Choose a method. Use fixture details for planning, or paste lux readings for verification.
Use this table to understand how fixture count, factors, and area influence average illuminance.
| Scenario | Fixtures | Lumens each | UF | MF | Area (m²) | Avg lux |
|---|---|---|---|---|---|---|
| Seedling bench | 4 | 3,000 | 0.65 | 0.85 | 8.00 | 829 |
| Herb rack | 6 | 2,800 | 0.60 | 0.80 | 6.00 | 1,344 |
| Veg bed | 8 | 3,500 | 0.60 | 0.80 | 10.00 | 1,344 |
| Fruit zone | 10 | 4,000 | 0.55 | 0.75 | 12.00 | 1,375 |
Average Illuminance (Eavg) is estimated by distributing useful light over the growing area.
- TotalLumens = Fixtures × LumensPerFixture
- Area is the bed footprint in square meters.
- Eavg (lux) = (TotalLumens × UF × MF) ÷ Area
UF (utilization factor) approximates optical and room losses. MF (maintenance factor) accounts for dirt and lumen depreciation. For measured readings, the calculator uses the arithmetic mean and uniformity ratios.
- Pick a method: use fixture method for planning, measured method for validation.
- Enter your area: choose rectangle, circle, or custom area for the bed.
- Add lighting data: fixtures, lumens, and realistic UF/MF values.
- Optional targets: add a lux target or stage preset for guidance notes.
- Calculate: results appear below the header and above the form.
- Export: download a CSV or PDF report for records and comparisons.
Light mapping for greenhouse beds
Average illuminance is most useful when it represents the canopy, not the aisle. Create a simple grid over the bed, then take readings at the same height as leaves. A 3×3 grid is a quick start for small benches, while larger zones benefit from 5×5 points. Recording more points reduces the risk that one bright hotspot hides dim corners. Take readings after lights stabilize for ten minutes, and avoid casting your own shadow across the sensor during every point measurement session.
Converting lamp ratings into usable lux
The fixture method starts with total luminous flux and then applies real‑world losses. Total lumens equals fixture count times lumens per fixture. Utilization factor approximates how much light reaches the target after beam angle, spacing, and reflections. Maintenance factor reduces output for dust, aging lenses, and lumen depreciation. Dividing useful lumens by bed area gives an estimated average lux.
Setting targets by crop stage
Target lux depends on crop, photoperiod, and temperature, but working ranges help planning. Seedlings often perform in the 5,000–10,000 lux band, vegetative growth commonly uses 10,000–20,000 lux, and fruiting zones may run 20,000–40,000 lux. Use the target field to verify whether your plan clears the minimum, then adjust fixture quantity or area coverage.
Interpreting uniformity and variation
When you paste readings, the calculator reports min, max, standard deviation, and uniformity ratios. Min/avg highlights how weak the darkest point is compared with the overall level. If min/avg is below about 0.50, plants at the edge can stretch or lag. Improve uniformity by raising fixtures, widening spacing, adding reflective sidewalls, or using diffusers.
Using exports to improve repeatability
Lighting changes slowly as seasons shift and equipment ages. Exporting a CSV or PDF after each adjustment creates a baseline you can compare week to week. Keep notes about mounting height, cleaning dates, and any shade cloth used. Over time, these records make it easier to standardize grow zones, troubleshoot uneven growth, and justify upgrades with measured results.
1) What is illuminance, and why does it matter for plants?
Illuminance (lux) describes how much visible light reaches a surface. In grow areas, it helps you spot underlit zones that can slow growth, stretch seedlings, or reduce uniformity across a bed.
2) Where should I measure lux for the measured method?
Measure at canopy height, with the sensor facing upward. Use a consistent grid, keep your body out of the light path, and record readings after fixtures warm up and output stabilizes.
3) What UF and MF values are reasonable starting points?
For many hobby greenhouses, UF around 0.55–0.70 and MF around 0.75–0.90 are practical starting ranges. Increase UF with reflective surfaces and optimized spacing; lower MF if fixtures are dusty or aging.
4) Can I compare different lamp types using lumens?
Lumens are useful for rough planning, but they do not fully represent plant-usable photons. For accurate comparisons between spectra, confirm with measured lux maps and, when possible, a PAR/PPFD meter at the canopy.
5) What uniformity should I aim for?
As a rule of thumb, min/avg near 0.70 or higher supports more even growth. If you see low uniformity, raise fixtures, increase overlap, or adjust layout so edge points are not heavily shaded.
6) Why might the PPFD estimate differ from my PAR meter?
Lux-to-PPFD factors depend on spectrum, sensor response, and optics. A single factor cannot match every light. Treat the estimate as a planning aid, and rely on your PAR meter for calibration-quality decisions.