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I want to build the brightest lamp, for it to be seen at daylight, should I use 3 2500 mcd LEDs or 1 6000 mcd?

And also Why?

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The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths

(Wikipedia)

Therefore, 7500 cd (3x2500 cd) is more visible than 6000 cd.

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But my question is that if it really adds up when you put 3 together –  João Paulo Apolinário Passos Dec 3 '12 at 17:35
    
@JoãoPauloApolinárioPassos: Consider three 1 cd LEDs, each emits n watts per steradian, if you put them next to each other and point them all in the same direction they will each project that number of watts into that same unit of solid angle, the total number of watts projected is the sum. 3.n. –  RedGrittyBrick Dec 3 '12 at 22:38
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In terms of overall light output then you just sum the mCd - it's just power. However if you want that light to do something useful then it's how much of the light can you focus on the target?

A single LED at the focus of some reflector will put more of the light into the beam than a cluster of three - simply because all three can't be perfectly at the focus and some of the light from each member of the cluster will just hit one of the other LEDS.

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