@92 wrote:
When I turn auto brightness off the light also goes off. Enable auto brightness and light comes back on. So I would say it's the auto brightness causing the red light. Confirmed on my XPERIA XZ
Now, start a telephone call and you will see that the red light comes on again, regardless of your auto brightness setting. In addition to that, even with auto brightness tuned off, the red light sometimes comes on intermittently for no apparent reason.
Ambient light sensor and proximity sensor in Xperia XZ is the same physical device. I.e. it is the same sensor that serves two functions. Ambient light measurement is a passive function: it does not need and additional illumination by any "red light". Proximity detection, on the other hand, is an active function: it works by detecting IR light emitted by an additional IR diode and reflected by user's skin back into the sensor. This IR illuminator diode is exactly what you see as dim red light.
It looks like Xperia XZ cannot power-manage these two functions independently, meaning that once you enable auto brightness, the entere sensor gets powered up, including the IR illuminator (even if the latter is completely unnecessary).
That's all there is to it.
So, again: the dim red light that you see is a part of proximity sensing function. The fact that it appears to be affected by auto brightness setting is just a purely "parasitic" connection, a quirk of Xperia XZ hardware. (I wonder what's the impact on battery life from this.)
