LED  specification  testing formula ?   - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 21, 2010, 09:25:23 pm
content: I got recently from ebay about 50 white LED's described as 15.000 Lumen its one .Basically any seller on ebay or not, he can baptize any bag of LED's at the named power that he likes,as there is no coding on the LED's body ..And I wonder , is there any formula ? practical test ? that we can verify , that we got what we payed for ?

author: djsb
author_rank: Super Contributor
timestamp: April 21, 2010, 10:06:29 pm
content: Hi,You could use a light meter and test the light output at several different voltage levels. A data sheet for another device could be used as a reference. If the datasheet for device X says the output is Y lumens at Z volts you could test the ones you bought at the same voltage/s.Or are you looking for a mathematical formula? Even if you had a formula you would still need the raw measured data (light level, forward voltage and current and maybe temperature) anyway.David.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 21, 2010, 11:16:51 pm
content: Hi , I mostly care to see if there is an practical way ... example : voltage By specific milliamperes draw = (amount of lumen .)Another example ... do we know how many milliampere one LED of 15.000 needs to work ?The 12.000 Lumen how many milliampere ?The 10.000 ? 8.000? 5.000? and so on .

author: EEVblog
author_rank: Unknown
timestamp: April 22, 2010, 02:41:15 am
content: Quote from: Kiriakos-GR on April 21, 2010, 11:16:51 pmNo, that is not possible. All LEDs have different lumens/watt (lumens/current) figures.Without a proper light meter the only thing you can do is pass the maximum rated current through the LED and compare it with another LED or torch that has a known output, like a well speced CREE for example.Dave.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 23, 2010, 11:34:13 pm
content: Quote from: EEVblog on April 22, 2010, 02:41:15 amWhat this one would cost .. ?Or is it possible , to build your own digital lumen meter , with common parts.Or use an old type exposure meter, from the ones that was used with old film cameras ?There must be a way..

author: desolatordan
author_rank: Regular Contributor
timestamp: April 24, 2010, 01:46:19 am
content: I think you'll have trouble getting numbers to match the datasheet (if you really are verifying) unless you spend a good amount on proper equipment.

author: djsb
author_rank: Super Contributor
timestamp: April 24, 2010, 08:51:24 am
content: I don't know what country you live in but MAPLINS in the UK sell them for £33http://www.maplin.co.uk/Module.aspx?ModuleNo=46557They also do mail order.David.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 24, 2010, 09:16:44 am
content: This is the source that I got them from ...http://shop.ebay.co.uk/giorgio11185/m.html?_nkw=18000mcd+white+5mm&_sacat=0&_trksid=p3911.m270.l1313&_odkw=18000mcd+white&_osacat=0&bkBtn=Size: 5mmLens Color : Water ClearForward Voltage (V) : 3.2~3.6Forward Current (mA):20View Angle: About 25 degree.Luminous Intensity: 18000mcdAnd here is a nice link for all the LED maniacshttp://led.linear1.org/why-do-i-need-a-resistor-with-an-led/There must be a way to build even a primitive LED tester ..I will say more as soon I found the missing links, of my fresh born idea .

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 24, 2010, 09:30:23 am
content: Quote from: djsb on April 24, 2010, 08:51:24 amThanks for the TipStill those are Calibrated to standard incandescent lamp at color temperature 2856K ,so its good enough only to compare two LED's, than accurately measure the output.(Plusthey must be the same color !! even for simple comparison )Found some Cheapys on ebay , for 13 Pounds ( free shipping) ,thats about 15 EUR or 20$ US .http://shop.ebay.co.uk/tomtop_auction/m.html?_nkw=Digital+Light+Lux+Meter&_sacat=0&_trksid=p3911.m270.l1313&_odkw=&_osacat=0&bkBtn=

author: Unknown
author_rank: Unknown
timestamp: April 24, 2010, 09:49:05 am
content: Note that simple light meters only measure lux, and this highly depends on the position of the LED, beam spread and, if applicable, reflector design. To match the datasheet specs, you need to measure lumen with an integrating sphere. I don't believe there's any $30 solution to measure lumen. The guys atCPFprobably have a lot more experience measuring LEDs.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 24, 2010, 11:56:37 am
content: Quote from: alm on April 24, 2010, 09:49:05 amI agree up to a point , but to make your own primitive LED luminous tester ,it takes just a strew + two LED's + an worthy in millivolts DMM .And this setupits my idea, that came up , by just adding factors in the equitation.a) As frequency of light ... ( Only two identical LED's can work at the specific color temperature)b) What will happens if we set the one as transmitter and the second as receiver ?c) Is it possible the LED diode , to convert light back to electricity ?d) If it produces electricity , how much it would be ?f) If I get any results , those will stand as reference to any 18.000 mcd LED !!On my setup I set the DC voltage to 3.2V on the "transmitter" , and got 400mV in return !!The most amazing part was that the "transmitter" LED as soon it got "warm up" in 2 minutes time,it drop the operation current at half !! from 28mA to 13-14mA ..I just created my primitive LED lightness tester , with zero cost.The true technicians , they do not pay for such things, usually they get only payed for them. LOL

author: Unknown
author_rank: Unknown
timestamp: April 24, 2010, 02:14:45 pm
content: Quote from: Kiriakos-GR on April 24, 2010, 11:56:37 amThis measures lux, just like those cheap ebay things. Lux is just the number of photons per surface area (like flux), lumen is lux integrated over the sphere around the light source, and is not easy to measure.Quote from: Kiriakos-GR on April 24, 2010, 11:56:37 amThis is indeed a simple way to measure lux, but probably not very linear. A light source with a very narrow bundle will produce a higher reading on a lux meter for the same amount of lumen, as long as you're within that narrow beam. Again, you're likely to find some improvised methods of approximating lumen at CPF.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 24, 2010, 03:46:13 pm
content: It all about the positioning of the LED's in the strew , so to be facing its other at the full View Angle: About 25 degree.At this point we will get the maximum reading.The LED by it self are all ready an half sphere ..Plus with internal mirror ..How can you tell that its does not do linear scaling ?Any way my idea works as is , and now I am only thinking to create also ,something as an little box ( tester ) , with white + green + red high quality LED's with mirrors,as receivers .So to be able to test and compare any LED ..

author: djsb
author_rank: Super Contributor
timestamp: April 25, 2010, 10:46:14 am
content: Hi,What are you trying to achieve?1/ Functional testing.2/ Comparing a batch of LED's you've bought against each other?3/ Comparing a batch of LED's against a known reference LED. Where is the reference you are comparing against?The LED's you have bought have are surposed to have an output of 18 thousand microcandela (MCD) which is 0.018 Lux. Unfortunately the tester I refered you too at Maplins is not sensitive enough, my mistake.Diodes are non linear deviceshttp://en.wikipedia.org/wiki/DiodeSo this will make it difficult to take a direct reading by using the LED as a photodiode. Some kind of conditioning circuit would be needed to linearise the diode output and to get a reading that in some way relates to the true light/intensity level.These types of conditioning circuits are covered in books such as THE ART OF ELECTRONICS. It's also better if you use a known device for the photodiode as other wise your working in the dark (pardon the pun).The method your using is usefull to check that all the diodes are broadly the same and that the seller has not mixed in a few low (or higher) ouput devices.Just my tuppeny worth.David

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 25, 2010, 03:52:21 pm
content: QuoteDavid this " tuppeny worth " its out even from the Google translations..I am an practical man ... and I will even show an small denial to books , at list in this case.I have test the 18000mcd and got 400mVI will just spent , few cash , getting few of its rating , compare them , write down the outputs in mV.And then I will have my own personal comparison chart ..That for example it could be like :18000mcd=40015000mcd= 32910000mcd= 505000mcd=3It does not matter David the linearity , it does not matter the reference diode .. I personally believe that there are just two or three, LED manufactures worldwide ..There is no point even using the word "reference ".Still I will call as "reference " the latest and greatest LED .This idea of my, its like an exercise for the mind ... I just started it , and any one who are interested ,he can start experiment with it.And my joy will be to see the response of others contributing to it .. like you do, but at not such high theoretical level .Grab your DMM and start measurements ..The all point are to verify those thousands of micro-candles (MCD) , some how.

author: kmossman
author_rank: Contributor
timestamp: July 18, 2010, 04:29:44 am
content: Quote from: alm on April 24, 2010, 09:49:05 amI think you over-state the complexity - just measure from a fixed distance, and use an appropriate formula to derive the lux.

author: kmossman
author_rank: Contributor
timestamp: July 18, 2010, 04:54:32 am
content: Quote from: djsb on April 24, 2010, 08:51:24 amWe have a very good LUX meter, digital, 3 ranges from 2 to 2000 lux, for just USD 15 or 10.30Eu or 8.62UK

author: SheldonGilbert
author_rank: Newbie
timestamp: August 01, 2013, 05:57:43 pm
content: Quote from: Kiriakos-GR on April 21, 2010, 09:25:23 pmWere you able to find any formula to test these lights? I got new lights and trying to check them out.. Please help if you can ..

author: SeanB
author_rank: Super Contributor
timestamp: August 01, 2013, 06:54:11 pm
content: If they are 5mm or 3mm then they run at a max of 20mA, so are going to be not terribly bright. You can overdrive them to around 100mA with a corresponding ultra short life, but they will be bright.