3
DEFINITIONS
AccuPAR LP-80
3
Definitions
The AccuPAR LP-80 uses several variables to calculate Leaf Area
Index, and displays values for these variables on the screen as mea-
surements are taken. This chapter will discuss these variables and
their definitions. If you are not familiar with them, we recommend
that you review this chapter before proceeding with measurements.
PAR
PAR (photosynthetically active radiation) is the radiation in the 400
to 700 nanometer waveband. It represents the portion of the spec-
trum which plants use for photosynthesis. Under a plant canopy,
radiation levels can vary from full sun to almost zero over the space
of a few centimeters. Therefore, reliable measurement of PAR re-
quires many samples at different locations under the canopy.
The AccuPAR measures PAR either manually or in unattended log-
ging mode. Intercepted PAR data can be used for determining im-
portant parameters of canopy structure and for the calculation of
LAI. An external point sensor may be used to collect instantaneous
above canopy PAR measurements when sampling under or within a
canopy. You also have the option of segmenting the probe to reflect
spatial changes in the plant canopy. This is useful when evaluating
discontinuous and irregular canopies, or to limit the number of active
sensors along the probe.
Tau (
τ
)
Tau is another variable in the LAI inversion equations. It is de-
fined as the ratio of below canopy PAR measurements to the most
recent above canopy PAR value. It is measured automatically by the
instrument, based upon the PAR readings you make. The current
τ
value is displayed and updated in the lower left corner of the screen
in the PAR menu. Chapter 9 provides further explanation of the
significance of Tau.
8
