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GLOSSARY
GLOSSARY
Cosine Response The cosine error for the external sensor for typical applications is <1%.
External Sensor An external PAR sensor allows the LP-80 to make simultaneous above-
and below-canopy PAR measurements. This makes multiple PAR measurements under the
canopy in variable light conditions without having to keep moving the instrument in and out
of the canopy to update the above canopy PAR reference. The external sensor is also the PAR
standard to which the probe is calibrated.
Fraction of Beam Radiation (
F
b
)
F
b
is the ratio of direct beam radiation coming from the sun
to radiation coming from all ambient sources like the atmosphere or reflected from other
surfaces. A fractional beam radiation value is necessary for calculation of LAI using PAR
data. The LP-80 obtains this value by comparing the above-canopy PAR measurement to the
calculated value of potential incoming solar radiation at the location and zenith angle. The
current calculated
F
b
is displayed and updated at the bottom of the screen in the PAR tab.
Leaf Area Index (LAI) or (
L
) LAI is the area of leaves per unit area of soil surface.
It is a valuable measurement in helping to assess canopy density and biomass. The
LP-80 calculates LAI based on the above- and below-canopy PAR measurements along
with other variables that relate to the canopy architecture and position of the sun. These
variables are the zenith angle, a fractional beam measurement value, and a leaf area
distribution parameter for the particular canopy.
Leaf Distribution Parameter (
χ
)
χ
refers to the distribution of leaf angles within a canopy
and is the ratio of the length of the horizontal to the vertical axis of the spheroid described
by the leaf angle distribution of a canopy. It can also be measured as the ratio of the
projected area of an average canopy element (a leaf, for example) on a horizontal plane to its
projection on a vertical plane. The default value for
χ
is 1.0, which assumes the canopy angle
distribution to be spherical. Onions are a good example of a strongly vertical crop. For onions,
χ
would be about 0.7. On the other extreme, strawberries, a crop with a strongly horizontal
nature, would have a
χ
value of about 3.
gives some typical values for
χ
. In some cases a range of values is given, indicating
the variability that is to be expected for
χ
in any canopy. Fortunately, the value of LAI
computed is not strongly dependent on the value of
χ
chosen. The LP-80 uses a value of
χ
= 1.0 as its default.
