Radiometry
Radiometry is the measurement of electromagnetic radiation in terms of physical power, without weighting it by the human eye’s sensitivity.
Radiant flux: The total rate at which radiant energy is emitted, transferred, or received, measured in watts. It is the power whose distribution across wavelength an SPD describes.
Radiant intensity: Radiant flux emitted per unit solid angle in a specified direction, measured in watts per steradian. It describes directional power when total radiant flux is insufficient.
Photometry: The measurement of light weighted according to the spectral sensitivity of human vision. Its eye-weighted quantities contrast with radiometry’s unweighted physical measurements.
Spectroradiometer: An instrument that measures radiant power as a function of wavelength. It measures spectral distributions in calibrated radiometric units.
Photon flux: The number of photons emitted, transferred, or received per unit time. Equal radiant powers can correspond to different photon rates at different wavelengths.
Radiant energy: Electromagnetic energy, measured in joules. Radiant flux measures how quickly this energy is transferred.
Radiance: Radiant flux per unit projected area per unit solid angle, measured in watts per square metre per steradian. It captures how much power travels from a surface in each direction.
Luminous flux: The photometric measure of visible light output, weighted by the standard human visual response. It parallels radiant flux but weights each wavelength by visual sensitivity.
Blackbody radiation: Electromagnetic radiation emitted by an ideal absorber in thermal equilibrium, determined by its temperature. Its known spectrum provides a reference for radiometric measurements.
Spectral radiance: Radiance specified per unit wavelength or frequency interval. It is the directional, area-normalized counterpart to a spectral power distribution.