A pyranometer measures solar radiation and is used in meteorology. It measures the density of fluctuations in the electromagnetic radiation spectrum and uses chemical or physical devices to measure the number of photons. The device can measure radiation levels from different angles, but is most accurate when the sun is directly overhead. Pyranometers are equipped with a glass dome to limit the response to ideal measurement parameters and provide safety shielding.
A pyranometer is an instrument designed to measure the power of heat and light from the sun. Mainly used in the meteorological field, solar radiation is identified with a pyranometer placed on a flat surface. The entire frequency spectrum of electromagnetic radiation projected by the Sun and received on the earth’s surface strikes sensors within the device, which measure the density of fluctuations in the entire 180-degree field surrounding the instrument.
A solar pyranometer works by measuring the number of photons, small units of light, that over time impact a chemical or physical device inside the instrument. Usually, this is most readily useful for identifying the wavelengths of the ultraviolet and visible light of the spectrum. Pyranometers are generally unpowered, as each of the system components reacts to or is directly affected by solar radiation.
Most chemical-based pyranometers use a solution of heat-sensitive, photoreactive chemicals that can measure the total level of electromagnetic radiation. The chemical itself requires a process by which radiation is identifiable from the absorbed light. This is known as quantum yield, and it produces a reaction due to lack of light, making every chemical in the pyranometer useful. Common examples of chemicals used in these tools include potassium ferrioxalate, monochloroacetic acid, and malachite green leukocyanide.
Physical types of pyranometers include bolometers, thermopyles, and photodiodes. Bolometers use a thin layer of metal attached to a heat sink that maintains a constant temperature, allowing the device to recognize solar radiation levels. Thermopiles convert heat into electricity through devices coupled in series, measuring the voltage output to determine the radiation. Photodiodes use a similar technique to convert light into current or voltage, enabling measurement.
The level of possible measurement in a pyranometer varies depending on the position of the Sun itself. Ideal conditions are provided by the Sun directly overhead, however, some measurements can be taken from different angles, as long as the impact of the radiation is identified. When the Sun is at a 90 degree angle from the instrument, no measurement can be made. Normal surface radiation, caused by the inclination of the Sun between 0.5 and 60 degrees, can be measured proportionally. However, when the Sun reaches its zenith, directly above the device, the measurement is perfectly accurate.
Most pyranometers are equipped with a glass dome to ensure correct directional attributes. This limits the response to 300-2,800 nanometers, the ideal measurement parameters. It also has secondary purposes in preserving the full 180 degree field and providing safety shielding.
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