Daily Light Integral (DLI) As A Measurement To Manage Light Levels

For commercial growers, managing light levels involves measuring a number of light attributes throughout the day and through the seasons. This blog post explores just one of these important attributes – DLI or daily light integral.  We’ve been approached by multiple growers regarding this topic.  This work is currently underway by our researchers at the Heliospectra Plant & Light Research Lab in Sweden. There are many reasons why commercial growers manipulate light levels - including: photoperiod control, minimizing crop stress, and optimizing photosynthesis. One thing we all agree upon - light matters.

Light measurements

Widely used units for light intensity measurements are lux, footcandles or micromoles of photons per square meter per second (µmol/m2/s). These, however, give an idea about intensity of instantaneous light, meaning the amount of light recorded at the time the measurement is made. In most greenhouse environments light levels change throughout the day. Therefore instantaneous measurements may not provide an accurate information of the total amount of PAR your plants receive during the course of a day. This is where DLIcome in.

Daily light integral (DLI)

Daily light integral (DLI) is defined as the amount of photosynthetically active radiation (PAR) delivered over a 24-hour period. It is measured as the number of moles (particles of light) per day (mol.m-2.d-1 ), and often abbreviated to “moles/day” (or m/d) in trade journals. To put it simply - the total quantity of light delivered over the course of an entire day. So, DLI is a useful measurement for describing the light in a commercial grow facility.

What DLI is needed?

But what DLI is ideal to grow high-quality plants? The answer depends on the crop or species itself, as requirements of plants vary greatly. But, an often used minimum target inside a greenhouse is 10-12  mol.m-2.d-1 for healthy crops.

The Importance of DLI in Greenhouse Production

DLI is an important variable to control in every greenhouse because it influences plant growth, development, yield and quality. Lowering or increasing the DLI the plant receives has a direct affect on the plant - affecting branching, rooting, stem thickness and flowering. This is specifically important in some part of the world where many plants are subjected to less than ideal light levels in the winter months. Therefore, growers often use supplemental lighting such as traditional High Pressure Sodium (HPS) lamps or  new generation LED lights for increased control of the light output.

Measuring DLI

Measuring DLI over a growth season and comparing the results could help you as a grower to decide which crop varieties work best for your location and optimize the use of supplemental lighting in the greenhouse. Measuring light levels every hour is not something all growers have time to do, but there are commercial alternatives available. By using a light sensor such as a Quantum PAR sensor connected to a data logger you are able to record light levels at set intervals and the total output for the day. From this info you can then calculate the DLI. If connected to a control system such as those from Argus, Hoogendoorn, or other similar products, you as a grower can easily monitor and automatically calculate the DLI, making sure your plants get the light they need.

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