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). Albedo is the ratio of reflected to incoming global
shortwave radiation and varies from zero to one. Materials with a high albedo reflect most solar radiation, and
materials with low albedo absorb most solar radiation.
An albedometer is an instrument designed to measure albedo, and consists of two pyranometers, which separately
measure incoming and reflected shortwave irradiance. The SP-722 upward- and downward-facing pyranometers
are blackbody thermopile pyranometers sensitive to most of the solar spectrum, thus eliminating spectral errors
associated with silicon-cell pyranometers. The upward-facing pyranometer is designed to measure incoming global
shortwave irradiance and combines an acrylic diffuser with the blackbody detector. The downward-facing
pyranometer is designed to measure reflected shortwave irradiance from terrestrial surfaces and combines a
sintered quartz diffuser with the blackbody detector. Both sensors have calibrations traceable to Class A blackbody
thermopile pyranometers traceable to the world radiation reference in Davos, Switzerland. Specifications for both
models compare favorably to specifications for World Meteorological Organization (WMO) moderate and good
quality classifications and specifications for International Organization of Standardization (ISO) Class C and Class B
classifications.
Typical applications of pyranometers include incoming and reflected shortwave radiation measurement in
agricultural, ecological, and hydrological weather networks and solar panel arrays. Albedo measurements indicate
the reflectivity of materials with respect to shortwave radiation, and are used to study climate and weather,
monitor bifacial solar panels, and understand heat retention in urban and architectural settings.
Apogee Instruments model SP-722 albedometers are two-component instruments, with individual upward- and
downward-looking pyranometers. Each albedometer consists of a thermopile detector and diffuser mounted in an
anodized aluminum housing. Each radiometer is heated (if desired, heaters can be turned on and off) to minimize
the effects of dew, frost, snow, and ice on the filter and sensor head. Analog signals from each albedometer are
measured with an onboard voltmeter and converted to a digital value. That value can then be accessed via a
Modbus RTU interface over an RS-232 or RS-485 serial connection. This eliminates the need for multiple analog
datalogger channels to make the four-component measurement of net radiation. SP-722 net radiometers are small
and lightweight to facilitate rapid and simple mounting.
