Using the Luminance key

The Luminance key creates transparency for darker values in the image, leaving brighter colors opaque. Like the Multiply and Screen keys, it is most effective when the image that you are keying contains highly contrasting dark and light areas. Use the Luminance key to create a subtle superimposition or to key out dark areas.

Adjust the following settings as necessary:

Threshold Specifies the range of darker values that will become transparent. Higher values increase the range of transparency.

Cutoff Sets the opacity of areas that have been specified by the Threshold slider. Higher values increase transparency.

Superimposing and Compositing > Using keys to superimpose and create composites > Using the Luminance key