Apogee SQ-212 Owner's Manual - Page 4

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Apogee SQ-212 Owner's Manual
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
s
, equal to microEinsteins per square meter per second)
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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
s
is preferred.
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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 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 PPFD measurement in indoor or outdoor
environments. SQ series sensors output an analog voltage that is directly proportional to PPFD under sunlight (e.g., model
SQ-215) or electric lights (e.g., model SQ-225). 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.