Radiometric ↔ Photometric Converter
Convert between radiometric (W) and photometric (lm) quantities for monochromatic sources using the CIE V(λ) function.
Radiometric quantities measure optical power in physical units (watts); photometric quantities weight that power by the CIE 1931 V(λ) photopic luminous efficiency function, which peaks at 683 lm/W at 555 nm and falls to zero outside 380–780 nm. The conversion factor K(λ) = 683 × V(λ) lm/W applies to monochromatic sources only — broadband sources require spectral integration. This tool converts between four radiometric/photometric quantity pairs: radiant flux (W) ↔ luminous flux (lm), radiant intensity (W/sr) ↔ luminous intensity (cd), irradiance (W/m²) ↔ illuminance (lx), and radiance (W/m²·sr) ↔ luminance (cd/m²). Enter wavelength and a value in either system to obtain the equivalent in the other. An interactive V(λ) chart marks the selected wavelength, and conversion is undefined — and flagged — for UV or IR wavelengths outside the visible range.