Using the Luminance-Based Keys
Luminance-based keys use luminance (brightness) levels to define the transparent areas. You can choose to key out either the brightest or the darkest pixels contained in the image. Luminance-based key types include luminance, multiply, and screen.
The luminance key isn’t the only key type that uses luminance to define transparency, but it’s probably the most commonly used (both in Premiere and in traditional editing suites). You may hear luminance keys generically referred to as luma keys.
To use the luminance key:
Select a clip in a superimpose track, and choose Clip > Video Options > Transparency (Figure 13.24).
Figure 13.24. Select a clip in a superimpose track, and choose Clip > Video Options > Transparency. ...