KNOWLEDGE CENTER
Learn how to plan supplemental greenhouse lighting around available daylight, crop light requirements and facility conditions. Explore how light intensity, operating hours, fixture layout and controls work together to support a practical lighting strategy.
WORKING WITH NATURAL LIGHT
Sunlight provides an important share of the light used by greenhouse crops, but the amount reaching the canopy changes with location, season and weather. Short days and prolonged cloudy periods can leave crops below their daily light target, even when the greenhouse appears bright to the human eye.
The greenhouse itself also affects light availability. Glazing, structural elements, screens and accumulated dirt can reduce the sunlight reaching the crop. Outdoor light data is therefore only a starting point; lighting decisions should reflect conditions inside the growing area.
Supplemental lighting helps fill the gap between available daylight and the crop’s light requirements. Planning begins with a crop- and growth-stage appropriate daily light integral (DLI) target—the total photosynthetic light received per square metre over a day. Comparing that target with the daylight available at canopy level helps establish how much additional light the fixtures need to provide.
MEASUREMENT CONDITIONS
Greenhouse PPFD may include sunlight, electric lighting or a combination of both. Before collecting measurements, define which contribution is being evaluated. Otherwise, changing daylight conditions can make results difficult to interpret or reproduce.
To evaluate fixture output and distribution independently from sunlight, collect the measurement grid after dark or when incoming solar radiation is negligible.
Keep all fixtures at the documented output level and maintain consistent curtains, screens and greenhouse operating conditions. The resulting grid represents the electric-lighting contribution at the selected canopy plane.
Daytime measurements represent the combined contribution of sunlight and electric lighting at that particular moment. Record the date, time, cloud conditions, curtain position and fixture settings with the results.
Because sunlight can change rapidly, a sequential handheld grid may combine spatial differences with changes that occurred while measurements were being collected. Total daily light conditions are more reliably evaluated through repeated or continuous measurements used to calculate DLI.
If sunlight increases or decreases while the operator moves across the greenhouse, measurements taken near the end of the grid may not be directly comparable with those collected at the beginning.
For daytime assessments, complete the grid as consistently as possible and document changing sky conditions. A stationary reference quantum sensor can also record light changes during the measurement period. If the objective is to evaluate electric-light distribution alone, measurements taken after dark provide a clearer comparison.
A single PPFD reading describes light intensity at one moment. DLI describes the accumulated photosynthetic light received throughout the day and is more appropriate for evaluating variable daily greenhouse light.
CROP LIGHT REQUIREMENTS
Begin with the crop’s total daily light requirement. Express this as a daily light integral (DLI), measured in moles of photosynthetically active photons per square metre per day (mol·m⁻²·d⁻¹).
The target includes sunlight and supplemental lighting combined. It should reflect the crop and its production stage, rather than being treated as a universal value for every greenhouse.
Record the crop, cultivar and whether plants are in propagation, vegetative growth, flowering or fruit production. A light target used for young plants should not automatically be applied to a mature production crop.
Use crop-specific research, breeder guidance or advice from a qualified crop consultant to select an appropriate DLI target. Document the source and check whether the growing conditions behind that recommendation are relevant to your facility.
Record the intended photoperiod and any crop-specific requirement for uninterrupted darkness. DLI describes daily light quantity, while photoperiod describes day length. Meeting the DLI target does not automatically mean the lighting schedule is appropriate for the crop.
ASSESSING AVAILABLE DAYLIGHT
The daylight contribution should reflect the light reaching the crop inside the greenhouse during the intended production season. Use canopy-level measurements where available, or a clearly documented estimate when preparing an initial lighting plan.
Use a suitable quantum sensor with a data logger, or a DLI meter, to record light throughout complete days at representative canopy-level locations. Keep the sensor level and clean, and document screen positions and relevant greenhouse conditions.
Collect records across different weather conditions and the production period being evaluated. A single midday PPFD reading cannot establish how much daylight the crop receives over an entire day.
For preliminary planning, combine local outdoor DLI data for the relevant month with an estimated greenhouse transmission fraction.
This fraction should represent how much daylight reaches crop level after losses through glazing, structural shading, screens and other obstructions—not simply the glazing material’s published transmission rating. Treat the result as an estimate and verify it with measurements when possible. Monthly averages can also conceal individual low-light days.
If outdoor DLI is 20 mol·m⁻²·d⁻¹ and the estimated fraction reaching the crop is 60%:
20 × 0.60 = 12 mol·m⁻²·d⁻¹ of daylight at crop level.
These values are illustrative. A 60% transmission factor is not a universal greenhouse assumption.
CALCULATING THE LIGHT SHORTFALL
Once the crop’s daily light target and available daylight are established, calculate the difference between them. This shortfall is the additional daily light that supplemental fixtures would need to deliver at crop level under the conditions being evaluated.
Subtract the daylight DLI reaching the crop from the total crop DLI target. All values must use the same units: mol·m⁻²·d⁻¹.
The “max(0, …)” expression prevents a negative result. When daylight already meets or exceeds the daily target, the calculated supplemental DLI requirement is zero.
Using the previous daylight estimate of 12 mol·m⁻²·d⁻¹ and an illustrative crop target of 20 mol·m⁻²·d⁻¹:
20 − 12 = 8 mol·m⁻²·d⁻¹ of supplemental light.
Under these assumptions, daylight supplies 12 and the fixtures supply the remaining 8, bringing the combined daily total to 20 mol·m⁻²·d⁻¹.
Illustrative planning example only. The target is not a recommendation for a particular crop, and the daylight value is an estimate rather than measured project data.
This result describes a daily light quantity — not a fixture wattage or an instantaneous PPFD. The next step is to determine the supplemental PPFD needed to deliver that quantity within the crop’s permitted lighting schedule.
LIGHT INTENSITY AND OPERATING TIME
The same supplemental DLI can be delivered through different combinations of light intensity and operating time. A higher supplemental PPFD delivers the required daily light in fewer hours, while a lower PPFD requires a longer operating period.
Enter the daily light shortfall in mol·m⁻²·d⁻¹ and the planned fixture operating hours per day. The result is the supplemental PPFD required at crop level, expressed in µmol·m⁻²·s⁻¹.
Use the PPFD supplied by the fixtures alone at crop level—not the combined reading from daylight and electric lighting. The result is the operating time per day needed to deliver the supplemental DLI.
For the illustrative supplemental requirement of 8 mol·m⁻²·d⁻¹, the following combinations provide approximately the same daily light quantity:
| Fixture operating time | Required supplemental PPFD |
|---|---|
| 8 hours per day | 278 µmol·m⁻²·s⁻¹ |
| 12 hours per day | 185 µmol·m⁻²·s⁻¹ |
| 16 hours per day | 139 µmol·m⁻²·s⁻¹ |
Values are rounded and assume constant supplemental PPFD while the fixtures operate. These are mathematical examples, not crop-specific lighting recommendations.
Fixture operating hours can overlap with natural daylight; they are not automatically additional hours after sunset. Keep the combined lighting schedule within the crop’s appropriate photoperiod and preserve any required uninterrupted dark period.
If fixture output changes through dimming, calculate the supplemental DLI for each output period separately and add the contributions.
FROM CALCULATION TO LIGHTING PLAN
The calculated supplemental PPFD establishes a light-delivery requirement, but it does not determine fixture quantity or spacing by itself. A lighting plan must show how the selected fixtures distribute that light across the cultivated area at the intended canopy height.
Identify the cultivated boundaries, crop rows, benches and aisles. Define the canopy-level plane used for the lighting calculation, and consider how crop height changes during production. Report results for the crop area rather than combining it with unrelated walkways or service spaces.
Evaluate fixture spacing using the selected fixture’s light-distribution data and the available fixture-to-canopy distance. These factors determine how neighbouring lighting footprints overlap. A spacing arrangement that works at one mounting height may produce a different distribution when the canopy moves closer to the fixtures.
Include structural members, screens and overhead equipment when reviewing the layout. Consider their effects on light reaching the crop, as well as the additional daylight shading created by the lighting installation. Coordinate fixture positions with screen movement, practical mounting locations and maintenance access.
Request a canopy-level PPFD map showing the electric-light contribution separately from daylight. Review the average, minimum, maximum and spatial pattern, including crop-area edges and positions between fixtures. Any reported uniformity percentage should identify its calculation method.
For detailed guidance on measurement grids, uniformity calculations and interpreting distribution maps, read Understanding PPFD for Commercial Growing.
DAYLIGHT-RESPONSIVE LIGHTING
A greenhouse lighting requirement changes as daylight increases, decreases or becomes interrupted by cloud cover. Controls help adjust supplemental lighting to these changing conditions while working within the crop’s daily light target and permitted lighting schedule.
A fixed timer controls when fixtures operate. Sensor-based controls can also adjust their output, while DLI-based strategies consider the accumulated light delivered throughout the day.
Define when supplemental lighting is permitted to operate, taking natural daylength and the crop’s required dark period into account. Automatic controls should work within these limits rather than extending the lighting period simply to reach a daily light target. DLI and photoperiod describe different aspects of the crop’s light environment.
With compatible dimmable fixtures and controls, supplemental output can decrease as daylight increases and rise again when daylight falls. This helps avoid unnecessary electric-light output during brighter periods. However, responding to the current PPFD alone does not establish whether the crop will receive its intended daily light total.
DLI-based controls track accumulated light from daylight and supplemental lighting against a daily target. Depending on the system, they can adjust fixture output or operating time within the permitted lighting window. Available fixture capacity and the remaining operating hours limit how much additional light can still be delivered.
Use sensor locations that represent the crop zone being controlled, and verify their readings against canopy-level measurements. Confirm that fixtures, drivers and controllers support the intended dimming and switching functions. During commissioning, check how the system responds to changing daylight, screen positions and lighting schedules rather than assuming that a dimmable fixture provides automatic daylight control by itself.
OPERATING COST PLANNING
Lighting electricity use depends on the system’s electrical input power and how long it operates. Start with the complete fixture input wattage, including its driver, rather than the wattage of the LED components alone.
For identical fixtures, multiply the fixture quantity by the input wattage and divide by 1,000 to obtain the total lighting load in kilowatts. If the installation contains different fixture types, calculate each group separately and add the results.
This formula applies when electrical input power remains constant during the operating period. If fixtures are dimmed or different lighting zones operate on separate schedules, calculate the energy used during each operating condition and add the results.
Multiply the calculated electricity use by the applicable energy price. If prices vary by time of day or season, calculate each period separately. This estimates the energy charge, not necessarily the complete electricity bill or the total cost of operating the lighting installation.
Consider 100 fixtures, each drawing 600 W, operating at that constant input power for 12 hours per day over 30 days.
Total lighting load:
100 × 600 W ÷ 1,000 = 60 kW
Electricity used over 30 days:
60 kW × 12 hours × 30 days = 21,600 kWh
At an assumed energy price of CAD 0.10 per kWh:
21,600 kWh × CAD 0.10 = CAD 2,160
This is an illustrative energy-charge calculation only. The fixture quantity, wattage, schedule and electricity price are assumptions—not a CultiLight project design, a crop recommendation or a current utility tariff.
For dimmed operation, use measured electrical input or manufacturer power consumption data rather than assuming that electricity use follows the dimming percentage exactly. For annual estimates, account for seasonal changes in daylight and operating hours.
Check the facility’s electricity tariff for demand charges, fixed charges and other applicable costs. Fixture purchase, installation, maintenance and changes to heating or cooling costs require separate assessment.
PREPARE YOUR PROJECT
A useful lighting proposal starts with a clear picture of the crop, greenhouse and production objectives. Gather the information below where available. If some details are still unknown, identify them as items to confirm rather than filling the gaps with assumptions.
Identify the crop, variety, growth stage and production system. Describe what the lighting project should support, such as winter production, propagation, a more consistent harvest schedule or an expansion of the cultivated area.
Provide the dimensions of the cultivated area, including crop rows, benches and aisles. A simple drawing with measurements and several greenhouse photographs can help communicate the layout and distinguish crop-production areas from surrounding spaces.
Share available crop-level daylight measurements, seasonal DLI estimates and any proposed DLI or supplemental PPFD targets. Identify which values are measured, estimated or still to be confirmed, and note the season or production period they represent.
Record the expected canopy height and available fixture mounting positions. Include information about greenhouse structure, screens, overhead equipment and access restrictions that could affect fixture placement, light distribution or maintenance.
Provide the available supply voltage, frequency and phase, together with any confirmed electrical-capacity limits. Identify existing lighting, dimming interfaces, sensors and climate-control equipment. Have the facility’s qualified electrical team confirm any uncertain supply information.
Describe the intended lighting window, seasonal operating pattern and required dark period. Indicate whether the project is a new installation, an expansion or a replacement, and share the expected timeline and any budget constraints.
LET’S PLAN YOUR LIGHTING
Planning a new greenhouse lighting installation or upgrading an existing system? Contact the CultiLight team to discuss your crop, facility conditions and supplemental lighting goals.
Whether you have detailed measurements or are still defining the project, we can help identify the information needed for the next step.
Already have your project details ready?