Overview

A radiometer is an instrument designed to measure radiant flux (power) or irradiance from electromagnetic radiation. Unlike photometric instruments that weight measurements by human visual response, radiometers quantify physical energy across specified spectral bands. They are used to assess total power, spectral components or time-varying radiation levels depending on design. For a broad context of the phenomenon being measured, see electromagnetic radiation.

Principal components and characteristics

Most radiometers convert incident radiation into a measurable electrical signal or temperature change. Typical elements include thermopile sensors, bolometers, pyrodetectors, thermistors and semiconductor photodiodes. Key characteristics are spectral response (which wavelengths they detect), sensitivity, dynamic range, response time and directional properties (cosine response). Instruments are often calibrated to provide readings in watts or watts per square metre.

Types and examples

  • Thermal radiometers: use temperature rise (thermopile, bolometer) and respond to broadband energy.
  • Photon detectors: semiconductor photodiodes and photomultipliers measure photon-induced currents in targeted spectral bands.
  • Solar pyranometers and pyrheliometers: specialized radiometers for global and direct solar irradiance.
  • Crookes or light-mill: a historical demonstration device that rotates in light but is not a precise radiometric instrument.

History and development

Simple radiometric concepts date to 19th-century studies of light and heat. The Crookes radiometer popularized visible effects of radiation, while instruments called roentgenometers historically tracked x-ray exposure. Advances in materials and electronics produced sensitive thermal detectors and solid-state photodiodes, enabling compact, fast and calibrated instruments for scientific and industrial use.

Applications and distinctions

Radiometers are used in meteorology, solar energy assessment, remote sensing, astronomy, building diagnostics and industrial process control. Devices labeled for ultraviolet detection or other bands specialize optics and filters; see an example of ultraviolet detectors at ultraviolet detector. Important distinctions include measuring radiant flux (total power, in watts) versus irradiance (power per area, W/m2), and the difference between absolute calibrated instruments and relative sensors. Calibration against recognized standards is essential for quantitative work.