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Instruments designed to measure the radiant flux (also known as radiant power) in a predetermined spectral range. Classical radiometers measure the thermal effect produced by radiation falling on the instrument, using a sensor that absorbs radiation and increases its temperature accordingly (frequently called a bolometer). The temperature increase is measured with a thermometer attached to the bolometer. Radiometers intended for the measurement of light radiation usually include a semiconductor diode that produces mobile electric charges when struck by photons (i.e., photodiodes) and a voltage or current meter; they are intended for measuring light in different ranges of the spectrum (e.g., visible, infrared, ultraviolet) within a given power range, accuracy, resolution, and/or other parameters. These instruments also include a user interface to access recent measurements, a unit zero function, available measurement units, and data export capability. To obtain accurate measurements, radiometers should be manufactured according to the source of light to be measured (e.g., lamp, LED, laser), the transmission medium (e.g., air, water, fiber optics), and the characteristics of the detector chosen. Power is typically measured in the unit watts and power density in watt/cm2.
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