PAR Charts

How to Read a Grow Light PPFD Chart

A manufacturer PPFD map is the most useful number a grow light maker publishes, and also the easiest to misread. Below is how to read one without being misled by the way it is drawn.

What a PPFD map actually shows

A PPFD map is a grid of readings taken across the fixture’s footprint at one fixed hanging height. Each cell is a PPFD value in µmol/m²/s — photosynthetic photons landing on one square metre per second at that spot. It is a snapshot of one fixture, at one power setting, at one distance, in one test space. Change any of those and the whole grid changes.

1. The hanging height is half the number

The first thing to find on a map is the height it was shot at. Most makers publish two or three heights; the Spider Farmer SF4000 map is drawn at 12 in and 18 in, and the Mars Hydro FC6500 map at 8, 12 and 16 in. The difference is large. On the SF4000’s own chart the center reads 1821 µmol/m²/s at 12 in and 1371 at 18 in — the same fixture, same power, six inches apart. A PPFD figure quoted without its height tells you almost nothing. See the sourced grids on the SF4000 data page and the FC6500 data page.

2. Read the units carefully

Three numbers get mixed up on spec sheets:

  • PPFD — µmol/m²/s, photons at a point. This is what a map should show.
  • PPF — µmol/s, the fixture’s total photon output. It is a larger number and says nothing about intensity at the canopy. The FC6500 spec sheet lists its PPF (1813 µmol/s) under the words “average PPFD,” which is a mislabel — the unit gives it away.
  • Lux / foot-candles — brightness weighted for the human eye, not PAR. Useful for rough estimates under white light, but not interchangeable with PPFD.

If a “PPFD” figure is in µmol/s, it is really PPF. If it is in lux, it is not PAR.

3. Center vs corners — find your plant’s cell

Every panel is brightest straight underneath and dimmest at the edges, and the gap is bigger than most people expect. The SF4000’s own 12 in chart drops from 1821 at center to about 270 at the 4×4 ft corners — roughly a seventh of the peak. The number that matters is the one over where your plant sits. A canopy filling a tent has plants in those corners, and they see the corner value, not the center.

Maps are often drawn as a square grid but sampled in a circle, which quietly leaves the corners out. If a grid looks suspiciously flat, check whether the true corners were measured at all.

4. Tent size and reflective walls

The same fixture reads much higher in a small, reflective space because the walls bounce light back onto the sensor. A map shot in a 3×3 ft tent will beat one shot in a 5×5 ft tent for the identical light. Manufacturer maps state the test footprint (the SF4000 map is a 4×4 ft tent, the FC6500 a 5×5 ft tent) — match it to your own space before trusting the numbers.

5. What a good map includes

A map you can trust tells you the hanging height, the tent size, the power setting, and enough grid points to show the center-to-corner fall-off. If all you get is a single center number or a bare “average PPFD,” treat it as a best case and assume the delivered light at the edges is well below it. For why these maps tend to read optimistically even when honestly drawn, see are grow light PPFD charts accurate, and for how the whole grid shifts with height see grow light PPFD at different heights.

Sources

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