Lux to PPFD: what a light meter reading actually tells your plants

A lux meter reads how bright light looks to a human eye; a PPFD reading is how many photons a plant can actually put to work. The two agree, even loosely, under exactly one condition — a white LED fixture, where PPFD in µmol·m⁻²·s⁻¹ comes out to roughly 15 times the illuminance in kilolux. Point the same lux meter at an HPS bulb, a fluorescent tube, or a red-blue LED panel, and that number stops meaning anything, because the conversion depends entirely on spectrum, not brightness.

That is the part a lux app tends to leave out. It will happily report a number for any light in the room, but the number only converts into something a plant cares about under one specific type of fixture. For real PPFD and DLI measurement — the readings that actually predict growth — PPFD and DLI for indoor herbs covers how to map a shelf properly with a sensor. The question below is narrower and more common: what does a lux number mean, and when can it be trusted at all.

Why lux and PPFD measure different things

Lux is a human-vision unit. It weights every wavelength by how sensitive the human eye is to it, and that sensitivity peaks in green light. A lux meter is, in effect, built to mimic your eye: it reports green light as valuable and treats deep red and blue as nearly invisible. Purdue Extension puts it plainly: foot-candles and lux are photometric units based on what the human eye detects, which makes them “focused on people and not appropriate for indicating plant photosynthesis.”

Plants do not see. Photosynthesis runs mostly on red and blue light, with green being the band chlorophyll reflects rather than absorbs — which is also why leaves look green in the first place. PPFD, photosynthetic photon flux density, ignores human perception entirely and counts every photon in the band Purdue Extension defines as photosynthetically active radiation — 400 to 700 nanometers — regardless of color.

Put those two facts together and the consequence is unavoidable: two lights can produce identical lux readings while delivering very different amounts of usable light to a plant. A fixture heavy in green and yellow can look bright to a meter and to your eye, while contributing relatively little to actual growth. A fixture rich in red and blue can read dimmer in lux while doing more photosynthetic work.

Purdue Extension puts a number on that gap using a foot-candle meter: multiply the reading by 0.20 to estimate PAR from sunlight, but only by 0.13 from an HPS lamp — a roughly 35% difference on the same meter reading, from two sources that can register identical lux. That is the cleanest sourced proof that a lux meter cannot be corrected into a PAR meter with one general rule; the correction has to know what kind of light it is reading.

The one estimate that holds — and only there

White LED grow lights are the exception, and only a partial one. Because most white LED chips are built around a similar phosphor-converted blue diode, their spectra are broadly similar to one another, which is why a rough conversion survives at all: PPFD in µmol·m⁻²·s⁻¹ is approximately 15 times the illuminance measured in kilolux. The figure comes from a single source — Sharakshane (2018), a bioRxiv preprint that regressed the estimate over a database of 205 white-LED spectra. It has not been peer-reviewed, which is worth knowing before you treat 15x as settled: it is the best estimate available, not a verified constant.

A 4-kilolux reading under a white LED panel, then, suggests a PPFD somewhere around 60 µmol·m⁻²·s⁻¹. That is an estimate for a quick sanity check, not a substitute for an actual PPFD reading — treat it the way you would treat a kitchen scale that has not been calibrated in a while: useful for noticing that something changed, not for a number you would defend.

This 15x multiplier applies to white LED only. It does not transfer to blurple red-blue fixtures, to HPS, or to fluorescent tubes. Each of those has a different spectral fingerprint: HPS and sunlight have their own source-specific factors from a foot-candle meter, but blurple LEDs and fluorescent tubes have no published factor at all. If you are comparing a fluorescent tube against an LED fixture, a lux meter cannot settle which one is delivering more usable light — you would need a PPFD reading from each.

Spectrum is the variable a lux meter cannot see through

If you are trying to understand why fixtures are described by their color mix, that is the short version: two panels marketed at the same lux output can have meaningfully different PPFD, depending on how much of that lux comes from red and blue photons versus green and yellow ones. HPS, fluorescent tubes, and red-blue LEDs each pull lux and PPFD apart by a different amount, which is why none of them gets the shortcut white LED gets.

Sunlight through a window is the clearest example

Point a lux meter at a bright windowsill and it can easily read similar to a white LED panel hanging over a seedling tray. That similarity is exactly why lux readings feel encouraging even when a windowsill setup is not delivering enough usable light — daylight’s spectrum and an LED’s spectrum are different enough that matching lux numbers do not imply matching PPFD.

This is one reason plants started on a bright-seeming windowsill still stretch toward the glass: the eye and the lux meter agree the spot looks bright, but the photosynthetically active portion of that daylight may be considerably lower than a comparable lux reading under a grow light would suggest, especially as the day goes on and the angle and cloud cover change. If you are weighing a windowsill against supplemental lighting, grow lights versus sunlight goes further into that comparison, and growing herbs without direct sunlight covers the setup question directly.

One reading rarely represents the whole shelf

Even where the white LED estimate applies, a single lux reading at one point tells you about that point, not about the tray. Beam angle changes how photons spread: a narrow-optic LED concentrates light into a tight, bright cone directly beneath it, while the corners of the same shelf can sit at a fraction of that intensity. A lux reading taken at the brightest spot, then run through the 15x estimate, will overstate what the rest of the canopy is actually getting.

The fix is the same whether you are working in lux or proper PPFD: measure at more than one point, at plant height rather than shelf height, and treat the center reading as the best case rather than the average case. The seedling-height version of this problem — how far a fixture should sit above trays in the first place — is covered in grow light distance for seedlings, since distance and beam angle interact directly with how even a shelf’s light actually is.

What to do with a lux number in practice

A lux meter or phone app still has a real, limited use: comparing positions under the same fixture, or watching for a bulb that has visibly dimmed with age. What it cannot do is tell you whether a shelf meets a plant’s light requirement, because that answer depends on PPFD and the hours a fixture runs, not on perceived brightness.

If you already have a lux reading under a white LED and want a rough PPFD from it, the 15x-per-kilolux estimate gets you a starting number. From there, converting that PPFD into a daily total — and comparing it against what herbs and leafy greens actually respond to — is a separate step. Daily light integral targets for herbs and leafy greens covers what those numbers should look like once you have something more reliable than a lux app to work from. And if the reading suggests the fixture itself is underpowered rather than just poorly aimed, the best grow lights for herbs is the place to compare options instead of chasing a better multiplier.

None of this makes a lux meter useless — it is often the only light meter someone already owns. It just answers a narrower question than it appears to. A number on a screen that says “4,200 lux” looks precise and complete. What it is actually reporting is how a human eye would judge that spot, filtered through a sensitivity curve a plant does not share. Treat it as a rough, fixture-specific hint, confirm anything that matters with an actual PPFD reading, and do not let a confident-looking lux figure stand in for one.

Sources

• Purdue Extension HO-238-W — Measuring Daily Light Integral in a Greenhouse
• Sharakshane, A. (2018), “An easy estimate of the PFDD for a plant illuminated with white LEDs,” bioRxiv preprint