UV Index & Photography

[colinbm]

UV Index & Photography

With the renewed threat of the Ozone Layer Depletion by some increased production of CFC's.

I have been wondering what is the relationship of the IV Index on photography & in particular UV sunlight photography ?

 

There are many affordable devices that measure UV Index & most weather reports now give the daily UV Index.

What does the UV Index mean in terms of UV transmission to ground level ?

I know that altitude also plays a part in the UV dose received.

 

Is there a one-meter-does-all, or do we need to measure UV A, B & C separately for our location ?

How much does location, latitude & altitude affect UV photography ?

How much does UV Index affect the results & comparing tests with UV sunlight photography with people located at different latitudes & altitudes, & is the UV different in the North & south hemispheres with different atmosphere compositions ?

 

Cheers

Col

[Andy Perrin]

We can only photograph UV-A, so I don't think the ozone layer thinning (which affects only UV-B and UV-C) matters with our equipment?

 

Wiki gives this description of a UV index calculation:

The UV Index is a number linearly related to the intensity of sunburn-producing UV radiation at a given point on the earth's surface. It cannot be simply related to the irradiance (measured in W/m²) because the UV of greatest concern occupies a spectrum of wavelength from 295 to 325 nm, and shorter wavelengths have already been absorbed a great deal when they arrive at the earth's surface. Skin damage from sunburn, however, is related to wavelength, the shorter wavelengths being much more damaging. The UV power spectrum (expressed as watts per square metre per nanometre of wavelength) is therefore multiplied by a weighting curve known as the erythemal action spectrum, and the result integrated over the whole spectrum. This gave Canadian scientists a weighted figure (sometimes called Diffey-weighted UV irradiance, or DUV, or erythemal dose rate) typically around 250 mW/m² in midday summer sunlight. So, they arbitrarily divided by 25 mW/m² to generate a convenient index value,^[7][8] essentially a scale of 0 to 11+ (though ozone depletion is now resulting in higher values, as mentioned above).

To illustrate the spectrum weighting principle, the incident power density in midday summer sunlight is typically 0.6 mW/(nm m²) at 295 nm, 74 mW/(nm m²) at 305 nm, and 478 mW/(nm m²) at 325 nm. (Note the huge absorption that has already taken place in the atmosphere at short wavelengths.) The erythemal weighting factors applied to these figures are 1.0, 0.22, and 0.003 respectively. (Also note the huge increase in sunburn damage caused by the shorter wavelengths; e.g., for the same irradiance, 305 nm is 22% as damaging as 295 nm, and 325 nm is 0.3% as damaging as 295 nm.) Integration of these values using all the intermediate weightings over the full spectral range of 290 nm to 400 nm^[7] produces a figure of 264 mW/m² (the DUV), which is then divided by 25 mW/m² to give a UV Index of 10.6.^[8]

Since the weighting factors are really really tiny for UVA, the UV Index may not be well suited to use by UVA photographers, although since UVB and UVC must be strongly correlated with the amount of UVA, it's possible we could get some rough indication from it?

[colinbm]

Thanks very much Andy

So even in the 'hole' over Antarctica, there is still no UVC getting to the ground level ?

cheers

Col

[Andy Perrin]

There is, but it wouldn’t do much for our cameras because they don’t show anything past UV-B.

[Cadmium]

Of course I don't think we will get much out of camera sensors below about 300nm, no mater what filter is used. So for photography, I rather doubt any UVB or C will play a part in reflected photography.

If you just want to measure UVA/B/C, then Solarmeter has quite a few various meters for specific and combined ranges, but if you want to measure each range separately, I don't think they have one that is 'switchable',

but they do have UVA, UVB, and UVC dedicated meters, and UVA/UVB, etc... and UV index, etc., etc., and if you own a lizard, they got that situation covered also. :D

https://www.solarmeter.com/

[colinbm]

Thanks again Andy

If the UV C is increasing in Antarctica, even though it maybe small, surely the UV A must increase significantly ?

Cheers

Col

[colinbm]

Thanks Mr Cadmium

I love lizards, but their Big cousins & those slippery relatives worry me though....;-)

Col

[Andy Perrin]

Colin, the UV-C is increasing only because of the ozone hole (as far as I know) and ozone absorbs UV-B and UV-C but not UV-A. Therefore if ozone is depleted UV-B and C increase but A is unaffected.

[colinbm]

Thanks very much Andy for your explanations.

Col

[Andrea B.]

The problem here where I live is that the online UV indices are "regional", that is, they are recorded at various weather station locations which may or may not be close to me or where I'm working. I am not able to always find a UV index nearby.

 

So I got a nice Solarmeter recommended by John Dowdy. I love it!!! I probably could take a reading off my Solarmeter and convert it to a UV index reading if I learn the methods. When I use my Solarmeter, I just aim it at the sun !! I've learned that the readings I get between 3.5-5.0 are very good for UV photography. I've also learned just how fast those measurements can change and that the change is not really apparent to the human eye.

[aphalo]

There is no solar UVC at ground level anywhere on Earth. The couple of published reports of UVC at ground level have been refuted. The explanation has been stray light from longer wavelengths being assumed to be UVC. The cause was the use of single monochromator spectrometers, which combined with array detectors with low sensitivity to UVC, results in increasingly amplified stray light as the wavelength gets shorter. UVB varies a lot with solar elevation angle, but it is so much weaker than UVA even at noon, that to notice anything in images the UVA would neef to attenuated with a filter. UVA also varies during the course of the day. Wavelengths longer than 350-360 nm vary more or less in parallel with VIS. Below 350 nm UVA starts to be proportionaly more at higher solar elevations than lower solar elevations.

 

[Andy Perrin]

Pedro- not even under the ozone hole? It is supposedly the ozone that blocks UVC, so if it is absent, shouldn’t the UVC get through?

[Cadmium]

I don't have the Solarmeter UVC model 8.0, so I can't go outside and test for that.

https://www.solarmeter.com/model8.html

One of those is useful for testing UVC sterilizing bulbs, which tend to have a fairly short UVC lifespan, far shorter than when the bulb actually burns out.

That is the only thing I can think of needing a UVC meter for. Maybe if you are using UVC for UVC induced fluorescence,

then you could test your bulb every so often to know if your bulb is still emitting UVC or not,

but using UVC for fluorescence is a lot of trouble and much more dangerous, and most people don't ever do that.

Note however, buying old UVC bulbs/lamps is a bad idea if you want UVC, used UVC bulbs would often be void of UVC when purchased used.

Notice how the model 8RP has a remote sensor. That is because with UVC you want to be physically removed and isolated.

https://www.solarmet...m/model8rp.html

 

There may be some small amount of UVC at ground level, some say they can detect, but of course this is of no concern to our UVA (+UVB?) photography:

[JCDowdy]

.... stray light from longer wavelengths being assumed to be UVC. The cause was the use of single monochromator spectrometers, which combined with array detectors with low sensitivity to UVC, results in increasingly amplified stray light as the wavelength gets shorter.....

 

And yet they persist in using such inadequate instruments, often despite being warned.

 

It find it particularity annoying how they then mislead others.

[Cadmium]

As far as any ground level UVC reading using the Solarmaster UVC model 8.0 or model 8.0-RP, it may be a bit of UVB band cross over.

If you view the sensitivity range graphs of the UVC model 8,0 and the UVB models 6.0 and 6.2 you may see some common ground.

So, just a guess, but that may explain why some people are measuring a small UVC ground reading.

Actually, by looking at the graphs, that idea might apply more to the UVB models, but who knows what might be crossing over in actuality with the UVC model.

Then again...?

 

UVC sensitivity graph

https://www.solarmet...l_8.0_Graph.png

 

UVB model 6.0 sensitivity graph

https://www.solarmet...l_6.0_Graph.png

 

UVB model 6.2 sensitivity graph

https://www.solarmet...l_6.2_Graph.png

[colinbm]

Thanks Steve, yes there is a lot of overlap with these instruments.

Col

[aphalo]

That's right, there is no UVC even under the ozone hole. There is enough ozone left in the ozone hole to absorb all UVC. Depending on solar elevation angle the observed tail of the solar spectrum at ground level is somewhere between 290 nm and 293 nm. Properly measuring the UVB region of sunlight is a difficult business, and measuring UVC even more. It is worrying that these erroneous measurements of UVC at ground level are getting so much attention, sometimes even in the usually reliable press. This is very close to my own research subject, and I have recently exchanged e-mails about this question with the members of the UNEP panel, including the developer of the TUV model. Do trust me, there is no UVC reaching ground level. I should probably write a more detailed argumentation at some point unless someone else writes it first, meanwhile, if you are interested in the difficulties in properly measuring solar UVB, you may want to read my article https://doi.org/10.19232/uv4pb.2016.1.14 (open access)

 

It is possible also to explore this question using the TUV model and setting the zenith angle (90 - solar elevation angle) and trying different ozone column values. Let's assume we have the worst of the ozone hole over Antarctica (about 100 DU) but at the Equator and at the time when the sun is highest in the sky. Even with the sun at the zenith and 100 DU of ozone, the simualted spectral irradiance at 280 nm is more than 6 orders of magnitude smaller than at 420 nm. So, in practice smaller than the noise floor of even the best double monochromator scanning spectroradiometers. Measurements done with such instruments confirm that the purported measurements of UVC using not as good instruments are too high to be anything else than measurement errors. It is too easy to trust measurements when one is not familiar with how the instruments used work and manufacturers do not give strong enough warnings.

 

Broadband UVC sensors and meters are meant to be used for measuring radiation emitted by germicidal and similar lamps, not sunlight. Single monochromator array spectrometers suffer from stray light that is about three orders of magnitude below the signal in the UVB, and even worse in the UVC. The tail of the response spectrum of UVC broadband sensors extends into the UVB enough for UVB to interfere, as mentioned above by Cadmiun. Given the steepness of the solar spectrum in the UV region, one would need sensor response curves expressed in a log scale and measured in a way that the zero level is well defined. A good example of how using a log scale can be illuminating can be found in the pages of sglux describing there UVC sensors https://sglux.de/en/product-category/sic-uv-photodiodes/uvc-photodiodes/

 

As you have probably already noticed, I am upset by how the idea that there is UVC at ground level is gaining credibility. I hope I can convince at least some of you, that we should not accept these claims at face value. :)

[colinbm]

Thanks very much Pedro for this explanation.

Out of interest, with the UVC germicidal lamps, how are the bugs destroyed, is it the DNA that is broken that prevents them multiplying or is the broken DNA fatal to the bug ?

Col

[Andrea B.]

If the DNA chain is broken in a bacterium by the UVC light, then the bacterium can no longer reproduce and soon dies.

 

I think I've read that it is harder to kill viruses with UVC light, but that it can be done.

 

In those organisms having mitrochondria in their cells, the breakdown of any mitrochondrial DNA would likely prevent certain celluar energy functions from taking place and the cell would die.

[colinbm]

Thanks Andrea....I have long wondered ;-)

Col

[dabateman]

Well its slightly more complicated than just that. It depends on the number of double strand breaks. Our cells and bacteria have DNA repair mechanisms (protein families) that can find nicks in DNA or miss alignments or faults. Some bacteria even have double strand break repair mechanisms. To kill something you need lots of energy and lots of double strand breaks. Think about radiation treatment to kill cancers. High energy lots of breakage.

The sensitivity of fungi, bacteria to uv has ranges and its not the best way to kill. Gamma irradiation, is much better and used to sterilize medical equipment and drugs.

I would say UV is not really best way to kill something. It is however a good way to cause an undetectable miss alignment, leading to cancer or out of control repair. Remember its not targeted, so what you hit may be not needed or critical for function. Thus why it can be complex.

 

[dabateman]

Also think about Andrea's virus comment. A virus is a chunk of protein (prion) or naked DNA (virion) or mix of DNA with protein coat. Why does UV have no effect? Because its not doing any thing. It waits for a host to take it up and then problems start with host mechanisms getting off track.

The same is for us, if we handle the problem quickly and without panic, its easily resolved. If instead you run around like a crazy person, well then you've become a crazy person and the problem will get out of control.

[colinbm]

Wow, thanks Da Bateman

Col

[dabateman]

I think only in residential homes would you really see uv being indicated for germiciadal action. Its too inefficient and far too costly. Also its only a means to disinfect or sanitize. It will not sterilize something.

My personal ranking would be:

Uv, alcohol, VHP tied with scrubing with detergent and water. These all will not sterilize only disinfect. The alcohol hand sanitizer actually contains bacteria. There is a maximum USP limit on it. VHP, is vaporized hydrogen peroxide.

 

To sterilize you need ethylene oxide, gamma radiation or pressurized steam heat. The old moist steam 122C for 20 to 40 minutes is still one of the best ways to kill something.

This is now geting into what I actually do. So I will stop now unless people are still having a hard time sleeping.

[colinbm]

Ha ha Thanks Da Bateman

Col