Candles,Lumens, 5mm LEDS and SMT LEDS

[marcopolo]

Why are 3-10mm LED's always specced in candels/candela and SMT LED's specced in lumens.?

They both have optic domes so I don't understand why an aboulute measure of light output ie Lumens cannot be used for both?

I can't even finda formula to covert candles to Lumens. I think candles measures brightness per unit area so surely there would be a formula to convert.

I was looking at some very cheap e-bay 10mm LED's that claim 220,000 mcd (0.5W) and thought they would be good dotted around the ceiling of my garden shed. I know what a lumin looks like coming out the front of my bike light but I havn't got a clue what a candle looks like!! (no funnies about wax sticks please!!!)

Marco..

 

[2xTrinity]

Why are 3-10mm LED's always specced in candels/candela and SMT LED's specced in lumens.?

They both have optic domes so I don't understand why an aboulute measure of light output ie Lumens cannot be used for both?

I can't even finda formula to covert candles to Lumens. I think candles measures brightness per unit area so surely there would be a formula to convert.

I was looking at some very cheap e-bay 10mm LED's that claim 220,000 mcd (0.5W) and thought they would be good dotted around the ceiling of my garden shed. I know what a lumin looks like coming out the front of my bike light but I havn't got a clue what a candle looks like!! (no funnies about wax sticks please!!!)

Marco..

Candela = Lumens / Solid Angle (in steradians)

Note that lumens/Area (in square meters) is lux -- this is for intensity incident on a surface. For point-sources, lux measured at 1m away from the source is the same as the candela intensity of the source, so you may sometimes see "Lux @ 1M" as a synonym for candela. However, for real light sources (not true points) this may not be very accurate.

For examlpe, an omnidirectional light source will be emitting light into a full sphere, or 4pi radians. If it does so with an intensity of 1 candela, it will be 4pi lumens.

Lumens are generally used for bare unfocused LEDs, like power LEDs, or light sources that are meant to be omnidirectional. Lights that are directional, like 5mm LEDs, or spotlights, usually report beam intensity in candela.

In general, the narrower the angle, the higher the candela. Converting between these isn't as simple as plugging in a simple equation, as the "candela" reported by most 5mm LEDs (assuming it is actually an honest number to begin with) only refers to maximum intensity. For most LEDs there won't be an abrupt transition from max intensity to zero intensity, but a gradual transition. To caluclate lumens from candela, samples taken at different angles (eg, the beam profile) needs to be known.

IMO it would be nicer if manufacturers just gave the lumens in addition to candela for this reason.

see White LED Lumen Testing Thread, for more info on this, and measured lumens of several 5mm LEDs.