High key lighting helps to improve contrast ratios. Containing a lot of whites and light tones, high key lighting creates a much more lighthearted tone than film noir. Characters’ faces can be seen completely, reflecting their mostly transparent attitudes. Nothing needs to be hid in movies with high key lighting, as the light exposes everything in frame. The image from the lighthearted film The Wizard of Oz below represents the typical high-key lighting found in studio set movies of the 1940s and 50s.
On the other hand, low key lighting relies more on shadows, dark colors, and an overall reduction on lighting. Usually only a portion or half of a character’s face can be seen. The shadows attempt to cover up the extensive lies and mischievous acts of individuals in these films. White tones and colors are rarely used, and when they are, it is used to emphasis a bit of purity or good in the world.