- Radiometry Definition: Radiometry is defined as the technique of measuring electromagnetic radiation across all wavelengths, including ultraviolet, infrared, and visible light.
- Radiant Energy: Radiant Energy (Qe) is the energy carried by electromagnetic radiation, while Radiant Flux (ф) is the radiant energy transmitted per unit time.
- Microwave Radiometry: Microwave radiometry is a method to measure the thermally caused electromagnetic radiation from matter above zero Kelvin, using antennas and detectors.
- Brightness Temperature: The radiation received by a microwave radiometer is expressed as brightness temperature, which is almost weather independent.
- Photothermal Radiometry: Photothermal radiometry is a technique that uses optical excitation to produce thermal waves and radiometric detection to measure IR radiation, crucial for material inspection without contact.
What is Radiometry?
Radiometry measures radiant energy and related quantities without weighting them for human vision. In optical metrology, it covers ultraviolet, visible and infrared electromagnetic radiation. Photometry covers visible radiation and weights it by the standard response of human vision. The term radiometry is also used in radio and microwave measurement.
Matter above absolute zero emits thermal electromagnetic radiation. Its spectrum depends on temperature and emissivity. Radiometric instruments measure quantities such as radiant flux, irradiance, radiant intensity and radiance rather than one generic intensity.
Radiant Energy (Qe) is electromagnetic energy, measured in joules. Radiant Flux (ф) is the rate of transfer of radiant energy, measured in watts.
Radiant Intensity is radiant flux from a source per unit solid angle in a stated direction. Its SI unit is watts per steradian.
The table compares common radiometric and photometric quantities. On the photometry side, the entries labelled “Irradiance” and “Radiance” should read illuminance and luminance. The correct photometric symbols are not necessarily the same as the radiometric symbols shown.
| Radiometry | Photometry | ||||
| Technical Term | Symbol | Unit | Technical Term | Symbol | Unit |
| Radiant Energy | Qe | J | Quantity of Light | Q | lm s |
| Radiant Flux | ф | W | Luminous Flux | F | lm |
| Radiant Intensity | Ie | Wsr-1 | Luminous Intensity | I | cd |
| Radiant Emittance | Me | Wm-2 | Luminous Emittance | M | lm m-2 |
| Irradiance | Ee | Wm-2 | Irradiance | E | lx |
| Radiance | Le | Wm-2 sr-1 | Radiance | L | cd m-2 |
What is Microwave Radiometry?
Microwave radiometry passively measures natural microwave emission from a scene. The measured radiation depends on physical temperature, emissivity, frequency, polarisation, viewing geometry and the intervening atmosphere, so calibrated measurements can be used to infer surface or atmospheric properties.
An antenna collects radiation from its field of view. Receiver electronics amplify and filter the noise-like signal, compare it with calibration references when required and estimate power over a known bandwidth and integration time.
Microwave radiance is often reported as brightness temperature: the temperature a reference blackbody would need to produce the measured radiance at that frequency under the stated convention.
Microwaves often pass through cloud and fog better than visible or infrared radiation, but the measurement is not weather-independent. Atmospheric water vapour, cloud liquid water, precipitation and surface conditions can change brightness temperature.
In the figure, the antenna receives radio-frequency power from the scene. A Dicke switch alternates the receiver input between the scene and a reference so that gain changes can be reduced or measured; it does not by itself establish the full calibration.
In a superheterodyne receiver, a mixer translates the selected radio-frequency band to an intermediate frequency using a local oscillator signal. Direct-detection and other receiver architectures are also possible.
The signal path uses a low-noise Amplifier and bandpass filter to set receiver sensitivity and bandwidth. A total-power detector integrates one band, while a spectrometer divides the received power into frequency channels.
What is Photothermal Radiometry?
Photothermal radiometry is a non-contact method for studying thermal and subsurface properties. A controlled optical source heats the sample, and an infrared detector measures the resulting change in surface thermal emission.
The absorbed optical power produces a time-dependent temperature field that diffuses through the material. The measured infrared amplitude and phase, or transient response, depend on thermal diffusivity, conductivity, layer thickness, defects and experimental geometry.
With periodic excitation, a lock-in measurement extracts signal amplitude and phase at the modulation frequency. With pulsed excitation, the detector records a broadband thermal transient over time.
Interpretation must account for the heating spot, optical absorption, infrared emissivity, detector band, surrounding temperature, surface roughness and heat losses. These effects cannot generally be ignored when accurate material properties are required.
The figure shows one photothermal radiometry setup. A Laser diode supplies intensity-modulated light. Absorbed optical power heats the sample, and the surface temperature response follows the sample’s thermal properties.
Suitable optics collect the emitted infrared radiation and focus it onto a detector. Some measurements use a cooled detector to reduce noise, but cooling is an instrument choice rather than a requirement of the method.
Mercury cadmium telluride (MCT) provides infrared detection. Liquid nitrogen may cool an MCT detector to reduce noise.
