Created by our team of experts, the P.L. Light Systems glossary provides an overview of common terms related to supplemental lighting. Here, we aim to further growers’ understanding of these terms in order to make an informed decision when purchasing horticultural lighting.

If you believe a definition is missing from the list below, please contact us.


Daily Light Integral (DLI) refers to the cumulative amount of PAR light received in a 1 square meter (10.8 sq. ft.) area each day, or mol·m–2·d–1. Similar to measuring rainfall, DLI cannot be determinedfrom an instantaneous reading. Just like a rain gauge is used to measure the total amount of rain that was received in a particular area during a 24-hour, DLI measures the accumulation of PAR light over the course of a day.

High Intensity Discharge (HID) lamps use an electric arc between two electrodes to produce an intensely bright light. Mercury, sodium or metal halide gas acts as the conductor.

Light Emitting Diodes (LEDs) are semiconductor devices that produce visible light when an electrical current passed through them. LEDs are a type of Solid State Lighting (SSL).

Light Intensity refers to the amount of mols being delivered to the plant at the canopy level.

Photomorphogenesis can be defined as light-regulated plant development, morphology and/or cell structure and function.

Photoperiodic Lighting refers to the number of hours of light in a 24-hour period (photoperiod) that controls flowering of both short-day and long-day crops. For example, when sunrise is at 6am and sets at 8pm, there is a 14 hour photoperiod (actually a bit longer as plants perceive light at twilight). Scientific evidence suggests that it is actually the uninterrupted period of darkness that controls flowering responses.

Micromoles is a measurement of PAR photons delivered per square metre per second (μmol/m2/s).

Mols is a measurement of PAR photons delivered per square metre per day.

Night Interruption Lighting is the practice of interrupting the dark period in the middle of the night, so that plants will perceive a short night length and, therefore, a long day photoperiod.

Photosynthesis is defined as “the process in green plants by which carbohydrates are synthesized from carbon dioxide (CO2) and water (H20)—using light as an energy source.” It is essentially an energy transfer reaction. The carbohydrate serves as an energy source that is stored in the plant and can be moved to the roots , fruit, leaves, or any place in the plant where growth is occurring. It is used as a building block in growth and to provide energy for general plant development. Photosynthesis takes place only in the green part of the plant and only when all the elements required to facilitate the process (light, water and carbon dioxide) are present.

Photosynthetic Active Radiation (PAR) is a term that refers to the amount of light within the 400- to 700-nanometer wavelength range. This is the range of light within the visible light range that is used by plants for photosynthesis. Since plant photosynthesis is the main use of supplemental light in the greenhouse, light sources are often rated on the amount of PAR light delivered to the plant surface.

Photosynthetic Photon Flux (PPF) expresses the total amount of PAR produced by a light source per second, expressed by micromoles per second (μmol/s). Although PPF is an important metric to calculate the efficiency of a luminiare, it does not tell us how much light is distributed to the crop canopy.

Photosynthetic Photon Flux Density (PPFD) is a measurement of the amount of PAR that is distributed to, and arrives at, the crop canopy. PPFD is measured in micromoles per square meter per second (μmol/m2/s). It is important to note that PPFD is a ‘spot’ measurement on the crop canopy, it is important that the average PPFD at several ‘spots’ are taken under a defined height.

Supplemental Lighting refers to artificial lighting used by growers to increase the DLI in their facility in the absence of adequate natural light.