How can I be sure that my UV LED is outputting true 365nm UV light? - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: May 30, 2022, 09:45:57 pm
content: Is there a way, without buying a multi-thousand-dollar UV spectroscope or spectrometer, of testing my UV LED components (or UV light sources using UV LEDs), to make sure they are actually outputting true 365nm UV light? Many UV LEDs (and light sources that use them) use the cheaper 400nm type LEDs, while still calling them "UV LEDs". 400nm is technically on the UV-VIS border, but it also is strongly visible compared to what a true UV light source should be, and this can obscure the visible light emitted by fluorescent objects being illuminated by the LED. Unfortunately it can be hard to tell if the purple glow of a UV LED is due to it being the main emission of a low power 400nm LED, or it being spectral spill over of a high power 365nm LED. I know that that LEDs don't have super sharp cutoff out of the intended emission band. An LED with a center wavelength at 365nm, and a FWHM bandwidth of 20nm (10 nm on each side of the center wavelength), still is going to have some weaker emission outside of that 20nm bandwidth. And if it's a high power LED like a 1W LED, then that out-of-band emission could be visible if the part of that emission crosses over into the visible spectrum with a high enough brightness.So to know if my UV LED is truly a 365nm LED, I'm going to need a way, hopefully a cheap way, of checking that my LED actually has a peak wavelength of 365nm. I know spectroscopes/spectrometers that can detect UV light are very expensive, so that's out of the question. Is there some kind of fluorescent UV detector card, with a chemical composition that is fine-tuned to fluoresce only from light in a very narrow bandwidth centered at 365nm? If so, that might help determine what type of LED (or LED based light source) it is that I'm using.

author: james_s
author_rank: Super Contributor
timestamp: May 30, 2022, 09:49:00 pm
content: You could buy name brand parts from a reputable source and trust what the datasheet tells you. Otherwise I think it's going to be difficult to tell, many things will fluoresce to some degree from a multitude of wavelengths.

author: analityk
author_rank: Regular Contributor
timestamp: May 30, 2022, 11:43:42 pm
content: Any LED are sensitive for light that it emmit and generally for any light with shorter wavelength. In this case you can use another one UV-LED as photodetector. If UV LED connected to high impedance DVM show any voltage does it mean there have to be UV light source that shine on your detector LED. Any light with longer wave that (let it be) 380 nm have not to enought energy to polarize this 365 nm diode.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 31, 2022, 06:14:18 am
content: First thought would be to use the photoelectric effect. If the wavelength is too long you won't see any photoelectric effect, no matter how high is the intensity of the light.Find a metal that exhibits the photoelectric effect at 365nm but not at 400nm (no idea if there is one), then improvise an electroscope and see if you can knock out charges from that metal.

author: lordium
author_rank: Regular Contributor
timestamp: May 31, 2022, 07:19:54 am
content: First thing that came to my mind is an old experiment from my olden school days... (slit) diffraction. Maybe possible to make a DIY simple setup to get enough resolution (razor and a tape measurement).

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 31, 2022, 07:23:05 am
content: There's a method that involves an initial expense, but it's a recoverable expense. US paper money in denominations above $5 all have a fluorescent strip embedded in them. They fluoresce in different colors. What would be useful to you is that the $100 bill strip fluoresces with a reddish color and doesn't fluoresce under 400 nm UV, but it does under 365 nm UV. The other denominations do fluoresce under 395 nm or 400 nm.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 31, 2022, 07:36:58 am
content: Quote from: lordium on May 31, 2022, 07:19:54 amProbably the easiest DYI method.A CD or DVD disk can be used as a calibrated diffraction grate.

author: voltsandjolts
author_rank: Unknown
timestamp: May 31, 2022, 09:58:50 am
content: Put anoptical filterin front of it.

author: wraper
author_rank: Unknown
timestamp: May 31, 2022, 10:16:10 am
content: Suitable detection cards certainly do exist.https://kpt.net.cn/en/uv-detection-card/#

author: dietert1
author_rank: Super Contributor
timestamp: May 31, 2022, 10:50:22 am
content: Another cheap solution is to buy LEDs with the desired spec. Last year i got one of those cheap lamps for UV hardening of finger nail decoration and it erased EPROMs within 5 or 10 minutes. I think it came from China via Amazon and was about 20 or 30 €.Regards, Dieter

author: eliocor
author_rank: Unknown
timestamp: May 31, 2022, 12:55:53 pm
content: as @voltsandjolts already wrote, use a bandpass filter:https://www.edmundoptics.com/c/optical-filters/610/#27587=27587_d%3A%5B365.00%20TO%20365.00%5Dthis one is not too much expensive:https://www.edmundoptics.com/p/365nm-cwl-10nm-fwhm-125mm-mounted-diameter/20133/

author: Ben321
author_rank: Frequent Contributor
timestamp: May 31, 2022, 05:39:44 pm
content: Quote from: The Electrician on May 31, 2022, 07:23:05 amI've read that the $100 bill strip simply has a very weak fluorescence, and that this is for any wavelength. You simply need a very powerful UV source and visible light blocking filter in front of the source (to make sure that the fluorescence of the bill's strip isn't obscured by the visible output of the UV source), in order to see the $100 bill fluorescent strip.

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2022, 06:22:51 pm
content: Quote from: lordium on May 31, 2022, 07:19:54 amI thought of that, but the challenge here is that the desired wavelength is invisible to the eye. I suppose a diffraction grating in combination with a phosphor could work.

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 31, 2022, 06:32:07 pm
content: Quote from: Ben321 on May 31, 2022, 05:39:44 pmI have a $100 bill right here in front of me. Its strip exhibits reasonably strong flourescence with 365nm but not with 395nm. My UV source is not particularly strong; it's a little flashlight powered by a single AA cell. It is filtered to block visible and that helps. The visible light output from the 365nm LED (viewed through UV blocking safety glasses) is rather whitish, whereas the visible light output from a 395nm LED is distinctly purple.I'll post some pictures of the glowing strips in US bills later today.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2022, 06:32:39 pm
content: I have a vague recollection of doing exactly that (grating plus phosphor) in physics lab.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 31, 2022, 06:52:33 pm
content: Quote from: james_s on May 31, 2022, 06:22:51 pmOr use another LED as a photodiode, and slide it across the screen to detect where the invisible peaks and troughs are.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 31, 2022, 08:00:08 pm
content: taken from:http://www.diyphysics.com/2012/04/12/telling-apart-365-nm-from-395-nm-ultraviolet-leds-and-flashlights/

author: Ben321
author_rank: Frequent Contributor
timestamp: June 01, 2022, 01:41:46 am
content: Quote from: The Electrician on May 31, 2022, 06:32:07 pmI have a UV LED flashlight that has a mostly white glow, with a slight purple tint to it when viewed directly (done by diffusely scattering the light off of a white surface that contains no fluorescent material, so as to avoid exposing my eye directly to the intense UV of the LED itself), or with no purple tint at all when wearing UV blocking glasses. I couldn't be certain what type of LED it was based on visible color. I ended up doing an experiment.Using a plastic handheld spectroscope, and a full-spectrum (UV-VIS-NIR) modified digital camera that I had bought, I got a couple pictures through the spectroscope. First of a fluorescent lamp (the old long type that had a broad fluorescent spectrum, not the new CFL type with multiple narrow peaks). Despite the phosphor absorbing the UV in the fluorescent tube, some of the visible mercury spectral lines still pass through, and I was able to use those (with known wavelengths) to calibrate the picture of the fluorescent lamp through the spectroscope. The second picture, of the UV LED in my UV flashlight, I held had managed to hold the spectroscope very close to its original position relative to the camera (some stabilization came from holding it directly up against the camera lens). I used Gimp with layers to write annotations such as marking the spectral lines of the mercury calibration source. I measured the pixel count between the 2 spectral lines at 546nm and 436nm. This gave me the nm/pixel scale factor that I needed. I then measured the number of pixels from the 436nm mercury line to the unknown UV LED line, and multiplied this difference in pixels by the scale factor to get the wavelength difference, which I then subtracted from 436nm to get the apparent wavelength of the UV LED (404.96nm). However, when I compared the pictures, I noticed a 14 pixel offset between the spectroscope light inlet of the one picture, and the light inlet of the other. This corresponded to a wavelength difference of 13.16nm. That is, the wavelengths in the UV LED spectroscope picture are 13.16nm shorter than their apparent wavelength. So I subtracted 13.16nm from to compensate for this offset, and got a wavelength of 391.8nm.So this is definitely not a 365nm LED, even if it does say so on the side of the flashlight. And yes, it does say specifically 365nm on the side of the flashlight. So I got ripped off.My only other guess is that they may have used some cheap plastic, instead of glass, for the lens, and the plastic is absorbing the 365nm line of the LED even if the LED itself is emitting 365nm UV light.

author: Ben321
author_rank: Frequent Contributor
timestamp: June 01, 2022, 02:23:58 am
content: And I repeated the experiment again, this time after first popping out the plastic lens on the front of the flashlight. Yes it actually was plastic like as I thought. Unfortunately the full-spectrum-modded camera + spectroscope that I had been using, still didn't reveal any sign of a 365nm spectral line. I compared the new test picture to the previous test picture in Gimp, and the spectrum was the same. The cheap spectroscope I have doesn't contain any plastic optics between the slit and the grating, though the grating is a plastic (not glass) grating, so it's possible that the grating is absorbing 365nm UV light, but it's so thin that it's likely that a high intensity UV source like an LED would still get SOME 365nm through unless the grating was made with a special UV absorbing material. So I think the logical conclusion unfortunately is that the seller of the UV LED flashlight that I bought was simply lying when they printed "365nm" on the side of the flashlight. They know consumers are getting smarter about the tech, and won't just buy a flashlight labled "UV LED" anymore, and are specifically looking for UV flashlights that emit will into the UV spectrum, at about 365nm. So to entice these more careful buyers, the sellers are now outright LYING about the wavelength their flashlight emits and calling it 365nm, when it actually is much closer to 390nm.

author: Ben321
author_rank: Frequent Contributor
timestamp: June 01, 2022, 02:44:58 am
content: Can someone here buy and check the "DARKBEAM UV 365nm Light" being sold on Amazon now, to see if that's truly 365nm? I'd like to know if it is, before I spend my money on it, as I need a true 365nm flashlight to replace my fake 365nm UV flashlight.

author: james_s
author_rank: Super Contributor
timestamp: June 01, 2022, 04:07:00 am
content: Why would somebody here buy a flashlight just to check it for you? Why don't you ask one of the youtubers that get paid to produce content evaluating various products?

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 01, 2022, 05:50:43 am
content: Quote from: Ben321 on June 01, 2022, 02:44:58 amI don't know about the DARKBEAM unit, but I have this high power Alonefire flashlight:https://www.amazon.com/Alonefire-Flashlight-Rechargeable-Ultraviolet-Blacklight/dp/B08B638TZM/ref=sr_1_6?crid=LYJ1HW78A3XI&keywords=alonefire+365nm+uv+flashlight&qid=1654061672&sprefix=alonefire+365%2Caps%2C200&sr=8-6This puts out so much 365nm UV that if you shine it on the back of your hand, it feels warm!They also have a smaller 365nm flashlight:https://www.amazon.com/Alonefire-Flashlight-Portable-Rechargeable-Blacklight/dp/B09T6QFZSZ/ref=sr_1_15?crid=LYJ1HW78A3XI&keywords=alonefire+365nm+uv+flashlight&qid=1654061672&sprefix=alonefire+365%2Caps%2C200&sr=8-15The Alonefire flashlights are really 365nm.I checked a $100 bill with a 395nm LED with and without visible light blocking filter, and there is no fluorescence, but all my 365nm flashlights do make the embedded strip fluoresce brightly reddish (not a pure red).The 395nm LED does make all the other bills strips fluoresce, but much less bright than the 365nm light.

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 01, 2022, 06:13:18 am
content: Here a couple of pictures of 3 bills. The first is the result of illuminating them with my high power Alonfire 365nm flashlight. This is just what my camera produced; I didn't have to adjust the image to get what you see:This next image is the same 3 bills but illuminated with 395nm without visible blocking filter. The camera was just barely able to get this image. I had to manually retouch the image to get it to look this good. Notice that the $100 bill shows no fluorescence of its embedded strip:

author: taste_tester
author_rank: Regular Contributor
timestamp: June 12, 2022, 01:28:35 am
content: Quote from: Ben321 on June 01, 2022, 02:44:58 amSo convenient that we have the internet to beg strangers to spend their money to satisfy your curiosities at no financial risk to yourself, despite the product being on a platform with free returns! Sheesh!