Brightness very high, cannot adjust it

What make and model system?
Why have you not added this to your initial question?

The PC was assembled by a small company - it is not a widely-known make and model.
I would be happy to provide further information about the components if you can tell me how to obtain it from the system.

Thanks for considering this problem.

What video chip do you use?
What monitor?

lspci

00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)

The monitor is a Samsung SyncMaster Series 3 TA350

Is there a control on the screen to reduce brightness?

Not on the screen, but using the remote control Menu => Picture mode => Brightness has a slight influence on the brightness.

I turned down the Backlight from the monitor/TV menu.
My eyes are no longer sore, the system is fine.
It remains the case that the slider in System settings => Screen => Brightness has no effect, but this is not a big problem with the backlight turned down.