At first glance it might sound stupid, but hear me out…
My current usage
I noticed that when ambient light is bright enough to read, I usually leave the backlight turned off. When I am in a completely dark area, like outside at night in an area without streetlights, I usually leave the backlight on with a very dim glow. Simple enough.
When I am in lit rooms but it is nighttime, having the backlight off is OK until I am suddenly in a dark place and I need to find where to touch to re-enable the backlight. In those cases, a very dim glow is also good: In the lit rooms, the glow is just about invisible because of the relative intensity of light and our logarithmic perception of brightness. In the darkness, the glow suddenly appears even though it was on the whole time. I just don’t see it when there is ambient lighting, kind of like a tritium watch dial.
Power consumed?
However, this obviously uses more power. I’m not sure if the amount is negligible at the absolute dimmest setting I am able to set. If it is negligible, I don’t need adaptive brightness at all. If it does use substantially more power, then I only need adaptive brightness to turn the backlight on or off. Let’s assume the power is non-negligible and that adaptive brightness is desirable to continue the other thoughts about adaptive brightness.
Why the current behavior is wrong
The current behavior of adaptive brightness appears to be inversely proportional to ambient light. It is easy to think that this makes sense since the eink panel has the “opposite” brightness requirements of a traditional display panel. That’s not quite right. The mapping of screen brightness to ambient light is only the opposite of a traditional display when the backlight is completely off. If the backlight is on, then the brightness requirements are similar to a traditional display. I obviously don’t want the backlight to be super bright if I am in an exceptionally dark area. Likewise, if the environment is only slightly unreadably dark then a brighter screen can be good.
The corrected behavior
This suggests that the mapping of screen brightness to ambient light should result in the brightness being 0 for all ambient brightness values above a threshold. Below the threshold, there should be a brightness curve that dims more with darker environments, and brightens with less dark environments. Even this isn’t necessary. I would suggest a much simpler system. The adaptive brightness toggle should simply enable an ambient brightness threshold to disable the backlight when outside during the daytime. Anything below the very bright threshold will turn on the backlight to the user-defined (and fixed) brightness setting.
Adaptive brightness responsiveness
The very high threshold must be very high to resolve the final issue of the latency of brightness changes. During the day outside, there’s virtually no reason for me to ever turn the backlight on. However, even in a well-lit room at night, I may suddenly turn the lights off or move somewhere that is dark and it takes a moment for the phone screen to react. In these cases, the tritium watch dial visual is best. Leave the backlight on a dim glow that is invisible in indoor lighting, and then the issue of suddenly responding to changes in brightness doesn’t matter. It’s effectively infinitely responsive.
btw
As a side note, I virtually never raise the brightness above 12 steps of brightness on the slider, and in very dim areas I set it to 1 or 2 steps above completely off. The vast majority of the brightness slider is unused by me.