PAR Simulator

1. PPF vs. PPFD: What's the Difference?

Two terms appear constantly when comparing grow lights: PPF and PPFD. They are related, but they describe different things.

• PPF (Photosynthetic Photon Flux): The total amount of PAR photons emitted by a light source each second. It is measured in micromoles per second (µmol/s).
• PPFD (Photosynthetic Photon Flux Density): The amount of PAR reaching a square meter of plant canopy each second. It is measured in micromoles per square meter per second (µmol/m²/s).

A fixture can have a high PPF but still produce relatively low PPFD if that light is spread over a large area. PPFD is therefore especially useful when estimating how much usable light actually reaches the leaves.

2. Why Hanging Height Changes PPFD

As a grow light is raised, its beam spreads across a larger area. The fixture is still producing roughly the same total PPF, but those photons are distributed over more space. The average PPFD at the canopy therefore decreases.

Lowering the fixture usually creates a smaller, more intense footprint. Raising it increases coverage but reduces average intensity.

• Lower height: smaller coverage area and higher estimated PPFD.
• Greater height: larger coverage area and lower estimated PPFD.
• The goal: find a height that provides enough intensity while still covering the canopy evenly.

Real fixtures do not distribute light perfectly evenly, so the center of the grow area may receive more light than the edges. The simulator gives you a useful average rather than a complete PPFD map.

3. How Beam Angle Changes Coverage

Beam angle describes how widely a fixture spreads its light. A narrow beam concentrates photons into a smaller footprint, while a wide beam spreads them across a larger area.

• Narrow beam: more concentrated light and a smaller footprint.
• Wide beam: broader coverage with lower average intensity.
• Fixture height matters too: the same beam angle covers more area as the light is moved farther from the canopy.

The simulator combines your selected beam angle and hanging height to estimate the circular footprint of the light, then divides the fixture's PPF across that area.

5. Understanding Your PPFD Result

The number produced by the simulator is an estimate of average PPFD at canopy level. It represents light intensity at that moment, not the total amount of light the plant receives over an entire day.

A higher PPFD means more photosynthetically active photons are reaching the canopy each second. Whether that is desirable depends on the plant, its stage of growth, the number of hours the light operates each day, temperature, water, nutrition, and other growing conditions.

For example, vegetable seedlings commonly perform well under roughly 200-400 µmol/m²/s. Mature plants and high-light crops may require substantially different levels, so always consider the requirements of the species you are growing.

6. PPFD Is Not the Same as Daily Light

PPFD tells you how much PAR is arriving each second. Plants, however, experience that light over many hours. The accumulated amount of PAR received during a full day is called the Daily Light Integral (DLI).

This means two plants receiving the same PPFD can receive different daily amounts of light if one fixture runs for 10 hours and another runs for 16 hours.

For now, use the PAR Simulator to compare fixture intensity and coverage. When planning an entire growing schedule, also consider how many hours per day the light will operate.

7. Getting Better Results From the Simulator

The quality of the estimate depends on the information you enter. For the most useful result:

1. Find the fixture's actual PPF. Check the manufacturer's specification sheet for a value in µmol/s. Do not confuse PPF with wattage, lumens, or efficiency expressed in µmol/J.
2. Measure from the fixture to the canopy. Use the actual distance to the tops of the plants rather than the height of the fixture from the floor or growing table.
3. Use the manufacturer's beam angle when available. If none is published, experiment with several reasonable values to see how much the estimated footprint changes.
4. Compare several heights. Try the same fixture at 20, 30, 40, and 50 cm to see the tradeoff between intensity and coverage.
5. Verify important setups with measurements. A PAR or quantum meter at canopy level provides much better information about the real fixture than a geometric estimate alone.