Why does my system dim and go grey sometimes?

[norman]

Out of curiosity, does this dimming happen when working with the battery
only or when connected to the mains electricity? I ask because I have
the Dell Inspiron 1525 which I use regularly throughout the day,
connecting to the mains and, except when I first had the machine, I do
not experience dimming or fade other thsn when the power management
setting kicks in. 

Norman