Get-WmiObject -Namespace root/WMI -Class WmiMonitorBrightnessMethods).WmiSetBrightness(5,20)

'$monitor=Get-WmiObject -Namespace root/WMI -ClassName WmiMonitorBrightnessMethods', '$monitor.WmiSetBrightness(0,50)'

'$monitor=Get-WmiObject -Namespace root/WMI -ClassName WmiMonitorBrightnessMethods; $monitor.WmiSetBrightness(0,50)'

import subprocess

proc = subprocess.Popen(['powershell', '$monitor=Get-WmiObject -Namespace root/WMI -ClassName WmiMonitorBrightnessMethods; $monitor.WmiSetBrightness(0,50)'])
proc.wait()