Not every photon drives photosynthesis equally. The McCree action spectrum gives the relative quantum yield of CO₂ fixation per absorbed photon across the visible range — red and blue are used efficiently, green less so, and far-red weakly on its own but it boosts red-driven photosystem II via the Emerson enhancement effect.
Relative quantum yield φ(λ) (McCree, simplified per band):
Blue (450nm) φ ≈ 0.72
Green (550nm) φ ≈ 0.78
Red (660nm) φ ≈ 1.00 ← peak efficiency
Far-red (730nm) φ ≈ 0.32 (+synergy bonus with red present)
Weighted photosynthetic flux:
PPFD_eff = Σ φ(λ)·I(λ) + Emerson bonus (up to +12% when
far-red and red are both present)
Daily Light Integral (mol·m⁻²·day⁻¹):
DLI = PPFD_total(μmol·m⁻²·s⁻¹) × photoperiod(h) × 3600 / 1e6
Growth index = PPFD_eff normalized against a target
(≈ 900 μmol·m⁻²·s⁻¹ effective, the ISS Veggie/APH range)
- R/G/B/far-red sliders — set each LED channel's photon flux; the panel glow and canopy hue update to match the actual mixed light color.
- Photoperiod — hours/day the panel is on; combined with total PPFD it sets the Daily Light Integral (DLI), the standard metric controlled-environment growers use to plan yield.
- Growth index — the McCree-weighted effective PPFD, normalized against a healthy target; it drives how tall and saturated the canopy grows over simulated days.
- ISS Veggie preset — mimics NASA's Veggie/APH chambers, which run mostly red LEDs with a smaller blue fraction (~4:1) plus a little white/green for crew visual inspection.
Real-world relevance: LED spectral tuning is exactly how NASA's Veggie and Advanced Plant Habitat on the ISS, and vertical farms on Earth, squeeze the most growth out of every watt of electrical power — a scarce resource on a spacecraft.