ATN’s Guide to Choosing a Thermal Rifle Scope

When deciding to upgrade and use a thermal scope for hunting, it can be bewildering to choose between scopes, given the sometimes impenetrable specifications.

To help you select the best thermal imaging scope for your hunting needs, we asked our ATN experts to explain some of the terms and highlight what you should look for.

Understand your application

While the cost of thermal imaging technology has reduced greatly, the price of a thermal scope reflects the quality of its components and its additional capabilities. Therefore, it’s important to understand what you need rather than want, as the higher the scope’s specification, the greater the cost.

The following table shows the tradeoffs you need to make between build quality, capabilities, and cost.

As you can see, if your needs demand a higher specification thermal scope, it comes at a cost, pushing you into professional or military-grade hardware costing two to three times that of commercial scopes. Here are the specifications you need to consider before making a purchase.

Thermal detector

The thermal detector is the heart of a thermal rifle scope, with the quality of the presented image depending on three important factors.

Resolution

Just as in digital cameras, sensor resolution greatly impacts image clarity. However, you’ll seldom see the high-resolution figures we’re used to for normal digital devices. This is because the infrared energy the sensor detects has a much larger wavelength than the light we can see, requiring larger pixel elements.

The large pixel size means the resolution will be lower when compared to a similar-sized digital camera sensor. Achieving higher resolutions requires larger sensors at a greater cost, leaving high-resolution thermal imaging for military and law enforcement applications.

Typical resolutions used for amateur and professional hunting are:

• 160 x 120
• 320 x 240
• 384 x 288
• 640 x 480

With higher sensor resolutions, greater detail is rendered in the final image, but with a narrower field of view and a lower magnification level than a lower resolution sensor using the same lens. As the sensor price is approximately 50% of the scope’s cost, a scope using a 640 x 480 sensor can be three to four times the cost of one using a sensor resolution of 160 x 120.

A term you’ll often see following the resolution numbers is a measurement, like 12 µm or 17 µm. This number is the distance, in microns, between each pixel. The lower the number, the better the image quality.

Noise Equivalent Temperature Difference (NETD)

The NETD rating of a thermal sensor is the sensor’s sensitivity to temperature changes. With NETD, the higher the number, the greater the noise and image blur. Low numbers mean less noise and greater definition between different temperatures, providing a sharper image.

The NETD rating is in milli-Kelvin or mK. In some thermal scope specifications, you’ll see sensors rated around 60 mK, yet treat this figure as an upper limit, with premium scopes having NETD figures around 25 or 30 mK.

However, while NETD is important, a lot also depends on the software algorithms used, so don’t solely choose your scope based on NETD figures. We’ll talk about software and algorithms shortly.

Refresh rate

The refresh rate refers to how often the image renews per second, shown as either Hz or frames per second (fps). Any movement will be jerky or dragging when viewing a moving object with a slow refresh. Most good-quality thermal scopes will have a refresh rate of 50 to 60 Hz to give smooth motion.

Image Quality & Algorithm

You might have a thermal detector with high resolution, low NETD, and a fast refresh rate, but if the converting software that creates the image is poor, you still end up with a mediocre picture. The thermal algorithm is a calculation that takes all the inputs from the sensor and optimizes them to provide an optimum image.

When choosing between several thermal scope options with similar hardware specifications, there are four characteristics you should look for to choose the scope with the best thermal algorithm.

Noise

More digital noise equals blurry and foggy images. While having a great NETD-rated sensor is a good start, the software processing the output from the sensor should remove as much residual noise as possible.

Noise can make an image look “snowy” as if viewing the image through a snowfall. It also appears as a light fog. Check each scope to ensure a clear and clean image.

Edge detail

Check an object’s edges to judge the quality of the edge line. A smooth and continuous edge indicates a good sensor and a good algorithm. Lower-quality sensors or poor algorithms will paint an edge as pixelated and jagged.

Infill detail

Poor algorithms or low-quality sensors will lack identifiable details on an object, showing it only as a bright spot or highlighted. For safety and functionality, the best algorithms display contrasts, body textures, and facial detail that allow accurate identification.

Background

The better the algorithm, the greater the background detail you can discern. Safety requires clearing the background by reviewing what’s around and behind your target. Check each scope under consideration to see how well trees and other features are delineated. How fine and sharp is the image? Is it blocky, or can you see fine detail?

Magnification and FOV

Magnification refers to the number of times the scope enlarges an object compared to the same object viewed by eye over the same distance. The field of view is the size of the horizontal area you can see through the thermal scope. The FOV measure is either given in degrees or width in meters at a defined distance.

Your hunting range and target size dictate the magnification, while target size and agility will guide your field of view (FOV) decision. Unfortunately, the two are linked and inversely proportional. That means that as your magnification increases, your FOV reduces.

Lower magnification scopes give you a wide field of view, easily allowing you to track game on the move. High magnification is great for longer distances and detail, but if it narrows your field of view, so a target can make a small movement and disappear from the viewing area, making it difficult to relocate.

Detection Range

Manufacturers publish something known as a DRI range. It comprises three ranges at which the scope allows you to detect, recognize, and finally identify a target. The DRI criteria is a standard derived in the 1950s to give the military an idea of the capabilities of a thermal device.

The DRI distances are in meters using a human-sized object as the standard. It’s important to understand that the term “detect” only means that you know something is there. You are unable to identify what it is. “Recognize” gives the range at which you can distinguish enough details to differentiate between a human and an animal or another object. The term “identify” refers to the distance at which you can distinguish between a fox and a hare or a man and a woman.

Note that these distances are optimal and that poor weather can reduce them. Therefore, when selecting a thermal scope, the detection range is of little use in guiding you on real-world applications. Derating the identification and recognition ranges to suit the atmospheric conditions you normally hunt will ensure you buy a scope that meets your performance expectations.

Software performance, scope durability & ingress protection

We’ve all experienced digital devices with poor user experience or design. Therefore, testing the controls and software when deciding on a thermal scope is important. Are image controls easily accessed and operated with intuitive menus? Is zeroing the scope easy, and how rapidly does it become operational after powering it up?

Check on the durability of each scope by researching the manufacturer’s IP or ingress protection rating. This rating consists of a number, with the first digit indicating the level of resistance to solid particles like dust, the second the level of resistance to water, and the third digit, if there is one, referring to the resistance to mechanical impact. While there is a specific measure for each number, in general terms, 0 means no protection, and 9 means high protection.

Most thermal scopes designed for general use should be at least resistant to water to withstand the weather. If you shoot heavy caliber weapons, check whether the scope has hardened electronics to resist the extreme shocks that occur during recoil. Thermal scopes designed for military use are tested to extreme levels to ensure durability and long life; however, the higher the durability rating, the higher the cost.

Reticle

Hunters have favored reticle patterns, so it pays to check that the thermal imaging scope you’re considering can offer your option. Most quality thermal scope manufacturers will offer a range of user-selectable patterns. Additionally, for the serious hunter, check to see whether the manufacturer offers a dynamic mil-dot reticle to accommodate the different ranges you shoot over and the game sizes you pursue.

Battery longevity

Finally, check the manufacturer’s claims on battery life. You don’t need to be halfway through your hunting day to have your scope run out of power. A quality scope should offer at least ten hours of use on one charge, with premium products almost doubling that. Also, check the options for extending the scope’s use with additional external battery packs.

Best ATN Thermal Scope

Our experts have selected several thermal imaging scopes from ATN’s extensive range for your consideration.

ATN MARS 4 384 2-8X

The MARS 4 384 2-8X is an advanced thermal scope from ATN’s premium range, offering a variable magnification from 2 to 8 times with a field of view from 12 to 9.5 degrees. It sports a fourth-generation high-resolution sensor of 384 x 288. The sensor refresh rate is 60 Hz to ensure smooth image movement when tracking moving game.

The MARS 4 range has electronics hardened against recoil, with the body constructed of hardened aluminium alloy to cope with heavy weapon recoil. Battery life exceeds 18 hours with USB-C charging capability. The DRI range is 960, 480, and 300 meters for detection, recognition, and identification, respectively.

The 384 2-8X is resistant to water and has many functions, including a smart range finder, ballistic calculator, and recoil-activated video. Bluetooth and Wi-Fi support dual video streaming to an onboard micro-SD card and an external device. Offering multiple reticles and color options, the 2-8x has a smart Mil-Dot reticle that adjusts with magnification and allows the variance between hash marks to be programmed.

ATN MARS 4 640 1-10X

From the same MARS 4 premium range comes the 640 1-10x, with a 1 to 10 times variable magnification and a 32 to 25-degree FOV. The sensor resolution in this scope offers outstanding clarity and detail with a 640 x 480 sensor having a NETD of below 25 mK. The sensor refresh rate is 60 Hz, and the DRI ranges are 830, 350, and 225 meters for detection, recognition, and identification.

As with all MARS 4 thermal scopes, you can choose white hot, black hot, or color mode for your image. Designed to cope with heavy recoils from high-caliber weapons, the 640 1-10X offers the same multiple reticle patterns and colors as the 2-8x, with the smart programmable Mil-Dot reticle. Battery endurance on this model exceeds 16 hours with USB-C charging.

For optimal scope adjustment and use, the 1-10x boasts ergonomic controls and offers a one-shot zero feature. Recoil-activated and dual streaming video capability using Bluetooth or Wi-Fi are also standard.

ATN MARS 4 640 4-40X

The MARS 4 640 4-40X is ATN’s longest-range offering. It includes all of the features of the previous two thermal scopes but with an impressive magnification range of 4 to 40 times at an 8.3 to 6.2 FOV. The 4-40x is fitted with ATN’s generation 4 sensor at 640 x 480 resolution, refreshing at 60 Hz and with a NETD below 25 mK. The detection, recognition, and identification ranges are 3300, 1450, and 800 meters, respectively.

The ballistic calculator on the 4-40x is a standard option on all MARS 4 thermal scopes, providing environmental information superimposed on the display. You can also program weapons profiles into the calculator to adjust for different loads or weapon use. The warranty is three years on all ATN premium thermal rifle scopes.

Conclusion

Thermal imaging technology can be confusing, and it’s difficult to compare one manufacturer’s claims against another. We hope the information presented here has helped give insight into the most important terms and how they might impact your normal hunting profile. With an understanding of thermal rifle scope terminology, you are well-placed to choose a product that suits both your hunting needs and your wallet.