Lux, Foot-Candles, and Daily Light Integral: What Your Phone Actually Measures

Open a light-meter app over a garden bed and it hands you a number, usually in lux. That number feels precise — four or five digits, updating in real time as you tilt the phone — but precise and meaningful aren't the same thing, and lux on its own doesn't actually answer the question a gardener is asking, which is closer to "will a plant have enough to eat here." Getting from a raw reading to a useful answer means understanding four related but genuinely different units, and being honest about how rough the whole chain of conversion really is.

Lux: a measurement built for human eyes, not leaves

Lux measures illuminance the way the human eye perceives it — weighted heavily toward the green-yellow wavelengths our vision is most sensitive to, and largely blind to the fact that chlorophyll actually absorbs most efficiently at the red and blue ends of the visible spectrum. It's the unit used for office lighting standards and camera exposure, which is exactly why your phone can report it: the sensor and the software behind it were built for photography and screen brightness, not horticulture. That's not disqualifying — lux still correlates reasonably well with how much total light is available — but it means a lux reading is a proxy for plant-usable light, not a direct measurement of it.

Foot-candles: the same idea, older units

Foot-candles are an older, imperial-system measurement of the same basic quantity as lux, and some light meters — particularly older photography meters — report in foot-candles instead. The conversion is fixed and simple: one foot-candle equals roughly 10.764 lux, so a reading of 1,000 lux is about 93 foot-candles. Neither unit is more "correct" than the other; they're the same measurement in different-sized units, the way miles and kilometers both measure distance.

PPFD: closer to what a plant actually uses

Photosynthetic Photon Flux Density (PPFD) measures something meaningfully different: the number of photons of photosynthetically active radiation — roughly the 400–700 nanometre band a leaf actually uses — landing on a square metre each second, in micromoles (µmol/m²/s). This is the number a dedicated PAR meter reports, and it's the figure growers and horticultural researchers actually work with, because it's tied to the wavelengths plants use rather than the wavelengths human eyes are sensitive to. Very few home gardeners own a PAR meter, though, which is why a practical shortcut exists: for ordinary daylight, 1 lux is commonly estimated at roughly 0.0185 µmol/m²/s of PPFD. That factor isn't a law of physics — it shifts with the light's exact spectrum, which changes with cloud cover, time of day, and whether the light reaching a spot is direct, reflected, or filtered through leaves — but it's a reasonable, widely-used estimate for outdoor daylight specifically. It is not a safe conversion to reuse for indoor grow lights or other artificial sources, whose spectra can differ substantially from daylight.

DLI: the number that actually predicts plant performance

Daily Light Integral (DLI) is PPFD integrated over an entire day — the total number of photosynthetically active photons a square metre receives from sunrise to sunset, expressed in moles per square metre per day (mol/m²/day). This is the single most useful number in the whole chain, because it captures both intensity and duration in one figure, and duration turns out to matter enormously. Our light level calculator takes a lux reading and the number of hours it's sustained, and returns the PPFD, DLI, foot-candle equivalent, and a shade category in one step, doing the unit conversion so you don't have to do it by hand each time.

Worked example: a bright, partly-open porch bed

Take a spot that reads 15,000 lux on a phone app, sustained for about 7 hours of a summer day — a fairly typical reading for a bed getting bright, all-day indirect light with some direct sun at the edges. Running that through the calculator: PPFD comes out to 277.5 µmol/m²/s, DLI to 6.99 mol/m²/day, and the foot-candle equivalent to about 1,393.5 fc. At a DLI just under 7, that lands the spot squarely in the partial shade band (roughly 4–10 mol/m²/day) — bright enough for many sun-tolerant shade perennials, but not the partial-sun territory a vegetable garden would want.

Worked example: two very different "shady" spots

Now compare two spots that would both get casually described as "shady." The first, a north-facing bed with open sky but no direct sun, reads 3,500 lux over about 6 hours: PPFD of 64.75, DLI of 1.4 mol/m²/day, foot-candles around 325.2 — deep shade territory, just under the 1.5 threshold. The second, tucked under a dense evergreen, reads only 800 lux over roughly 5 hours: PPFD of 14.8, DLI of just 0.27, foot-candles about 74.3 — also deep shade, but at barely a fifth of the DLI of the north-facing bed. Both spots earn the same one-word label from a casual glance ("shady"), and both land in the same DLI category, but the evergreen bed is receiving dramatically less usable light in absolute terms — a distinction that matters if you're choosing between two genuinely deep-shade-tolerant plants and want the hardier of the two for the darker spot.

Worked example: why duration can matter more than intensity

Here's the clearest illustration of why DLI, not lux alone, should drive a plant decision. Take a single lux reading of 6,000 and run it at two different daylight durations. At 3 hours a day, that reading produces a DLI of 1.2 mol/m²/day — deep shade. At 9 hours a day, the identical lux reading produces a DLI of 3.6 — comfortably into full shade, nearly three times the total light. Nothing about the intensity changed; only how long it lasted. This is exactly why a single quick reading taken at whatever moment you happen to walk past a bed can mislead you — a spot that reads dim at 6,000 lux for a passing glance might actually be gathering that same brightness for most of a long summer day, which adds up to meaningfully more total light than the raw number alone would suggest.

How accurate is any of this, really?

Worth saying plainly: none of these readings are laboratory-grade. A phone's light sensor isn't calibrated the way a dedicated meter is, different phone models and apps can disagree with each other on the same spot, and the lux-to-PPFD conversion factor is itself an estimate for average daylight, not a measurement of your specific light's spectrum. Treat every number in this piece — and every number your own phone gives you — as indicative rather than precise. What the method is genuinely good for is relative comparison: the same phone, the same app, used consistently across several spots in your own yard, will reliably tell you that one bed gets meaningfully more light than another, even if neither absolute number would hold up to a research-grade instrument. That relative comparison is exactly what you need to decide where to put a moisture-loving fern versus a tougher, drier-tolerant groundcover.

Getting a more consistent reading in practice

A few habits narrow the gap between "indicative" and "actually useful." Hold the phone level and at roughly the height your plants will sit, rather than up near your face — a reading taken at eye level over a low groundcover bed can differ meaningfully from what the groundcover itself actually receives, especially near a wall or hedge edge. Take readings on an ordinary clear day rather than an unusually hazy or unusually bright one, and avoid the first and last hour of daylight, when the sun's low angle and any nearby obstructions can produce a misleadingly extreme reading. If you're comparing several beds, take all the readings within the same short window of time — light changes throughout the day even in an open sky, and comparing a morning reading in one bed against an afternoon reading in another isn't really a fair comparison.

It's also worth taking more than one reading per spot before trusting it. A single lux number can be skewed by a passing cloud, a gust that shifts a branch out of the way, or simply where exactly you're standing within a bed that isn't perfectly uniform. Two or three readings a few minutes apart, averaged, smooth out most of that noise without adding much time to the process.

What this doesn't measure

It's worth being clear about the boundaries of this whole method, too. None of lux, foot-candles, PPFD, or DLI say anything about soil moisture, root competition, or how a spot's light changes through the year as deciduous trees leaf out and drop their leaves — all of which matter just as much as light level for whether a plant actually thrives. A single reading is a snapshot of one day's light, not a complete picture of a bed's growing conditions. Treat it as one solid input among several, not the whole answer.

Turning a reading into a plant decision

Once you have a DLI and a category, the practical next step is feeding that category into a plant choice — our companion piece on going from a light reading to a plant list walks through exactly that pipeline with its own worked example, and the shade plant selector takes the category alongside your hardiness zone and soil moisture to shortlist real plants. For a table that ties measured light straight through to plant groups, spacing, and watering in one place, see our shade level reference.