One fixture centred over the shelf, plants growing. But by the end of the week you notice: leaves at the edges are smaller, colour is paler, growth is lagging.
Quick Glossary
- PPFD (Photosynthetic Photon Flux Density) — the photon flux reaching the surface at canopy level
- Lighting uniformity — the ratio of minimum to maximum PPFD across the growing area
- PPFD map — a matrix of measurements taken at multiple points across the entire growing area
Why PPFD Drops from Centre to Edges
Any point or linear light source has a beam angle — and beyond that angle, intensity drops sharply. In ideal conditions, PPFD follows an inverse-square relationship with distance: twice the distance from the source means four times less light.
The real picture is more complex: side reflectors, secondary optic angles, reflections from walls and shelves — all of these shape the actual PPFD distribution across the zone. Manufacturers provide photon distribution maps for their fixtures — but these show distribution in open space without accounting for reflections in a specific room.
Two shelves positioned 20 cm lower under the same fixture already receive different PPFD due to the difference in distance. Plants on the bottom tier of a vertical rack with a single overhead light source may receive 30–60% less than the top tier.
How to Create a PPFD Map: Practical Method
A PPFD map is a measurement grid across the entire growing area. Minimum requirements for uniformity assessment:
For a shelf or tray up to 1 m²: measure at 9 points — a 3×3 grid (corners, mid-points of each side, centre). Record every value. Find the minimum and maximum. Uniformity = min/max × 100%.
For areas from 1 to 4 m²: use a 4×4 or 5×5 grid — 16 or 25 measurements. Step between points: 30–50 cm.
For large areas: step 50–100 cm, minimum 20 points. Results can be plotted on a diagram with isolines — shadow zones are immediately visible.
Measure with all grow lights on and no other light sources active. Hold the sensor horizontally at canopy height or at the planned plant placement height. If the room has side windows or other lighting — measure at night or block out the other sources.
Typical Shadow Zones and Where to Find Them
Corners of the growing area. A centrally placed fixture always under-delivers to the corners. At 50–60% uniformity at the edges, plants there will yield roughly half as much. For rectangular areas, linear fixtures along the long side or multiple point sources work better than a single central fixture.
Lower tiers in vertical systems. With a single overhead light, each tier below receives less. Check the PPFD map on each tier separately — the difference between top and bottom can reach 70%.
Zones between fixture rows. With multiple parallel fixtures, "valleys" of reduced PPFD can form between them. Optimise fixture spacing and mounting height to close these gaps.
Zones beneath rack structural elements. Metal profiles and cross-members cast shadows on plants below. When designing a rack, keep cross-members narrow and avoid positioning them directly above plants.
Perimeter areas without side reflectors. Dark walls and no side reflectors mean a portion of light goes "into the wall." White walls or reflective film (Mylar) on the side panels of a rack returns up to 80–90% of lateral light back to plants and improves edge uniformity without increasing fixture power.
How to Improve Uniformity Without Increasing Power
Raise the fixture. As the distance from the fixture to the plants increases, centre PPFD decreases but distribution becomes more even — the light cone widens. Downside: overall PPFD drops and must be compensated with more power or longer photoperiod.
Use multiple smaller sources instead of one large one. Two 240 W fixtures instead of one 480 W unit, spaced appropriately, deliver significantly better uniformity at the same total power draw. Overlapping light cones between fixtures fill in the gaps.
Secondary optics and lenses. Some LED fixtures offer adjustable beam angles through interchangeable lenses — wide angle for short mounting distances, narrow for greater heights.
Reflective surfaces. White paint or mirror-finish material (Mylar) on the side walls of a rack returns up to 80–90% of lateral radiation back to the plants. Especially effective on narrow racks where lateral output is significant.
Three Mistakes That Cost the Most
Measuring PPFD only at the centre and treating it as "the zone reading." The central point under a fixture is typically the maximum. Average PPFD across the zone may be 30–50% lower. Designing the system around the centre reading and then discovering that 40% of the area is under-lit wastes both time and money spent on fixtures.
Not re-measuring PPFD after changing fixture height or rack configuration. Raising the fixture by 10 cm or changing a reflector alters the distribution. A PPFD map made at one configuration does not represent the new one. Any change — a new measurement at a minimum of 9 points.
Ignoring uniformity when calculating DLI and sowing density. At 60% uniformity, edge plants receive 40% less DLI than the centre and will be ready for harvest 2–4 days later. With uniform sowing across the whole zone, harvest becomes uneven. Either improve uniformity, or account for it when planning harvest and arrange sowing according to light zones.
How to Know Lighting Uniformity Is Acceptable
- A PPFD map has been created for the growing area
- Uniformity (min/max) is no lower than 75–80%
- Plants across the entire area grow at the same rate and reach comparable marketable condition before harvest
- If there is a noticeable difference between centre and edges — measure first, then change the configuration