17
Light Management
Natural sunlight is the best source of light for measuring
plant reflectance because both wavelengths are present in
approximately equal quantities and the quantity of light
remains relatively constant. With artificial light sources,
the user must be aware that different light sources emit
light having different qualities. For example, florescent
lights emit more blue light and incandescent lights emit
more red light. The light sensors in the CM1000 NDVI
meter are sensitive enough to detect the cooling of the
filament in an incandescent light bulb as the 120 volt AC
current alternates from positive to negative and back again
at the rate of 60 Hz. The sensitivity of the meter to light
quality enables the meter to estimate plant health.
When using artificial light, the CM1000 NDVI meter
senses light quantities using a time period that takes into
consideration the 60 Hz alternating current used by
electric power companies in the U.S. The meter must be
reconfigured to be used in Europe and other regions where
50 Hz AC power is used. Compared to light generated
with AC current, DC light sources provide a steady light
output. In some situations, it may be easier to use a DC
light source than to concern oneself with the frequency
qualities of AC.
It is important to develop a standard method for taking
readings with the CM1000 NDVI meter. Ideally, the sun
is always at the user’s back and the line between the
sample and the sensors is approximately parallel to the
sun’s rays. The ambient light sensors are behind the white
Teflon dots in the black rectangular box on top of the
yellow case.
The Teflon dots diffuse the ambient light
and protect the ambient light sensors. The user can tilt the
black box so that the ambient light sensors are directly
facing the sun, thereby enhancing the capture of ambient
light.
