What Are the Units of PAR—and Why Do They Matter?

The Core Unit: µmol/m²/s

PAR (Photosynthetically Active Radiation) is measured in:

Micromoles per square meter per second

(µmol/m²/s)

But what does that actually mean?

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What’s a Micromole?

• A mole is a scientific unit that measures the number of particles—in this case, photons (light particles).
• 1 mole = 6.022 × 10²³ photons
• So, 1 micromole (µmol) is one-millionth of a mole, or 6.022 × 10¹⁷ photons.

In simple terms:

1 µmol/m²/s means that 602,200,000,000,000,000 photons are hitting every square meter of plant surface every second.

That’s how scientists measure the amount of light usable by plants.

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Why This Matters for Growers

Humans see brightness in lux or lumens, but plants don’t “see”—they absorb photons. Measuring light in micromoles gives a biologically meaningful measure of how much photosynthesis-driving energy is hitting the plant.

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PAR vs Lux

Metric	For Whom	Measures
Lux	Humans	Brightness (weighted to green/yellow light)
µmol/m²/s (PAR)	Plants	Photon quantity (400–700 nm range)

Even if two lights appear equally “bright” in lux, they can deliver very different PAR values—especially if one has more blue/red wavelengths that plants love.

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Daily Light Integral (DLI)

DLI is the total daily accumulation of PAR:

• Unit: mol/m²/day
• Formula:
  DLI = (Average PAR × light hours × 3600) ÷ 1,000,000

It’s like a daily rainfall total, but for light. Your plants need a certain number of moles of light per day—not just intense bursts, but consistent delivery.

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Use the Right Tools

At EvoDevice, our meters provide real-time:

• PAR (µmol/m²/s)
• DLI (mol/m²/day)

So you can finally stop guessing—and start growing with confidence.

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Summary

• Micromoles count the actual number of photons usable for photosynthesis.
• Plants don’t care how “bright” a light looks—they care about how many usable photons they receive.
• That’s why µmol/m²/s is the gold standard for plant lighting—not lux.