How to Calculate Vertical Farm Daily Light Integral
Daily light integral (DLI) translates lighting system performance into a crop-centric metric: the total mol of photosynthetically active photons delivered per square metre each day. Vertical farms rely on DLI to tune recipes, coordinate energy purchasing, and benchmark fixture performance. This guide codifies the definition, shows how to capture and process the required measurements, and embeds a calculator so horticulture teams can evaluate lighting scenarios rapidly.
Use this walkthrough alongside energy benchmarking resources such as the energy use intensity guide and cost planning tools like the LED savings calculator to align agronomy targets with financial objectives.
Definition and agronomic context
DLI is the integral of photosynthetic photon flux density (PPFD) over the lit photoperiod. Because PPFD captures instantaneous light intensity (µmol/m²/s), integrating over time produces mol/m²/day—a unit that correlates strongly with biomass accumulation, flowering, and pigment development. Vertical farms use DLI to differentiate between vegetative and generative stages, calibrate dimming schedules, and manage plant stress.
Each crop has a recommended DLI band. Leafy greens often thrive around 12–17 mol/m²/day while fruiting crops may require 20 mol/m²/day or more. Staying within the band avoids wasted energy and prevents photobleaching. Because stacked farms have many microclimates, a consistent calculation method ensures comparable results across tiers.
Variables, symbols, and units
Record PPFD at the canopy for each tier, ideally using calibrated quantum sensors. Photoperiod should reflect scheduled lighting hours after accounting for dark periods and sunrise/sunset transitions programmed into control systems. Express dimming and transmission losses as percentages so they can translate into multiplicative factors.
- P – Average canopy PPFD during the photoperiod (µmol/m²/s).
- H – Photoperiod duration (hours/day).
- d – Scheduled dimming fraction (unitless, 0–1).
- ℓ – Optical or transmission loss fraction from lenses, glazing, or obstructions (unitless, 0–1).
- DLIbase – Unadjusted daily light integral before modifiers (mol/m²/day).
- DLIeff – Effective DLI after applying dimming and loss factors (mol/m²/day).
- η – Overall photon delivery efficiency equal to (1 − d) × (1 − ℓ).
If you manage HVAC-coupled lighting waste heat, coordinate with thermal models or calculators such as the thermal storage sizing tool to understand how lighting adjustments interact with cooling loads.
Formulas and conversion steps
Convert PPFD and photoperiod into DLI by multiplying PPFD (µmol/m²/s) by the photoperiod (hours), multiplying by 3,600 seconds per hour, and dividing by 1,000,000 to obtain mol. Then apply dimming and loss factors to reach the effective DLI experienced by the plants.
DLIbase = (P × H × 3,600) ÷ 1,000,000
η = (1 − d) × (1 − ℓ)
DLIeff = DLIbase × η
When lighting schedules include sunrise or sunset ramps, compute a weighted-average dimming fraction that reflects the ramp duration. Store both base and effective values—base DLI helps evaluate fixture specifications, while effective DLI reflects crop experience.
Step-by-step workflow
Step 1: Capture reliable PPFD measurements
Map PPFD across the canopy using a grid or mobile sensor. Average readings to derive P, and record variability so agronomists can spot zones needing light redistribution. Recalibrate sensors quarterly or whenever fixtures are replaced.
Step 2: Confirm photoperiod programming
Extract the programmed lighting schedule from the control system. Ensure H matches actual runtime by checking controller logs—unexpected outages or maintenance may shorten the photoperiod and reduce DLI.
Step 3: Quantify dimming and losses
Document scheduled dimming percentages for energy savings or stress mitigation. Estimate transmission losses from any glazing, lensing, or canopy shading. For racks with adjustable curtains or reflective surfaces, update ℓ whenever physical changes occur.
Step 4: Calculate DLI and compare with crop targets
Apply the formulas to compute DLIbase and DLIeff. Compare the effective value with crop stage targets. If the result is below the recommended band, consider increasing photoperiod, boosting PPFD, or reducing dimming.
Step 5: Document decisions and iterate
Record parameter changes in your agronomy management system. Track yield and quality metrics alongside DLI adjustments to refine recipes over time. When energy costs shift, run additional scenarios linking DLI outcomes with facility energy budgets.
Validation and monitoring
Validate PPFD readings against independent sensors and manufacturer specifications. Cross-check calculated DLI with plant responses; if crops show stress despite being within target ranges, inspect sensor placement or recalibrate dimming assumptions. Track efficiency η over time to detect fixture degradation or dust accumulation.
Integrate DLI monitoring with facility dashboards so operations can react quickly to anomalies. For example, connect lighting data with HVAC load analysis to pre-empt hot spots and align with your thermal planning playbooks when evaluating cross-system impacts.
Limitations and scenario exploration
The DLI framework focuses on total photon delivery and does not capture spectral composition. Red-to-blue ratios, far-red supplementation, and UV treatments also influence plant morphology. Layer spectral analytics on top of DLI to ensure recipes meet cultivar requirements.
Additionally, the model assumes constant PPFD during the lit period. Dynamic lighting strategies that respond to transpiration or energy prices require time-series calculations. Use the DLI workflow as a baseline, then augment with sub-hourly analytics when implementing advanced controls.
Embed: Vertical farm daily light integral calculator
Adjust PPFD, photoperiod, and modifier assumptions in the embedded calculator to generate consistent DLI outputs for agronomy reviews, investor updates, or utility incentive filings.