AT BF5, User Manual, Page: 48 / 50

48  Glossary BF5 User Manual v 2.3
Glossary
Azimuth angle - the horizontal angle between the sun, or a light source
simulating the sun, and North, increasing in the direction NESW. The BF5
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.
Beam Fraction Sensor (BFS) - A device for measuring Direct and
Diffuse light above the canopy. Types BF3 and BF5 consist of seven PAR
sensors and an acrylic dome with stripes. They also measures sunshine
hours.
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
.
Illuminance - radiation in the visible range (400nm – 700nm) measured
with the same spectral sensitivity as the human eye.
It is measured in units of lux or klux (=1000 lux).
The normal daylight maximum is about 150 klux.
PAR - Photosynthetically Active Radiation is visible light of wavelength
400nm - 700nm, with uniform sensitivity to the number of photons
received, regardless of their wavelength. It is measured in units of

mol.m
-
2 .s -1 (micromoles per square metre per second) or formerly

E (micro-
Einstein).
The normal daylight maximum is a little over 2000

mol.m -2 .s -1 .
SunData - the software used to drive the SunScan probe and calculate
and store the results.
It runs on a PDA running Windows Mobile 5 or later.
SunScan probe - the long light sensitive wand and handle used for light
readings within the canopy.
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.