LED Brightness, Candelas & Lumens

This page started as a humble 'Kit Knowledge' page, but has been booted upstairs to 'Advanced', as I realise that the explanation has reached dangerous levels of complexity. I'm not even totally sure I've got all this right. Feel free to get in touch...

LEDs are bright, but how bright?

Mostly we see LEDs have a quoted figure of xyz mcd, where mcd are units called milli-candelas. (Like milli-metres) they represent thousandths of... yes, you guessed it - candelas.
A candela is, roughly, the equivalent of the light emitted from a candle.
However, a candle emits light all round, but an LED usually focuses it in one concentrated area.

An old-style LED that just glows without giving any appreciable light might be 50 mcd. A modern, bright 5mm LED (the basic LED we sell) will have a brightness of between about 5,000 and 12,000 mcd.

Our 10mm Hi-power LEDs have a brightness of 180 - 220,000 mcd, whilst 'stars' far exceed this, though their brightness is rated differently (see lumens, below).

However, the candela scale is logarithmic, which basically means you need to multiply the number of candelas by 10 in order to get a doubling in brightness.
So when you are comparing LEDs with, say, 8,000 mcds to those with 10,000, as far as your eyes go, there's no difference. For the second LED to be twice as bright as the first (8,000 mcd) it would have to be rated at 80,000 mcd.

This also means that a 10,000mcd LED (=10 candelas) is roughly twice as bright as a candle when you look at it directly. There is no guarantee that it will be anywhere near as bright when viewed from an angle.

This is then further complicated by the fact that our eyes perceive different colours at different brightnesses, even though a light meter might indicate they are the same. Blue and green LEDs (the blue end of the spectrum) always look brighter than reds and yellows (the red end) for the same given mcd output.

And then there's beam angle... if an LED throws all its light into a narrow beam and the lightmeter is placed at a standard distance, it will give a high reading of mcd against a similar (light) power LED that emits a wider cone of light.
So we need to be aware of the beam angle of our LED as well as the mcd.

When UV LEDs are rated with a light meter most of the beam is invisible, so isn't measured, which gives a very low mcd rating - in fact they are usually rated by the power they use instead.

And a word on Lumens (Lm), which the more powerful LEDs are usually rated in.
1 candelas equals 4π lumens, or, roughly 12.5 lumens.
However, lumens represent the amount of total light emitted in every direction. A 12.5 lumen light gives out roughly 1 candela in every direction, so is equivalent to a candle in three dimensions... ... BUT high powered LEDs don't usually emit in 3 dimensions like a candle, and even if they do they also focus their light to some degree. So 12.5 lumens is the amount of light given out (number of photons) which is equivalent to an average 3D 360° candle, but is concentrated into a smaller beam to a greater or lesser extent.

If this gives you some insight into light ratings, that's good. However, on a practical level, we try to source the brightest LEDs we can and test them so that you can be confident you are getting the best LEDs we can buy.