Daily Light Integral (DLI) is the amount of Photosynthetic Active Radiation (PAR) photons accumulated in a square meter each day. In other words, it is a measurement of how much light is reaching a particular area, in terms of PAR, over a 24-hour period. DLI is expressed as mols of light per square meter per day (mol·m2·d) and can be measured by a sensor, such as a light quantum sensor, connected to a data logger. It is important to note that DLI is measured in mols while PAR is measured in μmols. As such, calculations need to be made when determining the PPF needed to achieve a specific DLI.
DLI is most often used when describing how much light plants will need each day. This is especially useful for greenhouse applications due to the nature of using ever-changing sunlight exposure. Keeping an eye on DLI helps the greenhouse grower maintain the light needed for their crop given their specific region. Below is an example of how the DLI changes across the USA from month to month.