The above symptom is associated with, but not limited to, the following systems:
Note: Some ThinkPad LCDs will automatically dim when the AC Power is unplugged. This is to save battery power and is normal.
Windows 8 / 8.1 /10
Note: If moving the slider doesn't change the brightness, try updating your display driver. Here's how: Select the Start button, enter (type) Device Manager, select Device Manager from the list of results, expand Display adapters, right-click (or press and hold) the name of the driver (each manufacturer has a different name), and then select Update Driver Software.