For more than a century, photographers have been capturing spooky stuff on film: semi-transparent figures standing in the cemetery, for example, or glowing clouds of "ectoplasm" above a seance table, or orbs floating in a forest, or arcs of light encircling someone's head. Ghostly pictures play a key role in the plot for the horror flick "Shutter" - and the movie's producers are asking people to upload their own spirit photographs. Is there anything substantial behind the spookiness?

Even the people who take spirit photography seriously admit that most cases can be explained away as hoaxes or optical glitches. "I'd say 95 percent of it is just crap," said Barry Taff, a psychophysiologist who played a part in one of the most celebrated cases of the past 50 years.

Back in 1974, Taff and other researchers were called in to investigate the case of a single mother in Culver City, Calif., who said she was being tortured by a demonic supernatural being. The investigation team saw flashes of light and snapped pictures. "At one point we had over 25 people in this woman's bedroom," including several photographers, Taff said.

One photograph taken during the case appears to show an arc of light, with the woman cowering underneath. "Whatever this image was, it was in space," Taff said. "Well then, what was it? ... Is it coming from her, or is it her with something coming from us?"

The case stirred quite a controversy, and spawned the 1982 horror film "The Entity" - which starred Barbara Hershey and credited Taff as a technical adviser.

"The Entity" is Taff's biggest claim to fame, but he's seen plenty of spirit photography before and since. "It's one of the elements we've been investigating over the last 40 years," he said.

Taff figures he's taken on about 4,000 cases over those four decades, "and the majority don't go beyond one interview." In most cases, he has seen a correlation between reports of spooky visions and not-so-spooky explanations: a history of epilepsy or seizures, for example, or a tendency toward psychological instability.

"If we don't experience something and/or measure it in terms of video or still film, or if we don't measure it with our instruments, it's just not worth our time," he said.

But he's not willing to write off spirit photography entirely. In addition to the "Entity" case, he's seen some linkages between high magnetic readings and optical anomalies - the sort of thing studied by Canadian-American psychologist Michael Persinger. Taff thinks electromagnetic phenomena, coupled with our own bodies, may explain some of the reports about poltergeists, and perhaps even the "Entity" case.

"No dead people, no ghosts," he said. "This appears to be coming from us - living people."

That's why he's still investigating cases, when he's not involved with his "day job" as co-owner of a medical-device company. He declined to identify the company because of the stigma associated with the spooky business, but said "I think we're getting closer" to the day when such research might seem respectable.

"We're at a point now, after 40 years, that I believe we might be able to electromagnetically drive the phenomenon, whatever it is," he said. "If we can trigger it, rather than sitting around like a bunch of clowns, then we can document it."