Lux, PAR, and Foot-Candles: A Plain-English Guide to Plant Light

Ask a plant store employee how much light a monstera needs and you'll get "bright indirect." Ask a horticulturist the same question and you'll get "around 200 PPFD, with a daily light integral of 10–12 mol/m²/day." Same plant, very different answers.

The gap between casual plant advice and measurable plant light is confusing for most hobbyists — and it matters more than any other environmental variable for long-term plant health. Here's a plain-English breakdown of the three units you'll run into, what each measures, and how to turn a light reading into a decision about where a plant should sit.

Lux (and why your phone's light meter is lying to you)

Lux is a measure of perceived brightness to the human eye. It's built on a weighting curve — the photopic luminosity function — that emphasizes the wavelengths we see best, the greens and yellows in the middle of the visible spectrum, and all but ignores the deep reds and blues at the edges.

That weighting is the problem. Plants run photosynthesis mostly on red and blue light; green is partly reflected, which is why leaves look green in the first place. A lux reading therefore systematically overstates how useful a given light is to a plant, because it counts most heavily the very wavelengths the plant is throwing back. Two lights that read identical in lux can drive very different amounts of photosynthesis if their color mix differs.

If you've ever downloaded a "light meter" app for your phone, lux is what it reports — the phone's sensor is tuned to human vision, not plant vision. That's why plants don't always thrive in a spot your app rates as "bright."

PAR and PPFD (the numbers that actually matter)

PAR stands for Photosynthetically Active Radiation — light between 400 and 700 nanometers, the band plants can actually use for photosynthesis. PAR is a range of wavelengths, not a single number, which is why you never see "how much PAR" written as one figure.

PPFD (Photosynthetic Photon Flux Density) is the number: how many photons in the PAR range land on a square meter each second, measured in µmol/m²/s. Crucially, it counts photons rather than perceived brightness, so a red photon and a green photon carry equal weight — much closer to how a leaf sees the world. PPFD is the unit on commercial grow-light spec sheets and in research papers.

Rough targets by plant type:

• Low-light tolerant (pothos, snake plants, ZZ): 50–150 PPFD
• Medium light (most tropicals, philodendrons, calatheas): 100–250 PPFD
• Bright indirect (monstera, fiddle leaf fig, alocasias): 200–400 PPFD
• Full sun lovers (succulents, citrus, most vegetables): 400–800+ PPFD

The University of Georgia's greenhouse research extension publishes detailed PPFD tables for commercial production if you want species-level data.

DLI: The Daily Total That Predicts Growth

PPFD tells you light intensity at a moment. DLI (Daily Light Integral) tells you how much total light a plant received over the whole day, measured in mol/m²/day — essentially PPFD added up across every hour the light was on.

DLI is what actually predicts growth. A plant getting 300 PPFD for 12 hours fares better than one getting 500 PPFD for 4 hours — even though the second peak is higher — because the total daily light delivered is greater. This is why duration matters as much as intensity, and why a bright but brief flash of afternoon sun can look impressive on a meter yet do little for a plant.

Typical DLI targets:

• Most houseplants: 6–12 mol/m²/day
• Herbs and leafy greens: 12–17 mol/m²/day
• Flowering/fruiting plants: 20+ mol/m²/day

Typical Indoor Light, by Location

The single most sobering thing about measuring indoor light is discovering how little of it there usually is. Outdoors on a clear summer day, direct sun runs on the order of 100,000 lux — very roughly 2,000 PPFD. Almost nothing indoors comes close. As a rough guide (daylight, Northern Hemisphere):

• On a sunny south-facing sill, right at the glass: can reach 10,000 lux or more while the sun is on it — genuine bright light, but only for the hours the sun is there
• Beside an east- or west-facing window: a few thousand lux during its sunny stretch, much less the rest of the day
• A north-facing window: rarely above 1,000–2,000 lux, and no direct beam at all
• Three or more feet back from a bright window: often just a few hundred lux — light falls off far faster with distance than it feels like it should
• The interior of a room, away from windows: 100–500 lux, which is effectively deep shade to a plant

Convert those figures (very roughly, lux ÷ 50 ≈ PPFD) and the lesson lands hard: a "bright" spot a few feet from the glass may deliver well under 100 PPFD — short of what a monstera or fiddle leaf fig wants — which is why so many houseplants merely hang on rather than grow. The fix is usually simple once you can see the number: move the plant closer, choose a brighter exposure, or add a grow light.

Foot-Candles (you'll still see this in older books)

1 foot-candle ≈ 10.76 lux. It carries the same perceived-brightness bias as lux, just in imperial units. If you have an older gardening book, this is the unit you'll find. Convert to lux and mentally discount how useful the number is, for the same reason lux overstates things.

Why Continuous Measurement Changes the Answer

Light levels in a home change dramatically across the day and across the year. A spot that reads 400 PPFD at noon in July might only hit 80 PPFD at the same hour in December, as the sun drops lower and takes a shallower path across the sky. Cloud cover can knock a reading down by 70% within minutes.

A one-time reading captures one instant of that moving target and tells you almost nothing. A continuous reading — sampled through the day, every day — shows you the whole arc: when the light arrives, how high it climbs, and how long it holds.

That's why PlantSense uses a dedicated PAR sensor rather than a standard lux sensor: it responds to the 400–700 nm band that actually drives photosynthesis, not to perceived brightness. It samples that light about every 90 minutes and charts the day's arc, reporting the peak as a lux value you can compare against a plant's needs. Watching where that daily peak lands — and how long the light stays high before it falls off toward evening — tells you far more than one glance at a corner that merely looks bright.

The Practical Takeaway

If you want to use the light info in care guides:

1. Don't trust phone lux readings as a stand-in for plant light.
2. If a care guide lists a PPFD range, that's the useful number.
3. If you only have access to lux, a very rough conversion for typical daylight is lux ÷ 50 ≈ PPFD — but that factor depends on the light's spectrum, so it doesn't hold for LEDs, fluorescents, or grow lights. For those, trust a PAR meter or the manufacturer's measured PPFD, not arithmetic on a lux figure.
4. For long-term health, the daily total matters more than the peak, so favor a spot that stays bright for hours over one that spikes briefly.
5. Judge a spot by its daily pattern, not a single reading: a peak that reaches the plant's range and holds beats a higher number that lasts ten minutes.

Better light data is the fastest way to end most plant-care guessing. It turns "I think this spot is bright enough" into a number you can act on.