Uubuntu screen too dark on iMac (2011)

Hi I installed Ubuntu on an old iMac and the screens are too dark. Tried changing setting, and the settings are maxed out at 20.
Can anyone suggest a cure?

Most iMacs have a keyboard with built-in brightness controls. Try pressing the “brightness up” key (usually F2) to increase the brightness of the screen.

Did that it is at the max. Also when I lower that key it doesn’t get darker either , even though the brightness control looks like it is adjusting the light level does not change.