4
INTRODUCTION
Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total
radiation between 400 and 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.
Sensors that measure PPFD are often called quantum sensors due to the quantitated 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.
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.
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 SQ series quantum sensors consist of a cast acrylic diffuser (filter), photodiode, and signal processing
circuitry mounted in an anodized aluminum housing, and a cable to connect the sensor to a measurement device. SQ-500
series quantum sensors are designed for continuous PPFD measurements in indoor or outdoor environments. The sensor is
potted solid with no internal air space. The signal from the sensor is directly proportional to radiation incident on a flat
surface where the radiation comes from all angles of a hemisphere.