National Physical Laboratory

Unit of the month – candela

SI countdown - candela

Following the recent decision, taken by measurement scientists from around the world, to redefine the International System of Measurement (SI) units, on the 20th of each month we will be looking at one of the seven SI base units. You'll be able to find out where it's used in everyday life, how it's defined now, and the changes that will come into force on 20 May 2019.

20 November 2018 metre (m)
20 December 2018 candela (cd)
20 January 2019 ampere (A)
20 February 2019 kelvin (K)
20 March 2019 second (s)
20 April 2019 mole (mol)
20 May 2019 kilogram (kg)

On 25 November 1882, Gilbert and Sullivan's opera 'Iolanthe' opened at London's Savoy Theatre, and inadvertently started a Christmas tradition we still observe today.

In a theatre first, the Swan United Electric Light Company was commissioned to create miniature lights which twinkled from wreaths worn by the lead fairies. In an age when electric lighting was still cutting-edge, the tiny lights – powered by battery packs hidden in costumes – amazed audiences. The term 'fairy lights' was born. A year later, Edward Johnson, a colleague of Thomas Edison, put fairy lights on a Christmas tree for the first time.

Fairy lights are a good way of visualising the candela, the SI unit of luminous intensity. The light from a single clear indoor fairy light is approximately one candela. Both traditional tungsten filament fairy lamps and modern LED versions have similar luminous intensity, despite LEDs consuming a tenth of the electricity.

Perhaps unsurprisingly, the candela was originally based on the light of a candle; a sperm wax candle in the UK. But a definition based on something so variable was not good enough for measurement scientists. The candela, or the 'new candle', replaced candle-based definitions in 1948. In 1979, the modern definition of the candela was adopted:

The candela is the luminous intensity, in a given direction, of a source that emits
monochromatic radiation of frequency 540×1012 hertz and that has
a radiant intensity in that direction of 1/683 watt per steradian.

The candela is the only SI base unit linked to human perception. Light can be measured in terms of the power of its combined frequencies hitting an area (measured in watts). However, the eye cannot see all light colours equally well. Luminous intensity therefore measures light, weighted by a model of human eye sensitivity to different frequencies.

The eye is most sensitive to yellow-green light, approximating the peak frequency of solar radiation reaching the earth's surface, and less sensitive to red and blue light. The relative visual brightness of different colours of fairy lights changes according to the ambient light level.

The definition of the candela allows for this effect, and can be used to calibrate instruments which use the eye sensitivity model to measure luminous intensity.

The candela will effectively stay the same from 2019, as it is already defined in relation to other base units, though accuracy will be improved by updates to the second and metre. The new definition will be:

The candela is defined by taking the fixed numerical value of the luminous efficacy
of monochromatic radiation of frequency 540×1012 Hz, Kcd, to be 683 when expressed
in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where
the kilogram, metre and second are defined in terms of hc and ΔνCs.

The candela has become particularly important in the past decade as we have seen rapid innovation in energy-efficient lighting, and a need for reliable ways to compare the brightness of different light sources, and to reassure consumers.

This includes our fairy lights. Coloured tungsten fairy lights are even less energy-efficient than clear ones as most light is absorbed by the coating used to create the colour. LEDs use different semiconductor materials to create colours with more visible light per electrical watt. Luminous intensity allows us to compare the visual appearance or each.

The lights most people hang on their trees this Christmas will be LEDs. As with most modern lighting, the incandescent bulbs that amazed 19th century opera fans have been superseded by energy efficient alternatives. We have the candela to thank.

Find out more about Redefining the SI units and the candela

Find out more about NPL's work in Optical Radiation & Photonics

Last Updated: 21 Jan 2019
Created: 20 Dec 2018


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