Sometimes, the automatic ambient lighting adjustments in Chrome OS just stop working without giving you an explanation. That explanation is actually pretty simple, though, as is the solution to the problem.
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More new Chromebooks are shipping with ambient light sensors. They work similarly to the sensor on your phone or tablet, allowing the display—and backlit keyboard, if you have one—to automatically adjust to the lighting conditions around you. It’s a nice addition and it works well…most of the time. The problem is that the automatic adjustments to ambient light will stop working if you make any manual adjustments to your backlighting. And the only workaround is restarting your Chromebook. Let’s take a look at why this happens.
Understanding Ambient Light Sensors
First off, let’s take a look at how ambient light sensors work. We’re not going to get super technical here—just a basic understanding of what’s going on.
The light senors usually hang out somewhere at the top of the device (regardless of whether it’s a tablet, phone, or laptop)—typically near the camera. If you look closely on the top bezel of your phone, for example, you’ll like see a few little void areas—one of these is likely the ambient light sensor.
It’s worth keeping in mind that not all phones have these—especially if they fall into the “affordable” price spectrum. The same goes for tablets. But if you’re rocking a modern, premium smartphone, the sensor should be pretty easy to spot. The same rule applies to Chromebooks (or other laptops that have this feature).
That sensor monitors the ambient light wherever you are, then adjusts the display brightness and keyboard backlight accordingly. For example, in a dark room, the display dim to be easier on your eyes and the keyboard brightens so you can see it better. The opposite happens in sunlight or a bright room.
How Ambient Light Sensors Work on Chromebooks
Chromebooks are a little bit different from your smartphone. Most modern smartphones allow you manually adjust the display brightness and use the ambient sensor at the same time. The phone uses your preferred brightness as a baseline, then adjusts up or down accordingly as the environment changes.
Chromebooks don’t really work like that, because the way they judge brightness isn’t nearly as granular.
By that, we really mean Chromebooks only use a few discrete settings. At boot, the system automatically sets the display brightness to 40%, then adjusts accordingly as the system starts. After that, it checks a few more variables—like general lighting and whether the system is on AC power or battery—then sets the brightness to specific parameters based on what it finds. This is all controlled by a deamon called “powerd”—the Chrome OS Power Manager.
If the lighting in the area is more than 400 lux—a unit in which light is measured in a given space—and the system is on AC power, the brightness is automatically set to 100%. On battery power, it goes to 80%. If the lux is lower than 400, it’ll set to 80% on AC power and 63% on battery. Devices that don’t have light sensors will default to the “less than 400 lux” settings.
This is a pretty basic setup. You won’t notice a gradual response to subtle environmental changes like you do on your smartphone. That said, brightness will change instantly as you change the power state: plug in the Chromebook, and the brightness goes up. Unplug it, and the brightness goes down.
Keyboard backlighting works in much the same way, though there isn’t a hard and fast rule for how it gets set at boot. This is device-specific from what we can tell, but it’s also less important to understand since it’s not as dramatic a difference compared to the display brightness.
So, Why Did the Automatic Backlight Quit Working on My Chromebook?
Since Chrome OS handles brightness differently than other devices, as soon as you adjust the display brightness, it assumes that’s where you want it to be and disables automatic brightness.
In fact, this setting is so aggressive, it will disable automatic brightness even if you manually adjust the keyboard backlight. So, if you manually change the brightness of your screen or keyboard, automatic brightness gets disabled.
Note: We’ve seen a little confusion around the different brightness settings. In Chrome OS, you control keyboard backlight by holding the ALT key while using the screen brightness buttons.
In previous builds of Chrome OS, the automatic brightness setting would actually survive rebooting, so the last-used brightness level got re-applied at boot. That feature was removed in the latest build and the system now uses the guidelines we talked about in the previous section when determining the appropriate brightness levels at boot.
In the end, the only way to re-enable automatic brightness is to reboot the system. If you want to keep it enabled, you then have to avoid making manual adjustments.
Yes, it’s a simple solution—albeit a slightly annoying one. But it does help to understand why things work the way they do. At least Chromebooks start quickly, so there’s that. In the future, we’d love to see a more mobile-like approach to automatic brightness. We should be able to enable/disable it on the fly and make manual adjustments without disabling the automatic settings. And we’re not really against the set-in-stone 100%, 80%, 63% settings for display brightness. Having your laptop adjust constantly to changing light conditions could get way more annoying on a laptop that a mobile device.