Pure 660nm Test. Spectrometer vs PAR Meter Under Single-Channel Red

I ran a quick test today under a pure 660nm deep red LED channel. No white, no blue, no blend.

Why?

Because narrow band red is where spectral mismatch usually shows up in PAR meters.

Setup:
• 660nm LED only
• Single channel active
• Measured with lab spectrometer + PAR meter

Spectrometer:
Peak: 661nm
Calculated PPFD: 52.7 µmol/m²/s

PAR meter reading:
52.4 µmol/m²/s

Deviation was minimal.

This is interesting because 660nm sits near the edge of where some PAR sensors start rolling off. Under single-peak red, you can sometimes see larger discrepancies depending on sensor design.

For hydro growers running heavy red ratios in flowering:

If your meter under-reads or over-reads red, your DLI math will drift quickly.

That can translate into:
• Incorrect flowering intensity
• Misjudged stretch control
• Inefficient power usage

Curious how many of you are actually measuring PPFD under single-channel testing vs blended spectrum?