Put very simply: thermal imaging allows users to view a visible image of normally invisible heat. Everything around us is emitting infrared radiation, which we are unable to see with the naked eye — a thermal imager displays that on a screen.

The first units were very basic and will show you a simple image showing very hot objects against a cool background at close range. As technology has improved, we now have units available that are capable of showing a huge amount of information, presenting images like some of those shown here.

As it’s not using visible light like your normal binoculars do, these can be used any time of day or night, as long as there is a contrast in temperature to pick up on.

They aren’t an X-ray, however; if something is hiding behind a tree then you won’t see it’s there until it peeps around the side!

The applications for thermal imaging technology are vast and ever-growing. There has been a huge movement in using thermal technology for monitoring and ringing — rather than using a torch to locate ground roosting/feeding birds, thermal is now being used. This gives a much clearer indication on how many individual birds are present, and in some circumstances the species can be determined, too. It helps users work out a plan of action to swiftly and safely capture these birds with nets, allowing them to be ringed whilst all-the-while minimising the disturbance of other birds and animals within the same vicinity.

Thermal also greatly improves the reliability of bird counts by minimising disturbance. Curlew nesting sites can be monitored from afar and you can note (conditions depending) not only where the nests are, but when they hatch and when they fledge. Cliff faces full of various nesting species can now be counted in a clearer, safer way than ever before.

Thermal is speeding up the process of checking nest boxes, too – something particularly useful when you’re monitoring bird of prey boxes, which normally require ladder access. If you check early morning, before the sun heats up the outside of the box, you can see which ones have residents by the heat they have transferred to the wood. (However, we have been reliably informed that although you expect the heat source to be chicks, they can also be squirrels!) The ability to record is useful when observing a larger number of subjects.

For instance, filming bats leaving their roosts through a thermal imaging device will allow the user to slow the footage down, facilitating a much more accurate count; and, again, depending on conditions and user knowledge, you can easily identify different species.

This is the hardest part of thermal imaging — working out which device is best suited to your requirements!

A professional ecologist or a licenced ringer is generally going to need a higher-quality lens, faster processor, and better sensor in order to gather the information they need. All of these listed above would require thermal imaging units at the higher end of the scale in cost; as a general rule the more expensive the unit, the better the clarity and definition you will be presented with. This is important for those who wish to identify species or carry out observations across greater distances. The larger units have bigger batteries and therefore longer battery life – very useful when working in remote locations in which charging may not be convenient or possible.

 The hobbyist birder, however, would probably have different requirements — entry level thermal would certainly still show you where birds are, but without the level of detail; not a problem if you want to locate them with the thermal and then get a clearer image with your binoculars or spotting scope. They also prefer the smaller units — when carrying binoculars, spotting scopes, and even a camera, something that will fit into your pocket easily may be more suitable.

There are lots of components that make up a thermal imaging unit, and it’s the combination of these components that makes each unit different.

The sensor and the lens size are the two most important components, and these determine how usable they are in the field. You can see a very simplified guide below, or a more comprehensive guide can be found here:
https://www.pulsar-nv.com/glo/support/thermal-imaging-technologies/2232/

There are three main aspects to look at when trying to understand how good a sensor is.

SENSOR RESOLUTION: This is how many pixels (the sensitive element) make up the sensor. Two numbers indicate how many pixels there are both across and down. Thermal devices suitable for birding usually start at 320x240, but the better units are fitted with 640x480 sensors, and are ever increasing. The bigger this number, the greater the amount of information the thermal can read and therefore relay.

NOISE EQUIVALENT TEMPERATURE DIFFERENCE (NETD Rating): This is the thermal sensitivity of the unit; it shows the differences in temperature that the sensor can read and is measured in millikelvin (mK). In bad weather/fog/drizzle the temperature becomes blurred and harder to read. The lower this number, the greater the level of detail this device can achieve in worse weather.

PIXEL PITCH: This is the distance between the centre of two pixels and is measured in microns (µm). The smaller the distance between pixels indicates that the pixels are smaller and can therefore more of them can fit within the sensor. A smaller pixel pitch results in better image quality and a physically smaller sensor.

Most thermal imaging lenses are made from germanium and come in several different sizes. The size and shape of the lens determines important factors like field of view, magnification, and detection range.

It is important to work out at what distance you will need to view out to, as well as to determine what size species you are looking for to help you decide which is best for your needs. They are usually noted in the model name, i.e., the Pulsar Axion XM30F has a 30 mm focal length, and the Pulsar Helion 2 XP50 Pro has a 50 mm focal length.

As a general rule, the bigger the focal length, the higher the magnification — resulting in more detail at greater distances, but with a narrower field of view. And the smaller the focal length, the lower the magnification with a wider field of view. This isn’t always the case, though, so please check each units specification.

FIELD OF VIEW: Most thermal imaging devices will present their field of view in degrees. The higher this number is, the larger the visible field of view will be. If you’re looking for birds at close range, a wider FOV will be more beneficial; but, if you’re looking for smaller species at long distance, you’d be better with a narrower FOV.

MAGNIFICATION: This will be the native magnification of the unit. The higher this magnification is, the better when looking further afield and vice versa. Most devices also offer a digital zoom function — this shouldn’t be relied upon, as it is merely enlarging the current image; a bit like zooming in on a photo on a phone. The more you enlarge, the more distorted it will become.

DETECTION RANGE: This is often listed for each unit, and is measured by how far away a human sized object can be seen in the best possible conditions. You wouldn’t be able to tell it’s a human, though, as it would show as a couple of dots on your display screen. This makes it hard to use this distance to judge the realistic range on small species. As a general rule of thumb, the smaller the species and the further away it is, the greater the specified detection range would need to be for it to be useful. Just be aware that a clear identification isn’t always possible, and relies on good knowledge and close conditions. Experienced spotters can usually narrow it down to a family. This is only important when using a thermal at night; in the day time identification will be clarified with binoculars.