Practical light checks at home
Lux vs PPFD: what a phone light sensor can and cannot tell you
Understand lux, PPFD and DLI before comparing grow lights. Learn why a single conversion factor or phone reading cannot provide a calibrated plant-light assessment.
LumaLeaf guide · Reviewed
The useful distinction
Use phone lux readings to compare positions under similar lighting. Use suitable plant-light measurement equipment when you need defensible PPFD or DLI values. A value labelled lux is not already PPFD, and a quick spot reading is not the total light received in a day.
Three terms that answer different questions
| Term | Unit | Question |
|---|---|---|
| Illuminance | lux (lx) | How bright is light at this point, weighted for human vision? |
| PPFD | µmol m⁻² s⁻¹ | How many photons in the usual PAR band reach an area per second? |
| DLI | mol m⁻² day⁻¹ | How much of that photon light reaches an area over a day? |
Iowa State University Extension explains light intensity, duration and these measurement terms. The usual PAR band is 400–700 nm. These units describe different measurements; none is a direct reading of plant health.
Why there is no universal lux-to-PPFD multiplier
The relationship depends on the light’s spectrum. A factor appropriate to one lamp is not automatically appropriate to daylight or a different grow light. Iowa State’s setup guidance uses a light-source conversion factor when estimating PPFD from lux. Such an estimate still depends on the assumptions and the original sensor measurement.
If you do not know which spectrum a factor represents, keep the original lux value rather than displaying a precise-looking PPFD result. When comparing two different lamp types, a higher lux reading alone does not establish which provides more useful plant-light photons.
What an Android sensor reading means
Google’s sensor explanation describes Android lux as light reaching the ambient light sensor. That is a measurement at the phone’s sensor position, not at every leaf. LumaLeaf reads the phone’s available light sensor; it does not turn the phone into a calibrated quantum sensor.
For repeat comparisons, use the same phone, sensing position and orientation. Record shadows, changing daylight and lamp settings. If a value is fixed, implausible or reaches a repeated ceiling, check the device and measurement setup before using it to make a care decision. Do not compare extra decimal places as though they prove extra accuracy.
Choose the next tool for the question
- Which of my two shelf positions is brighter? Repeat lux checks under comparable conditions and keep a named record.
- Does this grow light meet a particular PPFD target? Check a trustworthy PPFD map and the specified mounting conditions; measure at the canopy with appropriate equipment if the answer matters.
- What is the daily light total? Measure or integrate PPFD over time with a suitable method. A noon reading alone cannot represent changing daylight.
For steady, known PPFD, DLI can be calculated from PPFD × light-hours × 0.0036. For example, a constant 100 µmol m⁻² s⁻¹ for 10 hours gives 3.6 mol m⁻² day⁻¹. This arithmetic example assumes constant light; it does not convert a phone’s lux reading or set a target for a species.
Continue with another guide
- How to measure plant light at leaf height with your phone
A repeatable lux-check routine: find the sensor, measure where leaves sit, record the conditions and compare readings without treating one number as a diagnosis.
- Compare window and shelf light before moving a houseplant
Compare two real plant positions with the same phone and timing, keep a simple log, and avoid choosing a spot from one bright midday reading.