The SI Unit Candela

Abstract

The history of photometry, the measurement of light-and thus also the root of the basic unit for the luminous intensity, goes back to the seventeenth century. The theoretical foundations are provided by Johann H. Lambert in his treatise on photometry around 1760. With industrialization in the nineteenth century, there is an increasing demand for stable reference light sources for the measurement of light and for an reliable definition of the unit for light. But it is not until the middle of the twentieth century that the basic prerequisites had been created to use a common, internationally recognized unit, the Candela, as a measure of the visible light. The following article describes the basics and the boundary conditions that arise for the unit candela from the revision of the SI.

Single photon sources for quantum radiometry: a brief review about the current state-of-the-art

“…The optical radiant flux Φ of such a device would be exactly given by Φ = f h c/λ, where f is the repetition rate of the excitation laser, h is the Planck constant, c is the speed of light and λ the wavelength of the emitted radiation. Since c and h have no uncertainty, and the frequency and wavelength can be measured with uncertainties in the 10 -17 and 10 -12 range, respectively, the optical radiant flux could be determined with an unprecedented accuracy, in particular far below the current state-of-the-art achieved using the cryogenic radiometer, which has uncertainties in the 10 -5 range [8]. However, this requires a perfect source, i.e., a source with a quantum efficiency of 100% (i.e., each excitation leads to an emission of a photon), a perfect purity of the single-photon emission, i.e., g (2) (0) = 0, and a collection efficiency of the emitted radiation of 100%.…”

The Revised SI: Fundamental Constants, Basic Physics and Units

“…Many of the ideas and results, discussed and even born during the seminar, are reported in this Special Issue. In particular, the present issue includes invited reviews by world‐leading experts who consider the use of fundamental constants for the definition and realization of the base units of the new SI : Kelvin, Mole, Kilogram, Meter, Second, Ampere, and Candela . High‐precision experiments aiming for the determination of the fundamental constants are also in the focus of this Special Issue; see, for example, ref.…”