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UltravioletPhotography

[SOL] Solar Radiation Spectrum


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Update: 24 Jan 2018. Added approximate percentage amounts of uv/vis/ir in sunlight.

Update: 28 Mar 2018. Added link to solar irradiance calculator. Added link to ASTM solar irradiance data.

 

I am giving this chart its own topic for easy reference.

 

Note the relative lack of UV in the solar spectrum. Look to the left of the violet band beginning at (unmarked) 400 nm.

 

Added: It is estimated that the average percentage amounts of UV/Vis/IR in sunlight are 3/43/54%. This may vary by altitude, latitude and time of year (and also other factors).

 

post-4-0-97702500-1433605108.jpg

 


 

From Pedro Apahlo and John Dowdy: Here is a link to a solar irradiance calculator depending on elevation, time and location.

Quick TUV Calculator

 

 


From John Dowdy. Here is a link to an excellent reference for Solar Irradiance. You can download data charts in HTML, Excel or a text file. There are many other references and links listed.

Sponsored by the American Society for Testing and Materials (ASTM) (http://www.astm.org/)

 

Solar Reference: Reference Solar Spectral Irradiance

 


 

From Pedro Apahlo: I have developed a set of packages package for photobiological and UV and VIS radiation calculations. One of the most recently accepted at CRAN (Comprehensive R Archive Network) is 'photobiologySun'. This is a data-only package with example spectral data for sunlight and shade light. If interested, there is more information at https://www.r4photobiology.info/.

Just let me know if you have any questions.

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Bill De Jager

And that's with the sun high in the sky and atmospheric attenuation minimized.

 

I'm wondering about the solar corona at the eclipse next month. The coronal spectrum must be quite different than normal sunlight given the far higher temperature of the corona. UV ought to be dominant, albeit less intense than in normal sunlight. However, I hear that atmospheric turbulence more strongly affects UV than visible light and that could affect coronal photos taken in UV.

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I wonder what the spectrum would look like in the 'blue hour". Of course absolute levels would be lower, but what about relative distribution?

UV reception has been suggested to help some animals living at high latitude during low light conditions.

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  • 4 weeks later...

Answering a bit late as usual. ;)

There is a quick on-line calculator for the short end of the solar spectrum (earlier UV only, but current version UV and visible), developed by Sasha Madronich.

http://cprm.acom.uca...nteractive_TUV/

The quick interactive version is handy when wanting to estimate the solar spectrum at a given solar elevation, or at a given location and instant.

For spectral output select "OUTPUT OPTION 2" and "IRRADIANCE, SPECTRAL (W m-2 nm-1)".

The default wavelength range is 280 to 420 nm. The upper limit can be increased as needed up to around 1000 nm. The "increments" input is the number of wavelength steps, rather than step width.

The option related to using 4 vs. 2 streams for calculation should affect results only at very low solar elevation angles, when 4 streams may give a better estimate.

 

The underlying simulation model (TUV) is also available as source and is written in FORTRAN. This model also simulates atmospheric chemistry. The trickiest part is estimating the "clouds" input data.

 

When the solar spectrum at ground level is the main interest meteorologists tend to use the model libRadtran that in its latest incarnation (version 2) can simulate the whole solar spectrum from UV to IR. http://www.libradtran.org/ As far as I know there is no interactive version and it does require quite a few inputs.

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Adding to the list of experiments I want to do, I have access to a little Ocean Optics USB spectrometer, which has been calibrated for absolute irradiance, and it has a cosine corrector on the end of the fibre. I'd like to do some measurements of the spectra during the day. However I need some advice on the actual logistics of how to do it, to get consistent, useful data. Do I need to point it at the sun, do I need to avoid the sun, can I just stick the fibre 'up in the air', etc, etc? Also it is slightly limited in that it is mains powered, so I can't just disappear off into the middle of nowhere and take the measurements away from buildings, however I could potentially run it on the roof of my house to get early morning through to early afternoon data without any obstructions.
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The problem with array spectrometers is that stray light is not better than 1:000 in the UV region, usually worse. Array spectrometers are always single monochromator instruments and the UV pixels receive some visible light due to internal reflections and other imperfections. Not even the best Ocean Optics spectrometers can be used to measure the solar spectrum down to the shortest wavelengths. One can to some extent get around this problem by measuring the stray light and then subtracting it. The choice of filter for this is crucial, and one may need to compensate for the absorption by the filter of the radiation creating the stray light. We have been using polycarbonate, as much of the stray light in our spectrometer is from the IR, that is in part attenuated by the more common UV long-pass glass filters. Believe it or not, under clear sky conditions a simulation with one of the models I mentioned above, will provide a far better estimate of the UVB region of the solar spectrum than measuring it with a single monochromator spectrometer. So, if you do the experiment with the little Ocean Optics, take note of the exact time and geographical coordinates, so that as a second experiment, the measured and simulated spectra can be compared.

 

As to the direction in which to point the cosine diffuser, it just depends on what you are interested in. Most meteorological data is collected with the surface of the diffuser perfectly horizontal. However, if you want to know the spectrum of the light being received by a flower or a wall, you would need to position the surface of the cosine diffuser parallel to the surface of your interest. Obviously the spectrum of direct radiation from the sun is different to that of scattered radiation from the sky. Oversimplifying things, by changing the direction the cosine corrected diffuser points to, you change the mix ratio of light form these two sources. In my experience the most frequent source of errors is shading by the operator (or reflections from those wearing white lab coats) followed by shading by other obstacles. And in the near infrared reflection from vegetation can be a significant problem even when the vegetation is not shading the sensor at all.

 

For reproducible and comparable output: keep the surface of the cosine sensor perfectly horizontal, make sure you do not disturb the reading by keeping yourself below the level of the sensor or several meters away. Avoid other obstacles by either being far from them, or measure from the roof of a building or other elevated place. Around local solar noon, the unexpected disturbances are less, but when the sun is low in the sky measurement becomes very tricky because, 1) sensor levelling errors disturb the measurements a lot, and 2) it is almost impossible to avoid disturbances from the surroundings unless the sensor is situated on a high elevation.

 

If you point the diffuser straight into the sun, the spectrum you will measure will change less through the day (under clear sky) as direct radiation will predominate always, but it may take some effort to orient the diffuser correctly, and to find/model spectral data to compare to.

 

By the way, which model of Ocean Optics spectrometer will you use? All those I know of are USB powered, and can be used in the field tethered to a laptop. And the very small OM modules require only low voltage DC as power. Even if sold with a mains adapter, it should be feasible to use them with a small LiPo battery or a power bank depending on the voltage required.

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Thanks for the advice Pedro, very useful. It's an Ocean FX model, bought for something different, and is mains powered. I just thought as it was something I had to hand, I would try it. The need for mains power shouldn't be too much of an issue as I could run it on the roof of my house, using an extension lead.
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  • 1 month later...

When will we get consistent blue sky in the UK? Oh, sorry I forgot, we don't.... I had a play about with the OO FX this morning, took it outside and tried to get a spectra from the light in the back garden. 100ms integration time, with each spectra being an average of 50 scans, FX spectrometer with 25um slit, 600um fibre and cosine corrector. Calibrated for spectroradiometric measurements. The end of the cosine corrector was held horizontal about 1.5m off the ground. This was done just before 10AM on 14th Oct 2017, 51.4 degrees north, 0.5 degrees west, weather was overcast, varying from cloud, to varying degrees of hazy sunshine, as the clouds blew through. There was no proper 'blue sky', it is the UK after all. Anyway, thought I would share what I saw.

post-148-0-22554300-1507977858.jpg

Looks as though at least I can pick up differences as a result of cloud cover :), and as mentioned above in one of the posts, no wonder we struggle with getting light for UV images.

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Thanks Steve. Andrea, I 'believe' they are something to do with clouds/water vapour. However if anyone has a reference to back that up please post it up. Interesting to note that down at 293nm there is nothing - Andrea, that may well explain your difficulty getting good images with the 293 filter. Obviously a sunnier day, would I guess produce more UV down there, but even so, when comparing it to the region covered by the Baader U, it will be much much less intense.
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JMC, definitely it goes down fast beyond 350, but it's hard to tell if there's an actual spectral absorption line there with the graph scaled as it is. You'd have to plot just the 250-350 nm range to even tell, and probably it should be in direct sunshine without haze. I think water absorbs in that region, so there may be more there on a dry clear day.
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  • 5 months later...
Updated first post with two links to solar irradiance calculator and solar irradiance data.
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JMC, definitely it goes down fast beyond 350, but it's hard to tell if there's an actual spectral absorption line there with the graph scaled as it is. You'd have to plot just the 250-350 nm range to even tell, and probably it should be in direct sunshine without haze. I think water absorbs in that region, so there may be more there on a dry clear day.

Andy, I've just realised I never responded to this. Below is a zoomed in graph of the full spectra from above, and yes there is a lot of structure in the spectra between 300 and 400nm.

post-148-0-20544900-1522308895.jpg

 

The spectrometer is calibrated for absolute irradiance, and because of the the fact that the structure looks the same for all the conditions, I'm confident this is not noise. I do not have bright sunshine, with absolutely no cloud, to compare with unfortunately.

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Andy Perrin

Good excuse for a holiday! Field trip across the channel?

-

Yeah, it definitely looks like there's structure, but it would be hard to know which parts are due to water absorption and which are really due to absorption in the sun. The trend seems to be that the less haze, the deeper the line near 359 and 384 get? So those might be real. Also something in the 390s. Does this one use 20nm bins?

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Andy, the OO system I use measures multiple times per nm. It doesn't average over 20 nm bins. It does look like there is structure there, but I have no idea what it would be.
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  • 1 month later...

Solar Light Penetration into Sea Water

 

Institute for Environment and Sustainability of the European Commission

Here is the original but no longer working link:

http://ies.jrc.ec.europa.eu/uploads/fileadmin/Documentation/Reports/Global_Vegetation_Monitoring/EUR_2006-2007/EUR_22217_EN.pdf]

 

Read the following carefully:

The vertical axis is depth, in meters, at which the intensity of that wavelength is equal to one percent of the intensity at the surface.

PenetrationLightInotSeawater.jpg

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Solar Energy in Fiji in July

 

The chart has been obtained from the solar spectrum at the boundary of the earth atmosphere using the SMARTS 2.9.5 scientific simulation software. This simulator takes into account light absorption by various components of the atmosphere as well as scattered light from the sky.

 

If this does not convince you of the scarcity of UV light around 300 nm in the solar spectrum, then nothing will! (I do grant that Fiji in July is a rather rare location for most of us. I would suspect that the situation is much worse UV-wise where I live.)

 

SolarSpectrumFijiJuly.jpg

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Actinic lighting is often used as supplemental lighting for aquariums because it peaks at 420nm and penetrates water deeper than other lighting, good for corals, etc..

Also sometimes used in phototherapy of infant jaundice.

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I find it interesting that there is an inflection point at ~330 nm where UV irradiance finally drops off to practically nothing (in JMC's charts). Interesting because, this is also at about the peak absorbance wavelength of the melanin in our skin. Its almost as if nature made a judgement call and said, "This is as far down (in nm) as I need to go with pigment protection... anything below that is too weak to worry about". Or something like that ;).
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Interesting to learn, Mark. Thanks. :D

 

If the atmosphere is preventing UVC and much below 330 nm from ever reaching earth, then it certainly makes evolutionary sense that melanin is like that.

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