Retinal Scannes and their use...

Several questions:[ul]
[li]Are retinal scanners employed widely in security facilities around the world? Are they effective?[/li][li]Is there any intrinsic reason why, with how the technology works, that someone with the proper clearance has to be alive to ensure the machine recognises their retina pattern?[/li] i.e. can you, say, use the decapitated head of a person with clearance at the scanner without facing any problems?
[/ul]
I have no idea why I wish to know this, but, for some reason, I do. Thanks.

I live in Charlotte…they are used at Charlotte Douglass Airport. It is/was part of a testing phase the FAA was conducting. They were testing them at other airports as well. That’s really all I can remember from the news reports I saw.