The Fartela

Following on from the great interest in my discussion of the mole, I thought I'd move on to the Candela, a unit of light intensity. This is one of the seven base units of the SI system, and most people would assume that it is important.

Why, in this case, despite studying physics for longer than is healthy, have I never used the unit Candela? Or seen anyone who used the unit? Or derived anything from this unit? Or ever bought a 10 Candela lightbulb?



The Candela describes how bright a light source is. Whichever way you look at light, the important thing is that it transmits energy, and the energy in a certain time is a power, measured in Watts: Hence the 60 Watt lightbulb. A Watt is already defined in terms of metres, kilograms and seconds, so it would appear that light intensity doesn't need its own base unit. A pre EU-directive shopper may wish to disagree, though.

Imagine that you bought a 60 Watt light bulb, and found out that it didn't light up despite drawing the full 60 Watt. It might give you a warm feeling, or a sunburn, but if the wavelength isn't between 400 and 700 nanometres you won't see anything. This is why the Candela is needed: it combines the power emitted with the sensitivity of the human eye to find the perceived brightness of something.

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This seems fair enough, and most useful to know when buying a lightbulb, or looking for a new star to orbit your planet around. But why make it a base unit? The Candela is strictly only of any meaning to a human with working eyes. In the kingdom of the blind, how many Candelas does a lightbulb emit?

Seeing as the Candela describes the human perception (brightness) of an physical property (flow of energy), why not do the same for the other senses, and give them each an SI base unit. Sound is a wave, which transmits power, but humans only hear certain wavelengths (most people would say frequencies, but I'm trying to be consistent). Multiply the sound wave power by a human ear function, and you have a new unit: the Bella. Sounds for which the ear is sensitive would earn more Bellas than those at the limit of human hearing.

Smells are concentrations of certain molecules in the air, best measured physically as moles/cubic metre (although it may depend on the speed of those molecules, too, for all I know. Physicists don't worry much about smells, or personal hygiene). Multiply this concentration by a wuffiness factor which describes the sensitivity of the human nose and you have a new unit: the Fartela.

So why is the Candela a base unit? I think it is something to do with unification. This website lists lots of old units of perceived brightness: The Hefner Candle, the Violle, the Apostilb, the Blondel, the Footlambert, the Nit, the Skot and the Stilb. The world's photometric experts were probably so besides themselves with happiness when the Candela came into standard use that they made it a base unit.

Writing this, I've grown to like the Candela. It's useful, practical, and although it probably shouldn't be a base unit it surely deserves recognition in the google calculator.