(High bit-depth) The Luma Key effect keys out all the regions of a layer with a specified luminance or brightness.

Use this effect if the object from which you want to create a matte has a greatly different luminance value than its background. For example, if you want to create a matte for musical notes on a white background, you can key out the brighter values; the dark musical notes become the only opaque areas.

White background of original (top and left) is removed using Luma Key and composited over underlying layer (right).