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UltravioletPhotography

False colour spectrum


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I keep trying to get my head around what, if anything the false colours we see in a white balanced (against Spectralon) UV shot may correspond to.

 

I decided I didn't have a suitable prism to generate a rainbow but the noticed the colours produced on a typical CD or DVD.

 

These shots are meant to stimulate discussions.

 

Camera was full spectrum modified Pentax K-5, lens was Nikon Rayfact PF1054MF-UV 105 mm f/4.5

 

Visible light shot of CD in sunlight, 1/50s @ f/11 ISO 200 B&W UV/IR cut filter

post-28-0-35533000-1400896343.jpg

 

UV light of CD in sunlight, 1.0 s @ f/11 ISO 200, BaaderU UV-pass filter, white balance in Photoninja on Spectralon (through central hole in CD)

post-28-0-58483900-1400896358.jpg

 

Visible light shot of DVD in sunlight, 1/50 s @ f/11 ISO 200, B&W UV/IR cut filter

post-28-0-19904900-1400896373.jpg

 

UV light of DVD in sunlight, 0.5 s @ f/11 ISO 200, Baader U UV-pass filter, white balance on Spectralon

post-28-0-99358600-1400896388.jpg

 

The CD pair clearly show that the visible colours are removed by the Baader U filter in the UV shot, but there are some "false colours" which do seem to resemble some of those found in UV images of wildflowers.

 

The DVD appears to have a blue layer on the disc which biases the results compared to the CD.

 

I suppose those of our members who have been shooting UV for a long time will have done this for themselves.

 

Dave

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Welcome to the wonderful world of spectroscopy & spectrography Dave.

I only found this science a couple of years ago & I was truly amazed at what is to be discovered in wavelengths.

Particularly astro spectroscopy, using a simple SA100 diffraction grating.

Cheers

Col

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This is a cool experiment with the CDs.

 

Seems like with the Baader-U we get mostly blues, yellows and some very dark greens in UV False Colour Land - or maybe they are very dark cyans, not sure which. It is all a bit tricky nailing down the false colours because of little differences between editors, sensors, bayer filters, lenses, lens coatings and so forth. Then, of course, that colour list can change quite a lot once you stray from the Baader-U.

 

The greyed-blues always look lavender/purple to me. And greyed-yellows look more greenish. Never have gotten any bright greens with the Baader-U.

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This is a cool experiment with the CDs.

 

Seems like with the Baader-U we get mostly blues, yellows and some very dark greens in UV False Colour Land - or maybe they are very dark cyans, not sure which. It is all a bit tricky nailing down the false colours because of little differences between editors, sensors, bayer filters, lenses, lens coatings and so forth. Then, of course, that colour list can change quite a lot once you stray from the Baader-U.

 

The greyed-blues always look lavender/purple to me. And greyed-yellows look more greenish. Never have gotten any bright greens with the Baader-U.

 

I agree that this is a very interesting approach. Your comment also reminded me of a UV image that I had taken last year but that wasn't published, so far. I've started a new thread related to this here:

http://www.ultraviol...aader-u-filter/

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Well, I just rested the CD on my block of Spectralon then I could do the WB directly instead of calling up a Spectralon preset which I have stored in PhotoNinja.
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Bother !!! I just realised in my Eureka moment that I had jumped out of the bath without first assigning the sRGB profile to the files as I usually do (they were still in my working Adobe RGB)

 

So here are the sRGB versions (if you can detect any difference) just the two CD images

 

post-28-0-67776200-1400992525.jpg

 

post-28-0-77387200-1400992541.jpg

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There is a lot happening here too, that I don't fully understand either, like 1/2 waves, 1/4 waves & double wavelengths, mixed in with UV, Vis & IR.........

Col

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CDs and DVDs give nice gratings for cheap spectrometers

 

 

just to show the principle

 

the light should come via a small gap, here from a pizza cardbox

 

 

 

post-21-0-44402400-1401021838.jpg

 

 

the next shows the some first order(?) reflections taken with modified GH3, UV-Nikkor without any filter

post-21-0-26636800-1401021870.jpg

 

 

and now from the same position with the Baader-U on the lens

 

post-21-0-91324400-1401021900.jpg

 

 

Werner

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Nice demo, Werner !!

The UV false yellow & blue shows up "below" the visible.

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I got the idea to test it with a narrow gap from here: http://www.heise.de/hardware-hacks/links/1402116

 

this is a an additional info to a journal "c't Hacks",issue 2,2014

and there is a link "PDF-Bastelanleitungen", where you can download a zip-file with

a plan to build your little own spectrometer

In the journal, they show also some plan to build Tesla generator and they show the spektra of the sparks, some of which showing strong UV down to 200nm (but obviously measured with a "slightly" more expensive instrument.

 

Werner

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Excellent Werner,

 

That was what I was sort of getting around to after remembering the story of Newton discovering the spectrum by interposing a prism in a ray of sunlight through a slit in blind into a darkened room. Then along came Herschel and put a thermometer in the dark area off the red end and discovered IR.

 

I couldn't think of anything that would serve as a prism so the CD sort of had to do, but what I really wanted was to see what UV colours I could find beyond violet, which you have done very well.

 

Dave

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PS: "Light source" is sun light.

 

Looks like sun light .... through a window .... with a fair coating of unknown origin :D , perhaps attenuating nm < ~330.

If you take it outdoors you might see some UV-green band adjacent to the UV-yellow. Might depend on polymer absorbance?

 

- John

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The coating definitely comes from the sky, though I doubt, that it is star powder ;-)

 

 

Werner

 

(No sun since sunday, have to wait for further testing.)

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The coating definitely comes from the sky, though I doubt, that it is star powder ;-)

 

This whole planet including all of us are ultimately composed of star dust!

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.....The UV false yellow & blue shows up "below" the visible.

 

....why is the yellow/purple spectrum seen through baader-u at the red side?

 

I was wondering the same thing, why is the banding order reversed??

Offhand I would have expected to see the UV false color bands on the blue side of the visible color bands.

You mentioned multiple orders of refraction coming off this field expedient grating which may explain things.

- John

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