At first, it seemed no more than a curious coincidence. Professor Klaus Heinemann, a researcher for NASA, the U.S. space agency, was studying a collection of photographs his wife had taken at a gathering of spiritual healers when he noticed that many of them featured the same pale but clearly defined circle of light, like a miniature moon, hovering above some of the subjects.
Like most rational people, he assumed that the pictures were faulty. 'I presumed the circles were due to dust particles, flash anomalies, water particles and so on,' says Prof Heinemann.
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'But I was sufficiently intrigued that I returned to the room in which the pictures were taken, in the hope of finding an explanation - like a mirror in the background. None was forthcoming.'
Nor could he find any faults with his wife's camera. And as a scientist with considerable experience in sophisticated microscope techniques - examining matter down to atomic levels of optical resolution - his methods were nothing if not rigorous.
Still puzzled, Heinemann set out to discover what else might have caused the mysterious circles. He and his wife began taking hundreds of digital photographs at random events to see whether they could recreate the mysterious effect.
The answer was that they could make these shimmering 'orbs' appear again, but only - absurd as it may sound - if they 'asked' the apparitions to make themselves visible to the camera. And they found this method worked particularly well when the couple photographed spiritual gatherings.
What on earth was going on? Again, a maverick technical glitch seemed the obvious answer. Such anomalies happen frequently in digital photography. If you accidentally jog a camera while a picture is being taken, especially in dim light, you can easily get a double image.
But again, Prof Heinemann ruled out a technical fault. 'We were quickly able to eliminate the common problems associated with photography - such as dust particles, water droplets, reflections and a host of other likely causes.'
Yet the orbs still kept appearing. And the more images he took, the more he was able to study the bizarre properties of these shimmering lights.
Heinemann set up dozens of experiments using two cameras on static tripods under controlled conditions. His early experiments found that orbs can move very fast, up to 500mph or more.
Heinemann also found that during his numerous dual camera experiments, when he used twin cameras to capture an object from two different angles, a single orb shape would often appear - but only in one of the two images taken simultaneously.
It was as if the orbs somehow chose which camera to appear on, or whether to appear at all.
Eventually, Heinemann was left with only one conclusion: that he was witnessing some form of paranormal intelligence.
'There is no doubt in my mind that the orbs may well be one of the most significant "outside of this reality" phenomena mankind has ever witnessed,' says Professor Heinemann.
'Until now, there has been a huge amount of anecdotal evidence that the spirit world exists. I believe it's no longer anecdotal. Thanks to digital technology, we can see it for the first time. We are dealing with a non-physical - albeit real - phenomenon.'
The temptation, of course, is to dismiss such claims as bunkum. Indeed, many of Professor Heinemann's colleagues consider his research to be 'utter flaky nonsense'.
And yet a growing number of respectable scientists refuse to write off the possibility that these orbs, which are starting to appear on cameras around the world, just might offer a fascinating glimpse into the unknown.
Earlier this year, the world's first conference on orbs took place in Sedona, Arizona, where several scientists controversially stated that they believed orbs were indeed a genuine paranormal phenomenon.
Their conclusions, if correct, could have huge implications on the way we view the universe and our part in it. The experts say that just because something has not yet been scientifically proven, it doesn't mean that it is not real.
Professor William Tiller, a theoretical physicist who spent 35 years researching consciousness and matter at Stanford University in California, reminded the conference that what we see with our physical eyes comprises less then 10 per cent of the known universe.
This is because human vision operates only within a limited range of the electromagnetic spectrum. For instance, we cannot see radio waves, which carry huge amounts of information, yet we know they exist.
Similarly, Miceal Ledwith, a former professor of Theology, who for ten years was President of Maynooth College at the National University of Ireland, reminded the sceptics that when, in 1861, Dr Ignaz Semmelweis had claimed there might be some unseen link between surgeons who didn't wash their hands and the high rate of infection in childbirth, his mainstream colleagues ridiculed him.
Yet he had found the first evidence of what was later to become known as bacteria.
'Most great discoveries throughout history have been initially ridiculed,' Ledwith told the orbs conference. 'To my mind, there is no doubt that the orb phenomenon is real and deserves to be taken seriously. There are not just a few pictures of orbs, which could easily be faked, but hundreds of thousands from all over the world.'
To date, Ledwith, who was also a member of the International Theological Commission at the Vatican, has a collection of more than 100,000 orb pictures .
'They come in all sizes, ranging from a few inches to several feet across,' he says. 'Sometimes they appear alone, and at other times hundreds of them, in colours ranging from white to blue, green, rose and even gold.
'Over time, I realised that a flash seemed to be essential to capture them, even in daylight. I believe this is because we can see the orbs only through the process in physics known as fluorescence. The camera flash sparks this fluorescence process, making the orbs visible to the camera.'
Ledwith is still uncertain about what these orbs might actually be, but he has no doubt that they are some sort of paranormal apparition.
'I believe they could be many things. They may turn out to be the spirits of those who have passed on; or, as some spiritual teachers state, they might be spirits waiting to be born into a physical body,' says Ledwith.
'They may also be, or represent, a host of other intelligences - from nature spirits to beings of pure energy that have never been incarnated in a physical form. There are hundreds of different types of orb.'
Many of the scientists at the conference believe the orbs are plasma-like balls of energy - but an energy that can be detected by physical means, and which appears to have some control over its own shape and form.
It's certainly the case that they can often be photographed best at places of psychic significance. 'They definitely seem drawn to spiritualtype gatherings,' says Ledwith.
'We regularly see orbs near healers' hands or heads. Perhaps they assist in the spiritual healing process.'
Take the case of Anna Donaldson, a freelance photographer who was commissioned to take pictures of Keith Watson, a medium who had been drafted in to help solve the disappearance of Sarah Payne, the little girl who was snatched while playing near her grandparents' home in West Sussex seven years ago.
The shoot took place at the exact spot where Sarah was last seen, because the medium had suggested that he 'might pick something up'. Sure enough, when the pictures were developed, a mysterious glowing dot appeared in one of the crucial images.
'I didn't believe in any of this paranormal stuff,' says Anna, 'but I couldn't find any fault with the camera - if there had been, then all the images would have been tainted, not just one of them.'
Still sceptical, Anna had the film and images analysed for technical faults, but again no one could provide a logical answer - until a member of the Psychic Institute suggested that the pictures could indeed be evidence of 'auras'. In this case, Anna was told that the blueish colour of the orb suggested the presence of a very young soul.
Still intrigued, Anna arranged to photograph Watson again - at the exact spot from where another young child had disappeared, this time in Greece. To her astonishment, the photographs again showed the presence of a blue orb.
And when Anna repeated the shoot the next day, in a bid to rule out a trick of the light, she got the same result - only this time it was two orange orbs.
'So what I now had was pictures of orbs from three different cameras, in two different countries, on three different days - there's simply no way that could be a chance occurrence or a technical fault,' says Anna.
'I still don't know what to think about it, but I suppose because a camera can pick up an image at a shutter speed of 1/2,000 of a second, it's possible it can detect things the naked eye cannot see.'
Could it have been the spirits of the lost children?
Terri Caldwell, a healer from Belbroughton, in Worcestershire, is among those who are convinced that orbs are a visible manifestation of human spirits.
'To my mind, the orbs are the spirit world simply going about their business,' she says. 'I believe we are all spirits having a physical experience, and when we die our energy field which carries all the information about us continues on.'