PPFD, PAR, DLI, and Lux Explained
Grow-light specs throw around four acronyms that overlap just enough to be confusing. One is a region of the spectrum, two are real plant-light measurements, and one is designed for human vision and has no business in a grow room. Knowing which is which saves you from buying a light based on the wrong number.
PAR — the playing field, not a score
PAR stands for Photosynthetically Active Radiation, and it only describes a wavelength range: roughly 400 to 700 nanometres, the slice of the spectrum that drives photosynthesis. PAR is not a unit of measurement — you cannot say a light emits '800 PAR.' That is like saying a road has 'kilometres per hour.' What you actually want to know is how many photosynthetically useful photons land on your canopy, and that is where the real units come in.
PPFD — photons hitting the leaf right now
PPFD, or Photosynthetic Photon Flux Density, is the number of PAR photons striking one square metre every second, expressed in micromoles per square metre per second. This is the number you measure with a quantum sensor at canopy height, and it tells you what the plant is actually receiving at that moment. Typical indoor targets range from around 200 µmol/m²/s for delicate seedlings up to 600-800 µmol/m²/s for vigorous flowering plants, depending on the crop and whether you are supplementing CO₂.
DLI — the daily light budget
DLI, or Daily Light Integral, is PPFD accumulated over the entire photoperiod and expressed in moles per square metre per day. A light that delivers 500 µmol/m²/s for eighteen hours gives a DLI of about 32 mol/m²/day. DLI matters because a plant responds to the total light it collects over a full day, not just to a single intense midday snapshot. Most common fruiting and flowering crops need somewhere between 20 and 40 mol/m²/day depending on stage, and falling short of that consistently is one of the quietest yield-killers.
Why lux doesn't belong here
Lux measures how bright a light looks to the human eye, weighted heavily toward the green and yellow wavelengths our vision evolved to detect. A lamp packed with green output can score high on a lux meter while delivering almost nothing useful for photosynthesis, and conversely a deep-red heavy fixture can read low in lux while driving excellent growth. Many phone apps report lux because that sensor is built into every smartphone, but converting lux to PPFD requires knowing the exact spectrum of your light and even then the conversion is an estimate at best.
Key takeaway
PAR is the wavelength range, PPFD is the momentary intensity at the leaf, and DLI is the daily total — only those last two tell you what the plant actually gets.