I love orbs. They glow, they fly, they are completely mysterious - what's not to like?
Admittedly there is ONE thing not to like about orbs, and that is their tendency to be mimicked by floating bits of dust or flying insects. One reason I really like "Ghost Hunters" out of all the paranormal investigations out there is that Jason and Grant almost always rule out "evidence" of orbs for this reason.
Cameras don't work the same way our eyes do, which is why a lot of people see a glowing spot on a picture and assume that it's an orb. If you have ever noticed lens flare effects, those shiny dapples that happen when you point your camera too near the sun, the same effect is at work with "orbs."
On video, "orbs" can appear to dance and move across the screen. Typically these are simply dust motes floating around with the various drafts and air currents we never notice ourselves. Orbs can also zip past or bob and weave in a distinctively powered fashion. These are insects, which are more often caught on camera as "rods." The technical term for this habit of dust and insects to reflect light and appear on camera is "backscatter."
In very rare cases, orbs may be something else. In all the episode of "Ghost Hunters" I have watched, I can think of only one case where Jason and Grant allowed a bit of evidence to stand. The light was definitely being emitted from the object (in other words it was definitely glowing strongly, and not just reflecting ambient light). The light had a weird purplish cast. And the orb zipped past very quickly in a straight line, which isn't characteristic of the motion of an insect.
Better evidence of orbs is from first-hand accounts. People can see flashes of light in their eyes, which often arises from stress or a micro-migraine. This can also happen if you glance directly at a lightbulb and glance away, as most people know from an ill-timed glance directly at the flash of a camera!
But there are many credible cases where all these factors can be ruled out. Often because there are multiple witnesses.
Now admittedly, a lot of these orbs are probably instances of St. Elmo's Fire. This is a kind of floating ball of charged plasma, which usually happens during thunderstorms. It is usually a violet or bright blue color, and it forms off the tip of pointed things - like a ship's mast, during a lightning storm.
Will 'O The Wisp is another culprit. This is an ethereal flickering of light that happens around bogs and other bodies of water. Every culture has projected their own beliefs upon this phenomena, which is usually seen as being the spirits of the dead, of the otherworld, or of Faerie.
In truth, it is caused by swamp gases which spontaneously combust at very low temperatures, and give off a strange greenish flame. It could also be simply a reflection of ambient light upon a barn owl, as detailed in an article with the wonderful turn of phrase, "luminosity in Barn Owls."
Creative Commons-licensed image courtesy of Flickr user Lighthelper :)