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Getting more of the 380-400nm UV light

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#1 Reed F. Curry

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Posted 05 November 2018 - 01:05

Solar UVA light has a hard time getting to our camera's sensor, glass is not a friend to UV. For this reason we choose to optimize our filters and lenses for the maximal passage of UV light.

Solar UVA is most intense in the range of 380-400nm. Below is a representation of solar UV:
Posted Image
Please note the sharp drop in intensity of the UV from 400nm toward 320nm.

However, the usual UV-bandpass filters in use by modern UV-reflectance photographers fail to fully capture these wavelengths. All Schott UG*-based UV filters and Hoya U-3**-based UV filters have a very similar transmission curve which is not optimal for the wavelengths in the 380-400nm range.

Posted Image
The image above shows a UG11 1.5mm/S8612 1.5mm stack in dark blue, the SEU Gen2 in green, and the long UVA wavelengths missed by the UG11 stack in pale blue.

Of course, the users of the SEU Gen2 notice the coloration of their images are not what they are familiar with when using a Hoya or Schott stack, or a Baader U. However, the image is representing a more accurate rendering of the available solar UV light. That is, in the opinion of many, a positive result.

Edited by Reed F. Curry, 05 November 2018 - 01:06.

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Reed
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#2 Andrea B.

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Posted 05 November 2018 - 07:30

Here is an interesting UV solar spectrum chart which omits the IR and visible light portion. The steepest UV drop-off begins around 330 nm. There is more UV between 380 - 400 nm. But there's a fairly good supply between 360 - 380 also.

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#3 dabateman

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Posted 08 November 2018 - 05:53

Andrea,
That spectra is very interesting to see. I can now see why people have placed markers for UVB from 290 to 320nm, as that fits a curve nicely. Similarly UVA would fit very well from 320nm to 380nm with a gentle raise. This makes sense to me why some have the uv end at 380nm and others at 400nm. Right at 385nm the curve takes a shift and spikes up.
So we could mark the 385nm to 405 band as uv/violent.
Using my tight wavelength filters I see difference between the 390bp25, the 370bp15 and 313bp25 filters, with slightly different dark spots appearing in flowers. The 390bp25 has a typical uv bulls eye, but lacks the finner dots. The UVb filter has the most dots typically.

#4 Reed F. Curry

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Posted 08 November 2018 - 12:51

"uv/violent", interesting choice :)
We can call parts of the electromagnetic spectrum whatever we want. I think 360-362nm should be called "Bob". "Sue" is my second choice. Misunderstandings might arise, of course. "I got a lot of Sue in my shot yesterday" might raise eyebrows.

If we wish to communicate with others, it is practical to have a common language. The 320-400nm range as UVA has been around for many years. But there is always room for change. See https://sputniknews....change-lawsuit/ :)

But that is only relevant if we wish to communicate, not confound.


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Reed
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#5 dabateman

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Posted 08 November 2018 - 13:17

Sorry Reed,
Thinking about it now I can see how that may have made you upset using new terms that are not established. I have no bad intentions and have always thought of UV ending at 400nm.
With all the recent discussion on this though now I can understand why there is discussion. Especially considering Andrea spectra above. I can see how many curves can fit and ranges being applied.

I mostly use led and mercury vapour lamps for lighting. So for me 313, 334, 365 and 405 are the big lines. 334nm and 365 have some differences, but not as significant as 313, 365, 405.
David

#6 Andrea B.

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Posted 08 November 2018 - 16:39

David, I really think Reed was just being funny. He has a wry sense of humor - probably triggered by your typo 'uv/violent'.

*****

I think we all agree that we must standardize in science and other disciplines to ensure we are all discussing the same thing. Here on UVP I always try to follow the generally accepted 400nm mark for the division between UVA and visible, but I keep in mind that one does occasionally see that demarcation shifted to 380 or 390 in some texts or references.

It seems reasonably clear from that second "spread out" chart, that we are going to have some problems in reflected UV photography in sunlight starting around 320 nm. And also that 370 - 400 nm is going to be the easiest range to capture in sunlight. (...at least in Zacatecas, Mexico.....j/k... :lol: )

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I have intended for days now to look up the lines associated with those dips at about 375 nm and 382 nm. Anybody know?
Andrea G. Blum
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#7 Andy Perrin

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Posted 08 November 2018 - 17:47

Ultraviolent photography sounds dangerous!

#8 UlfW

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Posted 08 November 2018 - 18:37

View PostAndrea B., on 08 November 2018 - 16:39, said:

I have intended for days now to look up the lines associated with those dips at about 375 nm and 382 nm. Anybody know?

The 382nm dip seams to be the L line of the Fraunhofer lines (Fe).
https://en.wikipedia...raunhofer_lines

375nm - I have no idea.
Ulf Wilhelmson
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