Eight alternate black and white vanes reside within a glass sphere at the top of the pyrometer (also called heliometer); difference in solar absorption between vanes cause difference in electrical conductivity; low strength electrical signal transmitted to the Solar Radiation Recorder where electrical signals are amplified and integrated in the weather station. Stronger signals indicate more intense solar radiation and vice versa. It is important that the device be mounted perfectly flat to optimize the sensor’s 180-degree field of view towards the sky.
In order to attain the proper directional and spectral characteristics, the pyranometer’s main components are:
- A thermopile sensor with a black coating. This sensor absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nanometer range, and has a near-perfect cosine response. A thermopile is simply an electronic device that converts thermal energy into electrical energy.
- The glass dome that covers the black and white vanes acts to limit the spectral response from 300 to 2,800 nanometers (cutting off the part above 2,800 nm), while preserving the 180 degrees field of view. Another function of the dome is that it shields the thermopile sensor from convection and bird feces.
The black coating on the thermopile sensor absorbs the solar radiation. This radiation is converted to heat. The heat flows through the sensor to the pyranometer housing. The thermopile sensor generates a voltage output signal that is proportional to the solar radiation.
~ Steve Woodruff and Devin Lussier