xbrightness sets the overall brightness of a monitor via the XFree86
X server video mode extension (XFree86-VidMod‐Extension).

It uses the XF86VidModeSetGammaRamp function, which provides
a more flexible control than the old XF86VidMode‐SetGamma. The
latter function cannot change the endpoints of the gamma curve,
only the middle, so full brightness remains full brightness no
matter how you set the gamma.