In recent years green lasers have become inexpensive and easy to get hold of in the form of laser pointers. The human eye is much more sensitive to green list, so light from these kinds of laser appears to be much brighter than that emitted from red laser pens. Green laser light also scatters more in the air which makes the laser's beam visible in darkness.
A lot of green laser pointers come with a simple filter which scatters the light into multiple beams, creating a grid-like pattern of dots and many ghost hunters use this pattern to detect motion during paranormal investigations.
The technique was first seen in the US ghost hunting show, 'Ghost Hunters International,' the idea is that the dots scatter across the walls, floor and ceiling and if anything moves in front of the laser you will see a shift or discrepancy in the grid.
A similar technique was used in the 2012 movie, 'Paranormal Activity 4' but instead of visible green lasers, the family being haunted used an Xbox 360 Kinect which uses a grid of invisible infrared dots which can be picked up be a video camera in night-vision mode. In the movie the light grid revealed the shape of a young boy, a ghost which was invisible to those in the room.
You may think it's easier to just turn on the lights or use a camera in night-mode if you want to see movement in the room, and of course these are valid options, but the nature of the laser light means it's not susceptible to dust particles in the air in the same way as infrared night vision is. Dust, insects and other artefacts can pass through the beams without creating orbs or other light anomalies.
However, lasers produced a highly focussed and energetic beam of light which can reflect and bounce around unpredictably, so you should take care not to move the laser around in front of the camera as refractions in the lens could cause light flares. It's best to mount the laser to a camera tripod so it's fixed and there's no unnecessary movement.
The one downside with using a commercially available laser pen for this task is that they are only designed to be used to short periods as a pointer. The diode which produces the light gets very hot and there is no cooling system built in, so you'll need to give it a chance to cool down after about two or three minutes of continuous operation.
Failure to allow the laser to cool can result in permanent damage to the device but can also cause temporary fluctuations and irregularities in its output which can be seen as a twinkling effect across the grid, giving you false positives of ghostly movement.