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INTRODUCTION
All objects with a temperature above absolute zero emit electromagnetic radiation. The wavelengths and intensity
of radiation emitted are related to the temperature of the object. The atmosphere and terrestrial surfaces (e.g.,
soil, plant canopies, water, snow) emit radiation in the mid infrared portion of the electromagnetic spectrum
(approximately 4-50 µm).
Pyrgeometers are sensors that measure the net longwave radiation difference between the detector surface and
surface the detector is directed towards (typically, atmosphere or ground surface). Longwave radiation emitted by
the surface of interest can be calculated with the additional measurement of detector temperature, typically
accomplished via an internal thermistor or PRT.
Typical applications of pyrgeometers include measurement of incoming longwave radiation from the sky and
outgoing longwave radiation from terrestrial surfaces in atmospheric and energy balance studies. Incoming
longwave radiation measurements from pyrgeometers are an important input to frost prediction models.
Pyrgeometers are also integrated into net radiometers for measurement of net radiation at the land surface.
Apogee Instruments SL-510 and SL-610 pyrgeometers consist of a thermopile detector, silicon filter with a
diamond-like carbon coating (for weatherproofing), precision thermistor (for detector temperature measurement),
heater, 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 measurement of longwave radiation from the sky and terrestrial surfaces in outdoor environments. SL-
510 and SL-610 pyrgeometers output an analog voltage that is directly proportional to the longwave radiation
balance of the target (surface the detector is directed towards) and detector; the detector is sensitive to radiation
incident on a planar surface (does not have to be horizontal), where the radiation emanates from all angles of a
hemisphere. Longwave radiation incident on the detector is calculated from the radiation balance measurement
and detector temperature measurement.
Summary of Contents for SL-510
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