bandpass that permits the
full solar spectrum to pass through to the sensor. The glass dome also protects the sensor
from the elements.
The sensor is a high quality blackened thermopile. Heating of the
sensor by incoming solar radiation produces a signal in the µvolt range. Each
pyranometer has its own calibration factor. Along with the processing algorithm,
it is
programmed into a memory chip embedded into the sensor. Upon connection to the PMA2100 the
processing algorithm is loaded into the meter and the correct reading is immediately displayed
on the LCD.
Thermopile type radiation sensors exhibit the widest and most uniform
spectral response. It is limited mostly by the spectral transmission of the dome. They are
useful to monitor the total radiant power or solar radiation as well as artificial
sources. The sensitivity of these types of sensors is somewhat limited and measurements of
radiant flux below 10W/m2 are difficult.
Due to almost ideal flatness of the spectral response the pyranometer
is often used with a set of low wavelength blocking filters in order to measure radiant
power distribution in various bands. The difference between the total power and the power
measured with the filter is the radiant power in the band blocked by the filter. A
correction that accounts for energy reflected from the filter’s surface is necessary.
In solar radiation studies the diffuse component of the total radiant
flux can be measured by using a shadow-band casting a shadow on the pyranometer's
entrance. The difference between the total power and the diffuse power measured with
shadow-band is the direct component. For better accuracy a correction should be made for
the part of diffuse radiation obstructed by the shadow-band itself.
