# Calculating Total Lumen Output?



## cycleologist (Nov 28, 2007)

Need a little help from you experts out there. I thought I saw a similar post a few weeks about but I can't find it. 

Here's the question: is the calculated total lumen output of multiple LEDs simply the sum of lumen output for each LED? Example if you have two (2) 150 lumen LEDs does the total lumen output equal 300 Lumens?? If not how do you calculate total output when working with multiple LEDs? Thanks for the help


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## hootsmon (Feb 7, 2008)

cycleologist said:


> Need a little help from you experts out there. I thought I saw a similar post a few weeks about but I can't find it.
> 
> Here's the question: is the calculated total lumen output of multiple LEDs simply the sum of lumen output for each LED? Example if you have two (2) 150 lumen LEDs does the total lumen output equal 300 Lumens?? If not how do you calculate total output when working with multiple LEDs? Thanks for the help


Well I'm certainly no expert, but yes, lumens are additive. That is 150+150=300 lumens.

As for whether 300 lumens is *perceived* as being twice as bright as 150 lumens, I'm not at all sure. I seem to remember something about our eyes perceiving brightness on a logarithmic scale, which would mean 300 lumens appears just a little bit brighter than 150, to our eyes anyway.


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## emu26 (Jun 23, 2008)

Cycleologist its not as simple as that, sadly.

If you want useable light, and given where we are I mean useable as a bike light, you need to be able "focus" the light produced. Unfortunately there isn't a holy grail of optics and no matter what you choose to use as an optic, some of the output produced by the LED will be lost. Most quality optics will have a claimed efficiency, usually between 85% and 95% depending on manufacturer and type, ie reflector, collimator or aspheric. 
So your 150 lumen led, when used in reality, will be less than 150 lumens.

The other point worth noting is that it would appear that some LED manufacturers Bin their LEDs by maximum output and some by minimum output so read the data sheets carefully.

Also as Hootsmon said, what is an actual measurable output and what our eyes perceive that output to be are two completely different things. As an exmple of what I mean, one of my builds was two lights at the same time. One was a triple Q5 for myself, the oter a double Q5 for a friend. I used a mix of optics on my light, spot, smooth spot and medium, on the double I used different optics. Sorry I can't remember exactly what they were by I do recall I stayed away from the spot. When we are at lookouts along the trail my light will throw further than hers but when riding on trail it often appears as though hers is brighter.

Hope this helps


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