Stereo cameras are "normal" film cameras with two lenses and (usually) a single flash. The lenses and ergo: The exposures, are slightly offset so that when viewed through certain special "viewers", make the image seem three-dimensional.
Now, that's pretty cool, but the 3-D effect is not what makes this piece of equipment so valuable to "ghost enthusiasts", especially orb-hounds.
You see, with TWO lenses and TWO exposures taken at only slightly different angles but at precisely the same time would negate some (most) airborne particles reflecting a flash.
The concept is, if one frame shows an orb and it's only an illuminated particle, the other lens either should not see the particle at all, or show it with a relatively distinct change in it's brightness.
Now, this tool would not be infallible. In fact, if a large particle was... say... three two five centimeters from the lenses, dead center, and was capture by the flash, then it MIGHT appear as a "real orb" of equal appearance in the photo. The chances, however, in this happening are somewhere between slender and none... Not impossible, but highly improbable.
Now, we have seen situations where investigators were already on this right track, but not to the same degree of preciseness. Many have mounted multiple cameras on a tripod or other contraption and attempted (sometimes successfully) to take images at the exact same moment using all the cameras. Now, this is okay, but a stereo camera, used properly, would eliminate possible time-changes during the exposures and also, if there was/is a film defect or processing problem, it would negate this too as the two images would show the problems a little better to even the untrained eye.
Now, the REALLY nice bit about this is that stereo cameras run about $120 Canadian if you shop around so this is not a heavy-duty expensive toy.
So, in my guesstimation, if you have a stereo camera (film please... need those negatives and to be able to back-track the chemical processing) that snaps an orb on BOTH frames that appear identical and that orb IS obviously obstructed by another object (not another orb, but a chair or something like that...), then you either have a photo of a genuine anomaly or an impressive hoax.
There's MUCH more to come on this... but it will take some time... We're still slugging away with Dr. Lietzau's paper and getting ready to discuss this with our own resident experts... but it's coming!
Either way, if you're a believer in orb photos, or even a big-time ghost enthusiast, a stereo camera may not be a bad purchase for the here and now.