How Do Thermal Scopes Work? Thermal Scope Breakdown

The value of a thermal imaging scope, or even a thermal camera, is readily apparent to anyone. While we will be focusing on the hunting and shooting applications of thermal, they offer extensive advantages to many different industries. The basest argument for wanting thermal imaging is that it has a cool factor that can’t be beat! In the firearms world, thermal sees the most use from hunters. Nocturnal animals like pigs and coyotes would be almost impossible to find at night, but a thermal scope makes them stand out even better than full daylight. While more and more people are buying new thermal scopes, few completely understand how they work.

Thermal Science 

“Any sufficiently advanced technology is indistinguishable from magic.” This is a famous quote from writer Arthur C. Clark, and it’s appropriate because a thermal scope often does feel like magic. However, the technology behind them isn’t all that new, and the main reason we’re seeing more of them amongst outdoorsman is that they’re finally entering the realm of affordability. For example, Sightmark’s new Wraith Mini Thermal has a price tag at only $2,000, yet it is a quality thermal that has proven successful for me on multiple hunts. The short version of how a thermal scope works is basically a thermal sensor inside a scope’s housing detects heat, then it creates a digital display on a screen. Overlay a reticle on that image, then ta da! You have a thermal scope.

The long version starts with infrared radiation (IR), which is a type of radiant energy that is emitted from all objects in the universe with a sun being the most powerful source. It’s not visible to the human eye, but we can still detect it. It’s simply heat! Credit for the discovery of IR goes to British astronomer Sir William Herschel in the year 1800, who discovered it while conducting an experiment to measure the difference in temperature between colors in the visible light spectrum. Infrared radiation is a type of electromagnetic radiation that includes gamma rays, ultraviolet radiation, visible light, microwaves, radio waves and others. IR waves are longer than visible spectrum light waves, and the longest waves on the IR spectrum are what is felt as heat. There are short IR waves, too, but they don’t emit heat and are what you find in various wireless remotes. Once infrared radiation was discovered, credit for measuring it goes to American astronomer and physicist Samuel Langley, inventor of the bolometer. It paved the way for modern thermal imaging technology and is still the term used to describe modern thermal sensors. The first infrared camera was built in 1929 for the British army to detect aircraft, but the first IR camera that could produce a still image didn’t appear until 1947.

IR Scope Terminology 

Infrared radiation and IR detection has been a subject of study for some time, and while we could dive even deeper into its history, let’s fast forward to modern thermal scopes for hunting and shooting. To start, there are a lot of terms to define before we dive into the details of a modern thermal scope. If you’ve ever shopped for a thermal scope before, the spec charts often have several obscure terms and symbols, like µm, that are worth breaking down.

• Pixels: The microbolometer, which is the sensor that detects infrared radiation, can vary greatly in resolution quality depending on the number of pixels within the unit. The more pixels within a microbolometer, the better the resolution, which translates into a more detailed image of an object.
• Pixel Pitch and Microns: The pixels within a microbolometer have space between them, which is measured in microns (µm). The ratio of the number of pixels within a bolometer to the total area of pixels is referred to as the fill factor. The greater the fill factor, the more IR energy can be detected.
• Noise Equivalent Temperature Difference (NETD): Objects give off varying degrees of heat. Animals produce much more heat than plants, and NETD refers to the sensitivity of a scope to tell the difference. Remember, the more pixels a microbolometer has, the more it can absorb IR energy, giving it the ability to better differentiate between temperature differences. A scope with an excellent NETD rating has more sensitivity and can better tell the difference between two objects that are close in temperature levels. NETD is measured in millikelvins (mK), and the lower the mK rating, the better the sensor can differentiate temperatures. (For example: 25mK — Superior, 40mK — Excellent, 50mK — Good, 60mK — Acceptable, 80mK — Poor)
• Frame Rate: Once a scope’s microbolometer detects infrared radiation, the electronic processing unit processes the signal to be presented on the micro display seen through the eyepiece. The frame rate, measured in Hertz (Hz), refers to the number of frames displayed on the screen in one second. Higher frame rates reduce the effects of “lagging.” Lagging reduces your ability to view a scene in real time and can be incredibly frustrating. A high frame rate gives the image a smooth appearance, especially when scanning across a field.

Thermal Scope Objectives 

We could get into the weeds even more with modern thermal scopes, but the above-listed terms are the primary factors to consider when looking at thermal scope specs. One that was not listed, however, is the objective. Objective size is an important consideration that affects both the field of view and the detection range of a thermal scope. The thermal sensor within a scope can’t see through traditional optics-grade glass. In fact, thermal scopes cannot see through most types of glass, including windows in a house or vehicle. Thermal scope objective lenses are built from single-crystal germanium. While there are a few other materials that can work, optics-grade germanium is certainly the most prolific, but it is incredibly expensive. In fact, it almost always represents the largest cost in thermal scope manufacturing, so if scope has a bigger objective lens, it’s going to cost more. Is a bigger objective lens always better? It’s mainly subject to use scenarios. A thermal scope with a large objective, usually 50mm on the high end, has a greater detection range than a smaller 25mm or 35mm scope. However, that larger objective generally means the thermal scope will have a higher base magnification and with that comes a narrower field of view. If you’re out west looking for coyotes, odds are you will want a greater detection range and can sacrifice some field of view. If you’re pig hunting in Texas, you’ll want a greater field of view to see all the pigs in a field, and you wouldn’t need as long a detection range. So, if you’re fortunate enough to have a big budget, don’t assume the most expensive scope is the best. Make sure you know how far and what you’ll be shooting before you drop several thousand dollars on a scope.

Put It Together 

Once you put all the factors that go into a modern thermal scope together, it’s readily apparent that it’s much more complicated than buying a traditional scope. Factor out the objective size for a moment, and the perfect thermal scope has a high pixel rating, a low NETD factor and a display high frame rate. Thermal scopes with that perfect combination come with a hefty price tag, so for most of us, we must strike a balance based on our needs and budget. If you are seriously looking for a thermal scope, start by setting yourself a budget. Look at the scopes at or below that price range and look for the one with the best pixel, NETD, and display frame rate combination. YouTube is a useful tool when comparing thermal scopes, too. Many modern thermals have built-in video and image recording, and there are tons of videos online. If you’ve narrowed down your options to a few, odds are there are some hunts online with the scopes you’re considering.

Thermal scope video recording is a great segue into some other factors for thermal scopes. All the previously stated factors really just define the image quality of a thermal, but there are plenty of operational considerations, too. Battery life is the biggest consideration after image quality. Thermal scopes are only battery powered, so if you want great image quality with great battery life, that, too, adds to the price. It’s important to consider weight, size and mounting as well. Many thermals have a built-in Picatinny rail adapter, but there are plenty of tubular thermal scopes that require traditional scope rings to mount to a rail. One factor with thermals that isn’t readily apparent until you use one is the lack of depth perception when viewing targets. Regardless of the thermal quality or price tag, you’re still viewing a flat screen, which makes it difficult to judge distances. If you know you’ll be hunting or shooting in unfamiliar areas, consider a thermal scope with a built-in rangefinder. This adds cost, of course, but knowing the exact distance of a target means you can hold accordingly for a first-round hit. If you will be hunting or shooting mostly in familiar areas, a rangefinder isn’t as important since you should already know approximate ranges based on familiar terrain features.

Most of us can’t afford to have multiple thermals, so a thermal that allows you to hold several gun profiles is a huge bonus. Thermals like the new Sightmark Wraith Mini Thermal have that feature, which allows you to quickly swap between different rifles and maintain zero without major adjustments. Thermal scopes are addictive. Once you’ve used one, especially on a pig or coyote hunt, it’s hard to go back to a traditional hunt. While there are a few other factors to consider when looking for a new thermal, we’ve hit the most important factors to set you on the right path. Cost is certainly the biggest barrier to owning a thermal of your own, but like guns, thermal scopes can hold their value surprisingly well. Many thermal owners upgrade every three to five years, and they are often able to sell their old thermal for close to the initial purchase price. Invariably as one looks for night optics, night vision or digital night vision appear as an option, but these devices require an ambient illumination source to operate. Only a true thermal device can operate in complete darkness, and thermal works just fine during the day, too. When it comes to owning the night, go with a true thermal scope!

---