Apple Final Cut Pro 6 - Manual (Page 1185)

400 Part II Effects
Screen
Screen emphasizes the lightest parts of each overlapping image, except that the
midrange color values of both images are mixed together more evenly.
Blacks in either image allow the overlapping image to show through completely.
Darker midrange values below a certain threshold allow more of the overlapping
image to show. Whites from both images show through in the resulting image.
The order of two clips affected by the Screen composite mode does not matter.
Suggested uses: The Screen composite mode is especially useful for knocking out the
blacks behind a foreground subject and is an alternative to using a luma key. Screen is
mainly useful when you want the rest of the foreground subject to be mixed with the
background image as well, based on its brightness. It’s good for glow and lighting
effects, and for simulating reflections. You can also use the Add and Lighten composite
modes to create variations of this effect.
Top layer Bottom layer Screen composite