luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz

SI defining constant

Name	Symbol	Base units
luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz	Kcd	kg−1 m−2 s3 cd
The numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, symbol Kcd, is defined to be exactly 683 when expressed in the unit lumen per watt, symbol lm W−1, or kg−1 m−2 s3 cd.

Luminous efficacy

The luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd, forms the basis for the definition of the unit of luminous efficacy, the lumen per watt.

The definition of the candela implies the exact relation:

Inverting this relation gives an exact expression for the lumen per watt in terms of the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd :

Luminous efficacy is a measure of the brightness of a light source, as perceived by the human eye, relative to the light source’s radiant power. In effect it is the ratio of the luminous flux to radiant intensity.

Electromagnetic radiation of frequency 540 THz is in the green region of the visible spectrum. The human eye, when adapted for bright conditions, is most sensitive near this frequency. At other frequencies, a higher radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye.

Luminous intensity

The luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd, together with the Planck constant, h, and the caesium frequency, Δν_Cs, forms the basis for the definition of the unit of luminous intensity, the candela.

The definition of the candela implies the exact relation:

Inverting this relation gives an exact expression for the candela in terms of the Planck constant, h, caesium frequency, Δν_Cs and the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd :

Nature

In the SI, the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz is a technical constant, K_cd.

This constant gives an exact numerical relationship between the purely physical characteristics of the radiant power stimulating the human eye and its photobiological response defined by the luminous flux due to the spectral responsivity of a standard observer at a frequency of 540 × 10¹² hertz.