The physics side of light measurement, using radiant flux in watts, radiant intensity in W/sr, irradiance in W/m squared and radiance. Photometry takes the same geometry and weights it by the V-lambda curve to give lumens, candelas, lux and luminance. Radiometric units are needed for UV, infrared, horticultural and photobiological work, where the eye's sensitivity is irrelevant.
Grow lights are rated in micromoles, not lumens, so we need the radiometry, not the photometric report.
Do not mix it up with
- Luminous flux: Luminous flux is eye-weighted; radiant flux counts all optical power equally regardless of visibility.
Related terms
V-lambda curve
The standard curve of the eye's daytime sensitivity to each wavelength, peaking in green at 555 nm.
SPD
Spectral Power Distribution: a graph of how much power a light source emits at each wavelength.
Spectroradiometer
An instrument that measures light's power at each wavelength, from which color and photometric values are calculated.
Blue light hazard
Photochemical retinal damage caused by staring at very bright sources rich in blue light, roughly 400 to 500 nm.