What the ambient light sensor does
The ambient light sensor sits next to the front camera, usually hidden under the top of the screen. It measures how bright the room is so your phone can raise brightness outdoors and dim it at night. When it fails, auto-brightness stops reacting, the screen stays dim in sunlight, or brightness flickers up and down.
How this test measures light
- Chrome with sensor access: reads the light sensor directly in lux.
- iPhone and most Android browsers: browsers don't expose the light sensor, so the test uses the front camera, which sits beside it, to measure how much light reaches the top of the phone. It shows a 0–100 light level. The video stays on your phone and is never recorded.
A camera reading proves the top of the phone can see light changes. To confirm the sensor itself works, do the auto-brightness check below as well.
Check auto-brightness in 30 seconds
- Turn on auto-brightness. iPhone: Settings › Accessibility › Display & Text Size › Auto-Brightness. Samsung: Settings › Display › Adaptive brightness. Pixel: Settings › Display › Adaptive brightness.
- Cover the top edge of the phone with your palm for 5 seconds. The screen should dim.
- Point the top of the phone at a lamp or window. The screen should brighten within a few seconds.
Auto-brightness not working? Fixes
- Remove screen protectors or cases with a thick lip covering the top of the screen.
- Clean the top of the display. Sensors under the screen need a clear view.
- Reset learned brightness: toggle Adaptive brightness off and on, or on Android go to Settings › Apps › Device Health Services › Storage › Clear data (Pixel).
- After a screen replacement, some aftermarket displays block the sensor window. On iPhone, a non-genuine display can also disable True Tone.
- If the lux reading never changes, even when you cover the top, the sensor is faulty. Check the proximity sensor too, because the two usually share one module.
Frequently asked questions
Where is the light sensor on my phone?
It sits at the top of the front, next to the front camera. On most recent iPhones, Galaxy and Pixel phones it is hidden under the screen near the camera cutout.
Why does auto-brightness not work?
Check that Auto-Brightness or Adaptive brightness is on, remove anything covering the top of the screen, and reset learned brightness by toggling the setting off and on. If the screen never changes when you cover the top, the sensor is faulty or blocked by an aftermarket screen.
Why does the test use the camera?
iPhone and most Android browsers do not give websites access to the light sensor. The front camera sits beside it, so its brightness shows how much light reaches the top of the phone. Nothing is recorded or uploaded.