannot find where you say what polarizers you used in there, howver. We have discussed polarized UV on the board here in the past and I know you need polarizing filters designed for UV.

#3 JMC

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Posted 24 December 2017 - 17:08

Ah yes, you are right Andy (I must admit I expected that to be picked up on in the review of the paper, and no one commented on it). I do not mention the specifics about what they were. There's a good reason for that and it's to do with my work, and that this currently is something I'm still researching. What I can say is that they were not specifically designed for UV work, and despite that they worked just fine, removing the shine from the highly reflective films. Not all standard polarizers are the same... Without them, all I could see was a white reflection.

In designing the system, there was a lot of skepticism about whether this was even achievable, and I went through quite a lot of polarisers measuring UV transmission profiles. The plastic polarising films let virtually no UV. However, some of the earlier linear polarisers designed for lenses, let quite a lot of UV through, especially above 350nm. While I would love to have had specialised UV polarisers, these let just enough light through to be usable.

Edited by JMC, 24 December 2017 - 17:20.


#4 Andy Perrin

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Posted 24 December 2017 - 17:59

Well I hope you will eventually share with the board once you’ve gotten your publication out or whatever. This should not be secret information forever.

#5 Andrea B.

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Posted 14 January 2018 - 17:20

While I would love to have had specialised UV polarisers, these let just enough light through to be usable.

I was wondering about UV polarizers and found these which are really **crazy** expensive:
https://www.edmundop...t-UV-Polarizer/
I do not understand how to interpret the accompanying charts, so I need to read up on that.
Andrea G. Blum
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#6 JMC

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Posted 15 January 2018 - 11:22

Andrea, I think there is something wrong, or at least not clearly explained in the chart on the Edmund site. For the light transmission, the legend on the graph says %, but the scale would suggest it's between 0 and 1, so 0.3 would be 30% transmission. If it really is transmitting 0.3% then it's not doing its job very well. Also, the graph only goes down to 360nm. What's going on below there then? Does it drop immediately, or does it carry on transmitting the same.

I hate not just being able to put up a list of suitable filters to try, but at the moment I cannot as this is my job, and if I simply put up a good list then why would people work with me. What I will say is try some of your older linear polarisers designed for lenses and see what happens. Do not bother with the new plastic polarising film, that blocks everything below 380nm and pretty much everything below 400nm (well, it did on the ones I looked at). I think in the end I spent about £50 on some linear polarisers from a well known internet auction site, and that gave me enough of a couple of different manufacturers ones to cover a couple of large flash guns (I used 72mm filters for that) and the lens. I should most people have a few knocking around in their cupboard they could try.

To put it into context, I was able to see 30% transmission down to about 350nm with the ones I was using. Given unlimited budget I would try to specialist ones, but 'unlimited' is something I do not have at the moment. All I need is a lottery win then I can devote the rest of my life to interesting research.....

#7 JMC

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Posted 17 April 2018 - 11:35

I'm going to be talking about this work at a conference in San Diego in a couple of weeks. As part of it I wanted to show that the technique could be used for in vivo imaging as well as just looking a in vitro samples. As such this morning I took some images of sunscreen (SPF50) on my hand to try and create some visuals for my presentation at the conference, and I wanted to share them with the community here.

Sunscreen was applied in a square to the back of my hand. These were taken with my monochrome EOS 5DSR, and an EOS 40mm pancake lens (not ideal for UV, but I needed something wider and closer focussing than my UV lenses for these images). The flash was an ACS modified EOS 600RT flash unit. The filter on the lens a Baader U. There are 2 images, one taken with no polarisation and one which is cross polarised.

Firstly the non polarised (done at ISO500, f3.5 and 1/2s, with lighting from the flash).
Attached Image: 0I8A5140 non polarised cropped.jpg

Now the cross polarised one (done at ISO6400, f3.5 and 1/15s, with lighting from the flash).
Attached Image: 0I8A5138 cross polarised cropped.jpg

The exposure on the bare skin is about the same, which is what I was aiming for, and the shine from the sunscreen film (the white banding in the non polarised image) has been drastically reduced. I will add these were done in a bit of hurry this morning, as the presentation is currently being written, and if possible I would have spent another hour or so optimising the cross polarisation, but I think it shows the gist of the technique, especially as the lens was not ideal for UV.

These were not specialist UV polarisers. While I cannot divulge the specific makes at the moment, I will say they are just standard linear polarisers designed for use camera lenses (not plastic polarising film as that blocks all the UV). If you have any older linear polarisers, i your bags try them - the older ones are less likely to have UV blocking coatings. I was surprised by how many I tried actually let quite a bit of UV through, especially at 360nm and above, where the Baader U is letting the most light through.

And yes, I will be thanking the Ultraviolet Photography Forum in my Acknowledgments at the conference, as it has been an amazing source of information and inspiration.

Edited by JMC, 17 April 2018 - 12:22.


#8 Andrea B.

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Posted 17 April 2018 - 15:06

A very nice UV polarization example, Jonathan. Thanks for posting.

And we thank you for your thoughtful mention of UVP at the upcoming conference.
**********

I have a question prompted by the "shine", i.e., specular reflection, in the first image. If sunscreen works by absorbing UV rays, then why does the sunscreen also exhibit this specular reflection? Was it wet when that photo was made? Or do you think this happened because the sunscreen smoothed out the rougher skin area to create a "mirror-like" surface? I think this might be a question about the underlying physics here and not about sunscreen, per se. But just in case you get asked this at the conference, you will have prepared an answer. :D

BTW, Bjørn and I have photographed many members of the Papaveraceae which exhibit this same mix of absorption and specular reflection. In the case of petals the reflection is due to the petal surface's conical cells.

(Polarizers require longer exposures, obviously. The 2nd photo needs a bit more.)
(Use an extension tube to get closer with your UV-dedicated lenses.)
Andrea G. Blum
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#9 JMC

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Posted 17 April 2018 - 16:10

View PostAndrea B., on 17 April 2018 - 15:06, said:

A very nice UV polarization example, Jonathan. Thanks for posting.

And we thank you for your thoughtful mention of UVP at the upcoming conference.
**********

I have a question prompted by the "shine", i.e., specular reflection, in the first image. If sunscreen works by absorbing UV rays, then why does the sunscreen also exhibit this specular reflection? Was it wet when that photo was made? Or do you think this happened because the sunscreen smoothed out the rougher skin area to create a "mirror-like" surface? I think this might be a question about the underlying physics here and not about sunscreen, per se. But just in case you get asked this at the conference, you will have prepared an answer. :D

BTW, Bjørn and I have photographed many members of the Papaveraceae which exhibit this same mix of absorption and specular reflection. In the case of petals the reflection is due to the petal surface's conical cells.

(Polarizers require longer exposures, obviously. The 2nd photo needs a bit more.)
(Use an extension tube to get closer with your UV-dedicated lenses.)

Thanks Andrea, The sunscreen films are not just sunscreen chemicals, there is a lot of other stuff in there such as oils. The film is relatively smooth and I am assuming it is basically reflecting like a mirror from the surface of the film, as visible light reflects from oils on skin. Yes it was freshly applied - perhaps 10mins between applying it, and getting both of these done. Also sunscreens don't just absorb UV - they prevent a certain amount from reaching the skin. This can be through absorption by the UV filters, absorption and scattering by things like ZnO and TiO2, and reflection by the formulation in general.

Once I'm back from the conference I'll be able to share more on the polarisers, including some of the transmission spectra I've been able to measure for them.

#10 Andy Perrin

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Posted 17 April 2018 - 16:22

Andrea, JMC is right about the oils I think. My understanding is that sunscreen works by not transmitting UV rays to the skin, which means that they all get either absorbed or reflected. The point is that they should not make it through to the skin, so we don't care if the light is absorbed or sent off in some other, non-skin, direction.

Edited by Andy Perrin, 17 April 2018 - 16:23.


#11 Andrea B.

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Posted 17 April 2018 - 22:18

Aha, now I know that my personal experience with sunscreens affected how I asked that question! I realized that I always use oil-free sunscreens or blockers. So I was not thinking about oils at all. "-)

But any sunscreen, oil-free or not, will form that smoothing film which induces specular reflections. It is the word "film" which I was searching for. Sometimes we just need the vocabulary. :rolleyes: Thankee.
Andrea G. Blum
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#12 JMC

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Posted 14 May 2018 - 19:20

A quick update. I did give the talk at the conference. Unfortunately everything was rushed for time (earlier speakers over-running), so I had all of 15 minutes to try and cram everything in. It covered my work from my sunscreen paper, a brief mention about cross polarised in-vivo UV imaging, and a chart from a new paper which I have just had accepted for publication on the calibration of UV reflectance. Here's a copy of the talk in case it's of interest.

Attached File  ISBS Cross polarised imaging of sunscreens May 2018-1.pdf   2.21MB   63 downloads

#13 Andrea B.

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Posted 14 May 2018 - 19:36

Thank you Jonathan -- and thank you for the mention of UltravioletPhotography.com.

Your work is quite interesting.
I thought it was very cool that you created your own set of reflective standards to use in this work. The makers of Spectralon better be on the lookout for sales loss because of these nice, inexpensive standards !! :lol:

(And says Mom -- this paper also serves also as a reminder to all of us that we need to protect against our favorite UV wavelengths. Wear your sunscreen and put on your --> B) )



Now, I'm curious. Why was it necessary to place a polarizer also on the flash? The cross-polarization improved the removal of shine as compared to only a polarizer on the lens?
Andrea G. Blum
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#14 JMC

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Posted 14 May 2018 - 20:33

Thanks Andrea. Yes, using two polarisers, one on the flash and one on the lens, set at 90 degrees to each other is to try and eliminate specular reflection from the surface of the sunscreen film. It's commonly used in visible light photography for skin research, to remove shine and allow the actual colour of the skin to be more easily captured. One polariser will reduce it but not all of it.

#15 Andrea B.

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Posted 15 May 2018 - 02:04

Cool. This cross-polarization is new to me. Not that I hadn't heard of it, but really never learned anything about it. So, this is good to know!
Andrea G. Blum
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