Why Is PAR Limited to 400–700 nm?

Photosynthetically Active Radiation (PAR) is defined as the range of light wavelengths that plants use for photosynthesis—400 to 700 nanometers (nm). But why exactly this range? Why not include ultraviolet (UV) or infrared (IR)? Here’s the science behind it.

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1. It’s the Range Chlorophyll Absorbs Best

Photosynthesis depends mainly on chlorophyll a and b, the primary pigments in most plants. These pigments absorb light most efficiently in:

• Blue light (≈ 430–470 nm)
• Red light (≈ 640–680 nm)

Green light (500–600 nm) is less absorbed, which is why plants appear green—but even green light contributes to photosynthesis through deeper leaf penetration.

Light below 400 nm (ultraviolet) or above 700 nm (infrared) is mostly not absorbed by chlorophyll and doesn’t drive photosynthesis efficiently.

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2. UV and IR Can Be Harmful

• UV (<400 nm): Can damage DNA, proteins, and photosystems in plants.
• IR (>700 nm): Mostly contributes heat, which can stress plants but doesn’t directly aid photosynthesis.

That’s why these wavelengths are excluded from the PAR definition, even though they are part of the broader solar spectrum.

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3. PAR = What Plants “See”

PAR is essentially the “visible light” to plants. Measuring it helps growers optimize grow lights, adjust plant placement, and manage DLI (Daily Light Integral). This is why most light meters—including AquaHorti’s—focus on the 400–700 nm range.

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Bonus: What About ePAR?

Some modern research suggests that far-red light (700–750 nm) can enhance photosynthesis when combined with shorter wavelengths. This is called extended PAR (ePAR), and it’s gaining traction in advanced horticulture.

But for most growers, standard PAR (400–700 nm) remains the key benchmark.

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Conclusion

The 400–700 nm range is not arbitrary—it reflects the light spectrum most effective for photosynthesis and safest for plant health. That’s why PAR meters, grow light specs, and DLI calculations all focus on this specific range.