What are lumens? How many do I need? And where can I buy bat DNA?

Winter has arrived.

Yet again, and as in previous years, I have failed in my attempts to evolve my own rudimentary sonar navigation system. Which leaves me, like every other human cyclist, forced to rely on bike lights.

Yes, they’re necessary. Yes, they’re practical. Yes, they’re safe. But, my god, there aren’t many more tedious products on which to spend several dozen English pounds.

Helmets come a close second, I suppose.

Until I find a safe and morally neutral method for introducing bat DNA into my family lineage, I’m just going to have to participate in the same lumen-based arms-race as every other cyclist.

I remember a time when the concept of a lumen was not something I had to bother myself with. Bike lights were either bright enough, or they weren’t. They either floodlit the road ahead, or they glinted weakly and served no real purpose.

But now, we have to cross match our budget with the value we place on our lives, factor in the reliability of our life insurance policy, and decide how many lumens we can reasonably purchase.

But I smell a rat.

The brightness of things used to be measured in relation the number of standard candles it replicated. A lighthouse, for example, might house a light that shined with the brightness of a million candles.

What was wrong with that, and where did lumens come from?

A quick Wikipedia gives me the following definition:

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source.

Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a “luminosity function”) of the human eye’s sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square meter.

The lumen is defined in relation to the candela as

1 lm = 1 cd ⋅ sr.

A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens.

Now, I’m a human being of approximately average intelligence. But these are just words in no particular order. Some real, some made-up.

No sense can be made of this.

Come on science, admit it; there is no such thing as a lumen. It was invented by the bike industry to sell us stuff.

At least candle-power was something we could grasp. We could easily imagine the light involved when heading out for a bike ride with ten thousand wax candles festooning the frame of our carbon framed pride and joy.

We would light up the road, and people would see us coming.

It’s self-evident.

If we strapped on a little box emitting four hundred lumens would they see that? Or would we end up in a hedge, barely emitting enough light to attract rescue never mind light up the hedge in the first place?

No-one really knows.

But, we are where we are, and so I suspect most of us engage in the same merry dance; buy the highest number of lumens, for the cheapest possible price, sacrificing quality and durability for brightness.

And then, when it bites the dust after a single winter of riding, do the same thing again next year.