The dev is also working on a Sensor Mode, which will allow you to use an external light sensor that reads the ambient light periodically and sends it to Lunar for computing brightness and contrast values for each external monitor. Custom hotkeys can be set for brightness, contrast and volume and the brightness and volume keys on the Apple keyboard work out of the box. Manual Mode – The adaptive algorithm is disabled and the external monitor can be controlled by hotkeys or the Lunar UI.Location Mode – The brightness and contrast is adjusted automatically throughout the day, by using the sunrise, noon and sunset times for your location.Custom brightness and contrast values are computed for each external monitor by passing the built-in brightness through a curve algorithm that is completely configurable by the user. Sync Mode – The built-in display brightness of your Macbook or iMac is synchronized to the external monitor.The app doesen’t use a software overlay, it changes the actual brightness and contrast of the monitor

Here’s a free and open source macOS utility that allows you to change the brightness and contrast of external monitors connected to your Mac: Lunar.

Adjust brightness mac second monitor