In Windows 8.1 I can adjust the brightness to very "low" - till the screen seems to be off. Thats too much because with a touchscreen tablet I can't find the place on the screen to make it brighter when it is totally dark.
I use a sony vaio duo 11. It took some time to realize, that the screen is not defect. Only with FN-Keys it is possible to make it bright again.
you should check this.