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UltravioletPhotography

The green banana test with a 265 nm UVC LED


Stefano

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Could I not try the green banana test with my LEDs? Of course not! It is a must with UVC.

 

So, I took a greenish banana (not really green, but it still has some green) to test this. Green bananas work better than yellow ones, for some reason. It is surely due to chemicals present only in unripe bananas, as this is almost surely a chemical reaction caused by the energetic UVC photons. For testing, I chose the green areas of the banana.

 

For both tests the LED was run current-limited at 50 mA without an heatsink. The output power, if it is a third of the maximum, is around 3 mW.

 

First, I tried with the LED at some distance from the banana, and a plastic thing that blocks UVC to cast a shadow. I irradiated like this for one hour:

post-284-0-40735800-1609691674.jpg

 

post-284-0-85839000-1609691286.jpg

 

After one hour, I turned off the LED and took a look. There was no visible discoloration.

 

Since this was probably due to the very low intensity, I placed the LED much closer, and irradiated for one hour again:

post-284-0-79077400-1609691784.jpg

 

post-284-0-80861500-1609691796.jpg

 

And, after waiting for another hour, I took a look, and this time it was a success!

post-284-0-12725600-1609691849.jpg

 

post-284-0-11054300-1609691858.jpg

 

It must also be noted that this browning/discoloration effect sometimes takes hours or days to develop AFTER the initial irradiation. Kind of like sunburns. So I will update you tomorrow if I find some differences.

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Cool banana tes! :smile:

 

Electrical banana

Is gonna be a sudden craze

Electrical banana

Is bound to be the very next phase

 

They call it mellow yellow (Quite rightly)...

 

(Donovan, "Mellow Yellow")

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Electrical banana will be Stefano’s band name.

 

Stefano, now you must project an image on the banana and use it as UVC film.

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Stefano, now you must project an image on the banana and use it as UVC film.

That's actually a fun idea. With a pinhole and reflected light it would probably take days or weeks of exposure, much less with a lens but still a lot.

 

If I had a quartz lens I could use it to project an image of the chip on the banana and an hour of exposure would be enough.

 

You can use this technique to customize bananas. Put your name on it made with a UVC-absorbing material, irradiate it for enough time, and then you have your name "printed" on it.

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You know, that is actually might be a really cool idea.

I get tired of all the fruit labels, well actually they are pretty cool too, but the extra stickers they put on the organic stuff, which all seems hard to remove, and the adhesives we consume...

Maybe they could put that stuff right on the outside with no labels. Maybe the idea is too SciFi outer space future creepy?

Never mind... wild un-throughout idea.

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So this is how you mark your food in your house. In the old days we just spat on it to say it was ours.

 

I am impressed that you could run it for an hour without a heat sink. I have started to think about building something now.

 

Need 265nm led, heat sink (maybe copper), driver, then power source. Since these will be restricted to my basement it doesn't need to be battery powered. I might use a 12V plug in pack that I have extra from external hard drives.

 

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I am impressed that you could run it for an hour without a heat sink. I have started to think about building something now.

Not really impressive. The LED produced roughly 0.3 W of heat, which is not enough to heat it too much given it's (high) K/W (Kelvin/Watt) thermal resistance. It reaches the equilibrium temperature in a few minutes.

 

Without a heatsink it may have a thermal resistance of 50 K/W or even higher, while adding even a small heatsink significantly lowers it. The lower, the better.

 

265 nm LEDs are fun to play with. Probably safer than mercury lamps mostly because of the low output, of course still a bit dangerous (especially if you build an array with tens of them), but you have total control over them. You can dim them down to extremely low outputs if you want, something you can hardly do with mercury lamps. You can modulate them with an audio amplifier to send music with UVC light. And just the thought that simply running electrical current through a piece of semiconductor gives you UVC photons is amazing (although the "piece of semiconductor" is actually composed of several layers of different materials doped in difference ways, so quite complex).

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Instead of projecting, you could use something like a stencil placed right against the banana to produce an image.

 

OOH, OOH

 

Use a quartz lens to focus the led to a spot, replace the extruder in your 3D printer with this assembly, and use it to decorate the banana like an Easter egg. :wink:

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One day later, the spot made with the LED very close to the banana didn't change much, maybe just a bit darker, but I did see a big change in the first exposed area.

 

Yesterday, just after irradiation, I couldn't see anything. Some hours later, I started to see a shadow. Today, one day later, the shadow is even more apparent:

post-284-0-42988900-1609772165.jpg

 

post-284-0-85464200-1609772173.jpg

 

I just "sunburned" a banana. 4.68 eV of energy is more than enough to mess with molecules, usually not in a pleasant way.

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While the second burn (the images just above this post) is still darkening, I noticed the first intense burn feels smoother to the touch than the healthy banana skin, which is more "rubbery" and has a bit more grip. It is really mind-blowing that a semiconductor today is capable of this. Just run current through it, and you can literally kill stuff, and give you cancer (note that sunlight can also damage your DNA and UV light, from A to C is classified as a known carcinogen (source). UVC is just a "bit" nastier).
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While the second burn (the images just above this post) is still darkening, I noticed the first intense burn feels smoother to the touch than the healthy banana skin, which is more "rubbery" and has a bit more grip. It is really mind-blowing that a semiconductor today is capable of this. Just run current through it, and you can literally kill stuff, and give you cancer (note that sunlight can also damage your DNA and UV light, from A to C is classified as a known carcinogen (source). UVC is just a "bit" nastier).

"A bit" is to stretch things if you consider the needed energy dose or exposure time to be harmful for different wavelengths.

 

In the safety standard, IEC 62471 Photobiological safety of lamps and lamp systems, there are weighing factors used for calculations of for an exposure considered safe for different wavelengths.

The safe time is based on certain energy densities for different safety classifications.

 

Here some highlights, recalculated to a safe time factor based in the worst point at 270nm:

 

200nm. 33 times

254nm 2 times

270nm 1 times

300nm 3.3 times

340nm 3600 times

365nm 9100 times

400nm 33000 times

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Ulf,

That is most interesting. I wonder why they settled on 270nm as most evil?

 

The 20x change between 300nm and 310nm, also 303nm to 313nm is most striking.

 

So UVA is okish.

UVB is not good and UVC is bad. Nice summary.

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David, guess is that the shorter wavelengths are absorbed so quickly that they cause damage in the part of the skin (or eye) where it is not important. I know for skin, that short wavelength UV (200nm ish) if shone on the skin is absorbed so fast that it doesn't reach live tissue in significant amounts.

 

That curve is similar to the 'erythemal action spectrum' we use for looking at sunburn damage.

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The banana today, 2 days and a few hours later:

post-284-0-52153300-1609863722.jpg

 

I will keep this experiment going for a few days, then I will eat the banana.

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I will keep this experiment going for a few days, then I will eat the banana

 

I am shocked, SHOCKED, to find that innocent fruits and vegetables are being euthanized after being subjected to disfiguring scientific experiments.

Your banana split sir.

Oh, thank you.

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Scary to think that if I put my finger instead of the banana and do the same "experiment" I would sustain the same results, on me. Reminds me of X-rays, and the early experiments people did, only to then have painful burns and hair loss, and not to mention the long-term consequences.
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Not so early either. When I was a kid in the '50s, there were coin operated fluoroscopes in shoe stores so you could see the bones in your feet to see if your shoes fit well. I wanted to see the insides of my feet, but fortunately my mother (a nurse) wouldn't let me try them.
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I read about this curiosity once, nice to "see" someone who actually saw it.

 

Your mother knew the dangers and other people didn't? Knowledge wasn't widespread back then I guess. They were other times.

 

I don't know if I have it, but I should have an X-ray image of my right foot done when I was in middle school since I stepped on it the wrong way and probably "stretched" something inside (it then completely healed by itself, but I couldn't load my weight on it for a full week if I remember well, it was very painful). Luckily it was negative, no broken bones.

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Also, somewhat more relevant to this forum, there were mercury vapor germicidal lamps in child care areas. The ones I remember were at church. They were little shelf like fixtures sticking out from the wall a few feet higher than a doorway, shining upwards towards a high ceiling. But then again, this was a time when polio was unchecked by the future vaccine and was regularly striking down children. Only now do folks who were born later have the context to understand how people of the time might be willing to try anything that might help.
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