Lux and lumen measure light as the human eye sees it. Our eyes are most sensitive to green and yellow light. Grass works the other way around: it uses blue and red light best. So a lamp can look very bright to us and still give little useful light to grass. That is why grow lights are measured in photons, not in lux.
PAR is not a measurement. It is the range of light that plants use for photosynthesis: from 400 to 700 nanometres. All the other values below count the light within this range.
PPF (Photosynthetic Photon Flux) is the total amount of PAR light a lamp produces each second, in μmol/s. It tells you how strong the lamp is. It does not tell you how much of that light lands on the grass.
PPFD (Photosynthetic Photon Flux Density) is the amount of PAR light that lands on one square metre of grass each second, in μmol/m²/s. This is the most useful number to compare grow lights. Two lamps with the same PPF can give a very different PPFD on the grass, depending on their optics and how evenly they spread the light.
DLI (Daily Light Integral) is the total amount of light the grass receives in one day, in mol/m²/day. It combines light strength with hours of light. It helps you plan how many hours of extra light your pitch needs, on top of the daylight it gets.
Photon efficacy shows how efficiently a lamp turns electricity into PAR light, in μmol/J. A higher number means more light for the same energy.
| Term | What it describes | Unit |
|---|---|---|
| PAR | The light range plants use | 400 to 700 nm |
| PPF | Light produced by the lamp | μmol/s |
| PPFD | Light that reaches the grass | μmol/m²/s |
| DLI | Total light per day | mol/m²/day |
| Photon efficacy | Light per unit of energy | μmol/J |
