# mcd and lumen values help please!



## pina (Dec 22, 2011)

I am confused on something and requesting your assistance for the following; 


We selected a 5mm led for our design having 6000-8000mcd value at 30 beam angle. Using the calculator here; http://led.linear1.org/lumen.wiz the results are between 231 lumen to 308 lumen for 180leds placed on the board. 


However, I am comparing another 5mm led having 2900-8280mcd value at 60 beam angle and using the same calculator the lumen value is between 439 to 1255lm for 180leds which is more than 3 times better than previous leds. 


Our clients are complaning about the brightness of the unit and If I select the bigger beam angle with smaller mcd value leds, it seems there will be more light produced from the board. 


What do you think? Is that calculations correct above? 


Thanks in advance;


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## BvanderG (Dec 23, 2011)

This could well be the case, but you have to realize that those calculators make assumptions about the distribution of light from the led to different angles, and it's likely that those assumptions are at least somewhat off. If you really want to know the luminous flux (lumen output), you could ask the producer.

The reason why it's not surprising is that the 6000-8000 mcd is the intensity _straight ahead_, that is on the axis of the led. It goes down as you look from a larger angle. In a led with 30 degree beam angle, it's already down to half the 6000-8000 mcd when you look at a 30 degree angle. So in total there's not much light: only straight ahead and at small angles, but very little at larger angles.

The 60 degree led has less intensity, but over a much larger spatial angle, so in total it may emit much more light that the 30 degree led.


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## Kinnza (Dec 23, 2011)

You cant use that calculator, as it uses the average intensity of the beam, and you have the peak intensity at the optical axis.

In order to obtain a result closer to reality, you need to derate the figure of lm by a coefficient obtained from the LED polar curve. Anyway, usually it dont worth the hassle of calculating it, as 5mm LEDs claimed intensity usually dont have anything to do with reality. On those models (good brands) which such figure is reliable, usually there is a figure of total flux output too.

The only way of being sure about the flux output of 5mm LEDs is measuring them.


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## BvanderG (Jan 2, 2012)

Kinnza said:


> You cant use that calculator, as it uses the average intensity of the beam, and you have the peak intensity at the optical axis.


You're right -- it's even worse than that, they assume that the beam has this average intensity within the beam angle _and is completely dark outside_. So if you tell it 30 degrees and an average of 3000 mcd, it will make a calculation for a beam that has 3000 mcd within a 30 degree spatial angle, and 0 mcd everywhere else. That can create significant further inaccuracies. The beam angle is not "where the light stops", it is where the light reaches half its central intensity (i.e. where it's gone down from 6000 mcd to 3000 mcd). Obviously there can be a lot of light outside the beam angle, especially when it is small.

So don't use these online calculators; they all make these weird assumptions (same for the ones at ledrise.com and ledtuning.nl).


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