Radiometry is the measurement and study of electromagnetic radiation, including its intensity, energy, and wavelength. Photometry, on the other hand, is the measurement and study of visible light, which is a subset of electromagnetic radiation. While radiometry includes all wavelengths of electromagnetic radiation, photometry only focuses on the wavelengths that are visible to the human eye.
The most commonly used unit of measurement in radiometry is the watt (W), which measures the power of electromagnetic radiation. In photometry, the most commonly used unit is the lumen (lm), which measures the perceived brightness of visible light. Other units used in both radiometry and photometry include the candela (cd) for luminous intensity and the lux (lx) for illuminance.
Radiometric and photometric measurements are used in a variety of everyday applications. For example, in lighting design, photometric measurements are used to determine the appropriate brightness and color temperature of light sources. In medical imaging, radiometric measurements are used to detect and diagnose medical conditions. Additionally, radiometry and photometry are used in environmental monitoring, remote sensing, and many other fields.
Radiometric and photometric quantities are related through the concept of luminous efficacy, which measures the efficiency of converting electromagnetic radiation into visible light. Radiometric quantities, such as radiant flux and radiant intensity, can be converted into photometric quantities, such as luminous flux and luminous intensity, by multiplying them by the appropriate luminous efficacy value.
Scientists use a device called a radiometer to measure the energy of electromagnetic radiation. A radiometer consists of a set of vanes that spin when exposed to radiation. The speed of the spinning vanes is proportional to the energy of the radiation. Scientists can also use detectors, such as photodiodes and photomultiplier tubes, to measure the energy of specific wavelengths of radiation.