LUX Meter · Guide
How accurate is a phone light meter app?
Short answer: an uncalibrated phone light meter is usually within about 30–40% of a real lux meter — rough, but fine for comparing spots. Calibrate it once, and studies show the error drops to roughly 1–10%. Here's what the research actually found, and how to get the most accurate readings from your iPhone.
What the research says
Several peer-reviewed studies have tested smartphone light meters against calibrated reference instruments. The pattern is consistent: raw readings are off by a lot, but a simple calibration closes most of the gap.
Three Android apps on two phones (20–1,000 lx) were compared to a calibrated meter. Raw readings ran 28–38% high — but a single correction factor at mean illuminance cut the error to 0.84%. A calibrated phone could stand in for a low-end meter.
Rodriguez, Paviglianiti, Dumit & Pattini — Dos & Don'ts in Measuring Illuminance With Smartphones, Ergonomics in Design, 2025.iPhone and Android apps were benchmarked against a Testo 435 reference. The best uncalibrated result was an iPhone 11 Pro Max (~8% deviation); the worst device/app combinations were off by up to 113%. Conclusion: phones are unreliable where high accuracy is required.
Vujica Herzog, Dobaj, Buchmeister & Spindler — Comparison between Lux Meter Apps and Illumination Measuring Devices, DAAAM International Scientific Book, 2022.Attaching a diffuser dome over the sensor was tested across platforms and apps. The diffuser improved accuracy enough for some combinations (notably an iPad) to come within ~10% of the reference across lighting conditions.
Vaz, Pinto, Louzada & Monteiro — Illuminance measurement by mobile devices with a diffuser dome, Int. Journal of Metrology and Quality Engineering, 2021.Three smartphone methods were evaluated — the ambient-light sensor, the camera, and an external Bluetooth luxmeter. Uncalibrated error was high (~39%); per-device calibration brought it down to roughly 8–10%.
Gutierrez-Martinez, Castillo-Martinez, Medina-Merodio, Aguado-Delgado & Martinez-Herraiz — Smartphones as a Light Measurement Tool: Case of Study, Applied Sciences 7(6):616, 2017.Light-meter apps were assessed for workplace use. Readings were extremely variable with large deviations from reference levels; the authors concluded they are not appropriate for occupational lighting assessments.
Cerqueira, Carvalho et al. — Is It Smart to Use Smartphones to Measure Illuminance for Occupational Health and Safety Purposes?, AHFE (Springer), 2017.How to get the most accurate readings from your phone
1. Calibrate once
The single biggest improvement is calibration. If you have access to one trusted lux value — a real meter, or a spec-rated fixture — match your phone to it once. That stores a per-device correction factor that accounts for your specific camera and lens. This is exactly what turned 28–38% error into under 1% in the research above.
2. Use a paper diffuser for incident light
For incident light — the light falling on a surface — cover the camera lens with a plain sheet of white printer paper (not glossy). It spreads the light so the reading behaves more like a real lux meter's diffuser dome. Without it, you're measuring reflected light, which is fine for comparing spots but not for absolute values.
3. Hold steady and compare
Keep the phone still until the reading settles, and use the same phone and method each time. Even uncalibrated, a phone is very good at telling you whether one spot is brighter or darker than another — which is what most people actually need.
Lux reference chart: how much light do you need?
Room and task figures below follow the EN 12464-1 workplace lighting standard; home values are general guidance; plant ranges are approximate (lux is not a direct measure of PAR light for plants).
| Setting / task | Typical range (lux) |
|---|---|
| Moonlight | ~1 |
| Dim / mood lighting | 10–50 |
| Living room (ambient) | 100–300 |
| Reading | 300–500 |
| Office / desk work (EN 12464-1) | ~500 |
| Kitchen / detailed tasks | 500–750 |
| Fine / craft work | 750–1,500 |
| Overcast daylight | 1,000–10,000 |
| Plant · low light (approx.) | 500–2,500 |
| Plant · medium (approx.) | 2,500–10,000 |
| Plant · bright (approx.) | 10,000–20,000 |
| Direct sunlight | ~100,000 |
So, should you trust a phone light meter?
For everyday decisions — is this desk bright enough, which windowsill is best for a plant, is one room darker than another — yes. A phone is quick, always with you, and more than accurate enough for comparisons and setup. Calibrate it and add a paper diffuser and you're often within 10% of a dedicated meter. For regulated, safety-critical, or professional compliance measurements, use a certified lux meter — the research is clear that uncalibrated phones aren't appropriate there.
Measure light with your iPhone.
LUX Meter reads lux and foot-candles from your camera, with target presets for rooms, offices, and plants, one-value calibration, a paper-diffuser guide, and the five studies above cited right in the app. Free, private, no accounts.