Plant light, in plain terms
How much light is your plant actually getting?
“Bright, indirect light” isn’t something you can measure. This page explains what Gnode measures, how it turns that into a number your plant cares about, and how far you can trust it. Plain words come first. The technical terms follow for anyone who wants to look them up.
The short version
- Your eyes and your plants want different light. Eyes are best at seeing green and yellow. Plants mostly use red and blue.
- Light meters and phone apps measure brightness as people see it. That unit is called lux.
- Plants care about how much usable light lands on a leaf. That’s called PPFD. Add it up over a whole day and you get DLI, the day’s total.
- Gnode measures lux, then converts it. It multiplies by a factor for the kind of light you tell it you have. For most kinds of light the result is usually within about 20–30% of what a lab-grade meter would read.
Four words, in plain terms
Lux
How bright it looks to you
What a phone app or light meter reads. It’s weighted toward green light, because that’s the color human eyes see best, so it says more about how a room looks to you than about what a plant can use.
PPFD · µmol/m²/s
How much usable light is landing right now
Picture light as rain falling into a bucket. PPFD is how hard it’s raining at this moment. Technically, it counts the particles of light (photons) in the colors plants use, 400–700 nm, landing on a square meter each second.
DLI · mol/m²/day
The day’s total
In the bucket picture, DLI is how full the bucket is at sunset. A plant’s growth follows this daily total more than any single moment, so a spot with gentle light all day can beat one with a short blast of sun.
DLI = average PPFD × hours of light × 0.0036
VPD · kPa
How thirsty the air is
Not light at all, but it decides whether a plant can use the light it gets. Air that’s too dry or too damp slows a plant down. There’s a slider for it at the bottom of this page.
Why lux on its own isn’t enough
Two lights can read exactly the same on a lux meter and give a plant very different amounts of usable light, because each one is a different mix of colors. At the same 10,000 lux:
The purple grow light looks dim to you because most of its energy is red and blue, the colors your eyes undercount and plants use most. That’s why Gnode asks what kind of light your plant is under.
Showing our work
How Gnode turns lux into PPFD
-
The sensor reads lux
Gnode uses a TSL2591 light sensor. It has two channels: one sees visible plus infrared light, the other sees mostly infrared. The chip maker’s formula combines the two into lux. The sensor also adjusts its own sensitivity, so the same Gnode works in a dim hallway and on a bright windowsill.
-
You pick the kind of light
Each kind of light (sun, window daylight, household LED, grow light, and so on) has a conversion factor that depends only on its mix of colors. How bright the light is is already in the lux number, so the factor doesn’t change with brightness.
-
Multiply
PPFD ≈ lux × factor
That’s the whole calculation. The firmware does it on the device, and GnodeHub recalculates it with its own copy of the factors, so a corrected factor fixes every reading at once.
Worked example
A plant near a bright window reads 12,000 lux. Daylight through a window has a factor of 0.019:
12,000 × 0.019 = 228 µmol/m²/s, give or take about 25% (roughly 170–285)
If the spot averages that for 10 hours, the day’s total is 228 × 10 × 0.0036 ≈ 8.2 mol/m²/day. That’s comfortably enough for an average houseplant.
Try it
likely 171–285
likely 6.2–10.3
Assumes the lux reading is an average for those hours. Real light changes through the day, which is why Gnode takes readings every few minutes and GnodeHub adds them up.
Every factor we use
“Give or take” combines two things: the sensor’s own lux error (about 20%) and how much that kind of light varies. They’re combined the standard way for independent errors: √(20² + factor spread²). For daylight through a window, √(20² + 15²) = 25%.
| Kind of light | PPFD per 1,000 lux | Give or take | Where the factor comes from |
|---|---|---|---|
| Direct sunOutdoors, or a sunbeam straight through a window | 18.5 | ±22% | Thimijan & Heins 1983 |
| Shade or cloudy skyOpen shade, overcast days | 19.0 | ±25% | Daylight, bluer than direct sun |
| Daylight through a windowBright but not in a sunbeam. Low-E glass cuts infrared | 19.0 | ±25% | Daylight through glass |
| Household LED, warmOrdinary bulbs and lamps, 2700-3500K | 15.0 | ±25% | Rule of thumb, not yet lab-checked |
| Household LED, cool/daylight4000-6500K bulbs and shop lights | 14.5 | ±25% | Rule of thumb, not yet lab-checked |
| White grow lightWhite-looking grow LEDs with extra deep red | 16.0 | ±28% | Rule of thumb, not yet lab-checked |
| Purple (red/blue) grow lightLux badly undercounts these. A PAR meter is the honest answer | 40.0 | ±54% | Varies a lot by model |
| Fluorescent tube or CFLCool white tubes and compact bulbs | 13.5 | ±22% | Thimijan & Heins 1983 |
| Incandescent or halogenFilament bulbs. Lots of heat, little usable light per watt | 20.0 | ±22% | Thimijan & Heins 1983 |
| High-pressure sodiumOrange greenhouse lamps | 12.2 | ±22% | Thimijan & Heins 1983 |
| Metal halideMetal halide and ceramic metal halide | 14.1 | ±22% | Thimijan & Heins 1983 |
Sunlight’s factor is 1 ÷ 54: about 54 lux of sunlight per unit of PPFD, from Thimijan & Heins, “Photometric, radiometric, and quantum light units of measure,” HortScience 18 (1983). The household and white grow LED factors are common horticultural rules of thumb and are marked that way until SKL checks them against a reference quantum sensor.
Where the estimate stops being reliable
- The wrong kind of light is picked. This is the biggest source of error by far. Calling a purple grow light “sunlight” makes the number more than twice too low.
- Mixed light. A window plus a desk lamp is two kinds of light at once. Pick whichever is brighter where the plant sits.
- Very bright direct sun. On a summer windowsill the sensor can max out even at its lowest sensitivity. Gnode flags those readings as “at least this much” instead of passing them off as exact.
- Placement. The sensor faces one way. Leaves face many. Put it at leaf height, facing roughly the way most leaves do.
- It isn’t a quantum (PAR) sensor. A dedicated PAR meter counts photons directly and is the right tool if you need lab-grade numbers. Gnode is for knowing whether a spot is dim, decent or bright, and how that changes through the year, for a fraction of the price.
- Infrared isn’t far-red. The sensor’s infrared channel sees a broad band of near-infrared, not the narrow far-red band some plants respond to. We’re testing whether the infrared share of a reading can warn you that the light type you picked looks wrong. It never changes your PPFD number.
VPD · calculated from temperature & humidity
Is the air helping or hurting?
Plants drink through their roots and breathe water out through their leaves. That flow carries nutrients up from the soil, and how fast it runs depends on how dry the air is.
VPD (vapor pressure deficit) is the gap between how much moisture the air is holding and how much it could hold at that temperature. Too small a gap and the plant barely breathes out: nutrient uptake stalls and mold gets a foothold. Too large and it loses water faster than its roots can replace it.
Simplified estimate. Assumes the leaf is the same temperature as the air.
See it for your own plants
GnodeHub keeps the readings, adds up each day, and tells you when a spot is too dim for the plant you put there.
Visit GnodeHub Ask a question