Delta-T Devices SunScan SS1 User Manual page 34

the probe. Don't hold the shade too close to the probe - otherwise it will
cut out some of the diffuse light as well.
SunData looks at the readings from the photodiodes and uses the lowest
value to calculate the Diffuse component of the incident light. It uses the
highest photodiode values to calculate the incident Total, and uses these
two values to calculate and display the Beam Fraction:
The Beam Fraction reading is used to increase the accuracy of the
calculation of Leaf Area Index, as explained in the LAI Theory section.
However, this is a secondary effect, and you should not worry unduly
about the second decimal place for the Beam Fraction value.
Direct and Diffuse components
Assuming that you make your above-canopy measurements on the
SunScan (with or without a BF5), then the next table summarises whether
you need to measure the Direct and Diffuse components of the incident
light.
Type of study
Fractional interception
LAI
PAR mapping
Canopy type and BF5 practicalities
Canopy type is the next variable to be considered. As a general guide, the
above-canopy reference measurements should be made close to, or
above, the position of the SunScan probe.
Canopy type
Low
Low
High
High
High
34  What to Measure and How
Incident PAR
Total only
Yes
No
Yes
Options
BF5 connected, with
extension cables
or radio link
No BF5
Devise a portable BF5
mount. Use extension
cables or radio link
Use clearings to get out
from under the canopy. (No
BF5 required)
Use independent sensor for
above canopy PAR
Incident PAR
Direct & Diffuse components
No
Yes
Possibly
Comments
Radio link is generally best, long
cables need management
Slower. Needs slow-changing
light conditions.
Good when possible
Needs steady light conditions.
Clearing light may be partly
shaded
Needs slow-changing light, and
possibly time average readings.
LAI readings not available
SS1 User Manual v3.3