O mais provável é que a opção " brilho adaptável " do Windows 8 esteja ativada.
Siga estas instruções para desativá-lo (o link inclui capturas de tela]:
Open up the Windows desktop with Win+D, click the power management icon in the tray, and then choose More power options from the menu.
Note that you could also use the Metro search screen to get to the Power Options panel.
Once here, click the “Change plan settings” link on the selected plan.
Then click the “Change advanced power settings” link.
And now, browse down to Display –> Enable adaptive brightness, and change the settings there to Off.