Most wanted

Thermal imagers seem to be the most desired instrument for ghost research at the moment. They are 'most wanted', rather than 'most owned', because they are very expensive. Unless you are rich, have a sponsor or someone has donated one, you probably don't have one. The fact that they appear on TV ghost shows just adds to their extreme desirability.

So why do ghost researchers want these expensive bits of kit and what can they expect to see if they own one? The most obvious reason is that they hope that ghosts will show up in thermal imagers when they are not visible in normal light. Similarly, cold spots are an obvious target. Some ghost researchers speculate that ghosts manipulate energy to appear and that such anomalies may show up in thermal imagers.

Are these reasonable expectations? A cold spot can be a real area of lower temperature or just one that feels colder to people. Even if a cold spot really is physically colder, would a thermal imager reveal it? Probably not. Like IR laser thermometers, it picks up temperatures from objects rather than the air. So a cold pocket of air might not show up in a thermal imager. On the other hand, an imager might reveal cold surfaces that might be causing people to feel cold nearby.

What about ghosts? The idea that ghosts should show up in thermal imagers is more of a theory than something supported by evidence. Apparent figures and other anomalies have appeared in thermal imagers but there could be other reasons for these (see Anomalous Thermal Images' below). This doesn't mean ghosts can't appear in thermal imagers, just that there is no compelling evidence yet that they do.

How does a thermal imager work?

It is crucial that anyone buying a thermal imager understands what they are getting. Many people think they are getting a 'heat viewer' but it is not as simple as that. A much better way to consider a thermal imager, despite its name, is that it is like a camera. However, instead of recording visible light, it shows mid- and far-infrared.

Infrared is the bit of the electromagnetic spectrum next to light that extends towards longer wavelengths. Infrared (IR) is invisible to humans but still acts in a very similar way to light - it can be absorbed, emitted, reflected, refracted, diffracted, etc. It is important to remember this because you can get anomalous images created on thermal imagers created in exactly the same way as they are in ordinary light photos.

A digital camera can pick up near-infrared (though not very well) which is nearest in wavelength to light. Thermal imagers pick up longer wavelengths, further from visible length (which is what the mid- and far- prefixes refer to). At these wavelengths the world looks very different. Even in the near-IR some objects look brighter, darker or more or less transparent than in ordinary light (see photo, right). In mid-far-IR things look even stranger. Most objects just absorb or emit these wavelengths without reflecting them. We are used to seeing objects by reflected light but objects appear by virtue of emitting mid-far-IR in thermal imagers. The wavelength of this emitted radiation is characteristic of the temperature of the object (black body radiation) which is why it can be used to produce a 'thermal image'. The wavelength of the radiation is measured in an array of tiny sensors which is used to calculate an equivalent temperature for each point. This is used to build up a 'false colour' contour image of the scene showing the temperature of the visible surfaces.

How does the world look to a thermal imager?

Most surfaces in direct line of sight to the thermal imager should give a temperature reading. That's because they emit mid-far-IR, rather than reflect it. Since the image shows the temperature, rather than light reflected off it, there can be hot and cold spots that wouldn't appear in a normal photo. In addition, a surface may appear warm or cool after having been in contact with a hot/cold object which has since departed. For instance, a chair may show a warm image of a human for a while after they get up. Unlike light, heat moves slowly through a solid body.

As with an ordinary camera, the air is generally invisible. Indeed, thermal imagers will 'see through' smoke, mists or sprays, which is one of the reasons why they are so useful. For this reason, a 'cold spot' might not show up in a thermal image at all.

Not all surfaces show their true temperature. Metals, for instance, appear dark and, indeed, reflect mid-far-IR. Other surfaces that readily reflect mid-far-IR include glass, polished ceramics and stone. Any 'temperatures', or even images, on such surfaces may, in fact, be a reflection from another source. Imagine taking an ordinary light photo of a mirror and you will begin to appreciate the problem. Such reflective surfaces may even reflect radiation onto surrounding surfaces, warming them up. Any reflective object could also reflect radiation directly into the thermal imager lens, possibly causing strange, false images similar to lens flare.

Anomalous thermal images

Since mid-far-IR acts like light, the same kind of anomalies can occur as in ordinary light photos. Odd shapes can appear suggestive (eg. like faces or figures) of paranormal phenomena. Given that the objects look radically different to their appearance in light photos, this can be particularly problematic. So what may look like a dark ghostly figure in a thermal image may just be a tall, cylindrical metal object.

As in ordinary cameras, images can be out of focus in thermal imagers. Also, since mid-far-IR has a longer wavelength than light, images will always appear fuzzier compared to ordinary light video. Overall, this means that you should not expect too much detail in thermal images. Any small anomalies are probably nothing but artifacts. Because mid-far-IR ignores small particles in the air, at least you needn't expect orbs!

Because of these problems, it is recommended that another video camera (eg. night vision), pointing at the same scene, should always used in conjunction with thermal imagers. In this way it should be possible to check the identity of any strange shapes that appear in the thermal imager.

Don't rely on the temperatures shown

As with laser thermometers, the temperature reading depends on the emissivity of the objects being imaged. This is a property that varies from object surface to surface. Since the emissivity will typically vary from object to object in one image, it means the temperatures shown cannot be relied on to be accurate. What looks like a 10 C difference between two objects may only be 5 C or perhaps it's 15 C. A thermal imager can give a reasonable qualitative picture of surface temperatures but if quantitative accuracy is important, you'll need a thermometer as well.

Do I need a thermal imager?

This is entirely up to individuals or groups to decide. As with any instrument, you should understand what it does, how it works and its limitations. You should use the thermal imager in lots of ordinary situations, to see what to expect, before you take it to a haunted building. Unless you clearly understand the science behind the instrument and what it is measuring, claiming a paranormal anomaly from an imager would be ill-advised.
© Maurice Townsend 2007