Nowadays, candles are only occasionally used for their original purpose of providing light in case of power cuts. Nevertheless, I'm sure many of us have wondered how much light we can get from candles.
What are the units of brightness?
Brightness is the amount of light a light source emits. In technical language, we call it luminous flux, which is measured in Lumens. So, the basic unit of luminous flux is the lumen (lm), which determines how bright a light source is. Another standard unit of brightness is the candela, which is derived from the candle itself. Candela is a Latin word meaning candle. The luminosity of an average candle is 1 candela, so we can say that 1 candela is the equivalent of the light of an average candle. For comparison, the luminous intensity of a 100-watt lamp is 120 candela.
There are complicated mathematical formulae for measuring the candela and lumen, but nowadays the use of specialized photometric instruments is common. These can be turned towards the light source for a few moments to obtain the data in lumens, lux and even candela. If you don't have such a tool, you can download a light metering app for your phone, which calculates the intensity of the light source from the beam coming through the smartphone's rear camera. This solution is not as accurate as a dedicated tool, but it's certainly cheaper and in most cases it is enough to satisfy our curiosity about the subject.
Measured by any of the methods mentioned above, an ordinary candle can produce about 12 lumens. The brightness of a candle is of course affected by the wax used, the length and quality of the wick, and the burning temperature, but only to a very small extent, so this 12 lumen brightness is true for almost all candles.
To give this number some meaning, we have given below the luminous flux in lumens of some well-known light sources:
iPhone flashlight: 40-50 lumen
Standard 100-watt incandescent light bulb: 1500-1700 lumen
Good quality projector: 1500-2500 lumen
Car headlight: 2000-4000 lumen
Compared to these numbers, a modest candle flame may not seem overpowering, but it is enough to light up a small room and make it feel cosy.
What makes candlelight different from artificial light bulbs is that it emits a so-called warm white light, which is much closer in colour temperature to natural light and therefore much more pleasing to the eye. Perhaps this is why today, more than a hundred years after the discovery of electric lighting, we still like to light a candle every once in a while not with the aim of bathing our rooms in a flood of light, but with the aim of making them more homely.