Daily Light Integral (DLI)

Definition

Daily Light Integral (DLI) is the cumulative number of photosynthetically active photons (400–700 nm) delivered to a square metre of canopy across one 24-hour cycle, expressed in moles per square metre per day. It is a derived, time-integrated quantity: the canopy PPFD (VC-MET-104) flux is integrated over the photoperiod by CHLORA. Where instantaneous PPFD describes the rate of light delivery, DLI describes the total dose — the figure that actually correlates with biomass accumulation and flowering.

Why it matters

Plants integrate light over the day, not the second. A canopy can sit at an ideal PPFD yet accumulate too little DLI on a short photoperiod, or bank an excessive dose that pushes cultivars into stress or premature flowering. DLI is therefore the variable the cultivation schedule is written against: CHLORA trims fixture intensity and photoperiod through the day to land each cell on its cultivar DLI target, and a running DLI shortfall is an early predictor of slowed relative growth rate (VC-MET-013) well before the canopy looks different.

Formula

DLI is the time-integral of PPFD over 24 hours, with the µmol→mol and per-second→per-day unit conversion folded in:

DLI = ( Σ_t  PPFD_t · Δt ) / 1e6           [mol·m⁻²·d⁻¹]

  PPFD_t  canopy PPFD sample (µmol·m⁻²·s⁻¹), VC-MET-104
  Δt      sample interval in seconds (60 s nominal)
  /1e6    µmol → mol

Closed form for a flat photoperiod:
  DLI ≈ PPFD · photoperiod_hours · 3600 / 1e6

Gap handling:
  missing intervals (> 3 min) are filled by the last valid
  PPFD only up to 5% of the day; beyond that the running DLI
  carries a SHORT-INTEGRATION flag and confidence drops.

Inputs

Units & Scale

Reported in moles per square metre per day to one decimal place. DLI is itself a sum over time, so it is reported as a single closing value per 24-hour cycle plus a live running accumulator that resets at the photoperiod boundary. It is not summed across zones; facility roll-up is the canopy-area-weighted mean of zone DLIs. Confidence degrades to DEGRADED when more than 5 % of the day's PPFD samples were gap-filled (SHORT-INTEGRATION flag).

Sampling & Source

• Closed once per 24 h at the photoperiod boundary; a running accumulator updates each minute as PPFD arrives.
• Integrated by CHLORA from canopy PPFD (VC-MET-104) sourced from the Spectral Canopy Array.
• Gap handling: short PPFD dropouts are forward-filled up to 5 % of the day; beyond that the cycle is flagged SHORT-INTEGRATION.
• Target-tracking: a mid-day projected-shortfall alert fires if the run-rate will miss the cultivar DLI target by > 2 mol·m⁻²·d⁻¹.

Thresholds

Recent Trend

Zone-A closing DLI, last 14 daily cycles (mol·m⁻²·d⁻¹):

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — photoperiod & light-dose scheduling procedures.
• Light-dose recovery on shortfall CRIT — see Environment SOP set in the SOP Library.
• Projected-shortfall alert handling — Monitoring Systems.