Apogee Instruments SI Series Benutzerhandbuch - Seite 4
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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. Terrestrial surfaces (e.g., soil, plant canopies,
water, snow) emit radiation in the mid infrared portion of the electromagnetic spectrum (approximately 4-50 µm).
Infrared radiometers are sensors that measure infrared radiation, which is used to determine surface temperature
without touching the surface (when using sensors that must be in contact with the surface, it can be difficult to
maintain thermal equilibrium without altering surface temperature). Infrared radiometers are often called infrared
thermometers because temperature is the desired quantity, even though the sensors detect radiation.
Typical applications of infrared radiometers include plant canopy temperature measurement for use in plant water
status estimation, road surface temperature measurement for determination of icing conditions, and terrestrial
surface (soil, vegetation, water, snow) temperature measurement in energy balance studies.
Apogee Instruments SI series infrared radiometers consist of a thermopile detector, germanium filter, precision
thermistor (for detector reference temperature measurement), and signal processing circuitry mounted in an
anodized aluminum housing, and a cable to connect the sensor to a measurement device. All radiometers also
come with a radiation shield designed to minimize absorbed solar radiation, but still allowing natural ventilation.
The radiation shield insulates the radiometer from rapid temperature changes and keeps the temperature of the
radiometer closer to the target temperature. Sensors are potted solid with no internal air space and are designed
for continuous temperature measurement of terrestrial surfaces in indoor and outdoor environments. SI-100
series sensors output an analog voltage that is directly proportional to the infrared radiation balance of the target
(surface or object the sensor is pointed at) and detector, where the radiation balance between target and detector
is related to the temperature difference between the two.