Index

## DEFININGCONSTANTSYMBOL: K_cd

**PID**: si:definingconstantsymbol:K_cd:2019**Definition**: The luminous efficacy of monochromatic radiation of frequency 540 × 10^{12}*Hz*,*K*_{cd}, is exactly 683 lm/W.**Source**:*SI Brochure 9th Ed. 2019, p 127***Formatted**:`633 lm W`

^{-1}**Numerical Value**:*633***Unit**: lm W^{-1}**Unit in Base SI**: cd s^{3}m^^{2}kg^{-1}m^{-4}**Notes**- This value was fixed by the CGPM in 2018 (Resolution 1), with effect from 20 May 2019.
- The luminous efficacy of monochromatic radiation of frequency 540 x 10
^{12}*Hz*is a technical constant that gives an exact numerical relationship between the purely physical characteristics of the radiant power stimulating the human eye (*W*) and its photobiological response defined by the luminous flux due to the spectral responsivity of a standard observer (*lm*) at a frequency of 540 x 10^{12}*Hz*.

**Representations**- Text:
`K_cd`

- HTML:
`<em>K</em><sub>cd</sub>`

- Latex:
`K_{\rm{cd}}`

- Text:
**Related Definitions**- isConstantSymbolOf -> luminous efficacy