DaveO Posted May 24, 2014 Share Posted May 24, 2014 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 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) Visible light shot of DVD in sunlight, 1/50 s @ f/11 ISO 200, B&W UV/IR cut filter UV light of DVD in sunlight, 0.5 s @ f/11 ISO 200, Baader U UV-pass filter, white balance on Spectralon 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 Link to comment
colinbm Posted May 24, 2014 Share Posted May 24, 2014 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.CheersCol Link to comment
Andrea B. Posted May 24, 2014 Share Posted May 24, 2014 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. Link to comment
Nico Posted May 24, 2014 Share Posted May 24, 2014 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/ Link to comment
JCDowdy Posted May 25, 2014 Share Posted May 25, 2014 Spectralon through the central hole? Ingenious!! Link to comment
colinbm Posted May 25, 2014 Share Posted May 25, 2014 Spectralon through the central hole? Ingenious!!Good one Dave :DCol Link to comment
DaveO Posted May 25, 2014 Author Share Posted May 25, 2014 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. Link to comment
DaveO Posted May 25, 2014 Author Share Posted May 25, 2014 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 Link to comment
colinbm Posted May 25, 2014 Share Posted May 25, 2014 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 Link to comment
Alaun Posted May 25, 2014 Share Posted May 25, 2014 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 the next shows the some first order(?) reflections taken with modified GH3, UV-Nikkor without any filter and now from the same position with the Baader-U on the lens Werner Link to comment
Andrea B. Posted May 25, 2014 Share Posted May 25, 2014 Nice demo, Werner !!The UV false yellow & blue shows up "below" the visible. Link to comment
Alaun Posted May 25, 2014 Share Posted May 25, 2014 Just above the blue, there is also a small, sligthly brighter line popping up Werner PS: "Light source" is sun light. Link to comment
Alaun Posted May 25, 2014 Share Posted May 25, 2014 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 2,2014and there is a link "PDF-Bastelanleitungen", where you can download a zip-file witha plan to build your little own spectrometerIn 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 Link to comment
DaveO Posted May 26, 2014 Author Share Posted May 26, 2014 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 Link to comment
JCDowdy Posted May 28, 2014 Share Posted May 28, 2014 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 Link to comment
Alaun Posted May 28, 2014 Share Posted May 28, 2014 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.) Link to comment
JCDowdy Posted May 29, 2014 Share Posted May 29, 2014 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! Link to comment
Johan Posted May 29, 2014 Share Posted May 29, 2014 Curious, why is the yellow/purple spectrum seen through baader-u at the red side? Link to comment
JCDowdy Posted May 30, 2014 Share Posted May 30, 2014 .....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 Link to comment
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