Horticultural lighting terms guide

It can be challenging to know which horticultural lighting terms and measurements to focus on when researching commercial grow light options. Continuing from our previous post on Lumens, Lux and PAR, let’s dive deeper into the light measurements important for plant growth so that you can make the best lighting choice for your production.

Photosynthetically Active Radiation (PAR)
PAR is the waveband of the light spectrum that plants use for photosynthesis. PAR is not a measurement of light, but rather represents the waveband in the light spectrum which is used by plants for photosynthesis. That’s why we say, “PAR is for plants”.

Lumens are a measurement of the amount of light that’s visible to the human eye. Lux is the measurement of the intensity of lumens (exe. 1 lux equals 1 lumen per square meter). These measurements shouldn’t be used in horticultural applications since they don’t measure the light that plants use for photosynthesis. That’s why we say, “Lumens are for humans”.

Plants perceive light as photosynthetically active radiation (PAR). Humans perceive light as lumens.

Photosynthetic Photon Flux (PPF)
PPF is the amount of PAR a light source produces. PPF is measured in microles per second or μmol·s-1. This is an important measurement to look for when comparing horticultural light fixtures. PPF tells you how much light your fixture will produce that your plants can use for photosynthesis.

Photosynthetic Photon Flux Density (PPFD)
PPFD is the amount (also referred to as intensity) of PAR light that lands on a square meter each second. PPFD is measured in micromoles per square meter per second (μmol·m-2·s-1). In other words, PPFD is how much PPF is hitting each square meter of your crop at any given second. It is important to look at PPFD when planning your grow light installation. Adjusting your fixtures’ height and density will affect the PPFD at your crop canopy.

PPFD is the amount of PAR photons that land on a square meter per second.

Daily Light Integral (DLI)
DLI is the amount of PAR that lands on a square meter surface over a 24-hour period. DLI is measured in moles of light per square meter per day (mol·m-2·d-1). You can think of this as the accumulation of PPFD over a full day. In human terms, you can think of DLI as a plant’s daily caloric intake of light.

Learn more about DLI in LumiGrow's article on Understanding Light Using Daily Light Integral.

Photosynthetic Photon Efficacy (PPE)
PPE measures a light fixture’s efficiency for converting electrical energy into PAR light. This is expressed as micromoles per Joule (μmol·J-1). PPE is an important measure for comparing the efficiency of different manufacturers’ fixtures.

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