8
Cosine Response
Mean cosine response of eleven Apogee silicon-
cell pyranometers (
error bars represent two
standard deviations above and below mean
).
Cosine response measurements were made
during broadband outdoor radiometer
calibrations (BORCAL) performed during two
different years at the National Renewable Energy
Laboratory (NREL) in Golden, Colorado. Cosine
response was calculated as the relative difference
of pyranometer sensitivity at each solar zenith
angle to sensitivity at 45° solar zenith angle. The
blue symbols are AM measurements, the red
symbols are PM measurements.
Directional, or cosine, response is defined as
the measurement error at a specific angle of
radiation incidence. Error for Apogee silicon-
cell pyranometers is approximately ± 2 % and
± 5 % at solar zenith angles of 45° and 75°,
respectively.
