The Luminance Key node creates a matte based on the luminance of an image. A front clip can be connected to the Luminance Key node and will process a result clip that can be used as a matte. In the Modular Keyer, this node can be placed only on a matte branch (blue).
There are two parameters available:
These settings can be animated and are available as channels. See Creating a Key by Setting the Luminance.