- 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 is a technique to measure electromagnetic radiation for any wavelength. Photometry is similar to radiometry, but photometry is related to visible light signals only, while radiometry includes signals of any wavelength – like ultraviolet, infrared, and visible light.
Radiometry detects radiation from materials and substances. According to Planck’s law, all materials radiate energy as electromagnetic waves. Radiometry measures the intensity of this radiation.
The energy carried by electromagnetic radiation is known as Radiant Energy (Qe). Radiant energy transmitted per unit time is known as Radiant Flux (ф).
In the radial direction, the radiant energy radiated from a point source per solid angle per unit time is known as Radiant Intensity.
The below table shows the comparison of various technical terms related to photometry and radiometry.
| 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 is used to measure the thermally caused electromagnetic radiation of matter at a physical temperature above zero kelvin (0 K). This measurement of radiation helps to determine the properties of matter.
Microwave radiometry uses antennas and detectors to observe various scenes. Microwave radiometers receive electromagnetic radiation emitted by matter.
The amount of radiation revived by a microwave radiometer is expressed as the equivalent body temperature and this temperature is known as brightness temperature.
Brightness temperature distribution is almost unaffected by weather conditions.
As shown in the above figure, the radiofrequency signal is received by the antenna. A Dicke Switch is used for calibrating the high-frequency signal.
Then the high-frequency signal is converted into the intermediate frequency with the help of a local oscillator signal.
After that the signal pass through the Low Noise Amplifier (LNA) and bandpass filter. The signal can be detected in full power mode by splitting into multiple frequency bands with a spectrometer.
What is Photothermal Radiometry?
Photothermal radiometry inspects materials without contact. It uses thermal waves to influence the thermal emission from heated surfaces.
Photothermal radiometry detects and measures IR radiation. It involves two functions: optical excitation to produce thermal waves and radiometric detection of the emitted heat radiance.
The first scheme is used to monitor the narrowband amplitude and phase of the signal for the modulated signal. The second scheme is used to monitor a wide band of sample’s thermal response on the pulsed excitation.
The temperature measurement affected by spatially concentrating the irradiance. And also consider various measuring conditions that affect the IR flux like optical material properties, the absolute temperature of surrounding, surface roughness. Generally, these factors are ignored in photothermal applications.
The above figure shows a compact photothermal radiometry setup. In this experiment, a Laser diode is used to generate intensity modulated light. By the absorption of light heat is generated.
In photothermal radiometry, a cooled IR detection device is used to monitor thermal response and variation of temperature. By suitable optics, the emitted IR radiation is imaged on detection.
A liquid nitrogen or MCT- Mercury Cadmium Telluride is used as a cooled IR detection device.
