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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

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(PPFD): photon flux in units of micromoles per square meter per second (µmol m

, 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

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expressing PPFD as µmol m

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 variable. 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 SQ-100X series quantum sensors consist of a cast acrylic diffuser (filter), interference filter,

photodiode, and signal processing circuitry mounted in an anodized aluminum housing, and a cable to connect the

sensor to a measurement device. Sensors are potted solid with no internal air space, and are designed for

continuous PPFD measurement in indoor or outdoor environments. The voltage signal from the sensor is directly

proportional to radiation incident on a planar surface (does not have to be horizontal), where the radiation

emanates from all angles of a hemisphere.