Apogee Instruments MQ-650 Manual del usuario - Página 4
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INTRODUCTION
Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and, historically, is defined
as total radiation across a range of 400 to 700 nm. PAR is almost universally quantified as photosynthetic photon
-2
-1
flux density (PPFD) in units of micromoles per square meter per second (µmol m
s
, 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 photosynthetically active radiation (ePAR) sensor detailed
in this manual uses a detector that is sensitive to radiation from 380-760 nm, which allows it to measure photons
from UV and Far-red.
Quantum sensors are increasingly used to measure PFD underwater, which is important for biological, chemical,
and physical processes in natural waters and in aquariums. When a quantum sensor that was calibrated in air is
used to make underwater measurements, the sensor reads low. This phenomenon is called the immersion effect
and happens because the refractive index of water (1.33) is greater than air (1.00). The higher refractive index of
water causes more light to be backscattered (or reflected) out of the sensor in water than in air (Smith,1969; Tyler
and Smith,1970). As more light is reflected, less light is transmitted through the diffuser to the detector, which
causes the sensor to read low. Without correcting for this effect, underwater measurements are only relative,
which makes it difficult to compare light in different environments. The immersion effect correction factor for
Apogee ePAR sensors (model MQ-610 and SQ-610 series) is 1.25. The MQ-650 ePAR meter is designed for
underwater measurements, and already applies the immersion effect correction factor to the meter's readings
through firmware. The meter consists of a waterproof quantum sensors attached via waterproof cable to a
handheld meter. Note: The handheld meter is not waterproof, only the sensor and cable are waterproof.
Apogee Instruments MQ-650 meters consist of a handheld meter and a dedicated sensor that is connected by
cable to an anodized aluminum housing. SQ-600 series ePAR sensors consist of a cast acrylic diffuser (filter),
photodiode, signal processing circuitry mounted in an anodized aluminum housing, and are potted solid with no
internal air space. MQ series ePAR meters provide a real-time PFD reading on the LCD display, that determine the
radiation incident on a planar surface (does not have to be horizontal), where the radiation emanates from all
angles of a hemisphere. MQ series ePAR meters include manual and automatic data logging features for making
spot-check measurements or calculating daily light integral (DLI).