Sony PCG-XG18 Primary User Manual page 26

Slide the LCD lock lever in the direction of the arrow, and lift the
cover.
Press the power button of the computer until the green power
indicator light turns on.
If necessary, adjust the brightness controls for the LCD display. To
decrease the brightness, press Fn+F5 and then the down or left arrow
key. To increase the brightness, press Fn+F5 and then the up or right
arrow key.