On Bicycle Lighting 2

The second in a number of items vainly intended to clarify the muddy waters of bicycle lighting terminology.

If you have a basic physics grounding and/or have researched buying a bike light lately you are probably a mite confused right now. All the manufacturers seem to rate their lights with different methods whether that is watts, candela, lumen or lux and it just muddies the water for the punter on the street.

To clarify my stance early on, I work for Amba-Marketing, the UK distributor for Busch & Muller and Supernova. I bought my first B&M light in the late 1990s (Thank you SJS Cycles) nearly a decade before I left the military and entered the bike world . Over the last 20 years I have used lights from B&M, Smart, Axa, BLT and Cateye for both battery and dynamo. I was given a B&M Ixon IQ by Amba the first winter they were on the market as a sales bonus/trial. I will not hide my respect of B&M's lights they have served me well on tours, club rides and some mostly unlit rural commutes. From my first halogen fuelled Oval to my current LED Oval+ with my trusty Seculite or D-Toplight depending on the bike, I'm now a kid in a sweetshop genuinely unsure what to replace the Oval with.

You only have to visit a cycling forum to discover a variety of differing opinions on what light to have. The trouble is once you get past, “It says 300 million lumens and only cost me a tenner” what do you actually know? So in the second of a number of blogs on terminology today's subject is;

Lumen

The Lumen is the SI unit of luminous flux, ie how much visible light is emitted by a source. Note visible and emitted it is what's pumped out not what we get back. In simple terms it is to light sources what pints are to beer, but like beer all outputs are not the same. While a pint of beer seems black and white without the ABV it is not as helpful as you might think. The same with lumen. It's not how big it's what you're doing with it that counts. 100 lumen, bomb burst across the sky is less use to us as cyclists from our cycle light than 100 lumen focused where we want it. It is an objective measure of effective luminous output from a light. We need to know how big the area and how far from the light that is.

As with the candela the lumen as an indication of your light needs definition. We don't know how or where it affects our vision merely what there is coming out of the source. We do not know its colour spectrum or useful spread.

The relationship 'twixt lumen and candela can be seen on last weeks diagram as, here comes a difficult sum AmbaTechGuy will fail to answer but expects to see on tee shirts soon:

1 cd·4π sr = 4π cd·sr ≈ 12.57 lumens

Put into simpler terms: Luminous flux (in lumens) is a measure of the total amount of light a lamp puts out. The luminous intensity (in candelas) is a measure of how bright the beam is. If a lamp has a one lumen bulb and the optics of the lamp are set up to focus the light evenly into a one steradian beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of two candela. The resulting beam is narrower and brighter.

Confused? It's just physics but it's beginning to show the limitations of the information on the box and we're not quite half way through my peripatetic ramblings yet. Meanwhile a picture is worth a word or twenty,