Ambient light sensing via LED response

[NoiseEHC]

> But why do you say you would need 1 mV accuracy ?   Bright sunlight  
> is far stronger than
> the light sources he used.
>   
I am not an engineer so forgive me if I am saying something stupid, but 
is not the goal to switch off the backlight if and only if there is no 
difference between the switched on and switched off state? I mean that 
this implies that you have to measure the mV at a light level when you 
cannot see the difference (what can be a much fainter light than the sun).