Converting between PPFD, lux and foot-candles matters most when you don't yet own a quantum meter, but you still want a rough read on how much light your plants are getting. Light provides the initial energy source for growth, and just as you'd measure the strength of a nutrient feed, you need some way to gauge the quantity of light reaching your canopy. This guide covers the proper conversion tables for going from PPFD to lux or foot-candles, plus an honest look at whether a smartphone app can actually stand in for a dedicated light meter.
💡 Why Light Quantity Matters
Light meters come in a few different forms, from simple lux meters through to quantum meters. Lux meters are the most common and widely available, but they measure lumens — a unit weighted to human eye sensitivity, not to how plants actually use light for photosynthesis. Quantum meters measure PPFD (Photosynthetic Photon Flux Density) directly, which is the metric that actually matters for plant growth.
Apogee Instruments makes some of the best-known quantum meters on the market, but the technology comes at a price that isn't always practical for a hobby grow. If a proper quantum meter isn't in the budget yet, converting a lux or foot-candle reading into an estimated PPFD is the next best option — with some important caveats covered below.
↔ Converting PPFD to Foot-Candles and Lux
The table below is sourced directly from Apogee Instruments' own PPFD-to-foot-candles conversion page. The conversion factor changes depending on your light source, because different lamp types emit different spectral distributions.
| Light Source | PPFD → Foot-candles | Foot-candles → PPFD |
|---|---|---|
| Sunlight | × 5.01 | × 0.200 |
| Cool White Fluorescent | × 6.87 | × 0.146 |
| Mogul Base High Pressure Sodium | × 7.62 | × 0.131 |
| Dual-Ended HPS (ePapillion 1000W) | × 7.11 | × 0.141 |
| Metal Halide | × 6.60 | × 0.152 |
| Ceramic Metal Halide (4200K) | × 6.01 | × 0.167 |
| Ceramic Metal Halide Agro (3100K) | × 5.45 | × 0.184 |
| Low Pressure Sodium | × 9.85 | × 0.102 |
Worked example: full sunlight at 2,000µmol/m²/s converts to 10,020 foot-candles (2,000 × 5.01). Working the other way, 10,020 foot-candles under sunlight converts back to 2,000µmol/m²/s (10,020 × 0.200).
📱 Can You Measure PPFD With a Phone?
A genuine phone-based light meter app is a reasonable stopgap, but it's worth understanding the limitation before relying on one: phone camera sensors and ambient light sensors aren't built to measure PAR, so any app-based reading is an estimate, not a lab-grade measurement.
A few real apps currently available include:
- Photone (iOS and Android) — uses the phone's camera sensor rather than the ambient light sensor, and includes PPFD, DLI, lux and colour temperature readings. The developer's own published comparison claims it out-performs other light meter apps for accuracy, though that comparison comes from the app's own website, so it's worth treating as a vendor claim rather than independent testing.
- PPFD Meter – Grow Light Meter (iOS and Android, by Candas Pamukcu) — uses the phone's built-in ambient light sensor, or pairs with a UNI-T UT383 BT Bluetooth sensor for more consistent readings.
- Grow Light Meter: PPFD (iOS) — a simpler app offering PPFD, PAR and DLI estimates from the phone's built-in sensor.
Whichever app you use, keep in mind that phone sensors and cameras can't capture the full PAR range or account for every light source's spectral quirks the way a dedicated quantum sensor can. Treat any app-based reading as a rough ballpark, not a number to fine-tune feeding schedules against.
💡 A-Grade Tip: If you're ready to move beyond estimates, a dedicated quantum meter such as the TREEGERS Quantum PAR Meter gives you direct PPFD and DLI readings without needing to convert anything.
Whether you're using Apogee's conversion tables with an existing lux meter, trying one of the phone apps above, or ready to invest in a dedicated quantum meter, matching your light measurement method to your budget and precision needs is the first step to dialling in your grow. Browse A-Grade Hydroponics' range of lighting tools and accessories to find the right measurement option for your setup.
