Let there be light

Last night we were plunged back into the 19th century by a power cut. No electric cooker, no lighting, no television. Out came the candles. We were only saved from total historical immersion by a fully-charged laptop which got used as a DVD player for the evening (and gave a fair bit of light too).

It’s amazing how the human eye is able to cope in very low light levels. With just a few candles, there was enough light to do some useful things, such as the washing-up. 

How much light does a candle give compared to a light bulb? A typical candle flame has a luminous intensity of 1 candela. (The magntiude and unit is no coincidence). Luminous intensity is a measure of how much visible light is emitted into a cone of a given size. Specifically, it means that one lumen of luminous flux is emitted by each candle into each steradian of solid-angle. (Solid-angle is the 3d equivalent of angle and is measured by steradians. A circle contains 360 degrees or 2 times pi radians, a sphere contains 4 times pi steradians). That gives us about 12 lumens out of each candle.

How does this compare with a light bulb?  Typical bulbs chuck out about a thousand lumens of luminous flux. When you next buy one, have a look on the packaging – it should be printed somewhere. The more lumens, the more light it gives. A thousand lumens is about a hundred candles worth.  Quite a difference. However, the eye is pretty good at coping with varying light-levels, and we were quite able to cope with a living area illuminated by four candles (for a while, anyway). 

The eye appears more impressive still when you consider how much illuminance is provided by the sun on a clear day. In this case, we are interested in how many lumens fall on each metre squared of ground.  This unit is called ‘lux’, which, for reasons best known to themselves, physicists feel the need to abbreviate, to ‘lx’. The sun, overhead on a clear day, provides us with about a hundred thousand lumens per metre squared of ground.  To do the same with light bulbs (about a thousand lumens each) would require a hundred light bulbs covering each metre squared of ceiling area. That’s one well-lit living area, and a large electricity bill.  If you were to try to get the same illuminance using candles, you’ll be talking about ten thousand candles per metre squared of area.  Yet whether it be a clear, sunny day outside, or a candle-lit house, the eye can still see. That’s pretty impressive stuff.

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