(The 'Annoying' One meets a nasty end)
(Picture courtesy of Sunnydale High School)

The ultimate weapon for fighting vampires. A single ray of sunlight turns them to dust (see what happened to the Annointed One). However it is a little difficult to use as vampires only come out at night. if someone can invent a way to bottle sunlight, they would become very very rich.
You could always rig up a complicated system of mirrors a la 'Indiana Jones', but that might be a little impractical.

 

Confuse the vampire into thinking it is night, then make him chase you outside. Or trick them like Buffy did in 'Welcome to the Hellmouth'. it's quite simple. Just pray it's not a cloudy day!

However, in Blade they did use an ultraviolet lamp to torture Pearl, so maybe it's the ultraviolet rather than the actual light that harms them.