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UltravioletPhotography

And yet, another rainbow


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We had a rainbow today, and I had to try some invisible light photography on it. My camera caught some drops of water, but nothing to worry about.

 

Cameras:

Samsung Galaxy A40, front camera (visible);

Full-spectrum Canon EOS M (UV, IR, full-spectrum);

Lens: Soligor 35 mm f/3.5.

 

Filters:

UV: ZWB2 (2 mm) + Chinese BG39 (2 mm);

IR: Hoya R72;

No filters for the full-spectrum image.

 

Visible light (f/1.7, ISO 40, 1/2370 s exposure). Image cropped to approximately match the others:

post-284-0-59301900-1622913872.jpg

 

Infrared (f/8, ISO 100, 1/250 s exposure):

post-284-0-08937700-1622913922.jpg

 

Ultraviolet (f/8, ISO 100, 1 s exposure):

post-284-0-28779400-1622913961.jpg

I have an image with more exposure, but the rainbow is less visible.

 

Full-spectrum (f/8, ISO 100, 1/1000 s exposure):

post-284-0-35398200-1622914024.jpg

The camera is very sensitive in full-spectrum. If I had the lens wide open, I would have overexposed at ISO 100. Also, the lens has quite a bit of chromatic aberration with a band this wide.

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Thanks both. Yes, I noticed too that the red bow in the visible image overlaps the infrared rainbow. I don't know if my phone camera is seeing some far red or the R72 filter leaks below 700 nm (it should cut there). I should investigate this further.
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Andy Perrin
I would suspect the phone cam of leaking unless you are using an no-name branded 720nm filter. We know the (real) Hoyas are okay! :grin:
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He says he only used the phone for Visible light.

" Full-spectrum Canon EOS M (UV, IR, full-spectrum); "

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Andy Perrin
Cadmium, because the red part of the rainbow overlaps in the visible and IR photos. I tried aligning the two images and confirmed it. So probably the phone is going a bit into the 700nm's. It seems unlikely the Hoya is transmitting below 700nm.
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Perhaps not, but even 720nm produced a good amount of false color, just think of all the 720nm images, swapped with blue skies, etc..
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Andy Perrin
The issue is that the rainbow is splitting up the colors like a grating would, so IR and red shouldn’t be in the same physical location. This has nothing to do with false color, the same issue would be there in a monochrome sensor. I don’t think the problem is the Hoya, I think that phone cam is not cutting off. Easy test would be to shine an 700nm LED at some PTFE and see if the phone can see it.
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...or just compare a shot with S8612 2mm in front of it to a shot with nothing in front of it (Schott BG39 2mm should work the same as Schott S8612 2mm, it cuts off at the same place as S8612 2mm, no leak).
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Andy Perrin

At first I thought that made sense but now I don’t know.

 

How would we tell a small IR leak in a vis photo without IR of known wavelength to verify with? We can’t say the phone has any particular filter glass.

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So, a few points:

- I bought the Hoya filter on Amazon from Hoya, I really think it's genuine (and it was also quite expensive);

- I can try to stack my (Chinese, not Schott) BG39 on the phone and see what happens (but I no longer have a rainbow);

- I can use a diffraction grating to see how deep the phone sees with and without the R72 filter. I could even try to see the Fraunhofer lines to have a wavelength reference, but I don't know if I can do that.

 

So I will try those experiments, and post here.

 

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Thanks Fandyus. Actually I am not the first one to do that, there are many other examples here on UVP.
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If you have a garden hose you can make a rainbow in your backyard. They just tend to be full circles and much smaller.
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If you have a garden hose you can make a rainbow in your backyard. They just tend to be full circles and much smaller.

Yes, I have it, I will give it a try if I can.

 

Initial experiments:

 

Solar spectrum with the Canon EOS M + Soligor 35 mm f/3.5 + Hoya R72 + 1000 lines/mm diffraction grating. I can't identify the other Fraunhofer lines, I would really appreciate if someone could do that:

 

f/8, ISO 100, 1/4000 s exposure:

post-284-0-33470700-1622979494.jpg

 

I tried it with my phone, but I could barely see a couple of lines, not very helpful.

 

My phone + R72 can see, so it does see a bit past 700 nm:

 

f/1.7, ISO 100, 1/100 s exposure (both images):

post-284-0-75011600-1622979674.jpg

 

post-284-0-98337800-1622979682.jpg

 

I then took two images with my phone, 1000 lines/mm diffraction grating, 12 V halogen lamp ran at 20 V, no filters and with the R72:

 

No filters (f/1.7, ISO 100, 1/8850 s exposure):

post-284-0-66181800-1622979900.jpg

 

R72 (f/1.7, ISO 100, 1/100 s exposure):

post-284-0-01101300-1622979910.jpg

 

BiColour stack (cyan: no filters, red: R72):

post-284-0-01555000-1622979954.jpg

 

Between the images there is a difference of 88.5 times (6.46 stops) of exposure, consider that.

 

A nice site to help find Fraunhofer lines: http://bass2000.obsp...solar_spect.php

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You could make a slit by sticking a needle in the floor or something a bit higher & get the lamp out of the picture So you don't over-expose the spectrum.
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I tried to make rainbows but I can't make them far away enough to be able to stack images from my camera and my phone, without artifacts. Also I am trying the needle technique, but it's quite hard to get a spectrum that way. If I will be able to do something useful I will post here.
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I tried to make rainbows when I first tried full spectrum about 10 or 12 years ago.

I had difficulty with a prism, then I found out about spectroscopy, which I had never seen & I knew nothing about. I have had a lot of fun since & learnt so much about light & colour.

I nearly got into astro-spectroscopy which is hugely interesting & so fascinating what can be learnt about a star thousands of light years away.

But I like my feather quilt too much, to sit up all hours at night.... ;-)

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A better spectrum:

post-284-0-97750400-1622984233.jpg

 

Please ignore the constantly-spaced "ripples", I don't know what causes them, they can be seen with the naked eye too.

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If you include the needle in the photo you can work out the wavelengths by measuring from the needle (slit) to a known line & then calculate the rest of the spectrum.

The spectrum is basically linear. I don't know how to do this myself, but I know it is easy for someone with brains.

You may find something on line, or one of the guys will help you.

There are apps that you can get for your phone that can calibrate the image for you.

I know it is possible, you just need some one to show you how it is done in a spreadsheet, or with the app.

Good luck & enjoy the fun with your first spectroscope.

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My lens is not wide enough for that, and the photo above was taken directly with the Sun partially "eclipsed" to make it narrower. Also a grating isn't linear, see here: https://www.ultravioletphotography.com/content/index.php/topic/3617-infrared-with-a-solar-panel/page__view__findpost__p__31708

 

A diffraction grating is approximately linear for wavelengths significantly shorter than the distance between the lines: https://www.ultravioletphotography.com/content/index.php/topic/3617-infrared-with-a-solar-panel/page__view__findpost__p__31757

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