INTRODUCTION
Solar radiation at Earth’s surface is typically defined as total radiation across a wavelength range of 280 to 4000
nm (shortwave radiation). Total solar radiation, direct beam and diffuse, incident on a horizontal surface is defined
as global shortwave radiation, or shortwave irradiance (incident radiant flux), and is expressed in Watts per square
meter (W m-2, equal to Joules per second per square meter).
Pyranometers are sensors that measure global shortwave radiation. Apogee SP series pyranometers are silicon-cell
pyranometers, and are only sensitive to a portion of the solar spectrum, approximately 350-1100 nm
(approximately 80 % of total shortwave radiation is within this range). However, silicon-cell pyranometers are
calibrated to estimate total shortwave radiation across the entire solar spectrum. Silicon-cell pyranometer
specifications compare favorably to specifications for World Meteorological Organization (WMO) moderate and
good quality classifications and specifications for International Organization of Standardization (ISO) Class C
classification, but because of limited spectral sensitivity, they do not meet the spectral specification necessary for
WMO certification.
Typical applications of silicon-cell pyranometers include incoming shortwave radiation measurement in
agricultural, ecological, and hydrological weather networks, and solar panel arrays.
Apogee Instruments SP series pyranometers consist of a cast acrylic diffuser (filter), photodiode, and signal
processing circuitry mounted in an anodized aluminum housing, and a cable to connect the sensor to a
measurement device. Sensors are potted solid with no internal air space and are designed for continuous total
shortwave radiation measurement on a planar surface in outdoor environments. The SP-420 outputs a signal that
is directly proportional to total shortwave radiation from the sun. The voltage signal from the sensor is directly
proportional to radiation incident on a planar surface (does not have to be horizontal), where the radiation
emanates from all angles of a hemisphere.
