All eyes on this high-tech triumph

Thermal optics have changed the face of hunting. Almost no one who is serious about stalking or vermin control in the UK now sets off without one in their arsenal. The acceptance of thermal devices in the hunting community has increased the market’s demand for new products. With the growth of the industry and the resultant economies of scale, prices have dropped, making thermal devices more and more affordable, further increasing demand. 

The mass adoption of these devices by professionals and amateurs alike has been responsible for shaping aspects of deer management policy from wildlife organisations. It has also sparked legislative changes across Britain and Europe regarding hunting with them at night. The broad application of this technology — with its clear advantages over traditional glass optics across both day and night — has established thermals as a must have. The ongoing rise in accessibility of this tech to the regular outdoorsman means the trajectory of its continued adoption by hunters doesn’t look like changing. 

Astronomer 

The history of the thermal optic began at the dawn of the 19th century when astronomer Frederick William Herschel was forced to stare directly at the sun for a study he was undertaking on behalf of the Royal Society of London. 

To be able to study the spots on the surface of the sun, Herschel was required to make his observations through darkened glass plates of different colours. He noted that the varying shades of the glass heated up differentially, which gave him the idea that temperature was related to colour — light’s wavelength. 

Further experimentation led to his discovery of electromagnetic waves beyond the visible spectrum, which still possessed the same qualities of dispersion and diffraction as light in the visible spectrum. He had discovered infrared radiation. 

One of the first commercial applications of thermal technology was the Infrared Eye. This was a device fitted to ocean-going vessels and used to detect the presence of icebergs and steamships in poor conditions by using a mirror and a thermopile — a device used for contactless temperature sensing. The Eye was patented in 1913, a year after the Titanic went down. 

The next meaningful advance in thermal technology was in 1929 when the Hungarian physicist Kálmán Tihanyi invented an infrared-sensitive camera that was used in British anti-aircraft defence. 

Various organisations continued to develop thermal cameras and in 1978, Texas Instruments was the first to fit thermal sights to a tank. It equipped the M60A3 tank with the enormously expensive and bulky TTS (tank thermal sight), a move that a host of other defence contractors quickly followed. 

In 1998, Raytheon produced the AN/PAS-13 thermal sight. This was one of the first sights to be mountable on small arms and was quickly rolled out across the US military. The initial AN/PAS-13 weighed up to 2.5kg and would have been a menace to manoeuvrability and rifle balance. 

As technology improved, the devices became smaller and towards the end of the first decade of the 21st century, various thermal sights and monoculars became available to the civilian hunting market. 

Evan Boulton, marketing executive at optical equipment distributor Thomas Jacks, said: “In 2012, Pulsar released its first thermal imaging monocular, the Quantum, making thermal-imaging monoculars available to the civilian market. The Quantum was groundbreaking. 

“Thermal-imaging devices have been a game changer in both professional and recreational markets, because in some stalking areas such as woodland, forest or clear fell, it can be almost impossible to spot or locate a deer through traditional day optics alone.” 

Thermal units are immensely complex and use delightfully exotic-sounding chemicals such as amorphous silicon or barium strontium titanate. They also involve all sorts of wonderful electronic wizardry such as “synchronous chopping”. To try to simplify some of the dense but necessary technical jargon, I asked the team at German optic manufacturer Zeiss if they could lay it out in simpler terms. 

The team explained that the key technical aspects to look for when buying a thermal were: focal length, noise equivalent temperature difference (NETD), resolution of both sensor and display, and pixel pitch. For digital systems, focal length is a combination of factors, but broadly it determines the width of the user’s field of vision. 

Zeiss says focal length is the second most important factor for thermal device selection after the sensor resolution. It urged those looking to buy a thermal device to identify realistically the precise purpose for which the device will be used. Using a device with a large focal length for stalking in woodland is less than ideal because the high magnification will be of limited use in close terrain. For this style of hunting, a system with a lower focal length — meaning a wider field of view — is preferable. Longer focal length is better suited to scanning open environments such as fields or mountains. 

Sensitive 

NETD describes how sensitive a sensor is in picking up differences in temperature. It is usually expressed in mK (milli-Kelvin), which describes the minimum temperature difference that will be registered. Zeiss says there is a misconception that a lower NETD will always give a better picture. In theory this is correct, but in real-world use you will seldom see the benefit of NETDs lower than 40mK. 

This is because the display in most devices is not capable of reproducing the granularity that the sensor is able to pick up. They say, as a rule of thumb, if your surroundings have a temperature difference of 10°C from the coldest to the warmest point, then a 40mK sensor is more than enough. 

The resolution of sensor and display describes how many pixels they have. In general, the higher the resolution the better the image.

Finally, pixel pitch, usually quoted in micrometres (μm), describes the distance from one pixel to the next pixel. This alone is not directly relevant, but together with the resolution of the sensor this defines the physical size of the sensor and that has an impact on the field of view of the whole system. For example, a system with a 35mm lens and a 17μm pixel pitch will have a wider field of view than a system with a 35mm lens and a 12μm pixel pitch, given that the sensor has the same resolution. 

Another misconception is that the smaller the pixel pitch, the better the sensor, when in fact it is the other way around. Sensors with a larger pixel pitch tend to produce a better, cleaner image signal than sensors with a smaller pitch, as the individual pixel has a larger surface area and can thereby receive a better signal. 

Sweet spot 

I asked Zeiss where the sweet spot between image quality and price point is. The team says systems offering a sensor larger than 640×480/512 have benefits in digital zoom, yet the standard image is not that much better than modern systems with 640 sensors, but the price point is significantly higher. 

The next step after the resolution increase is to focus on sensors with a smaller pixel pitch, although this can have a negative influence on the image quality, which needs to be compensated for by using increasingly sophisticated software. 

Zeiss concludes: “With a new era of sensors, we will probably see a slowing down of innovations, at least on the hardware side. More progression will be more costly, yet the benefit for the user only changes marginally.” 

HikMicro’s technical team also emphasised the importance of these base technical details. But they also urge customers to look closely at the product’s detection range, battery time and the ergonomics of the device’s soft functions. HikMicro advises buyers to choose a product with a smooth visual experience and no image freezing. It also suggests that customers check on the after-sale care offered by the brand. For instance, HikMicro organises roadshows across the UK at which its experts are on hand to talk about the products, and users can test and experience them free of charge before buying. 

Testing the product in real-world circumstances is essential prior to purchase. Both Zeiss and HikMicro warn that many devices produce clear, contrast-rich images, which look great when testing them indoors where there are a lot of straight lines. But when heading outside, where the finer details of nature need to be identified, many contrast-rich devices hide the subtle details that hunters need to see in order to be able to distinguish important differences. 

Thermal devices are of huge benefit when it comes to time spent on the hill versus effective shots taken. Devices such as clip-on thermals that attach to analogue optics offer enormous flexibility for hunting trips and wildlife management. 

Identify 

Paul Stewart, sales director at thermal expert Scott Country, says: “Thermal imaging has completely changed the hunting and stalking game, by providing users with the capability to detect, observe and identify species completely covertly. Finding wildlife in foliage is virtually impossible with traditional glass, but with a relatively inexpensive thermal the capabilities of finding quarry are radically transformed. 

“For very little these days you can buy a thermal imager that allows you to see something is there, which is often enough when scanning. You can find a heat signature and then stalk closer for a firm ID through traditional optics. Detection ranges are also important, but remember a detection range of 1,800m doesn’t mean you can tell Charlie from a muntjac at 1km away.” 

We are no longer entirely reliant upon weather, terrain or moon state to determine a successful and safe outing. If you plan to dip a toe into the world of thermal-imaging devices, do your best to make sense of the quoted technical terminology of products, be realistic about your primary uses for a new bit of thermal kit, and above all test and understand the equipment prior to purchase.