When you are using a multi monitor setup the ingame brightness setting might not work properly. This problem can also be caused if you have modified your Graphics Driver Color Settings. (Or are running new or a particular version of Windows 10, nobody really knows)
Even if the ingame brightness is working properly for you this can still be useful. If you set "Brightness=0.000000" in the "[WinDrv.WindowsClient]" section and apply the GammaOffset fix, you can ALT+TAB out of the game without having your desktop and other programs being overbrightened.
This also fixes some recording programs such as OBS or others requiring the need of a Gamma Color Correction Filter and prevents the dark footage of some recording programs.
#1. Open the "TacticalOps.ini" file in your TO's System directory with notepad.
#2. Press CTRL+F and search for "GammaOffset=" or manually find it at the bottom of the [OpenGLDrv.OpenGLRenderDevice] section.
#3. Change the "GammaOffset=" value to "GammaOffset=0.600000"
This will give an ingame brightness similar to putting the ingame brightness slider halfway. To increase the brightness set the value higher, for example "GammaOffset=0.700000".