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
density (PPFD) 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). 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.