Full-Spectrum LEDs Explained: PAR, PPFD & Efficacy

If you’ve ever wondered why two “200 W” lights grow very differently, the answer comes down to PAR, PPFD, and efficacy. Once you understand these three, wattage and marketing claims become much easier to ignore—and you can choose the right LED for your hydroponic setup.

Step 1 Understand PAR

Photosynthetically Active Radiation (PAR) is the slice of the spectrum plants can actually use for photosynthesis—roughly 400–700 nm. A good full-spectrum LED concentrates output inside this PAR window.

In practice that means:

• Enough blue (around 430–470 nm) to keep seedlings compact and leafy growth tight.
• Strong red (around 620–670 nm) to drive flowering and fruiting.
• A touch of green so light penetrates deeper into the canopy and colors look natural to your eye.

When you compare spectrums, ignore artistic color gradients and look for a clear spec sheet. You want a smooth curve focused in the PAR band—not big spikes outside 400–700 nm that waste power.

Step 2 Measure PPFD at the canopy

PPFD tells you how much PAR light actually reaches the canopy. It’s measured in µmol/m²/s and answers the question: “How many usable photons hit each square meter of plants every second?”

For real-world growing, PPFD matters more than watts. A well-designed 200 W fixture can beat a sloppy 300 W panel if it puts more light into the canopy and less into the tent walls.

How to read a PPFD map

• The numbers are PPFD in µmol/m²/s measured at the canopy height.
• Center values are usually highest; edges show drop-off. Aim for uniformity (min ÷ max) of at least 0.8.
• Height labels (12 ", 16 ", 18 ") show how raising or lowering the fixture changes intensity and spread.

Typical PPFD targets:

• Seedlings / clones: 150–300 µmol/m²/s
• Leafy herbs & greens: 300–500 µmol/m²/s
• Fruiting / flowering crops: 600–900+ µmol/m²/s (with good CO₂ and nutrition)

Dial in your PPFD

• Hot center (e.g., 1800+ µmol/m²/s): raise the fixture 2–4 ", or dim 10–20% to smooth the peak.
• Dark corners (<70% of center): lower the fixture slightly, add side bars, or widen panel spacing.
• Too low overall: lower the fixture, increase output, or add a second light to widen coverage.

Re-measure at the canopy after each change until the map looks even. Treat a phone lux app as a relative tool only; a proper PAR meter is the only way to get true PPFD numbers.

Step 3 Check efficacy (µmol/J)

Efficacy tells you how efficiently a fixture turns electricity into plant-usable light. It’s written as µmol per joule (µmol/J). Higher µmol/J means more photons for the same power bill.

As a rough guide:

• Older blurple LEDs and T5 fluorescents: often 1.0–1.5 µmol/J.
• Decent modern LEDs: around 2.3–2.7 µmol/J.
• High-end bar fixtures: 2.7+ µmol/J and up.

Two fixtures can both say “200 W,” but if one runs at 1.5 µmol/J and the other at 2.8 µmol/J, the efficient one can throw almost double the photons onto your canopy for the same wattage. That’s why efficacy should be the tie-breaker once spectrum and PPFD coverage look good.