PAR / PPFD (canopy)

Definition

PAR / PPFD is the Photosynthetic Photon Flux Density at the canopy plane: the count of photons in the photosynthetically active 400–700 nm waveband arriving per square metre per second, in micromoles. It is sensed directly by quantum sensors and the calibrated PAR channels of the Spectral Canopy Array, positioned at canopy height to report the light the leaves actually intercept — not the fixture output. PPFD is the instantaneous rate; integrated over the photoperiod it yields the Daily Light Integral (VC-MET-105).

Why it matters

Light is the energy budget of the whole operation. Below the canopy light-compensation point the plants respire faster than they fix carbon; above the saturation point, extra photons add heat and photo-oxidative stress without yield. CHLORA dims, boosts, and schedules the LED arrays against PPFD to hold each cultivar in its productive window and to co-meter CO₂ enrichment, since light and carbon are co-limiting. A drifting PPFD field — from a failing driver or a fouled lens — quietly erodes the Canopy Vitality Index (VC-MET-001) before any other channel reacts.

Formula

A directly measured signal. Zone PPFD is the spatially weighted mean of the quantum-sensor grid, cosine- and spectrally corrected at the node:

PPFD_zone = Σ_i ( w_i · q_i · g_i · cos_corr_i )

  q_i        quantum-sensor count (µmol·m⁻²·s⁻¹), node i
  g_i        spectral calibration factor (LED-matched cal)
  cos_corr_i cosine-response correction for incidence angle
  w_i        canopy-area weight of node i (Σ w_i = 1)

Lens-fouling guard:
  if grid mean drifts > 8% below fixture-commanded output
  with no dim command, flag LENS-FOUL and schedule cleaning.
Uniformity:
  report min/mean ratio; < 0.7 raises a uniformity advisory.

Inputs

Units & Scale

Reported in micromoles of photons per square metre per second, whole-number precision. PPFD is an instantaneous flux, not summed in time — temporal integration belongs to DLI (VC-MET-105). Spatial roll-up is the canopy-area-weighted grid mean with a min/mean uniformity ratio. Confidence degrades to DEGRADED when fewer than four grid sensors report or when measured PPFD diverges > 8 % from commanded fixture output (suggesting lens fouling or a failing driver).

Sampling & Source

• Sampled every 1 minute from the Spectral Canopy Array quantum-sensor grid.
• Spectrally calibrated to the installed LED spectrum; cosine-corrected per node.
• Stale handling: a sensor silent > 3 min is dropped from the weighted mean; below four live nodes sets DEGRADED.
• Calibration: semi-annual reference-lamp check against the cell SPD station; drift > 4 % raises a metrology work order.

Thresholds

Recent Trend

Zone-A canopy-grid PPFD, last 14 sampling roll-ups (µmol·m⁻²·s⁻¹):

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — lighting control & photoperiod procedures.
• Photo-stress shade response on high-PPFD CRIT — see Environment SOP set in the SOP Library.
• Fixture-fault alarm escalation — Monitoring Systems.