[CONFIGURATION] Can not modify screen brightness

[Bruintjedebeer]

I am interested in running Kubuntu 20.10 on my iMac. I started testing by booting of a USB drive. After changing to the proprietary binary driver for my graphics card, I got the screen brightness controls to work (both the slider in "Power Management" as well as the dedicated keys on my keyboard).

Encouraged I then installed Kubuntu 20.20 (standard install and all updated to the latest) on a dedicated 4T HDD (usb). So I can dual boot macOS or Kubuntu. Now I can't get the screen brightness controls to work, which is annoying since the brightness seems to be maxed. It strains my eyes. Reducing the colour temp helped a bit, but not enough. I tried all drivers (proprietary and open source) accessible via the driver manager to no avail. Eventually I would like to drop macOS all together.

Any ideas anyone?

This is my hardware:
iMac (27-inch, Late 2012)
2.9GHz Quad-Core Intel Core i5
24 GB 1600 MHz DDR3
128GB SSD SATA
3TB HDD SATA
4TB HDD USB
Apple ANSI (aluminum) keyboard
NVIDIA GeForce GTX 660M 512MB

[jglen490]

Open the Driver Manager and see if it will install the Nvidia video driver.

[Bruintjedebeer]

Open the Driver Manager and see if it will install the Nvidia video driver.

I tried all NVIDIA video drivers available in the driver manager. Neither of them resolved the issue
Since it worked when booting of usb stick, I assume it might be a configuration issue.