What is a Pyranometer?

A pyranometer is an instrument designed to measure the power of the heat and light from the Sun. Used primarily in the meteorological fields, solar radiation is identified with a pyranometer placed on a flat surface. The full frequency spectrum of electromagnetic radiation projected by the Sun and received on the Earth's surface impacts sensors within the device, which measures the density of the fluctuations in the full field of 180 degrees surrounding the instrument.

A solar pyranometer works by measuring the number of photons, small units of light, that impact either a chemical or physical device within the instrument over time. Usually, this is most readily useful in identifying the ultraviolet and visible-light wavelengths of the spectrum. Pyranometers are generally unpowered, as each of the components in the system either reacts to or is influenced by the solar radiation directly.

Most chemical-based pyranometers use a solution of photoreactive and heat-sensitive chemicals that can measure the total level of electromagnetic radiation. The chemical itself requires a process by which the radiation is identifiable from absorbed light. This is known as quantum yield and produces a reaction due to the lack of light, making each chemical useful in the pyranometer. The most common examples of chemicals used in these instruments include potassium ferrioxalate, monochloroacetic acid and malachite green leucocyanide.

Physical types of pyranometers include bolometers, thermopiles and photodiodes. Bolometers use a thin layer of metal attached to a heat sink which maintains a constant temperature, allowing the device to recognize solar radiation levels. Thermopiles convert heat into electricity through coupled devices in a series, measuring voltage output to determine radiation. Photodiodes use a similar technique to convert light into current or voltage, allowing measurement.

The level of possible measurement in a pyranometer varies depending on the position of the Sun itself. Ideal conditions are supplied by the Sun being directly overhead, however, certain measurements can be made from different angles, as long as the radiation impact is identified. When the Sun is located at a 90 degree angle from the instrument, no measurement can be made. Normal surface radiation, caused by the Sun being angled between 0.5 and 60 degrees can be measured proportionally. However, when the Sun hits its zenith, directly above the device, the measurement is perfectly accurate.

Most pyranometers are fitted with a glass dome to ensure the proper directional attributes. This limits the response to within 300 to 2,800 nanometers, the ideal measurement parameters. It also has secondary purposes in preserving the full 180 degree field and providing safety shielding.