Glossary
Azimuth angle -
the horizontal angle between the sun, or a light source
simulating the sun, and North, increasing in the direction NESW. The
SPN1 does
not
have to be aligned towards North for correct operation
(unlike most other devices).
Beam fraction
-
the fraction of Total incident radiation in the Direct beam.
Cosine response -
the response of a sensor in which the sensitivity to a
ray of light is proportional to the cosine of the angle of incidence of the ray
(measured from the perpendicular to the sensor surface).
Diffuse light -
light from parts of the sky other than directly from the sun,
from scattering in the atmosphere or reflection from clouds.
Direct beam -
light coming directly from the sun, with no scattering.
Usually treated as if it comes from a point source.
Energy -
radiation measured with equal sensitivity to the energy content
regardless of wavelength. It is measured in units of
W.
m
-2
.
The normal
daylight maximum is a little over 1000 W.m
-2
.
Sunshine -
the threshold for
bright
sunshine, defined by the WMO, is
120 W m
-2
of
Direct beam solar radiation, measured perpendicular to the
direction of the beam. It is defined this way in order to ensure historical
continuity with Campbell-Stokes recorders.
Total radiation -
the sum of Direct beam and the Diffuse light.
Zenith angle -
the angle between the centre of the sun and the point
directly overhead.
SPN1 User Manual v1.0
Glossary
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