If you are an astrophotographer, a keen stargazer, or attempting a Messier marathon, you know that to observe the faintest objects in the night sky, your eyes need to be dark adapted. That means you should avoid all artificial light.
Some astronomers cover their screens with a plastic film called 'Rubylith'. This turns the display red — many people believe red light is less harmful to your night vision than other wavelengths — but has little control over the brightness of the screen. Not only does Darklight mimic the Rubylith effect, it also gives you more power over the brightness levels of your Mac's display.
Darklight turns your display red — just like Rubylith — or green, and using the brightness control, you can darken the screen more than ever before.