Lunar is described as 'Intelligent adaptive brightness for your external display'. There are four alternatives to Lunar for Mac and Windows. The best alternative is Monitorian, which is both free and Open Source. Other great apps like Lunar are Simple Screen Brightness Adjuster (Free, Open Source), Brisync (Free, Open Source) and MonitorControl (Free, Open Source).
Monitorian is a desktop tool to adjust the brightness of multiple monitors with ease. Remarks The monitor name shown in main window can be changed for easier identification of monitors. To change the name, press and hold it until it turns to be editable.
Simple Screen Brightness is a small portable application written in C# that allows you to adjust the brightness for all of your connected monitors using a slider without having to use their difficult-to-use built-in menu systems.
Synchronize your external monitor brightness with built-in display. Application reads current brightness of your built-in display and passes it to external monitors via DDC interface. You can adjust brightness by menu in system status bar.