How to measure light for a houseplant

Take a comparable reading

1. Place the sensor near the leaves, facing the same way the leaf surface faces.
2. Record the unit, time, weather, direct-sun exposure, distance from the window, and whether a curtain or screen is in place.
3. Repeat at more than one time of day and on more than one day. Seasonal sun angle, clouds, nearby buildings, trees, and glass change the light that reaches a plant.
4. Use the result to compare locations or a broad plant group, then watch new growth and adjust placement gradually.

University extension guidance describes intensity, duration, and light quality as separate factors. A single lux or foot-candle reading only describes illuminance at one moment.

What window direction can tell you

In many Northern Hemisphere homes, a north-facing window tends to provide less direct sun; east-facing windows often receive morning sun; south-facing windows can receive stronger light for longer; west-facing windows can have intense afternoon sun. These are context clues, not fixed values. Latitude, season, buildings, trees, window size and coating, shades, and plant distance can change the pattern.

In Plantlight Match, choosing a direction changes the contextual note. It does not secretly change your entered reading or the light band. The meter reading is the primary input; the sun-exposure selection adds a caution about direct rays.

Lux, foot-candles, and plant light

Lux and foot-candles are illuminance units weighted for human vision. A lux reading does not identify the spectrum or directly report photosynthetic photon flux density (PPFD). Under grow lights, use a PPFD-capable meter or a reliable fixture specification when available. Do not treat a lux-to-PPFD conversion as universal.

Phone apps may use different sensors, cameras, and calibration methods. A peer-reviewed comparison found substantial error in non-calibrated phone readings; accuracy depends on the device, app, placement, and light source. For a rough comparison, use the same phone consistently. For closer measurement, use a calibrated lux meter.

Sources

• University of Minnesota Extension — Lighting for indoor plants and starting seeds
• University of Maryland Extension — Lighting for Indoor Plants
• Rodriguez et al. (2025), “Dos & Don’ts in Measuring Illuminance With Smartphones,” Ergonomics in Design
• University of Illinois Extension — Lighting