The light we see is known as "white light". It's really a combination of all visible colors of the spectrum. "Visible" being the key word. The total light spectrum is much larger than what we see. In fact visible light is only a tiny fraction of the entire spectrum.
Visible light is part of what is know as the Electromagnetic Spectrum. The spectrum starts with longer wave/lower frequency light on the left side which includes the infrared lights, radar, FM/AM and television signals followed by the narrow section that cab be seen with the naked eye called visible light, through to the shorter wave/higher frequency ultra violet lights, X-Rays and Gamma Rays on the right end of the spectrum, Yes, radio, television and radiation are all part of the same electromagnetic spectrum as visible light (they are all forms of light, but we can't see it with the naked eye).
Below are two diagrams that represent the full range of the EM spectrum:
It has long been theorized that entities, spirits, ghosts etc may be more visible in other light spectrums than the normal narrow range of visible light.
Infrared (IR) Light
Infrared (IR) light has long been used for night vision video cameras. IR light is invisible to the human eye so it does not disrupt a location the way shining a bright flash or searchlight would. It also doesn't affect investigator's night vision (that's why we also use red filters on our flash lights as much as possible). Camcorders and DVR cameras usually come with built-in IR illuminators and additional higher power illuminators can be purchased. Night vision viewers, monocular and binoculars also usually have IR illuminators built-in. There are also IR film cameras but they require special film and processing, and are generally more expensive and complex to operate. Today you can purchase a digital camera converted to IR for a fairly cheap price.
IR light is often confused with heat/thermal imaging. They are two separate things. While heat is a form of IR light, an IR camera is designed to see the light not the thermal and visa versa. Also, colors seen in IR can be very different from visible light. For example, what appears black in visible light is usually bright white in IR. And IR has an element of "x-ray vision". Some materials that do well to be opaque to visible light are transparent in the IR spectrum.
Left: Photo taken with a SLR Digital Camera | Right: Photo taken with a converted IR Digital Camera
Notice the differences in both pictures between the SLR Digital Camera (visible light) and the IR Camera (IR light)
Due to the easily obtained IR cameras and illuminators paranormal investigators have been using IR for years in their research. Many good images and video of potentially haunted or paranormal activities has been documented with IR light. The main draw back to using IR light is that it takes a really large IR illuminator to fully illuminate a big area. And IR cameras are very sensitive to small objects. Dust, pollen etc are frequently seen on IR cameras and can easily be mistaken for orbs.
Ultraviolent Light (UV)
UV (aka "black light") light hasn't received much review in paranormal research, probably because it isn't as easy to generate and use, as built-in IR illuminators. Some paranormal investigators have reported that certain brands of night vision camcorders are very sensitive to UV light. It has been reported that using a UV light source with the camera in night vision/night/shot mode illuminates much better then in regular IR night vision/shot mode. To take photographs in UV Light, you would have to have a digital camera and then have an added on UV dual bandpass lens. The Digital Camera that we use for UV photography is a Nikon SLR D50 with a Schott BG25 UV Dual Bandpass Lens. To date, however, we have not seen any significant improvement in collecting paranormal evidence by using UV light instead of IR light. Further research is being conducted.
Left: Photo taken with SLR Digital Camera | Right: Photo taken with SLR Digital Camera with a Schott BG25 UV Dual Bandpass Lens.
Notice the differences in both pictures between the SLR Digital Camera (visible light) and the UV Camera (UV light)
When a ghost/spirit manifests, it may appear in the light spectrum that we are unable to see with our naked eye. When using a Full Spectrum Digital Camera, this covers all spectrums of light being able to see the apparition that we are unable to see with the naked eye. Investigators believe that this type of camera is the best for capturing evidence of the paranormal because it covers all spectrums of light. Today you can purchase a digital camera converted to Full Spectrum for a fairly cheap price.
Left: Photo taken with a SLR Digital Camera | Right: Photo taken with a converted Full Spectrum Digital Camera
Notice the differences in both pictures between the SLR Digital Camera (visible light) and the Full Spectrum Camera (Full Light Spectrum)
During Long Island Paranormal Investigator's first meeting with TAPS (The Atlantic Paranormal Society) the topic of IR verses UV light was discussed with TAPS investigator Brian Harnois. Brian told LIPI that TAPS had experimented with UV light for paranormal photography and videography during their paranormal investigations. He said they too found little evidence that UV worked better than IR. Interestingly though, he did say that when investigating cases of alleged demonic hauntings TAPS found that UV light acted as a barrier to the demonic entity; The entity seemed to avoid crossing doorways and thresholds that were illuminated with UV light. Brian said research on this is continuing.
Based on research and discussions it appears that entities, ghosts, spirits etc. are more sensitive to being seen in IR light than UV light. This is in line with another theory that ghosts move very quickly through our space/time. A high-speed object would be better illuminated with a low frequency/long wave light source, than a high frequency/short wave source. It is also possible that whatever the composition of a ghost is (including ectoplasm) that it is more sensitive to reflecting IR light.
LIPI and other paranormal investigators are conducting additional research and investigation on this subject. Currently LIPI uses regular Digital Cameras, IR, UV, and Full Spectrum Cameras on all of our investigations.