Understanding PAR and DLI: Essential Light Metrics for Plant Growth

Whether you’re growing vegetables in a greenhouse, corals in a reef tank, or flowers under LEDs, light quality and quantity are key to success. But how do you know if your plants are getting enough light—and the right kind of light?

Two important concepts help answer this: PAR and DLI.

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What Is PAR?

PAR (Photosynthetically Active Radiation) refers to the portion of light (400–700 nanometers) that plants can use for photosynthesis.

• This range includes blue, green, and red light.
• It excludes UV and far-red wavelengths, which plants cannot use efficiently for photosynthesis.
• PAR is not the same as brightness. A room can feel bright to your eyes but still be poor in usable PAR for plants.

Measured in: micromoles per square meter per second (µmol/m²/s)
This tells you how much usable light is hitting your plant right now.

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What Is DLI?

DLI (Daily Light Integral) measures the total amount of PAR a plant receives over an entire day.

• It’s like counting how many “light particles” (photons) hit the plant during 24 hours.
• Think of PAR as the speed and DLI as the distance traveled.
• DLI is especially useful for tracking seasonal light changes or managing supplemental lighting.

Measured in: moles per square meter per day (mol/m²/day)

Different plant species require different DLI levels to thrive:

Plant Type	Optimal DLI (mol/m²/day)
Leafy greens	12–17
Tomatoes & cucumbers	20–30
Orchids & shade plants	6–12
Corals (reef tanks)	10–25

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Why Should You Measure Both?

Measuring PAR helps you evaluate light intensity in real time, which is useful when adjusting light placement or output.

Measuring DLI shows you how much light the plant receives throughout the day, critical for understanding whether your plants get enough total energy for healthy growth.

Using a device like the AquaHorti AH-PARDLI allows you to:

• Track daily PAR curves over time
• Log 180 days of PAR and DLI data
• Export results in CSV format for professional analysis
• Use outdoors or in humid environments (with waterproof sensor)

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Final Thoughts

Understanding PAR and DLI transforms how you manage plant lighting—from guesswork to precision. Instead of just “giving it more light,” you’ll know exactly how much and what kind of light your plants are getting.