radiometry

Radiometry is the science of measuring electromagnetic radiation, focusing on the physical power and distribution of light across wavelengths rather than human visual response. It encompasses infrared, visible, and ultraviolet radiation and applies to sources, detectors, and optical systems. Radiometric measurements aim to quantify how much energy is present, how it is emitted by sources, and how it is transferred through space and to surfaces.

Key radiometric quantities include radiant flux (power) and radiant flux density. Radiant flux, often denoted by Phi, is the total power carried by radiation. Irradiance, measured in watts per square meter, describes the radiant flux received by a surface. Radiant intensity, in watts per steradian, characterizes the power emitted by a source per unit solid angle, while radiance, in watts per square meter per steradian, describes how much power is emitted or received in a given direction per unit area. Spectral quantities—such as spectral radiance and spectral irradiance—describe power distribution as a function of wavelength. Radiometric measurements rely on calibrated detectors, including photodiodes, bolometers, and spectroradiometers, often using integrating spheres to capture diffuse flux.

Radiometry is distinct from photometry, which weights radiometric measurements by the human eye’s spectral response and uses units such as lumen and candela. Radiometry uses absolute physical units like watts and watts per steradian, enabling traceable calibration and comparison across instruments and wavelengths. Standards and traceability are maintained through reference sources (e.g., blackbodies) and calibrated detectors.

Applications span optical instrumentation calibration, remote sensing, astronomy, solar and climate studies, lighting engineering, and materials testing. The field intersects with radiative transfer theory and Planck’s law to relate source emissions to observed signals across wavelengths.