= Spectral luminous efficacy
{title2=$K(\lambda)=K_mV(\lambda)$}
<Spectral luminous efficacy> converts <radiant flux> at one <wavelength> to <luminous flux>. The <candela> convention fixes $K_{\mathrm{cd}}$ exactly at $540\times10^{12}\,\mathrm{Hz}$, not at an exactly specified <wavelength> of $555\,\mathrm{nm}$. <SI base unit: candela (cd)>{ref}
In <vacuum>, that <frequency> corresponds to $c/(540\times10^{12}\,\mathrm{Hz})\simeq555.1712\,\mathrm{nm}$, using the <speed of light>. In standard air it is approximately $555.017\,\mathrm{nm}$, close to the $555\,\mathrm{nm}$ maximum of the <photopic luminous efficiency function>. Thus $K_m$ is slightly larger than $683\,\mathrm{lm\,W^{-1}}$; using $K_m\simeq683\,\mathrm{lm\,W^{-1}}$ in ordinary calculations is a rounded convention. The exact anchor is $K(\lambda_a)=683\,\mathrm{lm\,W^{-1}}$. <luminous efficacy of radiation, for a specified photometric condition>{ref}
Back to article page
