How do you catch a ghost? Not with an iPhone—at least not yet. Paranormal investigators employ an unusual array of equipment: Favorite gadgets range from the simple—thermometers and carpenter's levels—to the complex and expensive—infrared thermal cameras and ultrasonic listening devices.

Thermometers and Thermal Scanners

Thermometers have been used for decades in paranormal investigations, as cold spots are said to be clues to the locations of supernatural beings, says Loyd Auerbach, founder and director of the Office of Paranormal Investigations.

Simple temperature gauges like the ones you might hang outside your back door can be set up in separate areas of a home or other locations to monitor temperature and detect sudden changes. But experienced ghost hunters employ digital thermometers with built-in alarms and memory to record minimum and maximum temperatures. The alarms can alert investigators to drastic temperatures changes, so that they don't need to remain in a single spot to monitor shifts. These gadgets, which sell for about $40, also show the historical range of normal temperatures.

Some ghost hunters also swear by thermal scanners—thermometers that use infrared technology to measure temperatures at a distance. Most of these gizmos look like bulky, plastic guns that you point at an area (perhaps across a room) to gauge temperature. But you need a surface against which the infrared laser can bounce. So if you're looking for a reading in the middle of a room, you must purchase an even more expensive scanner that works with a moveable probe and receiver. Typical point-and-shoot thermal scanners run from $100 to $200.

EMF Meters

TriField EMF Meter

Electromagnetic Field (EMF) meters measure levels of electromagnetic radiation—which ghost hunters believe apparitions emit. This radiation originates from a wide variety of sources, including the earth, people, electronics and power lines. Different EMF meters gauge radiation levels at different frequencies, so some investigators choose to carry multiple meters.

The meters often look like large, handheld computing devices with small displays and various knobs to adjust settings as you pace around an area.

One commonly used EMF meter, the Trifield Natural EM Meter, goes for $170, but some high-end industrial EMF meters will run you several thousand dollars.

Cameras and Video Recorders

Canon EOS Digital SLR Camera

Vince Wilson, author of Ghost Science: The Essential Guide to the Scientific Study of Ghosts and Hauntings, and Ghost Tech: The Essential Guide to Paranormal Investigation Equipment, calls his digital SLR camera the most valuable tool in his ghost hunter's arsenal—though other ghost hunters say the value of photographic or digital images in this pursuit is debatable. Some paranormal investigators believe that ghosts cannot be photographed at all and that the true value of photographic and video recording equipment is in capturing witness testimony and documenting locations. Wilson believes a combination of audio and video will eventually convince the public of the existence of ghosts.