# Need help with lumens, candela, view angles of LEDs



## bjcryss (Mar 14, 2012)

Hi ! My first post here is asking you to help me convert lumen to candela and vice-versa.
The problem is i would like to get the best lighting possible for my money. Some LEDs (especially the SMD type) have their luminosity measured in lumens , and the normal round headed LEDs we all know are measured in mcd(milli Candela), specifying the angle of the light also.
I found a good converter for this here http://www.compuphase.com/electronics/candela_lumen.htm but this does not help me all the way...
My question goes like this: if i have 1 SMD type LED with 100 lumen and 170 degrees angle, and i reflect it's light into a spot of 25 degrees angle(like a flashlight), HOW many normal round headed LEDs with 30.000mcd(30 candela) with 25 degrees angle do i have to put together to match the intensity of the focused light from the SMD type LED ?!? And how could i calculate this...?

It's a good question because i have to choose between the 2 types of LEDs and get my money's worth, the point being to get the most light possible (into a 25 degrees angle as the example i've given) for the money. 
Thanks to any smart guy that can tell me


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## BvanderG (Mar 15, 2012)

Calculators like that always have to make assumptions about the angular distribution of the light, so they will usually not give you the right answer.

Unfortunately there's no simple answer based on the things you have specified. We don't know the light distribution of each LED, we don't know the properties of the reflector. Are you planning to mount the leds opposite the reflector to shine the light into it? Or do you put the leds in the middle of the reflector, like in most torchlights?


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## AnAppleSnail (Mar 15, 2012)

High-power LEDs tend to maintain lumens better than most 5mm LEDs. There is some long-term testing on the boards here. In most portable lighting situations, the light distribution is far more important than overall output. The only times output beats pattern is when you're shining fixed lighting onto a fixed diffuser to create given light levels in a room.

Your 25 degree angle - most optics give "Width to half output." Imagine graphing the output of the light source with Output vs. Degrees off center. The halfway point is given as the beam angle. Most 5mm LEDs have built-in optics for fixed throw, but tend to produce a 'ring' of light at about 60 degrees off center. Optics for a single source in the same build size can give anything from a wide circle of light to a narrow spot, to various combinations of spot and spill.

For longevity, I say that high-power LEDs from a reputable manufacturer will always beat most 5mm LEDs UNLESS the 5mm LEDs are underdriven (10mA each or so).


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## bjcryss (Mar 15, 2012)

BvanderG, yes, I'm going to put the SMD LEDs just like in a flashlight, "in the middle of the reflector"

AnAppleSnail, I agree with you, I believe that going with the SMD type would be the wise choice, one reason being that some of them(especially which I will buy) have very good heat dissipation and transmission through an heat-sync application.

For curiosity, a real life example of comparison goes like this:
- 1 x SMD type LED, Osram GoldenDragon Plus(273lm max) ( 3.2-3.6V, light intensity: 273 lumen @1A max, forward current: 1A max, 170° viewing angle,color temperature: 6500K). Put in the middle of a reflector exactly like in a flashlight(hoping to get the same viewing angle as the 5mm LEDs i'm talking about lower). Here is it's datasheet. 

compared to:

- 15 x normal round headed 5mm white LED( 3.2-3.4V, UltraBright, light intensity: 35000mcd, forward current: 20mA, 25° viewing angle,color temperature: 6000K)(i don't have the datasheet)

The reason why i'm comparing 1 to 15 is because 1 x SMD costs the same as 15 x 5mm LEDs.
What's your opinion, which would you buy? 1 x SMD or 15 x 5mmLEDs ? Which will output the most light?


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## AnAppleSnail (Mar 15, 2012)

bjcryss said:


> Which will output the most light?


5mm LEDs suffer shorter life above about 20 mA, with the cheapies fading after just a few dozen hours. That means you need 50 of them to see 1 amp - to equal the Osram, 15 5mm LEDs would have to be 3.4x more efficient at producing light! (They aren't).

For any purpose where you can add a reflector or optic, I suggest a power LED (One not encased in epoxy, but built to dissipate heat). Even if there's no room for much in the way of optics, I'd put a flat fresnel lens (Like that on the Tikka XP) for some decent output.


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## bjcryss (Mar 19, 2012)

Thankyou ! This was a really big help, you are right, i haven't thought about considering the efficiency and doing the math according to it. It's all clear now, i'll buy the SMD types with the proper heat dissipation, and with a collimator lens, i believe the result will be good . Thanks a lot !


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