In horticultural lighting the delivered metric is not illuminance but photosynthetic photon flux density, and the variation across the canopy matters more than the average. A grow room at 400 µmol/m²/s average with a twenty percent hot spot grows crops of inconsistent quality, because the plants under the hot spot and the plants at the edge experience different light integrals and therefore mature on different schedules. This white paper sets out how PPFD is mapped, how canopy uniformity is computed and interpreted, how to convert PPFD to the daily light integral that actually drives growth, and how the geometry of the luminaire format determines the map before any fixture is chosen.
The reader is a controlled-environment agriculture engineer, a greenhouse operator, or a specifying engineer comparing grow-light formats for a commercial installation.
1. PPFD, DLI and why uniformity is a commercial issue
PPFD is the photosynthetic photon flux density at a defined plane, expressed in micromoles of photons in the 400–700 nm waveband per square metre per second. DLI is the integral of PPFD over the photoperiod, expressed in moles per square metre per day.
DLI = PPFD × photoperiod (s) / 1 000 000
The conversion is arithmetic, and the table below gives it directly for common combinations.
| PPFD (µmol/m²/s) | DLI at 12 h | DLI at 16 h | DLI at 18 h | DLI at 20 h |
|---|---|---|---|---|
| 150 | 6.5 | 8.6 | 9.7 | 10.8 |
| 200 | 8.6 | 11.5 | 13.0 | 14.4 |
| 300 | 13.0 | 17.3 | 19.4 | 21.6 |
| 400 | 17.3 | 23.0 | 25.9 | 28.8 |
| 500 | 21.6 | 28.8 | 32.4 | 36.0 |
| 700 | 30.2 | 40.3 | 45.4 | 50.4 |
| 900 | 38.9 | 51.8 | 58.3 | 64.8 |
| Crop class | Typical DLI target | Typical PPFD at 16 h | Implication for uniformity |
|---|---|---|---|
| Leafy greens, herbs | 12–17 mol/m²/d | 210–295 | Moderate tolerance, ±15 % acceptable |
| Tomato, pepper | 20–30 mol/m²/d | 350–520 | Tighter, ±10 % preferred |
| Cannabis, high-value floriculture | 30–45 mol/m²/d | 520–780 | ±8 % or better for grade consistency |
| Microgreens, propagation | 6–12 mol/m²/d | 105–210 | Small trays, edge effects dominate |
| Research and phenotyping | Specified per protocol | Specified | ±5 % or better, often ±3 % |
The last row is the demanding case. In a research or phenotyping installation, the biological conclusion depends on the light treatment being identical across the population, and an uneven light field is a confounding variable in the experiment. This is where uniformity moves from a commercial quality issue to a validity requirement.
2. Reading a PPFD map
A PPFD map is a contour or heat map of the photosynthetic photon flux density on a defined plane — the canopy top, the tray surface, or the bench — and it is the primary deliverable of a horticultural lighting design.
Four features determine whether the map is acceptable:
- Core value and its extent. The green contour should cover the entire productive area, not just the bench centre. If the productive area extends to the bench edge — as it does with a full tray of seedlings — then an edge roll-off of fifteen percent is already a uniformity failure.
- Edge roll-off width. With linear formats the roll-off is narrow at the ends and wider across the width. The width of the roll-off zone is the design variable that decides whether the outer tray row needs its own luminaire or a reflector.
- Hot spots. A single-bar format over a narrow bench can produce a distinct bright line beneath the luminaire. A hot spot above 115 percent of the average is a canopy stress risk in high-DLI crops.
- Cross-contamination between adjacent benches. In a multi-tier or multi-bench room, light from one bench’s luminaires contributes to its neighbour’s canopy. Ignoring this produces measured values higher than predicted on the bench edges and an apparent uniformity that is better than the design intent.
3. Uniformity metrics for a canopy
Horticultural uniformity is expressed in several ways, and the definitions are not standardised across the industry.
| Metric | Formula | Interpretation | Typical acceptance |
|---|---|---|---|
| U0 | PPFD_min / PPFD_avg | Darkest point versus average | ≥ 0.85 commercial, ≥ 0.90 research |
| U1 | PPFD_min / PPFD_max | Darkest versus brightest | ≥ 0.70 commercial |
| CV | Standard deviation / mean | Statistical spread | ≤ 0.10 commercial, ≤ 0.05 research |
| Max/avg ratio | PPFD_max / PPFD_avg | Hot-spot severity | ≤ 1.10 |
| Edge-to-centre ratio | PPFD_edge / PPFD_centre | Edge performance | ≥ 0.85 for full-tray use |
Horticultural uniformity targets are much tighter than the interior lighting targets discussed elsewhere. A U0 of 0.90 is routine in a well-designed grow space, whereas an interior lighting U0 target is typically 0.40. The reason is biological rather than perceptual: plants integrate light over time, and a plant in a low-PPFD zone grows measurably slower and matures later than its neighbour. There is no adaptation effect to compensate.
3.1 A worked uniformity calculation
The grid below comes from a nine-point measurement of a 1.2 m × 1.2 m quadrat under a linear bar installation at 150 mm above canopy.
| Grid point | PPFD (µmol/m²/s) | Deviation from mean |
|---|---|---|
| Centre | 472 | +5.6 % |
| North mid | 468 | +4.7 % |
| South mid | 455 | +1.8 % |
| East mid | 438 | -2.0 % |
| West mid | 434 | -2.9 % |
| NE corner | 415 | -7.2 % |
| NW corner | 412 | -7.8 % |
| SE corner | 405 | -9.4 % |
| SW corner | 402 | -10.0 % |
| Average | 433 | — |
From these values:
- U0 = 402 / 433 = 0.93
- U1 = 402 / 472 = 0.85
- CV = 24.9 / 433 = 0.058
- Max/avg = 472 / 433 = 1.09
This is a well-performing installation for a commercial leafy-green crop, and it is marginal for research use. The corners hold the minimum, which is the expected pattern for a square luminaire layout over a square quadrat: the minimum always occurs at the corner that is furthest from the largest number of luminaires.
4. Canopy height and PPFD decay
PPFD falls as the canopy grows toward the luminaire if the luminaire is fixed, and falls as the canopy grows away from the luminaire if the luminaire is fixed at the top of a tall space with the crop on the floor.
The shapes carry two lessons. First, the near-field region (below about 0.25 m) is where the inverse-square approximation breaks down and where a bar’s individual emitters resolve into distinct hot lines. Operating in the near field is how the high bar values in the blue curve are achieved, and it is also why the same luminaire at 0.1 m produces a strongly striped canopy. Second, the panel format loses PPFD more slowly with distance, because its larger emitting area puts it closer to the far-field regime; a panel is therefore the more forgiving format where the mounting distance cannot be controlled precisely, but it cannot deliver the highest intensities at the canopy.
The design implication for multi-tier vertical farming is direct: with fixed tier heights, the canopy-luminaire distance shrinks as the crop grows, so the PPFD delivered at harvest is higher than at transplant. A design that targets 400 µmol/m²/s at the transplant canopy distance will over-deliver by twenty to thirty percent at maturity unless the luminaires are dimmed or raised. Modern installations handle this with a dimming schedule that follows the crop cycle.
5. Comparing luminaire formats for canopy uniformity
The format determines the shape of the map more than the flux does. Each format produces a characteristic uniformity signature.
| Product | Format du fichier | Typical mounting distance | Map signature | Best-fit canopy |
|---|---|---|---|---|
| Z-01 T8 Plant Tube | Linear tube, 1.2 m | 0.15–0.4 m | Striped along axis, even across | Multi-tier racks, small benches |
| Z-02 T8 Integrated | Linear, integrated driver | 0.15–0.4 m | As Z-01, simpler retrofit | Retrofit of fluorescent racks |
| Z-03 T10 | Larger-diameter linear | 0.2–0.5 m | Broader cross-axis spread | Wider benches, two-row racks |
| Z-04 T12 | Largest linear section | 0.25–0.6 m | Flattest cross-axis of the tube family | Single-tier wide benches |
| Z-09 UFO Ruiguang E | Round point source | 0.6–2.0 m | Concentric rings, edge fall-off | Open floor, individual large plants |
| Z-10 Panel Plant Light | Flat rectangular panel | 0.2–0.8 m | Broad and flat, low peak | Full-tray propagation, research benches |
| Z-11 Octopus Plant Light | Multi-arm point array | 0.3–1.2 m | Multiple overlapping lobes | Irregular canopies, mother plants |
| Z-12 Plant Floodlight | Asymmetric flood | 1.0–4.0 m | Elongated throw, sharp cut-off | Greenhouse supplementary, high bays |
5.1 Uniformity comparison
| Product group | Typical U0 at nominal distance | Typical CV | Max/avg | Research-grade suitability |
|---|---|---|---|---|
| Z-01 / Z-02 single row | 0.78–0.85 | 0.10–0.14 | 1.20 | Not suitable alone |
| Z-01 / Z-02 multi-row rack | 0.88–0.93 | 0.05–0.08 | 1.10 | Suitable with careful layout |
| Z-03 / Z-04 multi-row | 0.90–0.95 | 0.04–0.07 | 1.08 | Suitable |
| Z-09 open floor grid | 0.82–0.88 | 0.08–0.11 | 1.15 | Marginal without overlap |
| Z-10 panel array | 0.92–0.96 | 0.03–0.06 | 1.06 | Best fit for flat-bench research |
| Z-11 multi-arm | 0.84–0.90 | 0.07–0.10 | 1.12 | Application-dependent |
| Z-12 greenhouse flood | 0.80–0.88 | 0.09–0.13 | 1.15 | With array overlap |
The comparison is representative and depends strongly on spacing, height and room reflectance; a single Z-01 tube on a wide bench will produce U0 below 0.70, while the same tube in a six-tube rack produces U0 above 0.90. The format sets the ceiling on achievable uniformity, and the layout determines how much of that ceiling is reached.
For a research or phenotyping installation, the panel format is usually the best starting point because it reaches U0 above 0.92 without demanding precise mechanical alignment. For a commercial multi-tier rack, the tube format is more economical and reaches comparable uniformity once the row count and the cross-axis spacing are set correctly.
6. Spectrum, far-red and the map
A PPFD map quantifies photons in the 400–700 nm waveband, which is the definition of photosynthetic photon flux. It deliberately excludes far-red radiation above 700 nm, even though far-red at 730 nm is photosynthetically active through the Emerson enhancement effect and is used for morphological control.
| Band | Longueur d'Onde | Included in PPFD | Function | Related product |
|---|---|---|---|---|
| Blue | 400–500 nm | Yes | Compact growth, stomatal control | All formats |
| Green | 500–600 nm | Yes | Canopy penetration | All formats |
| Red | 600–700 nm | Yes | Primary photosynthetic driver | All formats |
| Far-red | 700–750 nm | No | Phytochrome, stem elongation | Z-06 L2 ZW (780 nm), Z-07 L3 CK (730 nm) |
| Far-red | 750–800 nm | No | Not photosynthetically effective | Z-06 L2 ZW (780 nm) |
The practical consequences are two. First, a PPFD map is not a complete description of the light environment when far-red channels are in use, and a parallel measurement of phytochrome photostationary state is the appropriate complementary metric. Second, because far-red is excluded from the PPFD integral, adding a far-red channel changes morphology without changing the reported PPFD, which can confuse comparisons between installations. Report both when far-red is present.
7. Measurement practice
A PPFD map is only as good as the instrument and the protocol behind it.
| Requirement | Specification | Why it matters |
|---|---|---|
| Sensor type | Quantum sensor, cosine-corrected | Uncorrected sensors under-read at oblique angles |
| Spectral response | 400–700 nm, matched to plant response | Broadband meters over-read under deep-red spectra |
| Calibration | Traceable, within validity window | Absolute accuracy of every grid point |
| Grid spacing | 0.1–0.2 m for benches, 0.3 m for floors | Resolution against hot-spot and stripe features |
| Vertical position | Canopy plane, or a documented reference height | PPFD falls rapidly with distance |
| Obstruction | Sensor level, no shadowing by operator | Operator shadow is a common error source |
| Repeats | Minimum three per grid point, luminaires stable | LED warm-up and driver ripple |
| Warm-up | 30 minutes before measurement | Output settles after thermal equilibrium |
The most frequent error in field measurement is the operator’s own shadow. With a hand-held sensor over a low bench and a low mounting height, the operator’s body is unavoidably in the light path for the grid points nearest to them. A tripod-mounted sensor with a remote readout removes the error at trivial cost.
8. Common PPFD design errors
| Error | Consequence | Correction |
|---|---|---|
| Designing to average PPFD only | Edge canopy grows slower, harvest uneven | Specify U0 and CV alongside average |
| Using a broadband lux meter for PPFD | Over-reads under red-dominant spectra | Use a quantum sensor with the correct response |
| Ignoring mutual illumination between benches | Measured values exceed prediction at edges | Model the whole room, not one bench |
| Measurement at a height different from the design plane | PPFD appears higher or lower than design | Fix and state the reference plane |
| Leaving far-red channels out of the reporting | Morphology differs with no PPFD explanation | Report far-red separately and explicitly |
| Fixed luminaires with a growing canopy | PPFD drifts up by 20–30 % over the cycle | Dimming schedule or height adjustment |
| Nearest-plant assumption for point sources | Concentric rings, U0 fails at the edge | Add overlap or switch to a linear/panel format |
| Assuming parity between 400 V and 230 V drivers | Different efficacy and thermal headroom | Check the driver variant specified |
The dimensioning error at the end of the list deserves a note for project engineers: horticultural installations frequently run high total loads, and the choice between a 230 V single-phase and a 400 V three-phase distribution affects both the cable sizing and the achievable driver efficiency. Confirm the distribution before fixing the luminaire count.
9. Conclusion
A horticultural lighting design is judged on its canopy map, not its average. The working sequence is: fix the crop class and its DLI target, convert to a PPFD target for the planned photoperiod, choose the luminaire format according to the uniformity requirement and the mechanical constraints of the growing system, and then verify the layout by simulation with a grid resolution fine enough to reveal stripes and hot spots.
For multi-tier racks and small benches, the Z-01 and Z-02 tube formats combined with a Z-03 or Z-04 cross-axis arrangement typically deliver U0 of 0.88–0.93. For full-tray propagation and flat-bench research, the Z-10 panel format reaches U0 above 0.92 with less mechanical precision required. For open-floor and greenhouse applications, the Z-09 UFO, Z-11 multi-arm and Z-12 flood formats require deliberate array overlap to reach the same uniformity, and the design must account for the mutual illumination between adjacent areas. Where far-red channels from the Z-06 and Z-07 families are in use, report the PPFD and the far-red dose separately so that the two effects on crop morphology can be distinguished.
10. Referenced standards
- ANSI/ASABE S640 — Quantities and units of electromagnetic radiation in the plant Sciences
- IES LM-79 — Approved method: electrical and photometric measurements of solid-state lighting products
- DLC Horticultural Lighting Technical Requirements — DesignLights Consortium qualified products list
- EN 13032-4 — Light and lighting: measurement and presentation of photometric data, LED lamps and modules
- CIE 239 — Guide on the use of the photosynthetic photon flux and related quantities
- CIE 026 — CIE standard observer for photometry
- IEC 62471 — Photobiological safety of lamps and lamp systems
- GB/T 32655 — Terminology for plant lighting
11. Contact us
QUEENDOM supplies PPFD simulations and uniformity reports for the horticultural range, covering the Z-01, Z-02, Z-03 and Z-04 tube formats, the Z-09 UFO, the Z-10 panel, the Z-11 multi-arm and the Z-12 greenhouse floodlight. Provide the bench or rack dimensions, the canopy height, the crop class and the DLI target, and the engineering group will return a PPFD map, a uniformity summary and the recommended layout, together with the far-red configuration where Z-06 or Z-07 channels are included.
Related products and applications
The grow luminaires used in the PPFD mapping example are listed below.