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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

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photon flux (PPF): 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 expressing PPF as µmol m

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is preferred.

Sensors that measure PPF 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 PPF measurement over plant canopies in

outdoor environments or in greenhouses and growth chambers, and reflected or under-canopy

(transmitted) PPF 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. Sensors are potted solid with no internal air space, and are designed for

continuous PPF measurement in indoor or outdoor environments. SQ series sensors output an analog

voltage that is directly proportional to PPF under sunlight (e.g., model SQ-110) or electric lights (e.g.,

model SQ-120). 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.