Growers and specifiers work in two different currencies and lose money at the exchange rate. Fixtures are sold on PPF and PPFD; crops respond to daily light integral. The conversion between them is one multiplication, but getting the inputs right is what separates a lighting plan that hits its yield target from one that overspends on fixtures for the same delivered photons. This page gives the conversion, a calculator that returns DLI and the fixture count, and the crop target table our horticultural group uses in proposals.

1. What DLI, PPFD, PPF and efficacy each mean

Each of the four quantities describes the same photons at a different point in the chain, and the confusion between them is the most expensive error in horticultural lighting. A fixture emits photons; those photons spread out as they travel; a surface receives some share of them; and the crop integrates what it receives over the whole photoperiod. Skipping any step in that chain produces a number that looks plausible and is wrong by a factor.

Four quantities carry the whole conversation, and confusing them is the first cause of an over- or under-lit room.

The relationship between PPFD, photoperiod and DLI is a single identity:

DLI = PPFD x photoperiod (h) x 3600 / 1,000,000

The factor 3600 converts hours to seconds and 1,000,000 converts micromoles to moles. Because PPFD and photoperiod multiply, the same DLI can be delivered in many ways, and the choice between them is a crop-management decision rather than an arithmetic one.

2. Calculator

The calculator below applies one multiplication and one division. The multiplication converts PPFD and hours into a daily photon total; the division converts that total into a fixture count once the growing area and the per-fixture photon output are known. Everything else on this page exists to make sure the numbers going in are the right numbers, because the arithmetic itself is trivial and the inputs are not.

Enter the average PPFD at canopy level and the photoperiod. The calculator returns DLI and, if you also supply the growing area and the fixture PPF, the number of fixtures required.

The utilisation factor accounts for light that misses the canopy, wall losses and non-uniformity at the tray edges. Commercial indoor practice sits between 0.7 and 0.85; below 0.7 usually means the layout needs redesign rather than more fixtures.

3. The PPFD and photoperiod plane

The chart below is the calculator drawn as a map. Each curve is one DLI value, and the bands mark the ranges that commercial practice uses for the main crop classes. Read it from the crop to the axis: a lettuce target of 15 mol/m2/d at a 16-hour photoperiod calls for roughly 260 umol/m2/s, and the same target at 20 hours calls for only about 210.

The three axes of the chart are the photoperiod, the PPFD and the resulting DLI, and any design fixes two of them and reads the third. Practical projects almost always fix the photoperiod first, because the crop dictates it and the operating schedule is built around it, and then read the required PPFD from the target DLI. Where the resulting PPFD is beyond what the mounting height can deliver, the photoperiod is the variable that gets revisited, and that revisit is where a crop with a strict dark period runs into trouble.

Note that the curves bend less at long photoperiods than at short ones. Doubling the photoperiod from 8 to 16 hours does not halve the required PPFD, because the relationship between the hours and the required irradiance is linear only along a fixed DLI curve and the curves are drawn on a linear PPFD axis. Reading the band boundaries rather than a single point is the safer way to use the chart, since the target ranges themselves are bands rather than lines.

4. Crop target table

The ranges below are those used in our greenhouse and vertical-farm proposals, and they are the values the calculator checks against. Greenhouse practice treats the DLI figure as the combined natural plus supplemental total; sole-source indoor production supplies the whole amount.

A DLI target without a photoperiod is ambiguous, and a photoperiod without a target is a schedule rather than a specification. The table below pairs them, and the photoperiod column is as important as the DLI column because a crop that will not flower under a 20-hour day cannot simply be given more hours to reach a target. Where the two constraints conflict, the photoperiod wins and the PPFD has to rise to compensate, which usually means more fixtures or a shorter mounting distance.

The ranges are deliberately wide. Cultivar, growing medium, temperature and the last irrigation before harvest all shift the optimum within the band, and a commercial grower with a specific cultivar and a specific market grade will usually know the target better than any general table. The table exists to establish the order of magnitude and to catch a proposal that is out by a factor of two, not to replace local data.

5. From DLI target to fixture count

The chain from a chosen DLI to a purchase order is five steps, and each one has a trap that the calculator helps avoid.

The five steps below are the whole design, and each one is a place where a proposal quietly loses a few percent. Accumulated across the chain, the losses can reach thirty to forty percent between the catalogue figure and the delivered photons, which is the difference between a design that meets its target and one that under-delivers by the margin of a whole fixture row.

The utilisation factor in step three deserves particular attention because it is the most commonly overstated input. It combines the fraction of emitted photons that reach the canopy plane with the uniformity of their distribution. A well-designed overhead installation with reflective walls reaches 0.8 or better; an open installation with a dark floor and the fixtures at a low mounting height may not reach 0.6. Using 1.0 assumes every emitted photon lands usefully on the canopy, which no real installation achieves.

6. Photon efficacy and the electricity bill

The same delivered DLI costs very different amounts of electricity depending on the platform. The chart below shows why the choice of fixture generation, not just the fixture count, decides the operating cost of a room.

Multiplying the energy index by the DLI and the area gives the daily electricity for lighting; that figure, not the fixture price, is what determines payback. A worked example: a 100 m2 lettuce room at DLI 15 needs 1,500 mol per day. A platform at 3.0 umol/J delivers those photons for 139 kWh of lighting energy, whereas a platform at 1.6 umol/J needs about 260 kWh for the same photons. At a tariff of 0.15 per kWh the annual difference is roughly 6,600, and it repeats every year.

Photon efficacy decides the operating cost, and the difference between fixture generations is large enough to change the answer to a build-or-not question. A fixture at 2.6 micromoles per joule delivers the same DLI for roughly half the electricity of one at 1.4, so over a five-year project the electricity saved by the better fixture can exceed its purchase price. This is why photon efficacy, not fixture price, is the number that belongs in a capital comparison.

The chart below also shows why retrofitting an older installation is usually the cheapest capacity increase available. Replacing a first-generation fixture with a current one at the same photon output releases electrical capacity, which in a building with a fixed supply can allow more fixtures in the same room without any electrical upgrade at all.

7. Why the same DLI is not the same crop

Two rooms can deliver an identical DLI and produce visibly different plants. Three mechanisms explain the gap, and each has practical consequences.

Photoperiod sensitivity. Short-day crops such as chrysanthemum and cannabis need an uninterrupted dark period; delivering the DLI over 20 hours can suppress flowering entirely. Long-day crops behave the opposite way. The DLI figure must be reached inside the photoperiod the crop will accept.

Peak intensity and leaf stress. A very high PPFD for a short period can exceed the leaf’s photosynthetic capacity and waste photons while heating the canopy. A gentler intensity over a longer day delivers the same DLI with less stress, which is why extending from 12 to 18 hours at a lower PPFD usually beats a short, intense day.

Spectrum. DLI counts photons in the 400 to 700 nm band, but morphology, flowering and secondary metabolism also respond to far-red and blue. Two fixtures with equal DLI can still produce different plant architecture. Our far-red white papers cover this in detail.

Three mechanisms explain why two rooms with identical DLI figures produce different plants: the timing of the light, the intensity at which it arrives, and the spectral composition. All three operate at once, and a design that manages only the total will still be surprised by the crop.

The spectral mechanism is the subtlest of the three. Two photons carry the same energy for photosynthesis regardless of wavelength, so a red photon and a green photon count the same toward DLI, but they do not have the same effect on plant shape, flowering or secondary metabolism. A DLI achieved mostly in the green region produces a stretched plant with a poor internal structure even though the photon count is correct. Where morphology matters as much as biomass, the spectrum has to be specified alongside the DLI.

Peak intensity is the second mechanism and is often overlooked because it is invisible in an average figure. A canopy that receives 400 micromoles for part of the day and 100 for the rest can show the same daily total as one that receives 250 throughout, but the high-intensity period may exceed the photosynthetic capacity of the leaf and the low-intensity period may fall below the light compensation point, so a portion of the delivered photons is simply not used. Uniform intensity within the photoperiod is the safer design.

8. Checking the result against a real design

A vertical farm runs four tiers of lettuce. Each tier is 8 m2, the target is DLI 15, and the photoperiod is 16 hours.

Note how the final two rows move when the efficacy assumption changes. Because the fixture count is set by the PPF requirement, a higher-efficacy platform does not reduce the number of fixtures – it reduces the power each fixture draws and therefore the electricity bill. That is the mechanism that makes an efficiency upgrade pay back without changing the layout.

A worked example makes the chain concrete. The numbers below follow the five steps from the previous section for a four-tier lettuce installation, and the verdict column shows where a real design would be checked rather than assumed.

Note the utilisation figure of 0.8 in the second row. It is applied to the area conversion rather than left out, because a design that omits it under-buys by twenty percent and then discovers the shortfall as a dim edge row. Note also that the fixture count is derived from the photon requirement rather than from the number of fixtures that fit the rack, which is the reverse of how a room is often laid out and the reason so many installations are slightly under-lit.

9. Common errors

The errors in the table below are ordered by how often they appear in proposals we review, and the first three account for the great majority of under-performing installations. Each is a mismatch between an input and the condition it was measured under, not an arithmetic mistake, which is why a calculator alone does not prevent them.

10. Contact and layout review

QUEENDOM supplies the Z-01, Z-02, Z-03 and Z-04 grow tube families with published PPF and operating-temperature efficacy, and our horticultural engineers will convert a crop, an area and a tariff into a DLI-and-payback calculation and a fixture layout at no charge for project opportunities. Send the crop, the growing area, the ceiling height and the local electricity tariff.

Related products and applications

• Horticulture grow light (Z-01)
• Horticulture grow light (Z-02)
• Horticulture grow light (Z-03)
• Horticulture grow light (Z-04)
• Application overview: Lighting application solutions
• More technical papers: Lighting knowledge resources

Seasonal and Crop-Stage DLI Reference

The converter assumes the PPFD you enter runs for the photoperiod you enter, every day. In real greenhouses the outdoor daily light integral changes by an order of magnitude between December and June at mid latitudes, so the same fixture delivers a very different supplemental fraction across the season.

Outdoor DLI by latitude and season (typical clear-sky greenhouse values):

Crop targets below are intake values for mature canopies; seedlings and transplants typically need 30 to 50 percent less until roots establish.