Reading the numbers, and "bright indirect" decoded

Foot-candles, lux, PPFD and DLI: four ways to put a number on light, plus what "bright indirect light" actually means.

How light is measured, and what the numbers mean

You'll run into several units. The two you'll see most often are the foot-candle and the lux (Torres & Lopez, 2010), and they measure the same thing on different scales: one foot-candle is about 10.8 lux (Thompson & Taylor, 2008). Both count light as the human eye perceives it (Steil, 2023b). Purdue's extension guide calls foot-candles photometric units, based on the visible light the human eye detects, which is mainly green (Torres & Lopez, 2010). In daylight, our eyes are most sensitive to greenish-yellow light (National Institute of Standards and Technology, 2019), so in our reading a foot-candle or lux reading gives those colors extra weight. A plant uses light across the band from blue to red (Liu & van Iersel, 2021), and Missouri's extension guide advises using light measurements only as a guide (Trinklein, 2017). That's usually what a phone light-meter app reports.

The unit that counts what a plant uses is PPFD, photosynthetic photon flux density: the number of photosynthetically active photons landing on a square meter each second, counted in micromoles per square meter each second (µmol·m⁻²·s⁻¹; a micromole is simply a fixed count of photons) (Torres & Lopez, 2010; Steil, 2023b). DLI, the daily light integral, adds PPFD up over a whole day into moles per square meter per day, so it captures both intensity and duration in one number (Torres & Lopez, 2010). Neither has an imperial version. Virginia Tech's extension guide calls DLI among the most useful measures of how much light a plant receives (Stallknecht, 2025), and Faust and Logan have mapped outdoor DLI across the United States month by month (Faust & Logan, 2018). Lesson 8 puts it to work.

In sunlight, you can roughly convert a foot-candle or lux reading into PPFD. Virginia Tech gives about 5 foot-candles for every unit of PPFD (Stallknecht, 2025), which works out to about 54 lux. So a reading of 1,000 foot-candles (about 10,800 lux) in sunlight is roughly 200 µmol·m⁻²·s⁻¹. The factor depends on the light source (Torres & Lopez, 2010), so we use it for orientation only. Any reading from a meter, even a rough one, tells you more than your eye does.

Units-ladder diagram: lux (eye and phone, green-weighted) leading to PPFD (photons per second the plant uses) leading to DLI (the day's total), as three stacked rungs with eye, leaf, and sun-clock icons. Walnut line on cream.

Fig. 5. Three ways to measure light. Foot-candles and lux measure brightness as the eye sees it (what a phone app reads), PPFD counts the photons a plant can use, and DLI adds those photons up across the whole day.

"Bright indirect light," decoded

Iowa State's extension service defines bright indirect light as light that casts a distinct shadow, but without the sun's rays falling directly on the leaves, and notes that most houseplants, even those that tolerate low light, grow and look their best in it (Steil, 2023a). So "indirect" doesn't mean dim. The New York Botanical Garden's houseplant guide puts bright indirect light up to about 5 feet (1.5 meters) from a south-facing window, with direct sun in the first 2 to 3 feet (LuEsther T. Mertz Library, n.d.). A curtain cuts a lot of light: Nebraska's extension guide counts sunlight filtered through a curtain as low light (Baxendale et al., 2013).

Putting numbers on the light levels

There's no agreed number for "bright indirect," and extension guides draw their lines in different places. The University of Florida's midday bands are a practical place to start (UF/IFAS Gardening Solutions, n.d.). We've added the PPFD each band works out to in daylight, using Virginia Tech's figure of about 5 foot-candles for each unit of PPFD under sunlight (Stallknecht, 2025).

Light level	Foot-candles (lux)	PPFD in daylight, µmol·m⁻²·s⁻¹	Where you'll find it
Low	25–100 (about 270–1,100)	about 5–20	Little natural light, often far from windows
Medium	100–500 (about 1,100–5,400)	about 20–100	Near a window, but no direct sun
High	500–1,000 (about 5,400–10,800)	about 100–200	Near a window, any direct sun softened by a sheer or by trees outside
Direct sun	over 1,000 (over 10,800)	over 200	Right at the glass, with nothing between the plant and the sun

In our reading, Iowa State's bright indirect light, with a distinct shadow but no sun on the leaves, spans the upper part of Florida's medium band and its high band. Other guides would label some of those spots differently. Clemson's extension guide, for example, counts anything from 150 to 1,000 foot-candles as high light (Shaughnessy & Pertuit, 1999). So treat the category names loosely and the numbers as the thing to compare.

Under grow lights

The daylight column above doesn't carry over to grow lights, because the conversion depends on the light source. Virginia Tech gives about 5.0 foot-candles per unit of PPFD for sunlight, but 6.9 for cool-white fluorescent tubes, 6.6 for metal halide lamps and 7.6 for high-pressure sodium lamps (Stallknecht, 2025). LED grow lights vary with their mix of colors. Because foot-candles and lux count light as the human eye perceives it (Steil, 2023b), and in daylight the eye is most sensitive to greenish-yellow light (National Institute of Standards and Technology, 2019), we'd expect red-and-blue LED panels, which have become more common (Weisenhorn & Hoidal, n.d.), to read low on a lux meter compared with the light a plant actually gets. Iowa State's advice is to set whatever app or meter you use for the kind of light you're measuring; meters that measure PPF or PPFD directly, such as quantum flux meters, can be expensive (Steil, 2023b).

Under lights you control the hours as well as the brightness, so it's easiest to work backward from a daily light integral. Iowa State's formula is DLI = PPFD × hours × 0.0036 (Steil, 2023b). Turned around and applied to Iowa State's DLI targets, it gives the PPFD to aim for at the leaves:

Plants (Iowa State DLI target)	Lights on 12 hours a day	Lights on 14 hours a day
Foliage houseplants, low light (3–6)	about 70–140	about 60–120
Foliage houseplants, medium light, and some flowering houseplants (6–10)	about 140–230	about 120–200
Flowering houseplants, succulents, and herbs and vegetables grown for their leaves (12–16)	about 280–370	about 240–320
Herbs, fruits and vegetables, very high light (18–30)	about 420–690	about 360–600

PPFD in µmol·m⁻²·s⁻¹. These figures are our arithmetic on Iowa State's targets, not tested recommendations for any one plant. Measure at the level of the foliage, in several spots, and average them, as Iowa State suggests; a fixture gives more light the closer it is to the leaves (Steil, 2023b).

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

Your phone's light-meter app gives you a reading at the windowsill. Why treat it as a relative guide rather than exactly what the plant experiences?

Because foot-candles and lux are weighted to the human eye, which is most sensitive to greenish-yellow light, so they measure brightness as you see it rather than the photons a plant uses. PPFD counts those photons, and DLI adds them up over a day. A rough meter reading still tells you more than your eye does.

A care tag says "bright indirect light," so you tuck the plant into a dim corner. What did you get wrong?

"Indirect" doesn't mean dim. Bright indirect light casts a distinct shadow, but the sun's rays don't fall directly on the leaves. Up to about 5 feet back from a south-facing window is a typical example.

At midday your phone app reads about 3,000 lux beside a plant near a window, with no sun on the leaves. Roughly which light level is that, and about what PPFD?

About 280 foot-candles, which puts it in the medium band (100 to 500 foot-candles). In daylight that's roughly 55 µmol·m⁻²·s⁻¹ of PPFD, dividing lux by about 54. Under a grow light the same reading could mean a different PPFD, because the conversion depends on the light source.

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