Light Sensor Test

How to test the light sensor

1. Open this page on your phone. If Light sensor API says Available, tap Read light sensor to see the reading in lux.
2. Otherwise tap Start camera light meter and allow the camera. Cover the front camera, then point it at a lamp; the brightness should drop to near 0% and rise high.
3. Work through the auto-brightness check: turn auto-brightness on, cover the top of the phone, then shine a light on it, and answer each step.
4. Read the verdict, then the fixes below if the screen did not follow the light.

What the results mean

The ambient light sensor measures how much light falls on the front of the phone, in lux. The phone uses it to set the screen brightness and, on some models, the keyboard backlight and the colour tone of the display.

Most browsers do not let websites read the sensor directly, for privacy, so the lux reading usually shows "Not exposed". That does not mean the phone lacks the sensor. The camera light meter and the auto-brightness check test the same thing from the outside: if the screen dims when covered and brightens in strong light, the sensor works. The camera meter shows relative brightness, compensated for exposure on phones whose camera reports exposure time and ISO; it is not a calibrated lux meter.

Where the light sensor is

The sensor sits on the front of the phone, usually next to the front camera or the earpiece. On phones with a notch or a hole-punch camera it is often hidden under the display or in the notch, so a dark screen protector, a thick case lip or a dirty corner of the screen can block it. If auto-brightness reacts only when you cover a particular spot, that spot is the sensor.

Why auto-brightness seems wrong but the sensor is fine

Modern phones learn from you: every time you drag the brightness slider, adaptive brightness adjusts its curve. After a few corrections in a dark room it may stay dimmer than you expect elsewhere. Resetting the learned settings usually fixes it (on Pixel phones: Settings > Apps > Device Health Services > Storage & cache > Clear storage > Reset adaptive brightness; on other Android phones and iPhone, turn auto-brightness off and on again). More in auto-brightness not working.

Common problems and fixes

The screen does not dim when covered

Check that auto-brightness is on, then clean the top of the screen and remove any screen protector or case that covers the sensor. Restart the phone. Some phones only adjust slowly; keep the sensor covered for 5 to 10 seconds.

Brightness jumps up and down

A flickering light, a hand near the top of the phone or a sensor half covered by a case causes jumps. Move the case and test again. On Android, resetting adaptive brightness clears a bad learned curve.

I want to see the raw sensor value

Samsung phones show it in the test menu: dial *#0*#, tap Sensor and look at the light sensor lux. Other Android phones show it in their hardware test menus or in sensor apps from the Play Store. iPhones do not show the raw value.

Auto-brightness stopped after a screen replacement

The sensor may be on a flex cable that has to be moved to the new screen, or covered by the new frame. Take it back to the repair shop and ask them to check the light and proximity sensor flex.