Video: How we measure brightness

The candela is one of 7 basic units of measurement. Australia’s Chief Metrologist, Dr Bruce Warrington from the National Measurement Institute, explains how we measure brightness.
Screenshot from video showing a tea-light candle on a computer monitor

Hello again, I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

I’ve been talking about the seven basic units from which every measurement is made. And today it’s the candela, which measures the intensity or brightness of light.

Up to the middle of last century, many countries had their own standards for light output, often based on the brightness of a standard candle or ‘candlepower’. In 1948 the international community adopted a new unit, based on the light glow from molten platinum, and called it the candela.

A common candle emits about 1 candela; the light on your mobile phone about 10; a 40 Watt light bulb around 100; and a lighthouse thousands or even millions of candela. These measurements are of the total light output; the apparent brightness also depends on the size of the source.

This is some of the equipment we use at the National Measurement Institute for measuring and characterising light. Measurements like this are important for a whole range of applications, from the safety of lasers and ultraviolet lamps through to making sure that traffic lights can be seen on a bright, sunny day.

I hope this has been illuminating – and stay tuned for more on the other units of measurement!