Lux, PPFD, DLI: Which Light Number Actually Matters
Lamp makers advertise watts and lumens – the plant gets nothing out of either. Here's which of the three light numbers decides your yield, how to measure it wi…
Every lamp box lists watts and lumens. Both tell you what the lamp pulls from the wall and how bright it looks to your eye . What actually arrives down at the plant, and what the plant can do with it, isn't on that box. That's why people buy a stronger lamp and still don't harvest more. 🌱
Three numbers clear this up – and only one of them decides your yield.
Lux: the number for your eye
Lux measures brightness the way a human perceives it. Our eyes are most sensitive to green, so the measurement is weighted towards green. For photosynthesis, green is the least interesting part of the spectrum – plants mainly work with red and blue.
The result: a lamp that looks richly green can deliver plenty of lux and little usable light, and a red-blue looking one the other way round. Lux still isn't worthless – every meter and every phone app spits out this value, and with the right conversion factor it gets you surprisingly far. But there's a better way now, more on that in a moment.
PPFD: what actually arrives
PPFD counts the usable light particles landing on a surface per second – only the ones between 400 and 700 nanometres, the range the plant can actually use. That's the honest number for how much light arrives at a given spot.
The catch: PPFD is a snapshot at exactly one spot . Directly under the centre of the lamp you'll often measure twice what you get in the corner of the tent. So a single PPFD figure doesn't describe a grow yet.
DLI: the number that decides your yield
DLI is the amount of light that adds up over the whole day : PPFD times hours of light. And that's the one that counts.
Think of rain. PPFD is how hard it's pouring right now. DLI is how much is in the bucket by the evening. A short downpour and a steady drizzle all day can fill the same bucket – which is exactly why a weaker lamp running longer can deliver the same light as a strong one running short.
In practice: in veg, run time is your cheapest lever. You reach the same DLI with less lamp power simply by lighting longer. In flower that lever disappears, because the 12 hours of darkness are fixed – there, intensity is all you have.
Why the conversion stays an estimate
The calculator converts lux to PPFD by multiplying with a factor. That factor depends on the spectrum, which is why you have to pick your light source:
- LED Full (full spectrum): 0.020
- LED White: 0.018
- HPS: 0.013
- MH: 0.016
- CFL: 0.014
- Sun: 0.018
Pick wrong and you are off by more than 50 % between HPS and LED. And even with the right pick it stays an approximation, because every lamp has its own spectrum.
Buying trap: if a cheap lux meter advertises a "PAR mode", it almost always still measures nothing but lux and multiplies internally with exactly such a fixed factor. You get the same result as this calculator gives you – except you paid for it. A real PPFD measurement needs a quantum sensor with a cosine-corrected head.
Better: measure PPFD directly with an app
Converting from lux is the old route. There are apps now that give you PPFD and DLI directly – the best known is Photone (Grow Light Meter), available for iPhone and Android.
The difference to a plain lux meter: Photone doesn't just scale up a lux figure, it evaluates your camera image and comes pre-calibrated for your specific phone model . The app was built to the ISO/CIE 19476 measurement standard. You select your light source and get PPFD, DLI, light colour and even ePAR including far-red, plus heatmaps across your growing area.
Important – without a diffuser the measurement is worthless. You need a sheet of white printer paper over the camera, exactly the way the developers describe it. That scatters the light evenly onto the sensor. Measure without it and you get random numbers, then wonder why.
The app is free to use, with a paid version for the full feature set.
And the honest caveat: it is still a phone sensor, not a lab instrument. For home growing that is more than accurate enough – what matters is that you always measure the same way : same phone, same diffuser, same light source setting. Then your readings are comparable to each other, and that is what actually matters when you grow.
Be careful with apps that just print "PAR" on the label. If an app doesn't ask for a diffuser and doesn't know your light source, it is almost certainly measuring plain lux and multiplying by a fixed factor – the same thing this calculator does, only without telling you.
How to measure properly
App or lux meter, the routine is the same:
1. Hold the device at canopy height , flat, sensor facing up towards the lamp.
2. Don't stand in the light. Your own shadow is the single most common measuring mistake. Stretch your arm out, keep your head to the side.
3. Measure at five points : once in the centre, four times in the corners. Measuring only under the lamp centre makes your grow look far better on paper than it is – light falls off noticeably towards the edges.
4. Take the avera …
Lux, PPFD, DLI: Which Light Number Actually Matters