I am trying to create a flashing lights effect to put over another clip in Premiere Pro, but am stuck with a black background.
In After Effects 6, I have several black solid layers each with a lens flare, set to screen (whatever that means), but they still show up as black as I can't see the checkerboard pattern through them. Unfortunately I'm a complete beginner to After Effects so can't understand many of the solutions that have been posted so far - would someone be able to give me a simple step-by-step guide of what to do?
You wont see the checkerboard if you toggle it. The layers are not transparent, but as you said, set to "Screen" mode - this is a blending mode that calculates new pixel values per pixel, depending on what mode you selected.
I suggest you create a new solid (any colour but black or white) and place it at the bottom of the layer stack. You should be able to see it through the flares.
Good luck on getting started - try this link:
To create an alpha chanel from a black solid that has Lens Flare applied use the free UNMULT plug-in. Google will take you there.
The other thing you can do is simply put your video below these black layers with the Lens Flair applied. That will work also.