Light meter
A phone reports illuminance in lux using the small ambient light sensor near its earpiece. It is good for checking whether a room is bright enough to work in, and not suitable for photographic exposure metering.
| Sensor used | Ambient light sensor |
|---|---|
| Unit | Lux (lx) |
| Useful range | Roughly 1 lx to several thousand lx, hardware dependent |
| Cannot do | Replace an incident light meter for photography, or serve as a calibrated instrument for lighting certification |
The mechanism, not the marketing.
The ambient light sensor is a photodiode sitting behind the glass near the earpiece, and its normal job is deciding how bright to make the screen. It reports illuminance — how much light is falling on that spot — in lux.
For orientation: a dim living room is in the low hundreds of lux, an office is typically 300 to 500, an overcast day outdoors runs into the thousands, and direct sunlight is tens of thousands.
What a phone cannot do here.
- Resolution is coarse, especially at the bright end. The sensor exists to drive screen brightness, a task that needs approximate values, so many phones report light in large steps rather than smooth increments.
- It measures light arriving at one small point behind the display glass, which has its own transmission characteristics. Point the phone differently and the reading changes, so where you hold it matters as much as the light.
- Its spectral response is not the human eye's. Two sources that look equally bright — an LED panel and daylight, say — can read differently.
Can I use my phone as a light meter for photography?
Not as a replacement for a real one. Photographic metering needs incident readings and an exposure value calibrated to film or sensor response. The ambient sensor gives illuminance at one point behind the glass instead.
Is it accurate enough to check office lighting?
It is useful for a rough check against a guideline such as 300–500 lux for office work. Anything that has to be certified or documented needs a calibrated lux meter.
Why does the reading jump in large steps?
Many ambient light sensors quantise their output coarsely, because their real purpose is adjusting screen brightness rather than measurement.
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