Grow Light CheckUnits explained
Lux and PPFD measure different kinds of brightness
Lux weights light for human vision. PPFD counts photosynthetically active photons reaching an area. A conversion is always dependent on the light’s spectrum.
Published and reviewed by Grow Light Check · Updated
When lux is useful
A phone lux reading can help compare positions under the same broad-white lamp. It can reveal that one shelf is darker than another or help restore a light to a previous setting. It should not be treated as a universal plant-light measurement.
Why there is no universal conversion
Human eyes are most sensitive to green light, while PPFD counts photons from 400 to 700 nanometers more evenly. Two lamps can therefore produce the same lux but different PPFD. Grow Light Check uses an approximate conversion only for ordinary broad-white LEDs and carries extra uncertainty.
| Light source | Lux conversion? |
|---|---|
| Ordinary white LED with known CCT | Useful as a rough estimate |
| Strong red/blue “blurple” light | Do not use a generic conversion |
| Unusual or tunable spectrum | Use spectral data or a PAR meter |
Foot-candles
Foot-candles and lux are both human-vision measures: one foot-candle is approximately 10.764 lux. Converting either to PPFD still requires an assumption about spectrum.
Use the result as a starting point
A listed wattage is electricity use, not plant-level brightness. Reflectors, lenses, fixture dimensions, spectrum, barriers, and the illuminated area all affect the result. Manufacturer PPFD maps or a reading from a cosine-corrected quantum meter are stronger evidence than wattage alone.