measure for measure From candle to candela Yuqin Zong sheds light on photometry’s fundamental unit. he candela (cd), the unit of luminous the average human eye for specified visual steps need to be taken to redefine the candela intensity, is the International System of conditions (photopic, scotopic and mesopic based on fundamental quanta (photons) TUnits (SI) base unit for photometry — vision). Using the definition of the candela to meet the increasing needs of quantum the science of measuring light as perceived and the CIE spectral luminous efficiency photometric and radiometric measurements. by the human eye. The standards used for functions, the luminous intensity of any This view also expresses that human vision is defining and measuring luminous intensity, light source can be obtained. photon-based — the interaction of photons a measure of the luminous power emitted In 2009, in response to advances in with photoreceptor molecules in the eye. by a light source, in a particular direction, quantum-based optical technologies and In the end, there was no unanimous per steradian, have evolved along with their emerging metrological applications, agreement within the CCPR concerning the the development of technologies and the Consultative Committee for Photometry 2009 proposal, and it was ultimately rejected. It application needs, from candles, gas lamps and Radiometry (CCPR) proposed to is still unclear whether the ongoing debate on and incandescent lamps to Planck radiators. add the following explanatory text to the the explanatory text for the reformulation of The candela was first defined in 1948, and reformulation of the candela2: “Thus we have the candela will eventually reach a consensus reformulated in 1967 as “the luminous the exact relation Kcd = 683 lm/W. The effect and whether a redefinition of the candela intensity, in the perpendicular of this definition is that the candela is based on photons will be considered in the direction, of a surface of the luminous intensity, in a given near future. Meanwhile, the CCPR decided 1/600,000 square meter of a Y direction, of a source that emits to include photon-number-based quantities M A black body at the temperature L monochromatic radiation of (for example, photon intensity, photon flux A 12 / of freezing platinum under a frequency 540 × 10 hertz and and photon irradiance) and their realizations K C pressure of 101,325 newtons O that has a radiant intensity in in the recently published mise en pratique — T S per square meter.” R that direction of 1/683 watt guidelines on how the candela and its related E P 4 U Subsequent advances in S per steradian. This radiant units can be realized in practice . radiometry (the measurement © intensity corresponds to a In any event, Kcd is the new defining of optical radiation power) photon intensity of (683 × 540 × constant for photometry, without uncertainty, brought new capabilities to the field 1012 × 6.62606896 × 10−34)−1 photons representing the luminous efficacy of of photometry, offering alternative ways per second per steradian.” (The constant monochromatic radiation of frequency 12 to realize the candela in addition to using Kcd involves the lumen (lm = cd sr), the unit 540 × 10 hertz, and is recommended by high-temperature Planck radiators, which are of luminous flux, a measure for the total the CCPR to be one of the seven defining expensive and difficult to operate. Therefore, amount of visible light emitted from a source. constants used to scale the entire SI system3,5. in 1979, the current definition of the candela The unit ‘photons per second per steradian’ is A new SI brochure on the definitions was adopted1: “The candela is the luminous used for photon intensity.) The last sentence of the SI units is now being prepared and intensity, in a given direction, of a source that of the explanatory text, which did not alter in is expected to be published in 2018, with emits monochromatic radiation of frequency any way the current definition of the candela, a definition of the candela adjusted to the 540 × 1012 hertz and that has a radiant was a suggested compromise between standard format for the definitions of all intensity in that direction of 1/683 watt per proponents of a radiant-intensity formulation seven base units, that is, expressed in terms –1 3 steradian.” The value 1/683 W sr was chosen and those of a photon-intensity formulation . of its defining constant, Kcd. ❐ to maintain continuity with the previous The radiant-intensity side’s argument definitions based on candles and Planck is that the current definition serves well YUQIN ZONG is at the National Institute radiators. Indeed, an ordinary wax candle the practical needs of the classical world of of Standards and Technology, Gaithersburg, produces nearly 1 cd of luminous intensity. lighting and the majority of practitioners Maryland 20899, USA. The 1979 definition is given for only one of photometry and radiometry3. Such a e-mail: [email protected] frequency of optical radiation. To determine viewpoint implies, however, that direct References the luminous intensity of a light source realization of the candela using current 1. SI Brochure: The International System of Units 8th edn (BIPM, 2006). emitting monochromatic radiation with photon-number-based techniques is difficult 2. CCPR Report of the 20th Meeting to the International another frequency or a broad spectrum of and has much larger uncertainties than Committee for Weights and Measures (CIPM) (BIPM, 2009); http://go.nature.com/OXwCDR frequencies, the International Commission the present method of using a cryogenic 3. Zwinkels, J. C. et al. Metrologia 47, R15–R32 (2010). on Illumination (CIE) has defined a set radiometer — an electrical-substitution 4. SI Brochure Appendix 2: Mise en Pratique for the Definition of of weighting functions — called spectral radiometer that is operated at an extremely the Candela and Associated Derived Units for Photometric and Radiometric Quantities in the International System of Units (SI) luminous efficiency functions — that low temperature (typically, a few kelvin). The (BIPM, 2015). describe the relative spectral sensitivity of photon-intensity camp argues that proactive 5. Fischer, J. & Ullrich, J. Nature Phys. 12, 4–7 (2016). 614 NATURE PHYSICS | VOL 12 | JUNE 2016 | www.nature.com/naturephysics ©2016 Mac millan Publishers Li mited. All ri ghts reserved. .