Owner's Manual

THEORY OF OPERATION
Radiation that drives photosynthesis in plants is called photosynthetically active radiation (PAR). PAR is typically
defined as radiation in the 400-700 nm range, the visible wavelengths. Chlorophyll has distinct radiation
absorption characteristics and is a strong absorber of all photosynthetically active wavelengths. However, it does
not absorb quite as much green radiation as it does blue and red radiation, thus more green radiation is reflected
from or transmitted through leaves. This is what gives leaves their green color, and this feature shows up as a
characteristic hump near 550 nm in a leaf transmittance spectrum (or reflectance spectrum; leaf reflectance and
transmittance spectra are very similar). Chlorophyll absorbs very little radiation at wavelengths greater than 700
nm, thus there is a sharp transition from low to high transmittance near 700 nm (see leaf transmittance spectrum
in figure below). This characteristic feature of a leaf transmittance spectrum is called the red edge, where radiation
of wavelength greater than approximately 700 nm, termed near infrared (NIR), is not used for photosynthesis.
The Apogee Instruments MC-100 chlorophyll concentration meter exploits the different absorption characteristics
of chlorophyll at different wavelengths to determine relative chlorophyll content. Radiation from two light
emitting diodes (one centered at 653 nm and one centered at 931 nm) is passed through a leaf sample and
intensity is measured by paired detectors to determine transmittance (see inset in lower right-hand corner in
graph above). The ratio of transmittance at 931 nm to transmittance at 653 nm provides a measure of relative
chlorophyll content because 653 nm is a photosynthetically active wavelength within the chlorophyll absorption
range and 931 nm is a near infrared wavelength outside the chlorophyll absorption range, which serves as a
reference measurement to compensate for mechanical differences between leaves (e.g., tissue thickness). The