radiometers

Radiometers are instruments that measure radiant energy across the electromagnetic spectrum. They quantify quantities such as radiant power (flux), irradiance, or spectral radiance and are used in meteorology, astronomy, remote sensing, and industry. The term covers a broad range of detectors and systems, from simple calorimeters to sophisticated spectroradiometers.

Most radiometers convert incident radiation into an electrical signal or a measurable temperature change. Thermal radiometers—bolometers, calorimeters, and thermopile detectors—absorb energy and produce a signal proportional to the incident power. Pyroelectric radiometers respond to temperature changes caused by modulated radiation. Photodetectors such as photodiodes and photoconductors provide fast responses for specific spectral bands.

Common instruments include pyranometers and pyrheliometers for solar irradiance, infrared radiometers for surface temperature or atmospheric sounding, and microwave radiometers for atmospheric temperature and humidity. In astronomy and Earth observation, radiometers on satellites and telescopes measure radiance across near- and far-infrared, microwave, and submillimeter bands. Calibration typically uses blackbody references or integrating spheres with traceability to national standards.

Key considerations include spectral response, field of view, linearity, and temporal response. Radiometers are calibrated to convert signals into physical units such as watts per square meter, watts per steradian per square meter, or brightness temperature. The Crookes radiometer is a historic device sometimes called a radiometer, but it does not provide absolute radiometric measurements for scientific use.