Luminous efficacy
= 683 lm W−1
- Value
- 683 lm W−1
- Standard uncertainty
- exact
- Relative uncertainty
- exact
- Status
- Exact by definition (the candela, 1979)
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
Plain
LaTeX
Python
# luminous efficacy, CODATA 2022 (NIST)
K_cd = 6.83e2 # lm W^-1, exact
# SciPy: scipy.constants.physical_constants['luminous efficacy']What it is
Fixes how bright light looks to a person relative to its power: monochromatic green light of frequency 540 × 10¹² Hz, near the peak sensitivity of the eye, has a luminous efficacy of exactly 683 lumens per watt. It defines the candela and so ties photometry, the measurement of light as people see it, to the watt.
The candela has been defined through this value since 1979; the 2019 revision restated it as a fixed constant.
Equations
- at 540 THz
How the value changed, 1998–2022
| CODATA | Value (uncertainty) | Relative uncertainty | Change from previous |
|---|---|---|---|
| 2022 | 683 | exact | exact |
| 2018 | 683 | exact |
“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).