4
INTRODUCTION
Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total
radiation across a range of 400 to 700 nm. PAR is often expressed as photosynthetic photon flux density (PPFD): photon flux
in units of micromoles per square meter per second (µmol m
-2
s
-1
, equal to microEinsteins per square meter per second)
summed from 400 to 700 nm (total number of photons from 400 to 700 nm). While Einsteins and micromoles are equal (one
Einstein = one mole of photons), the Einstein is not an SI unit, so expressing PPFD as µmol m
-2
s
-1
is preferred.
The acronym PPF is also widely used and refers to the photosynthetic photon flux. The acronyms PPF and PPFD refer to the
same parameter. The two terms have co-evolved because there is not a universal definition of the term “flux”. Some
physicists define flux as per unit area per unit time. Others define flux only as per unit time. We have used PPFD in this
manual because we feel that it is better to be more complete and possibly redundant.
Sensors that measure PPFD are often called quantum sensors due to the quantized nature of radiation. A quantum refers to
the minimum quantity of radiation, one photon, involved in physical interactions (e.g., absorption by photosynthetic
pigments). In other words, one photon is a single quantum of radiation.
Typical applications of quantum sensors include incoming PPFD measurement over plant canopies in outdoor environments
or in greenhouses and growth chambers, and reflected or under-canopy (transmitted) PPFD measurement in the same
environments.
Apogee Instruments MQ series quantum meters consist of a handheld meter and a dedicated quantum sensor that is
integrated into the top of the meter housing (MQ-100) or connected by cable to an anodized aluminum housing (MQ-200
and MQ-300 series). Integrated and separate sensors consist of a cast acrylic diffuser (filter), photodiode, and are potted solid
with no internal air space. MQ series quantum meters provide a real-time PPFD reading on the LCD display and offer
measurements for both sunlight and electric light calibrations (menu selectable) that determine the radiation incident on a
planar surface (does not have to be horizontal), where the radiation emanates from all angles of a hemisphere. MQ series
quantum meters include manual and automatic data logging features for making spot-check measurements or calculating
daily light integral (DLI).
