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Daily light integral at the canopy
The three quantities, and why wattage is not one of them
Plant lighting is described by three quantities, and none of them is watts. Wattage describes power consumed, which tells you about your electricity bill rather than about your plants, and two fixtures of identical wattage can differ substantially in usable light output.
PPFD, photosynthetic photon flux density, is the instantaneous intensity of usable light arriving at a surface, in micromoles per square meter per second. It counts photons in the 400 to 700 nanometre range that photosynthesis can use. This is what a PAR meter reads, and it is the quantity that falls off with distance.
Photoperiod is simply how many hours light is delivered.
DLI, daily light integral, is the total quantity of usable light delivered over a whole day, in moles per square meter per day. It is the number that predicts growth, because plants integrate light over time rather than responding to intensity alone. A high intensity for four hours and a moderate intensity for sixteen can deliver the same DLI and broadly the same growth.
The conversion is simple arithmetic: DLI equals PPFD times hours times 3,600, divided by one million. The 3,600 converts hours to seconds and the million converts micromoles to moles.
Why the distance matters more than the fixture
Light intensity falls off with the square of distance from the source. Double the distance and intensity drops to a quarter.
In practical terms, an LED strip delivering 300 micromoles at 4 inches delivers roughly 75 at 8 inches and about 33 at 12. A grower who hangs a perfectly good fixture at 12 inches and gets leggy seedlings has a mounting problem rather than a fixture problem, and buying a bigger light will not fix it as efficiently as lowering the one they have.
This is also why manufacturer PPFD claims need reading carefully. Figures are frequently quoted directly beneath the center of the fixture at an unstated or very short distance, which is the single brightest point in the coverage area and bears little relationship to the average over a tray. Where a maker publishes a coverage map at a stated height rather than a peak number, that is a meaningful signal.
Coverage matters as much as peak intensity. One strip over the middle of a 1020 tray gives a bright center and dim edges, and the seedlings at the edges lean and stretch toward the light. Two or three strips spaced across the tray width give even light, which is why fixtures in this category are sold in multiples.
Converting a lux meter reading
PAR meters are the correct instrument but a lux meter costs a fraction as much and can be used with an approximate conversion, provided you understand what makes it approximate.
Lux measures illuminance weighted for human vision, which peaks in the green part of the spectrum. Plants use red and blue heavily and green less, so the relationship between lux and PPFD depends entirely on the spectrum of the light source. The conversion divisors are therefore source-specific: roughly 54 for daylight, around 74 for white LED at 5000 K, about 74 for fluorescent T5 and around 82 for high pressure sodium.
For comparing one position with another under the same light source, a lux meter is entirely adequate and will tell you exactly how much intensity you lose across a tray or by raising a fixture. For absolute values under mixed daylight and LED, where the effective divisor is somewhere between two figures, it is less reliable.
A phone light meter app is a further step less accurate again, because phone sensors are not calibrated and have their own spectral response, but for relative comparisons it is better than guessing.
Targets by crop and stage
The DLI a crop needs varies by roughly an order of magnitude across the range of things grown under cover, which is why a single lighting setup cannot suit everything.
Shade plants such as orchids and ferns want 4 to 8 mol per day. They are adapted to filtered canopy light and direct sun damages foliage. These are also the crops most easily satisfied by supplemental light, and the ones most often over-lit.
Seedlings and leafy greens want 10 to 17. This is the band where hobby supplemental lighting is genuinely economic, and it is why the propagation bench is where lights belong.
Fruiting crops at full growth want 20 to 30. Delivering 25 mol per day across even a modest greenhouse footprint requires a fixture investment and a power draw that does not make sense at hobby scale. In practice, greenhouse tomatoes and peppers depend on natural light, and supplemental lighting for them is a commercial rather than a hobby proposition.
The full table is in the DLI targets by crop chart. The practical conclusion is that a hobby grower should light the propagation bench properly and accept natural light for the main crop, adjusting what they grow in winter rather than trying to light it.
Photoperiod as a signal, not just an amount
Day length does two things at once, and confusing them causes problems that look mysterious.
As an energy input, longer photoperiod at the same intensity delivers more DLI and more growth, up to a point. Most vegetable and flower seedlings do better at 14 to 16 hours than at 12, and worse at 24 than at 16, because they need a dark period for respiration and normal development.
As a signal, day length triggers developmental changes. Several crops bolt in response to lengthening days rather than to heat, notably spinach and some lettuce varieties, so extending photoperiod on those crops can push them to flower and turn bitter. Many ornamentals flower in response to short or long days specifically. Poinsettias and chrysanthemums are short-day plants; several bedding flowers are long-day.
In a greenhouse this matters because supplemental lighting extends day length as a side effect of adding intensity. A 7 day timer that allows different schedules on different days is the tool for managing photoperiod deliberately rather than accidentally.
