Why Matching Readings at 450 nm and 660 nm Don’t Guarantee PAR Accuracy

When growers compare different PAR meters, a common test is to place them under a blue LED (450 nm) and a red LED (660 nm). If both meters show the same reading, it feels like a sign of accuracy. But the truth is more complicated.

---

1. PAR Covers 400–700 nm, Not Just Two Points

Photosynthetically Active Radiation (PAR) includes all photons between 400–700 nm.

• Blue (~450 nm) and red (~660 nm) are important, but they are only two narrow slices.
• A truly accurate meter must respond correctly across the entire spectrum—including green, yellow, and orange light.

---

2. Different Sensors Have Different Spectral Response

High-quality meters are calibrated with spectroradiometers to ensure their sensors follow the ideal PAR response curve.

• Two meters may agree at 450 nm and 660 nm…
• …but still disagree at 520 nm (green) or 610 nm (orange).
  This means the overall accuracy may be very different, even if spot checks look good.

---

3. Why Broad Calibration Matters

Plants respond to light hour by hour and wavelength by wavelength.

• White LEDs and sunlight contain broad spectra.
• If your meter is only correct at red/blue, it could underestimate or overestimate PAR under other light sources.

That’s why calibration against a full spectrum is the gold standard.

---

4. The Takeaway

Matching PAR readings at 450 nm and 660 nm is not enough to prove a meter’s precision.

• It only shows agreement at those points.
• True accuracy requires broad-spectrum calibration and testing under multiple light sources.

For growers who want reliable data, always look for instruments that are calibrated across the full PAR range—not just red and blue LEDs.

---

At AquaHorti, our meters are designed with this in mind: continuous spectral calibration and long-term data logging. That means you get readings you can trust—not just at two points, but across the full light spectrum your plants use.