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Is an Assignment of Colors to UV-Wavelengths possible?

31 replies to this topic

#1 WiSi-Testpilot

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Posted 10 December 2018 - 21:59

I took the spectra of a CMOS sensor and extended the curves to the UV range, see image. The DCC1645C sensor was taken as an example, which I found on the Thorlabs website.
The Baader U-Venus filter for example is open for UV light from 320 to 380 nm with a maximum at 350 nm. The overlays of the filter shape and the spectra indicates, that in a first approximation UV light from 380 nm downwards generates blue, while UV light from 320 nm upwards generates red. My UV images have a great amount of blue and red and a little bit of green. This is in accord with the shown spectra.
I think, if correct, this aspect is very interesting, since the UV photos indicate which wavelength is observed.
A spectrum of a CMOS sensor with a Bayer filter that is extended to UV would be interesting.
Best regards,
Wilhelm
Attached Image: CMOS&Filter.jpg

#2 Andy Perrin

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Posted 11 December 2018 - 00:13

NO DON'T DO THIS! We have endless wars on this topic! Please search the forum first and read the many many posts already on this topic before delving in!

Only madness lies here.

Edited by Andy Perrin, 11 December 2018 - 00:13.


#3 Cadmium

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Posted 11 December 2018 - 02:03

There was a group working on this on an Island, food was dropped in from time to time, but no word from them... (humor)
The angle of the lines can't be projected into the unknown UV range like that.

#4 Reed F. Curry

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Posted 11 December 2018 - 02:05

I hear the drums...
The naive are restless...


The drums have stopped!
Best regards,
Reed
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#5 Andy Perrin

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Posted 11 December 2018 - 04:47

Wilhelm, you really have no idea what a can of worms this question is. Probably hundreds of forum hours have been spent on far more sophisticated efforts to get a handle on it without much agreement or end result. Forum members have even been banned over the resulting arguments.

One of the better attempts to grapple with the issue have been Jonathan's experiments here:
http://www.ultraviol...ctral-response/
http://www.ultraviol...n-in-uv-and-ir/

which Jim then attempted to use to derive colors here:
http://www.ultraviol...er-meaurements/

but the colors were ultimately not a perfect match. While even this effort failed, I think it was going in the right general direction. It is impossible to work backwards, though, from the colors to the wavelengths, except in a very rough and imperfect way that leaves everyone unsatisfied. Like, if you see only blues in the images you take with a certain lens, then you can deduce that the bandpass is probably not very good.

Edited by Andy Perrin, 11 December 2018 - 04:50.


#6 Cadmium

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Posted 11 December 2018 - 06:07

Maybe you can buy the stuff in quart glass bottles, paint it on some UV transmitting glass, and test it with your spectrometer somehow?
https://www.fujifilm.../rgb/index.html

I have not priced the stuff, but I can only imagine that 3 quart glass bottles of that stuff is going to be a slightly expensive experiment... :P
You can ask them for the price.

Edited by Cadmium, 11 December 2018 - 06:27.


#7 Andy Perrin

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Posted 11 December 2018 - 07:26

Hee. I think Jonathan’s numbers are probably the best we are likely to get for some time. It would be good to reproduce that experiment, however. Especially with a different camera brand.

#8 Cadmium

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Posted 11 December 2018 - 08:11

:( Pout.... Don't you like paint, Andy?! You don't like to paint...
You... you weren't one of the island group, were you? :unsure:

Edited by Cadmium, 11 December 2018 - 08:20.


#9 Andy Perrin

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Posted 11 December 2018 - 08:23

*shoots you with a poison dart*

#10 Cadmium

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Posted 11 December 2018 - 08:47

Oh, jeez, thanks! :-/ No need for violence man.

#11 JMC

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Posted 11 December 2018 - 09:05

I saw this topic just before I went to bed last night, and I though I'd leave it until this morning to respond.

This subject always causes some intense discussion. As has been mentioned I went after measuring camera response in the UV, and even built my own equipment to do it. In short, yes, camera response can be measured - I've done it, and a few others have published on it (and I recently submitted a paper on it). It certainly isn't easy, but yes camera response curves are possible. As I have monochrome converted cameras too, I can even use those to back calculate Bayer filter transmission spectra in the UV. I have even tried approaching someone who makes Bayer filters to have them make me a sample of quartz with each of the filters individually on them so I can measure transmission spectra directly. After initial discussions it has all gone quiet, so they are either unable or unwilling to do it.

The most up to date plot on the forum of camera response in the UV is here in post #9;
http://www.ultraviol...dpost__p__21902

Can that response be used to predict colours for different wavelengths? Well, this makes the measuring of the camera response look easy.

Overall, the general colours you see can tell you something about the wavelengths present. If you get lots of blues in the raw composite file then there is likely to be a lot of the 390-400nm light for instance. Can it be used to say Pixel X is 365nm and Pixel Y is 368nm, no.

The threads mentioned above make interesting reads, if you have a glass of wine and want to chill out one evening.

Edited by JMC, 11 December 2018 - 14:13.

Jonathan M. Crowther

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#12 dabateman

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Posted 11 December 2018 - 14:59

Well lets not discourage anyone from testing. However, lets keep an open mind and be able to accept when we may have made a mistake or a breakthrough.
Generally what I see is less than and equal to 345nm is green. At 313nm its really green, more green than 335nm.
Above that til about 375nm is yellow. I like to think of it as golden. So a mixture of what ever colors make up yellow. Then above that is blue. 380nm to 405nm is blue with mixtures of purple or violent.
So you can stop there or try and tease it out more specifically.
What I want to test next is the color checker passport at all wavelengths that I have band pass filters for. As a follow up to the color chart MATLAB post I posted earlier.
But I think I want a new light. Cadmium may have been correct months ago recommending the 200 UVb 26W light by Exo terra. It looks to have the range I want. The temperature I want and lower IR.
My lucky herp mercury vapour lamp gets really hot, has a ton of IR. But has a really strong 313nm line and 365nm line. So has been useful.

#13 Andy Perrin

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Posted 11 December 2018 - 17:03

I want to discourage people from jumping into a complex subject without doing the background reading. After that, new work is fine.

Dabateman, that color checker is useless in UV. The spectrum of the blacks and grays are not even flat- Jonathan measured those awhile ago.

Edited by Andy Perrin, 11 December 2018 - 17:05.


#14 dabateman

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Posted 11 December 2018 - 17:48

@Andy,
Well I feel I may need to waste some shutter clicks to see. Also, I want to look at the color side. I am curious if the colors respond differently at different tight bandpasses. The red seems to have nice UVIVF as many here have shown.


#15 Cadmium

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Posted 11 December 2018 - 20:25

View PostAndy Perrin, on 11 December 2018 - 17:03, said:

I want to discourage people from jumping into a complex subject without doing the background reading.

With 'poison darts'?! :unsure:

I think you should splurge and buy the quart jars of paint, Andy. :)
I was being serious about that idea, if someone had the inclination.
However, it doesn't seem worth it to me.

Edited by Cadmium, 11 December 2018 - 20:29.


#16 Andy Perrin

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Posted 12 December 2018 - 01:06

Quote

With 'poison darts'?! :unsure:
Nah, with common sense. :rolleyes:

Honestly I'd love to see what tests of the paint or whatever would do, but I don't own a spectroscope and I'm sure those paints cost $$$$.
--

dabateman, there are tons of photos of color checkers already on the site? I mean, knock yourself out, but all they show is that the color checkers all reflect blue-ishly, without any yellows or other false colors that would be needed to use that MATLAB software. It is ridiculously hard to find a pigment that reflects shortwave UV. Only in flowers have I seen anything like that, and I almost wonder if it's structural color in their case...

Edited by Andy Perrin, 12 December 2018 - 01:11.


#17 DaveO

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Posted 12 December 2018 - 02:33

Here, to save you searching is what happens to a colour checker

Visible
Attached Image: _DO64284_v2.jpg

UV
Attached Image: _DO64286_v3.jpg

Dave

#18 DaveO

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Posted 12 December 2018 - 02:46

Here is a reflected spectrum of an Australian flower, a Hibbertia
Attached Image: Dyer 2012 Hibbertia Screen.jpg

A different Hibbertia exutiaces looks like this in Visible light
Attached Image: Hibbertia_exutiaces_Vis_Screen.jpg

and this in UV
Attached Image: Hibbertia_exutiaces_UV_Screen.jpg

Work that one out!

Dave

Edited by DaveO, 12 December 2018 - 03:40.


#19 Cadmium

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Posted 12 December 2018 - 02:59

Dave, No matter how I work it out, those are very strikingly beautiful comparison shots! Wow!

#20 Reed F. Curry

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Posted 12 December 2018 - 03:03

Here is another colorchecker in the UV --
Posted Image
Best regards,
Reed
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