Low key lighting is technically the complete opposite of high key lighting arrangements. The low key fill light is very low, creating a large amount of shadows. This highlights the dark and light parts positioned in the frame.
This lighting technique is commonly used and is more effective in film noir productions as well as gangster and crime films as it presents a very dark and mysterious atmosphere. Low key lighting is just seen as an overall darker image compared to the high key lighting.
When researching low key lighting I came across this example from the film “The Dark Night”
The shot begins with low key lighting from two visible sources. The lamp of the desk, and the florescent light behind Detective Gordon. The darkness behind The Joker, creates and off screen space. Ultimately a big dark shadow. Then to end this particular shot sequence I have presented above is Detective Gordon, standing in front of the police door, that florescent light that was once behind him is now directly above him, showing us a full visual of his face and expressions. It bounces off the wall, but is intended to create a large source of shadow on his character, as it is a very intense scene in the film.