Apogee SQ-644 Owner's Manual - Page 4

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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 almost universally quantified as photosynthetic photon flux

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density (PPFD) 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). However,

ultraviolet and far red photons outside the defined PAR range of 400-700 nm can also contribute to photosynthesis

and influence plant responses (e.g., flowering).

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. Sensors that function like

traditional quantum sensors, but measure a wider range of wavelengths can be thought of as an 'extended range'

quantum sensor.

Typical applications of traditional 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. The Extended Range PFD Sensor detailed in this manual uses a detector

that is sensitive to radiation up to about 1100 nm, well beyond the range of wavelengths that influence

photosynthesis and plant responses. This means this particular sensor should only be used for photon flux

density measurements under LEDs.

Apogee Instruments SQ-600 series Quantum Light Pollution 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-600 series sensors are designed for continuous photon flux density

measurements in indoor environments under LEDs. The SQ-640 Quantum Light Pollution models output a voltage

that is directly proportional to photon flux density. The voltage output by 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.