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-301X line quantum consists of a separated sensor bar with 10 sensors connected to a
hand-held meter via cable. The SQ-301X line quantum consists of the sensor bar with 10 sensors and pre-tinned
pigtail leads. The sensor housing design features an integrated bubble level to ensure level deployment. The
sensors consist of a cast acrylic diffuser (filter) and photodiode, and the sensors are potted solid with no internal
air space. The meter provides a real-time PPFD reading on the LCD display and offers 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 X series line
quantum meters include manual and automatic data logging features for making spot-check measurements or
calculating daily light integral (DLI).
