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Author Topic: How to measure the UV output of a UV LED (DIY style)?  (Read 11958 times)

Offline Escorpiuser

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I wanted to buy a bunch of UV LEDs by Internet. Some sellers on ebay offer items of 2000~3000 mcd (millicandela) while some others offer 400 mcd. It's a big difference. (The candela is a unit designed for visible light, but UV is invisible). Which one is more reliable in saying the truth? So, I decided to measure the UV output of a sample of UV LEDs by myself.

Looking for information I found the following post:

http://www.thenakedscientists.com/forum/index.php?topic=20305

A man wanted to help his son to find out a way to measure UV from a CFL, what is in some way the same thing that I plan to do (respect to the LEDs).

After reading this, I got in contact with Techmind, a contributor in that thread. Summarizing, he has asked me two questions:
* if I need very deep UV.
* what do I need the LEDs for.

(Now I decided to post here the subject just in case some others can take advantage of this information as I did with the thread before mentioned.

Of course, this is a project of my own, so I'm not in the idea of using expensive measuring intruments out of my reach).

Why do I want to measure UV? As I said, I would like to buy some LEDs based on their [UV output/price] rate. I'm pretty sure that the cheapests could work well for my project, but I want the 'optimum' ones. It has become more a challenge than any other thing.

And what project for? The project is to make an ozone generator, not very big. Something small enough to get into a PC power source box. I must say that I have made already one with an UV 'fluorescent' lamp, but the lamp crashed after short time because of its fragility. So, I decided to move myself to the LED grounds.

I don't need much ozone neither deep UV light, just 'close' UV light enough to produce ozone when a flow of air is passing through the beam.

So, I want to buy a bunch of UV LEDs from Internet (in the shops of my town they are much more expensive), but I want to be sure that I'm buying the right thing, I don't want to be conned thinking that I'm buying UV LEDs just because I "see" violet light getting out of them. The seller was unable to provide me a datasheet of the LEDs, but he has sent to me a couple of free LEDs to make my own checks.

Based on the materials I can use at home, I have thought the next experiment to "measure" (even relatively) the UV outputs:

* to open one tube edge of one CFL that got out of service.

* to put the LED inside the CFL tube pointing the beam outside the glass wall.

* to light the LED on.

* to take a picture of the illuminated point (from outside) in a dark room. I plan to use a DSLR camera with a tripod.

* to repeat with the other LED in the same conditions.

* to compare both pictures in a Photoshop type software.

What do you think? Will it work? At least, will I be able to check that the LEDs are yielding UV light? Should I diffract the light with a CD/DVD before take the photos?
How can I avoid the violet (visible) light to get to the lamp (to be sure that I only photograph the white light produced by the UV light)?.

Thank you for any help and/or information. Comments and suggestions are also wellcome.


 

Offline Bored chemist

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How to measure the UV output of a UV LED (DIY style)?
« Reply #1 on: 02/12/2011 18:24:00 »
AFAIK there are no UV LEDs on the consumer market that will produce ozone from air.
You need to get into the far UV (about 200nm or less) to do that.
 

Offline techmind

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How to measure the UV output of a UV LED (DIY style)?
« Reply #2 on: 02/12/2011 22:02:54 »
I wrote, in private correspondence:
Quote
You are right that the use of "Candela" (as mcd etc) is somewhat dubious for UV LEDs.

Candela is the actual output power weighted (according to wavelength) by the eye-sensitivity. This means that an X-candela UV led at (for example) 390nm may be pumping out far less photons than an X-candela UV LED at 360nm (where X is the same) ... as the eye is much less sensitive to 360nm than 390nm. The Candela weighting function is the CIE luminosity function, sometimes known as Ybar. Ybar on my webpage http://www.techmind.org/colour/
There isn't a well-defined cutoff when visible becomes UV - there's a continuum where the eye becomes progressively less sensitive, as you'll see from the Ybar function (on my webpage or linked tables).
It's probably dodgey/dubious to use any photopic measurement unit (Candelas or lumens) for wavelengths shorter than about 420nm. You'll notice that Philips' LumiLEDs are quoted in watts even for the deep blue rather than Candela/lumens for this reason.

(Similar arguments arise with deep red LEDs - 625nm (an orangey-red) tend to have higher brightnesses (mcd) than 645nm (deep red) again because the eye is less sensitive to the longer wavelength so the Candela rating for 645nm is lower for the same radiometric (light photons or light watts)).


The second thing to be aware of is that the manufacturers' measure of mcd (or Cd) is _in the main beam_... therefore if you have the same LED-chip in two LED-packages running at the same power and efficiency... the one with the optics giving the narrowest beam angle has the highest mcd-rating (even though the total number of photons given out summed over all directions is identical).
If you compared the solid-angle of the beamwidth you could roughly compensate for this effect in the specification.

Another question you need to ask yourself is what you want the UV for. Is there something specific you are trying to make fluoresce which needs a specific wavelength?
Conventional mercury UV "blacklight" tubes (fluorescent lamps without the fluorescent material) are normally filtered to emit primarily the 365nm wavelengh which is _relatively_ eye-safe. Shorter wavelengths (mercury tubes can also be filtered to pass ~305nm or 254nm) are significantly more dangerous for a given (watts) power.

Regardng eye-safety you need to be a bit careful with anything deep-blue ("Royal blue") or further into the UV at sufficient powers. The LumiLEDs/Luxeon Royal Blue if driven at a watt or so (eg 350mA) will easily give you temporary retinal burn (spots for a minute or two) if looked at directly even for a few seconds. 5mm UV LEDs driven at a few 10's milliamps are less concerning, but if you've got UV LEDs where you're passing 100~200mA or more then you really should be taking some care - certainly not looking directly at the LEDs ... even more so as the wavelengths get shorter than 390nm.

Wikipedia reckons that "LED efficiency at 365 nm is about 5-8%, whereas efficiency at 395 nm is closer to 20%"... that efficiency _ought_ to be quoted in watts of light out vs watts of power in. I don't know the accuracy of the figures, but they sound plausible. Thus a 395nm LED of a given input power will give out perhaps around 3x as much UV light power (in watts or photons) as a 365nm LED. But the 395 will be more visible as a dull violet than the 365 which will be more invisible on non-fluorescent surfaces - also the 365 will (for the same amount of light Watts, or photons) probably excite stronger fluorescence effects in general - if that's what you're looking for.


So..... not a simple answer, but hopefully you'll have a better understanding of the problem now :-)

At this time I had no knowledge of the ozone requirement.
UV LEDs stretch to shorter wavelengths all the time, but in the absence of any other specific information I have to agree with BoredChemist - that UV-generation requires very shortwave UV (shorter than 254nm for sure) and I too am not aware that LEDs go that short. If they did, they'd be very expensive, no doubt!

Be aware that high voltage discharge is another way of generating ozone, if that is the driving requirement...
« Last Edit: 02/12/2011 22:15:12 by techmind »
 

Offline techmind

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How to measure the UV output of a UV LED (DIY style)?
« Reply #3 on: 03/12/2011 21:00:32 »
Based on the materials I can use at home, I have thought the next experiment to "measure" (even relatively) the UV outputs:

* to open one tube edge of one CFL that got out of service.

* to put the LED inside the CFL tube pointing the beam outside the glass wall.

* to light the LED on.

* to take a picture of the illuminated point (from outside) in a dark room. I plan to use a DSLR camera with a tripod.

* to repeat with the other LED in the same conditions.

* to compare both pictures in a Photoshop type software.

What do you think? Will it work? At least, will I be able to check that the LEDs are yielding UV light? Should I diffract the light with a CD/DVD before take the photos?
How can I avoid the violet (visible) light to get to the lamp (to be sure that I only photograph the white light produced by the UV light)?.

Bearing in mind the LEDs won't be short enough wavelength to make ozone anyway...


In principle this method has merit, however
 - be careful opening/breaking fluorescent/CFL tubes - the mercury is regarded as hazardous, and the dust of the fluorescent material isn't good to breathe either
 - you'd need to reposition the LED accurately in order to get a reproducible and reliable measure - and have the LED a consistant distance from the phosphor. Also the method can't/won't really give consistent results for LEDs with different beam-angles
 - for your photos you'd have to be at the same distance, same exposure tiem, shutter speed etc
 - in Photoshop or whatever graphics program be aware that RGB values used on computers are not linear with respect to light levels. This is because of the so-called gamma-encoding. http://www.poynton.com/GammaFAQ.html and http://www.w3.org/Graphics/Color/sRGB (see equations 1.6 to 1.7b)   (Also your camera may use a slight variation on this strict coding to create that manufacturer's signature colour-appearance!)
- you don't need to split up the spectrum. I wouldn't worry about any small leakage of UV light as this will barely register in the camera compared to the visible light.

- note that even if carried out properly and the errors minimised, it only tells you about the relative efficacy of the UV light sources for exciting the specific phosphors used in the fluorescent tube you originally broke. The efficacy is likely to be very different for different wavelengths... so the answer may not be relevant to your original application.
« Last Edit: 03/12/2011 21:04:59 by techmind »
 

Offline Bored chemist

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How to measure the UV output of a UV LED (DIY style)?
« Reply #4 on: 03/12/2011 21:22:56 »
Do you realise you could just buy an ozone generator?
Not much fun, but a lot easier.
 

Offline Escorpiuser

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How to measure the UV output of a UV LED (DIY style)?
« Reply #5 on: 05/12/2011 21:26:24 »
Hey, excuse me for answering so late.

Thank you both for your comments.

I'll probably try the measure of the UV LEDs taking into account your considerations and publish here the results.

And I'll eventually buy some ozonizator, as Boredchemist suggests. :)
« Last Edit: 05/12/2011 21:29:22 by Escorpiuser »
 

Offline Bored chemist

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How to measure the UV output of a UV LED (DIY style)?
« Reply #6 on: 05/12/2011 21:31:15 »
Come to think of it here's a simple test, if you can't smell ozone then there's not enough of it to make any difference. Also I'm not sure how you would package a deep UV LED that could make ozone. All plastics are opaque at those wavelengths.
 

Offline Geezer

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How to measure the UV output of a UV LED (DIY style)?
« Reply #7 on: 06/12/2011 07:51:21 »
I am reminded of an April front cover on Electronic Design News (EDN), probably 1983, where they showcased a brilliant new technology, complete with colour photographs - the FED - Flame Emitting Diode.

I wish I had kept the front cover. If anyone still has it, please let me know. 
 

Offline Escorpiuser

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How to measure the UV output of a UV LED (DIY style)?
« Reply #8 on: 07/12/2011 00:28:10 »
I am reminded of an April front cover on Electronic Design News (EDN), probably 1983, where they showcased a brilliant new technology, complete with colour photographs - the FED - Flame Emitting Diode.

I wish I had kept the front cover. If anyone still has it, please let me know. 

joker!  ;D
« Last Edit: 07/12/2011 00:31:56 by Escorpiuser »
 

Offline Escorpiuser

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How to measure the UV output of a UV LED (DIY style)?
« Reply #9 on: 07/12/2011 00:30:37 »
How would I filter the (visible) violet component?
I'd use a yellow dye plastic (polypropilen or similar).
Comments?
 

Offline Bored chemist

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How to measure the UV output of a UV LED (DIY style)?
« Reply #10 on: 07/12/2011 06:54:22 »
You will have difficulty finding a dye that absorbs visible violet light, but not UV. Try woos glass.
 

Offline Escorpiuser

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How to measure the UV output of a UV LED (DIY style)?
« Reply #11 on: 07/12/2011 12:01:30 »
(...) Try woos glass.

Excuse my ignorance. Could you explain what "woos glass" is?
 

Offline Bored chemist

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How to measure the UV output of a UV LED (DIY style)?
« Reply #12 on: 07/12/2011 21:05:45 »
It's the result of typing "Woods glass in" a hurry.
Sorry about that.
http://en.wikipedia.org/wiki/Wood's_glass
It block most of the visible light though it's not that great at blocking violet.
There are special filters for this sort of thing, but they are not cheap.

As I have pointed out, the idea that this is going to produce ozone is dead in the water.
 

Offline Escorpiuser

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How to measure the UV output of a UV LED (DIY style)?
« Reply #13 on: 08/12/2011 23:05:19 »
I've been making some preliminary trails to accomplish the experiment.

The first observations have been:

* the first LED has reduced its luminosity (or whatever is the name) to about 50% after a few minutes of use. (It can be due to a shortcut in the circuit that has killed most of the visible light, perhaps also some of the UV component. It can also be a defective LED). I had already take several photos. As the LED was able to produce luminescence on a 50€ note, I thought it was able to still shed UV.

* the phosphor deposit is not regular on the edge of the tube that I have been using for the trials. I think that this is the main handicap to make good comparisons afterwards on the computer. I must look for another point to place the LED.

* the camera "see" the violet light as blue. Only when I have set up the white balance to 10000K it has begun to display the light as purplish, with a blue (not real) halo and white in the centre of the LED emitter.

* a lot of violet light is able to trespass the phosphor. That is the light mostly captured by the eye. I don't see white light produced by the UV-phosphor interaction, except the small centre of the LED, that seems to be white (to the eye and to the camera).

(I'll soon post some of the photos).
« Last Edit: 08/12/2011 23:44:37 by Escorpiuser »
 

Offline techmind

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How to measure the UV output of a UV LED (DIY style)?
« Reply #14 on: 08/12/2011 23:30:54 »
* the first LED has reduced its luminosity (or whatever is the name) to about 50% after a few minutes of use. (It can be due to a shortcut in the circuit that has killed most of the visible light, perhaps also some of the UV component. It can also be a defective LED). I had already take several photos. As the LED was able to produce luminescence on a 50€ note, I thought it was able to still shed UV.
That doesn't sound good. Perhaps you're over-running the LED and the heat is damaging it. Make sure you don't exceed the rated current, and if it's designed to be affixed onto a heatsink, then do so (using a suitable thermal paste)!

* the camera "see" the violet light as blue. Only when I have set up the white balance to 10000K it has begun to display the light as purplish, with a blue (not real) halo and white in the centre of the LED emitter.
Digital cameras invariably 'see' deep violet or near-UV as straight Blue (you find this in technical light photography, or even if trying to photograph flowers). This is because their colour-filters differ from those of the human eye and in particular because their 'red' filter doesn't leak a little violet (the eye perceives colour as if it's 'red' filter also slightly responds to deep violet).
 

Offline Escorpiuser

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Re: How to measure the UV output of a UV LED (DIY style)?
« Reply #15 on: 19/12/2011 01:27:10 »
That doesn't sound good. Perhaps you're over-running the LED and the heat is damaging it. Make sure you don't exceed the rated current, and if it's designed to be affixed onto a heatsink, then do so (using a suitable thermal paste)!

You are right. In spite of I can't notice any heat, I guess I've been overrunning the LED. I thought 4.5V would be fine (since blue and deeper need higher voltage than classic red and yellow LEDs), but today I've seen (in a page with similar LEDs) that they require only 3.2 to 3.6V. I still have another one to test (and the other 2 are untouched).

And I have to add a resistor.

I've parked the experiment for now because some domestic duties but I haven't given it up.

I'll keep you updated.
 

Offline Bored chemist

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Re: How to measure the UV output of a UV LED (DIY style)?
« Reply #16 on: 19/12/2011 12:44:52 »
There's never going to be much of a mass market for deep UV leds.
In order to convert oxygen into ozone the first thing that has to happen is that the oxygen in the air must absorb the UV light.
But, if that happens then the range of the light in air will be very short.
That's seriously going to restrict the applications of such a lamp.
Sure, you could use it for ozone generation or you could use it for other things provided that there's no oxygen in the environment, but I don't see that as mass market..
 

Offline Geezer

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Re: How to measure the UV output of a UV LED (DIY style)?
« Reply #17 on: 19/12/2011 18:20:18 »
You are right. In spite of I can't notice any heat, I guess I've been overrunning the LED. I thought 4.5V would be fine (since blue and deeper need higher voltage than classic red and yellow LEDs), but today I've seen (in a page with similar LEDs) that they require only 3.2 to 3.6V. I still have another one to test (and the other 2 are untouched).

And I have to add a resistor.


They are forward conducting diodes. You don't tell them what voltage to operate at - they tell you. The thing you must control is the current. Some diodes come with a built-in resistor, but if they don't have one, you must include one to limit the current.

To determine the resistor value, subtract the diode forward voltage from your power supply voltage and divide the result by the current you want. You can put a lot of diodes in series and use a single resistor if you want. Just add the diode voltages and treat them like a single diode with a large forward voltage.
 

Offline Escorpiuser

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Re: How to measure the UV output of a UV LED (DIY style)?
« Reply #18 on: 30/12/2011 01:14:05 »
This are the pics of the experiment (not the results yet). The results are taking a bit long because I took a lot of pictures and I wanted to make a "collage" of them, since only need a small part of every image (about 1/64) but the software that I'm using (Gimp) is a pretty bothersome or I'm not used enough to it. I'll post it later.
 

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Re: How to measure the UV output of a UV LED (DIY style)?
« Reply #18 on: 30/12/2011 01:14:05 »

 

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