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), the sum of photons from 400 to 700 nm in units of micromoles per square meter per second (µmol
m
-2
s
-1
, equal to microEinsteins m
-2
s
-1
). While microEinsteins 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. Daily total PPFD is typically
reported in units of moles of photons per square meter per day (mol m
-2
d
-1
) and is often called daily light integral
(DLI).
The acronym PPF is also used and refers to the photosynthetic photon flux. The acronyms PPF and PPFD refer to
the same variable. Both terms are used because there is not a universal definition of the term flux. Flux is
sometimes defined as per unit area per unit time and sometimes defined as per unit time only. PPFD is used in this
manual.
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 full-spectrum quantum sensors (model MQ-500 and SQ-500 series) is 1.25. The MQ-510 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).
