What is PAR & DLI?

Caring for your grow operation involves understanding how your plants operate so that you can select the right grow light to enhance your yield. PAR and DLI are two terms that measure the quantity and quality light that reaches your cannabis plants. Read on to learn how to utilize these two terms to optimize your farm.

What is PAR?

& Why Is It Important?

Photosynthetically Active Radiation (PAR) is the amount of light that most effectively stimulates photosynthesis in plants. Not all light can reach the cycle of photosynthesis, which requires light in the 400 to 700 nanometer (nm) wavelength range. Throughout the year, PAR changes per season and fluctuates depending on the location (specifically, latitudes) and time of day.

Each crop species has varying optimal light intensities, based on operation size, room temperature/humidity, room dimensions, and other factors, that maximizes photosynthesis and plant growth. Too much or too light can negatively affect the growth, quality, and yield of a crop.

Why is PAR Important?

Plants get their food from light converting water, carbon dioxide, and minerals into energy: a process also known as photosynthesis. To ensure that plants get this light, it is important to monitor PAR.

How is PAR measured? Millimoles of light energy per square meter.

PAR Values:

• PAR values range from 0 to 3,000 millimoles per square meter.
• At night, PAR is zero.
• During mid-day in the summer, PAR often reaches 2,000 to 3,000 millimoles per square meter.

How to Measure PAR

PAR can be measured by quantum sensors or PAR meters.

Some PAR sensors/meters measure the photosynthetic photon flux density (PPFD) of PAR. These sensors can be used with a light meter to measure instantaneous light intensity or cumulative light intensity.

What is PPFD? PPFD illustrates the amount of light that hits a defined surface area of plants. Do not confuse this with photosynthetic photon flux (PPF), which measures how much light reaches a plant within what time period.

What is DLI

& Why Is It Important?

Daily Light Integral (DLI) measures how much Photosynthetic Active Radiation (PAR) photons, or light intensity, reaches a specified area, over a 24-hour cycle. A photoperiod is the period in which plants are exposed to light (within those 24 hours). Thus, DLI indicates how much light a plant has received within the full photoperiod.

Why is DLI Important?

DLI influences plant growth, development, yield, and quality–for each stage of the plant’s growth. Routinely monitoring and recording of DLI received by crops can help to determine when plants need adjustment in light levels.

By keeping track of when supplemental lighting, or light intensity reduction is needed, throughout the various season and plant cycles, growers can maintain uniform light levels throughout the entire year, and in turn, yielding a higher quality, higher quantity crop.

How is DLI measured?

DLI is expressed as mols of light per square meter per day = mol·m2·d.

Tip: DLI is measured in mols, while PAR is measured in μmols. Keep this in mind when calculating the photosynthetic photon flux (PPF) needed to achieve a specific DLI. Growers can use PPFD, or how much light plants receive per second, to conduct a DLI reading.

How to Measure DLI

DLI may be measured by light meters, or light quantum sensors connected to a computer or data logger.

To learn more, contact our Grow Pros grow experts at

info@wegrowpros.com or 818-818-8088.