Daily Light Integral: Measuring Light for Plants

PQ-500 Package

The following graph illustrates the year-round variability of global horizontal irradiance at Apogee Instruments in Logan, Utah.

Seasonal Comparison of Solar Zenith Angles in Logan,UT

Based on this variability it’s not hard to imagine that certain greenhouse crops and even nonnative houseplants are experiencing less than ideal light levels in the winter months. Many greenhouses will provide supplemental lighting to better control the photoperiod that each crop receives. Since growth is determined by the total integrated amount of photons that a plant receives each day, this daily light integral (DLI) must be measured to accurately predict plant growth. A basic quantum meter can be used to measure the instantaneous photosynthetic photon flux density (PPFD) in micromoles of photons per meter squared per second (µmol m-2 s-1), but ultimately it is the sum total of moles of photons per meter squared per day (mol m-2 d-1) that must be quantified.

Under constant electric lights the conversion from µmol m-2 s-1 to mol m-2 d-1 is straightforward. It is determined by multiplying the instantaneous PPFD by the number of seconds in the photoperiod. One of the most highly used section of our website shows examples of this calculation: https://www.apogeeinstruments.com/conversion-instantaneous-ppf-to-integrated-ppf/.

It’s not possible to use a constant multiplier with variable lighting, so a quantum meter that has an integrating capability is necessary to determine the daily light integral under variable lighting. The Apogee MQ series meter is designed to measure and store this value each day. Once the unit is in LOG mode and deployed it will automatically log the DLI every. DLI collected by the MQ series meters can be viewed using the ApogeeAMS software that is included with the AC-100 Communication Cable. Click here for a video guide on how to set up log mode on the MQ meters, and use the AC-100 cable and AMS software. Other methods include using the logged data over a 24 hour period and calculating the average µmol m-2 s-1. Once you know the average over a 24 hour period you will multiply it by 0.0864 to get the mols/day. This factor is the total number of seconds in a day divided by 1,000,000. This method can be found here.


{Impatiens Walleriana Photo by http://www.hear.org/starr/}

Once an accurate measurement is made it is important to interpret that measurement – it's nice to know that those impatiens are getting 10 mol m-2 d-1, but is this enough to grow healthy plants? There are a number of online resources available that will provide optimal lighting levels for various plants and food crops. One was developed by Purdue University: http://www.extension.purdue.edu/extmedia/HO/HO-238-W.pdf. Pages 5-7 include a table listing species of plants and the associated range of acceptable daily light integral values.