Cost-effective tools for making research-grade measurements of daily light integral (DLI)
Daily Light Integral (DLI) is the total number of photosynthetically active radiation (PAR) photons (traditionally those between 400-700 nm) incident on an area over the course of a day. DLI is a function of both the intensity and duration of the photosynthetic light falling on the target over a day and is most commonly expressed as moles of light per square meter per day or mol m-2 d-1.
Specific plants and other photosynthetic organisms, like corals, require different daily light integral levels to achieve optimum growth. There are many resources available that list different DLI recommendations. Some of our favorite resources on daily light integral requirements for plants come from Dr. James Faust
and Dr. Erik Runkle
, and from Dana Riddle
for growing coral.
To make an accurate measurement of the DLI during a day with changing light levels, a high-quality quantum sensor must be used and coupled with a data recorder to log and sum all the measurements from the day. A quantum sensor, also known as a PAR or PPFD (photosynthetic photon flux density) sensor, measures the intensity of photosynthetic light incident on a target. Almost universally, photosynthetic light is defined as equal weighting of all photons between 400-700 nm. Newer research is showing that photons between 380-750 nm are also photosynthetic and should be counted towards DLI. Apogee Instruments sells meters to measure both of these ranges.
To determine DLI of an area illuminated by a light source with a constant output and known duration, an instantaneous PAR measurement can be made and used to calculate DLI:
DLI = PAR * (number of hours) * (3600 s hr-1 ) / 1,000,000 µmol
where PAR is the instantaneous measurements in units of µmol m-2 s-1, number of hours is the total number of hours the light is on during a 24 hour period, 3600 is the number of seconds in an hour, and 1,000,000 is the number of µmol in one mol. For example, if the instantaneous PAR from a specific light source is 600 µmol m-2 s-1 and the light is on for 12 hours, the DLI is 25.9 mol m-2 d-1:
DLI = 600 µmol m-2 s-1 * (12 hours) * (3600 s hr-1 ) / 1,000,000 µmol = 25.9 mol m-2 d-1
Calculation of DLI from an instantaneous PAR measurement relies on the assumption that the light source is constant while it is on. If there is variability in the light source, then calculation of DLI from an instantaneous PAR measurement will be incorrect.
Apogee offers a few different options for making research-grade daily light integral measurements
The PQ-500 package couples our SQ-500 Full-spectrum 400-700 nm quantum sensor with our new µCache device (microCache) at a promotional price. The µCache continually records the PAR levels measured by the sensor and then transmits the data to your smart device that is then beautifully graphed in the app. Raw data can then be exported to your computer via email.
These packages function similar to the PQ-500, but connects to the SQ-610 ePAR sensor, which measures photon intensity between 380-760 nm.
Apogee handheld meters are a little older technology than the µCache device and provide portable, self-contained portable measurements without needing a phone. The MQ-500 features a handheld meter connected to a Full-spectrum 400-700 nm sensor. The MQ-610 features a handheld meter connected to an ePAR 380-760 nm sensor. Measurements can be read in real time or automatically recorded over time. This data can then be downloaded to a computer for computing the DLI using an AC-100
These meters operate similarly to the MQ-500, but have a slightly lower-quality, lower-cost sensor that works well under broadband sources like the sun and HPS, but doesn't do as well measuring under LEDs.
Each of these options is shown and described in more detail below. If you still have questions about which option is right for you, please contact Apogee Instruments or one of our many fine distributors.