I have a T510 with nvidia nvs 3100m graphic card windows 7 professional 64 bit, and for the past few months the windows login screen brightness automatically dims and then after windows open within few seconds brightness is restored. It happens on an intermittent basis. I have updated my BIOS, windows and GPU as well, but that made no difference.
This can easily be adjusted via a setting change in Power Manager.
Open up Power Manager (right click on battery icon in task bar).
Switch to mode.
Go to the tab .
Choose the menu "Dynamic Brightness Control" and uncheck the checkboxes to disable this feature: