Thermal Imaging: A Hot Topic

Thermal Imaging: A Hot Topic


Various thermal imaging sights and imaging devices are available to both commercial and military markets that claim near miraculous capabilities. Manufacturers’ technical explanation of thermal imaging devices usually overwhelms most shooters’ ability to comprehend. This typically leads to their choice of devices being based upon the packaging claims, versus the actual technical capability of the device. This article will describe how thermal imaging technology works, along with its differences to night vision technology. It will further outline the future direction of thermal imaging and present some advanced thermal imaging sighting devices available to the shooter today.

So–what is thermal imaging? Let’s start at the beginning. Sir Frederick William Herschel discovered infrared radiation in 1800. Herschel studied visible sunlight by using a prism to display the spectrum in its rainbow of colors. By placing a thermometer on the different colors, he discovered that the colors all possess different temperatures. To his amazement, and quite by chance, he discovered that a thermometer placed past the red end of the visible color spectrum reported a notable rise in temperature. Herschel correctly deduced that the light spectrum wavelengths went well beyond what could be seen by the human eye. It is this invisible radiation within the infrared (IR) region of the electromagnetic spectrum, past the visible red in the color spectrum, that is captured by thermal imaging devices and made visible to the human eye.

Literally everything with physical mass known to science, whether alive or inert, emits infrared radiation to one degree or another. This is called “emissivity” (emitting), and unless you can achieve absolute zero (-273 °C), emissivity cannot be eliminated. Emissivity is a term that is often misunderstood and misused. It represents a material’s ability to emit thermal radiation and is an optical property of all matter.

Since everything around us emits IR radiation (heat) to varying degrees, thermal imaging devices “see” that radiation and provide the user an image based upon the contrasting differences in emissivity of the objects being observed. While Herschel discovered infrared radiation, it wasn’t until the 1960s that anyone learned how to exploit it for the purpose of target detection and sighting.

The obvious question at this point is “Why did it take so long?” The answer is easy. First, thermal imaging devices are very sophisticated photoelectric devices that use lenses made from rare and unique materials like germanium and zinc selenide. Before 1960, we didn’t possess the exotic materials and manufacturing processes necessary to perfect them. Furthermore, the transistor and the light-emitting diode came of age in the 60s, which led to the optoelectronic circuitry that allowed us to manipulate IR wavelength information and create a picture from it.

Secondly, the demand for thermal imaging cameras and sighting scopes doesn’t even come close to that for digital photography devices, as evidenced by the billions of smartphones and computers that today contain digital cameras. Demand drives development–not the reverse–so thermal imaging has been largely left to advance along its own path based upon its unique demand, and that has largely been for military applications.

Thermal imaging uses a special optical lens that focuses the infrared light emitted by all the objects within the field of view. Compared to the selection of optics on offer for the visible world, infrared optics are very limited. Glass is opaque to thermal radiation, so thermal imaging optics are made of exotics such as germanium, zinc selenide or sapphire. This makes optics expensive and limited in selection. IR optics are not readily interchangeable, even between similar models of thermal imaging devices. Their optics are all special, and that means costly.

As previously mentioned, infrared is part of the electromagnetic spectrum with wavelengths that are longer than visible light. Infrared is typically divided into near, mid, long and extreme and is measured in units known as microns or nanometers. Thermal technology detects infrared energy in the mid-wave infrared (MWIR) spectrum at 3–5 microns or long-wave infrared (LWIR) at 8–12 microns.

That all seems simple enough, but how does it work?

Day or night in any environment, every person, object and structure emits infrared (heat) waves. The IR device’s lens serves to focus infrared energy that is, in turn, collected and scanned by a phased array of infrared-detector elements. The detector elements create a very detailed temperature pattern called a thermogram. It only takes about one-thirtieth of a second for the detector array to attain all the necessary temperature data to construct the thermogram. This information is obtained from several thousand points within the field of view of the detector array, and it all happens extraordinarily fast.

The thermogram created by the detector elements is instantaneously translated into electric impulses expressed in hertz (Hz). The impulses are sent to a signal-processing unit, a circuit board with a dedicated and highly sophisticated microchip that translates the thermogram impulse information into data for the visual display. The signal-processing unit sends the information to the display, where it appears as various colors depending on the intensity of the infrared emission. More simply, the combination of all the impulses from all of the detector elements creates the image.

Most thermal imaging devices scan at a rate of 30 times per second. They can sense temperatures ranging from -4 °F (-20 °C) to 3,600 °F (2,000 °C) and can normally detect changes in temperature of about 0.4 °F (0.2 °C). There are some that are far more sensitive, but they are also far more expensive. Nonetheless, there are two types of thermal imaging systems: cooled and uncooled. Almost all man-portable thermal systems used for gun sights and observation are uncooled systems.

The cooled systems are cryogenically cooled, with most using either open-system or closed-system Dewar flasks containing liquid nitrogen (LN2). Dewars are specially constructed thermos bottle-like insulated cryogenic liquid containers. The open-system Dewars require hourly/daily monitoring and the replenishment of LN2 that “boils off.” The closed systems have sealed Dewars, therefore cryogenic replenishment is unnecessary. These systems have the IR elements sealed inside a container that cools them to below 0 °C. As you might imagine, cryogenically cooled systems are expensive to manufacture and maintain and are more susceptible to damage from rugged use. These systems are usually platform-mounted on; for example, aircraft, land vehicles, ships, permanent observation posts and medical facilities.

The advantages of a cooled system are the incredible resolution and sensitivity that result from cooling the sensing elements. Even the low-end cryogenically cooled systems can “see” a temperature difference as small as 0.2 °F (0.1 ° C) from more than 1,000 ft (300 m) away. This translates to enough detail to determine if a person is holding a handgun or a wallet at that distance. The high-end units provide even more clarity.

Uncooled systems are the most common type of thermal imaging device. The infrared-detector elements are contained in a unit that operates at room temperature. This type of system is completely quiet. It activates immediately (no system spool-up time), and it’s battery-powered. Comparatively, the uncooled system’s resolution is far less than the cooled system’s, but it is sufficient for man-portable military applications like weapon sighting scopes and small UAV drones for the detection of people or objects in absolute darkness and a number of commercial applications that range from industrial to medical.

In summary, the two types of thermal imaging technology each offer their own advantages, and both are built with exotic materials, making them expensive. Some recently developed uncooled systems are built with a vanadium oxide (VOx) detector that allows for a very small camera size, low weight, minimal power requirements and high resolution. Cooled systems often employ mercury cadmium telluride (MCT) technology that utilizes a highly efficient cryogenic cooler, enabling the detector to discriminate smaller differences in infrared emissions.

About now, you’re probably asking, “What is the difference between thermal imaging and night vision image-enhancement technology?” The answer is that they are different technologies, and they’re commonly confused because both are mostly used to see in the dark. When most people talk about night vision devices (NVDs) they’re actually talking about image-enhancement technology. NVD systems still require visible light, albeit very faint (such as starlight), which is collected and intensified, while their thermal imaging system cousins work in the total absence of visible light. The key difference is that NVDs rely on a special tube, called an image-intensifier tube, to collect and amplify near-infrared and visible light. NVDs must use an IR illuminator to be able to see in complete darkness, and that IR illuminator emits near-infrared radiation.

NVDs employ a conventional glass lens, called the objective lens, to capture ambient light (starlight/moonlight) and some near-infrared light. Near-infrared radiation is closest to the red band (but just beyond it, on the invisible side) of the visible light spectrum. The light gathered by the objective lens is focused into the image-intensifier tube that contains a photocathode. The photocathode converts the photons of light energy into electrons. As the electrons pass through the tube, additional electrons are released from atoms provided by a microchannel plate (MCP) in the tube. An MCP is a tiny glass disc that has millions of microscopic holes (microchannels), made using fiber-optic technology. This multiplies the original number of electrons by a factor of thousands.

At the output end of the image-intensifier tube, the electrons hit a screen coated with phosphors. These electrons maintain their position in relation to the channel they passed through, which paints a perfect image that perfectly represents the distribution of the original photons. The energy of the electrons causes the phosphors to reach an excited state and release photons. These phosphors create the green image on the screen that has come to characterize night vision.

The green phosphor image (white phosphor is now available too) is viewed through another lens, called the ocular lens, which allows you to magnify and focus the image. The NVD may be connected to an electronic display, such as a monitor, or the image may be viewed directly through the ocular lens. Now you know how NVDs work, so back to thermal imaging.

Like any other technology, there are teams of very bright people working to improve thermal imaging products with new materials and better optoelectronics. But since there are highly exotic materials and sophisticated manufacturing processes involved and the lens itself is uniquely designed for each individual camera, it may be many years before a decisive breakthrough drives the cost of thermal imaging down to that of today’s smart phones.

That said, there are some discriminators you might want to consider when choosing a particular thermal imaging device. First and foremost, you can’t judge the contents by their cover. The wavelength envelope the device sees is certainly very important, as are its operating speed and its ability to focus. Both athermalized lenses and manual focus lenses are offered by several manufacturers. An athermalized focus (AF) lens is one that will maintain focus over a wide temperature range. The lens is either immune to focus shift over temperature, or there are fixtures in the lens that maintain focus over a wide temperature range.

With a manual focus (MF) lens, you will see slight focus shifts as the field of view (FOV) temperature changes. This will be apparent at temperature changes of approximately 20 °C or more. You can manually adjust the lens to fix this focus shift, where the athermalized lens does this automatically. Athermalized lenses are generally larger lenses that are used for a fixed FOV.
Quality thermal imaging rifle sights often have a high price range (US$3,000–6,000, or more) due to the expense of the imaging technology and the bells and whistles it offers. Features like Bluetooth to your smartphone, GPS, recoil activated record capability, built-in IR illuminator and IR target spotting laser, runtime extender auxiliary battery packs, in-line rail mounting for in-line compatibility with conventional rifle scopes and built-in zoom magnification, all add cost.

In lower-end thermal imaging devices, accurate images can be easily hindered by differing emissivity and reflections from target-adjacent surfaces. Images can also be difficult to interpret accurately when based upon certain objects, specifically objects with erratic temperatures. That said, this problem is reduced in active thermal imaging, where an object is illuminated by an IR light. So camera accuracy is another discriminator. Most of the better thermal imaging devices have at least a ±2% accuracy threshold.

Where will thermal imaging likely go in the future? Since a thermal imager is an optoelectronic device, the attainment of fully autonomous artificial intelligence (AI) will play a dominant role in thermal imaging sophistication, especially in industrial, medical and humanoid robotic applications. The laws of physics will forever restrict some of the applications we can and can’t achieve with thermal imaging, but providing AI with thermal imaging sensors to explore its environment will give it super abilities. It will see in all environmental conditions. It will be able to map and navigate its path in total darkness. It will be able to conduct diagnostic analysis on nearly everything based upon unique heat signatures. It will measure precise speed and distance within its environment. For instance, it will see us as a series of nanosecond thermogram frames until it rapidly discriminates us from everything else in the surrounding environment and identifies us individually as humans.

Being able to see the forest for the trees and anything else walking, crawling or flying within it, with near perfect discrimination and the ability to instantaneously identify it through the use of cloud-based computer image identification technology, will mark a new era in thermal imaging capability. These capabilities and more will become reality within this decade.