When half of a character's face is dark and half is light - What is this called? And what is the intent?

Question

In many films and TV shows, just before a seminal moment (though not necessarily before a climax), the director will call for a character's face to be half in darkness, half in light.

I first noticed this technique in 'The Empire Strikes Back'. During the lightsaber scene between Luke and Darth Vader, there is a moment when Luke is pushed back, underneath a stairwell, and half of his face is in darkness, half is in light.

Last Sunday, I saw it again in 'Boardwalk Empire'.

Just before Eddy Kessler jumped off of the balcony, he looked in the mirror. Half of his face was in darkness, half was in light.

What is the name of this technique? And what is the director trying to get across?

Answer 1

According to an article on tvtropes.org, this is called "Face Framed In Shadow."

A face half-covered by shadow, very often through a partial Lightning Reveal. Good way to emphasize a character's sinister side. This can overlap with Hidden Eyes.

It is also sometimes called "chiaroscuro," which seems to be a bit more broad than just applying to the face.

A visual trope, using a stark contrast between dark and light in an image, usually for dramatic effect. Generally uses directional lighting and sharp shadows.