CANOPY-LEVEL LIGHT MEASUREMENT
Learn how to measure PPFD at canopy level, create a representative measurement grid, evaluate light uniformity and interpret distribution data in commercial growing environments.
BEYOND THE AVERAGE
Average PPFD provides a useful indication of overall light delivery, but it does not reveal how evenly that light reaches the crop. Two lighting layouts can produce the same average PPFD while having very different minimum values, high-intensity areas, low-light zones and canopy-level uniformity.
A meaningful evaluation therefore depends on where measurements are taken, how the measurement grid is arranged and whether the readings represent the crop’s actual growing area. This guide explains how to collect canopy-level PPFD measurements, calculate common uniformity ratios, understand the influence of mounting height and interpret a PPFD distribution map.
CANOPY-LEVEL MEASUREMENT
PPFD is measured with a quantum sensor and expressed in micromoles of photons per square metre per second (µmol·m⁻²·s⁻¹). The measurement represents the photon flux arriving at the sensor’s surface, making sensor height, orientation and position essential parts of the result.
For a relatively level crop canopy receiving overhead light, the sensor should be positioned horizontally and kept level at the height that represents the top of the active canopy. It should not be buried among leaves, tilted toward a fixture or shaded by the person taking the measurement.
The objective is not simply to obtain a reading. It is to collect readings under consistent, documented conditions so that different positions, layouts or operating scenarios can be compared meaningfully.
1. Use an appropriate quantum sensor
Use a calibrated, full-spectrum quantum sensor suitable for the sunlight and electric-light sources being evaluated. Keep the sensing surface clean and confirm that the instrument is operating correctly before beginning.
2. Define the measurement plane
Position the sensor at a consistent height representing the crop canopy. Record the sensor height, crop stage and canopy condition so the measurements can be repeated or compared later.
3. Keep the sensor level and unobstructed
For horizontal canopy measurements, keep the sensing surface level and facing upward. Ensure that the operator, meter, cable, crop supports and nearby equipment do not unintentionally shade the sensor.
4. Reproduce the intended operating conditions
Record fixture output, dimming level, lighting schedule, mounting height and the condition of greenhouse curtains or screens. Measurements intended for comparison should be taken under equivalent operating conditions.
MEASUREMENT GRID DESIGN
A PPFD measurement grid divides the cultivated area into a repeatable series of measurement locations. Its purpose is to capture the complete lighting pattern—including areas directly beneath fixtures, spaces between fixtures and lower-light locations near the boundaries of the crop.
The grid should represent the area where plants are actually grown. Walkways, service areas and uncultivated spaces are normally excluded unless they are intentionally part of the evaluation. The number and spacing of measurement points should be adapted to the size of the cultivated area, fixture arrangement, crop-row geometry and level of detail required.
Illustrative PPFD measurement-grid example. The appropriate grid dimensions, point spacing and measurement plane should be adapted to the cultivated area, crop architecture and lighting layout.
1. Define the evaluated boundary
Identify the cultivated area that the lighting system is intended to serve. Record its dimensions and exclude walkways or non-crop spaces unless those areas are intentionally being evaluated.
2. Establish evenly distributed measurement points
Divide the cultivated area into equal or consistently spaced zones. Place one measurement point within each zone so the grid captures locations beneath fixtures, between fixtures and near the crop-area boundaries. Avoid selecting only the brightest or most convenient locations.
3. Label every grid position
Use a simple coordinate system such as A1, A2 and B1 to identify each measurement point. This makes it possible to connect every recorded PPFD value to a physical location and reproduce the same grid during future evaluations.
4. Document the measurement plane
Record the sensor height, crop stage and canopy condition represented by the grid. All readings intended for direct comparison should be taken at the same measurement plane and under equivalent lighting and facility conditions.
BEYOND A SINGLE NUMBER
Average PPFD is calculated by adding all measurements within the evaluated area and dividing the total by the number of measurement points. It provides a useful summary of overall light delivery, but it does not show where high- and low-light conditions occur.
Minimum PPFD is the lowest value recorded within the grid. It helps identify the weakest illuminated location, which may occur near a crop area boundary, between fixtures, beneath structural shading or where fixture coverage does not overlap sufficiently.
Average and minimum PPFD should therefore be reviewed together with the complete distribution map. An unexpectedly low individual reading should also be repeated and checked for temporary shading, sensor positioning or another measurement irregularity before conclusions are made.
Average PPFD = Sum of all PPFD readings ÷ Number of measurement points
Minimum PPFD = Lowest PPFD reading within the evaluated grid
Two lighting layouts can produce the same average PPFD while delivering very different conditions across the crop. A well-distributed layout keeps most measurements relatively close to the average. A less-uniform layout may combine high-intensity areas with substantial low-light zones, even though its calculated average is identical.
Both illustrative layouts shown above have an average PPFD of 300 µmol·m⁻²·s⁻¹. Layout A has a minimum PPFD of 270, with its measurements remaining relatively close to the average. Layout B has a minimum PPFD of 150 and reaches the same average by combining substantial low-light zones with much higher-intensity areas.
This demonstrates why average PPFD should not be interpreted alone. The minimum value and complete spatial distribution provide essential context for understanding how evenly light reaches the evaluated crop area.
This is an illustrative comparison using representative data. The values shown are not measured or simulated results from a CultiLight project. This visual is technically useful because PPFD distribution maps reveal spatial conditions including edge losses—that a single average cannot communicate.
Additional technical example: The Illuminating Engineering Society discusses canopy-level PPFD distributions using heat maps and iso-PPFD plots. Illuminating Engineering Society
QUANTIFYING LIGHT DISTRIBUTION
A uniformity calculation converts a complete set of PPFD measurements into a ratio that describes how closely the lowest-light area compares with the rest of the evaluated crop area. The result is normally expressed as a percentage: a higher percentage indicates that the minimum PPFD is closer to the comparison value.
Different uniformity formulas provide different information. For this reason, a lighting report should always identify the formula being used rather than presenting only a percentage labelled “uniformity.”
Minimum-to-average uniformity compares the lowest PPFD reading with the average of all measurements in the grid. It provides a straightforward indication of how far the weakest measured location falls below the overall average.
Minimum-to-maximum uniformity compares the lowest and highest readings in the measurement grid. It describes the spread between the two extremes, but it can be strongly influenced by a single unusually high or low measurement.
Minimum-to-average:
270 ÷ 300 × 100 = 90%
Minimum-to-maximum:
270 ÷ 325 × 100 = 83.1%
Minimum-to-average:
150 ÷ 300 × 100 = 50%
Minimum-to-maximum:
150 ÷ 450 × 100 = 33.3%
Although both layouts deliver the same average PPFD, their uniformity results are substantially different. Layout A keeps the lowest measurement relatively close to the average and maximum. Layout B combines pronounced low-light and high intensity zones, producing considerably lower uniformity ratios.
A uniformity percentage is meaningful only when its calculation method and measurement conditions are known. Record the formula used, evaluated boundary, number and spacing of measurement points, sensor plane and operating conditions whenever uniformity results are reported or compared.
Uniformity ratios summarize a distribution, but they do not replace the complete measurement grid or PPFD map. Those visual records remain necessary for identifying where high-light and low-light conditions occur.
The Illuminating Engineering Society likewise distinguishes minimum-to-average and minimum-to-maximum ratios and notes that multiple uniformity metrics exist, making the calculation method important to identify. IES technical discussion
FIXTURE-TO-CANOPY DISTANCE
Mounting height is the vertical distance between the fixture’s light-emitting surface and the crop-level measurement plane. Changing this distance affects the size of each fixture’s lighting footprint, the peak PPFD beneath the fixture and the amount of overlap between neighbouring fixtures.
Closer to the canopy
At the same fixture output, a lower mounting position generally concentrates more light within a smaller area. This can increase peak PPFD directly beneath the fixture while creating stronger differences between high-intensity and low-light locations.
Farther from the canopy
Increasing the fixture-to-canopy distance generally spreads the light across a larger area. Greater overlap between neighbouring fixtures can produce a smoother PPFD distribution, but peak intensity at the canopy will normally decrease and more light may extend beyond the intended crop area.
Finding the appropriate height
The appropriate mounting height depends on the fixture’s light distribution, fixture spacing, crop geometry, target PPFD, greenhouse structure and cultivated-area boundaries. Mounting height should therefore be evaluated as part of the complete lighting layout rather than adjusted independently.
FIXTURE-TO-CANOPY DISTANCE
Record mounting height from the fixture’s light-emitting surface to the same canopy-level plane used for the PPFD measurement grid. Measuring only from the fixture to the floor does not describe the actual distance over which light travels before reaching the crop.
As the crop grows, the canopy moves closer to the fixture and the effective mounting height changes. Record the crop stage, canopy height and fixture position whenever measurements are collected or compared.
Simple distance rules should not replace a lighting calculation or measurement. Horticultural fixtures are extended light sources operating in multi-fixture layouts where optics, overlap, structural shading, reflections and crop geometry influence the final PPFD distribution.
The caution about extended light sources is important because point source assumptions become less reliable when the measurement distance is close to the physical dimensions of the fixture. IES discussion of near-field horticultural-lighting modelling
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.
REPORTING AND REPEATABILITY
A PPFD measurement becomes substantially more useful when the conditions under which it was collected are documented. Recording the grid design, equipment, lighting configuration and greenhouse conditions allows the results to be interpreted correctly and compared with future measurements.
Keep the individual grid readings—not only the calculated average, minimum or uniformity percentage. The complete dataset shows where differences occurred and makes it possible to review or recalculate the results later.
Record:
Record:
When measurements are repeated, use the same evaluated boundary, grid coordinates, sensor plane and fixture settings whenever possible. Differences in crop height, curtain position, daylight, dimming level or measurement location can change the results even when the lighting installation itself has not changed.
Retaining the original coordinate-level readings makes it possible to distinguish a general change in light output from a localized change caused by shading, crop development, fixture positioning or another site condition.
MEASUREMENT QUALITY
Small differences in measurement technique can produce substantially different PPFD results. A consistent procedure helps ensure that the completed grid represents the crop environment rather than changes in sensor position, operating conditions or sampling choices.
1. Measuring only beneath the fixtures
Sampling only the brightest positions can overstate average PPFD and conceal lower-light areas between fixtures or near crop boundaries.
2. Changing the measurement plane
Moving the sensor higher or lower between grid positions changes its distance from the fixtures. Keep every directly comparable reading at the same defined canopy-level plane.
3. Tilting or shading the sensor
The sensor should remain level and unobstructed. The operator, meter, cable, leaves or surrounding equipment should not unintentionally shade its sensing surface.
4. Ignoring changing daylight
Sunlight can change while a daytime grid is being completed. Record the conditions and avoid treating measurements collected under substantially different daylight levels as directly comparable.
5. Using an unsuitable or unverified sensor
Use a quantum sensor appropriate for the light sources being measured. Record its model and calibration information, and keep the sensing surface clean and in good condition.
6. Reporting only the average
An average PPFD value does not reveal the minimum, maximum or location of uneven conditions. Retain the complete grid and identify any uniformity formula used.
An unexpectedly high or low measurement should be repeated before it is accepted or excluded. Check the sensor position, measurement height, temporary shading and fixture operating status, then record the verified value.
Do not remove an unusual reading simply because it differs from the surrounding measurements. It may identify a genuine distribution problem that requires further investigation.
FIELD CHECKLIST
Use this checklist to maintain a consistent procedure from the initial grid design through the final reporting of results. Project-specific requirements may vary, but the same measurement principles should be applied throughout the evaluated area.
PUT THE DATA TO WORK
A representative PPFD grid provides the foundation for evaluating light intensity and distribution. The next step is to consider those measurements alongside the crop’s target light level, photoperiod, available daylight and complete facility layout.
Use the following planning tools to explore the results or provide CultiLight with the information required for a more detailed project assessment.
Calculate and compare grow-lighting metrics
Estimate the supplemental electric-light contribution that may be required to support a selected crop-level PPFD target.
Estimate Supplemental PPFDConvert PPFD and lighting duration into an estimated Daily Light Integral to better understand accumulated light over the photoperiod
Calculate DLIShare your crop, facility, lighting conditions and project objectives so our team can better understand your application.
Start the QuestionnaireOnline calculators provide preliminary planning estimates and do not replace a complete site-specific lighting assessment. Final fixture selection and layout should consider the full project conditions.
PROJECT-SPECIFIC SUPPORT
CultiLight works with commercial growers to evaluate crop-lighting requirements, facility conditions and fixture layouts. Share your project information with our team to begin a more detailed discussion.
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