F. Glossary
Beam fraction - the fraction of the Total incident PAR in the Direct beam.
Beam Fraction Sensor (BFS) - consists of two PAR sensors and a shade ring, used for measuring Direct and Diffuse light above the canopy.
Beer’s law - a general law describing transmission through an absorbing medium. The intensity falls off exponentially with distance through the medium.
Cosine response - the response of a sensor 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).
CSV (Comma Separated Variable) a file format intended for importing into spreadsheet or database programs. Fields are separated by commas, text is enclosed in quotes.
Data Collection Terminal - the Psion Workabout handheld computer, used for driving the SunScan probe, and presenting and storing the results.
Diffuse light - light scattered in the atmosphere. It is treated as coming from all parts of the sky with equal intensity i.e. a Uniform Overcast Sky.
Direct beam - light coming directly from the sun, with no scattering. Usually treated as if it comes from a point source.
ELADP - see Leaf Angle Distribution
Emulator - a setting in the SunData software that generates random results, regardless of whether a SunScan probe is connected. Useful for learning to use the software.
GMT - Greenwich Mean Time, also called Universal Time (UT). The standard time used for astronomical measurements and calculations.
Hemispherical response - the response of the sensor is equal for all light rays coming from above the plane of the sensor surface, independent of angle.
LAD - see Leaf Angle Distribution. LAI - see Leaf Area Index.
Leaf absorption - the fraction of intercepted PAR that is actually absorbed by the leaf. The remainder is reflected or scattered.
Leaf Angle Distribution - a way of describing the distribution of orientations in space of the canopy elements. We model this using the Ellipsoidal Leaf Angle Distribution, which describes the distribution of canopy elements as in the same proportions as the surface of an ellipsoid. Using this model, a wide range of different canopy types can be described by a single parameter, the Ellipsoidal Leaf Angle Distribution Parameter (ELADP), which is the ratio of the horizontal to vertical axes of the ellipsoid. An ELADP much greater than 1 describes a canopy of mostly horizontal leaves, an ELADP near 0 describes a canopy of mainly vertical leaves.
Leaf Area Index (LAI) - the surface area of leaf per unit of ground area (assuming leaves are flat, and including only one side of each leaf). Instruments like the SunScan cannot differentiate between leaf and stem, so could more correctly be said to estimate Plant Area Index.
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