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
-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). 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
-2
s
-1
is preferred.
The acronym PPF is also widely used and refers to the photosynthetic photon flux. The acronyms PPF and PPFD refer to the
same parameter. The two terms have co-evolved because there is not a universal definition of the term “flux”. Some
physicists define flux as per unit area per unit time. Others define flux only as per unit time. We have used PPFD in this
manual because we feel that it is better to be more complete and possibly redundant.
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.
Quantum sensors are increasingly used to measure PPFD 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 original quantum sensors (model MQ-200 and SQ-200
series) is 1.08. The MQ-210 quantum 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.
MQ meters consist of a handheld meter and a dedicated quantum sensor that is connected by cable to an anodized
aluminum housing. Sensors consist of a cast acrylic diffuser (filter), photodiode, and are potted solid with no internal air
space. MQ series quantum meters provide a real-time PPFD 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 quantum meters include manual and automatic data logging features for making spot-check measurements or
calculating daily light integral (DLI).
