a simple PowerShell script to adjust the screen display's brightness.

brightness.ps1

<#
    .SYNOPSIS

        Adjust screen display brightness.

    .DESCRIPTION

        This script adjusts the screen display's brightness by accepting
        a percentage value between 0 and 100 indicating the strength of
        the brightness the screen display's backlight should emit.

        A default value of 40% brightness is applied if nothing is passed.

    .USAGE

        C:\path\to\scripts\folder\brightness.ps1 70

#>

Param([Int16]$brightness=40)

If ($brightness -in 0..100) {
    (Get-WmiObject -Namespace root\WMI -Class WmiMonitorBrightnessMethods).WmiSetBrightness(1, $brightness)
}