I don't know if this is normal on the stock samsung rom because I've been using cyanogenmod for the last few months. But recently I flashed a stock jelly bean rom for sgs2 (XXMS1) and the light sensor doesn't work properly. When I download some app which shows output values from different sensors, such as "android sensor box" it always shows values of either 10, 100, 1000, or 10000 for the light sensor, which is pretty inaccurate (the same app would show very accurate values on cyanogenmod 10.1). It does that for any other app capable of showing light sensor output. However, when I use the built in sensor tool when you type *#0*# in the dialer app then it will show me quite accurate values only rounded by 5.
I also noticed that the automatic brightness feature is not working like it should, probably because of this. When the sensor shows 10 the screen will just go to the darkest possible mode and then it will go bright all of the sudden when it changes to 100. For example because of the lighting in my room, it will just keep changing depending on which way the screen is facing. It will just be a bit too bright and then suddenly become too dark. Has anyone ever had a similar problem? Why is the light sensor so inaccurate, even though it shows proper values in the sensor thing in *#0*# ?