Spectrum Technologies Field Scout CM1000 Operation Manual page 19

Like the ambient light sensors, the target leaf should
always be held perpendicular to the sun's rays to
maximize light reflectance into the meter's lens from the
leaf surface. All samples must be taken against the same
background. It is preferable to hold the leaf with one's
hand, not having anything immediately behind the leaf. If
that is not practical, a flat black background should be
used behind the leaf samples. With light-colored or
glossy backgrounds, one may not only be sensing the
reflective properties of the leaf but also the reflective
properties of the background shining through the leaf.
It is preferable to sample with the leaf still attached to the
plant. The effect on light absorption or reflection due to
the extreme water stress of detached leaves is not known.
Canopy-level measurements are recommended by
Spectrum Technologies for fine-textured turf grass only .
When making measurements on turf grass, an adequate
measuring procedure should include having the sun, the
target, and the meter in approximately the same
relationship with each other for each reading. The problem
with trying to measure an entire plant canopy is that not
all leaves in a coarse canopy are exposed to the same
ambient light. Because the light reflected from a canopy
is being reflected in many different directions and that
reflected light is being compared to the available ambient
light, it is absolutely necessary for the both the entire
sample area and the ambient light sensors to be subjected
to the same quantity and quality of light.
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