# The luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz

##### SI defining constant
Name Symbol Value Unit Expressed in terms of SI base units
the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz Kcd 683 lm W−1 kg−1 m−2 s3 cd

### Definition

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.

### Luminous efficacy

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.

### Defining constant

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

The luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, together with the Planck constant, h, and the hyperfine transition frequency of the caesium 133 atom, ΔνCs, forms the basis for the definition of the candela.