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How Do Thermal Scopes Work and What Are They? 

Brian Belko | Updated February 26, 2024 | Why You Should Trust Us | How We Earn Money
cover photo of how do thermal scopes work showing a hog under infrared light, article title, and logo

You’ve probably come across the words ‘thermal scopes’ quite recently and now want to know what these things actually do and why everyone is talking about this heat-sensing scope like it’s the next best thing to sliced bread.

Well, we’ve got that completely covered, and anything else you’ll want to know: what they do, how they work, where they’re most often used, who buys them, why people are buying this not-cheap scope, and which are the features to look for to get the best thermal scope for you!

Basically, we’ve got all the questions covered.

So, here’s the rundown and the only one you’ll need on everything about thermal scopes.

How Do Thermal Imaging Scopes Work? 

Thermal rifle scopes are devices that see infrared light (heat) instead of light that is visible to the naked eye. These scopes work by converting infrared energy (heat sources) into something our eyes can see on the light spectrum. (1)

Electronics inside the device, the heat sensor specifically, pick up heat emitted by a target before other systems process the information, which converts and transmits that information to a screen.

The digital screen can be as large as a TV set or as small as the one found in rifle scopes and can be in color or black and white.

One reason thermal imaging devices are so popular is their ability to “see” through fog, light brush, and total darkness. That’s why they’re highly regarded by hog and varmint hunters, as you can hunt at night without a spotlight (spotlights with a red or green filter can spook some animals).

What is a Thermal Scope?

Simply put, a thermal rifle scope is a thermal imaging camera with the addition of an adjustable reticle and the ability to mount onto something that shoots. The device fits a rifle, shotgun, pistol, or bow scope and uses internal electronics to convert a heat source into something visible.

For instance, you won’t be able to tell if a cup of coffee is hot from 100 feet away. However, a thermal scope will pick up the heat from the cup and convert that to an image.

Most commonly, the heat source shows up as black with a white background, white with a black background, or a shade of red for hot and shifting down to blue for cooler temperatures. However, some thermal imagers may have different color schemes.

And turns out that reflected light, which can affect a regular scope, will not affect the thermal image seen through a scope.

As for mounting, thermal scopes mount to your gun or crossbow the same as regular scopes. Some thermal imagers come with a built-in mount, and others require rings. 

Given the nature of the electronics and other considerations, most thermal scopes use 30mm or larger rings. Finding one that uses 1-inch rings is rare.

What sets these scopes apart from traditional devices is the internal electronics. Every thermal has housings for a battery or power port and internal components. This can be small or large, but it will be larger than the adjustment knob housing on a regular scope.

Now, if you’ve looked into some thermal imaging scopes already, you’ll know that thermal imaging is not cheap. If you believe in the adage that your glass should cost as much as the rifle, you need to double the scope cost. Even the low-end devices fall around the $1,000 mark, and the most expensive ones can hit 6 digits.

That said, mid-level and higher devices can create videos and still images to be downloaded and enjoyed later, and some can even stream live to your phones via Bluetooth.

Thermal vs Night Vision Scope

In the debate between thermal vs night vision scopes, hunters with experience using both devices almost always prefer thermal imaging scopes.

Why? Well, first, night vision:

  • Can only be used at night (as full sunlight will destroy the internal sensors in a NV device).
  • A few NV scopes do come with a pinhole cover, allowing usage during the day (however, knocking the cover off or accidentally removing it during the day while it’s turned on will ruin the scope).
  • Only a few NV scopes can be used during daylight with the NV electronics turned off.
  • Much more limited range than thermal imagers. The very best NV can see around 200 yards under ideal conditions.
  • Cannot see through weeds or fog.
  • Requires infrared light to work.

However, a benefit to NV:

  • Much cheaper than thermal. Prices dropped dramatically when thermal imagers became common for night hunting.

Meanwhile, thermal imaging scopes:

  • Can be used during the day and night. 
  • Its sensors are not bothered by ambient light (warning: don’t point it at the sun, as it could damage the sensors). 
  • Can be sighted in at night.
  • Thermal infrared also uses the infrared spectrum but processes the incoming wavelengths differently. Rather than enhance (what NV does), it changes the wavelength into visible light on the screen.

The drawback of thermal imaging?

  • More expensive.

When Do You Use a Thermal Scope?

The times when to use a thermal scope are basically whenever you want to, as they can be used both at night and during the day. Keep in mind you’ll always want to make sure it is legal to use a thermal in your particular state. Check out our full article on the topic for more info.

Why Are Thermal Scopes So Expensive? 

Thermal scopes are so expensive because of the microtechnology in their electronics. Making the circuits, the boards, and the sensors is delicate and expensive work. 

Demand also figures into the price. In fact, thermal units with practical uses outside of hunting can cost as much as a house.

Thermal devices are nowhere near as common as computers, laptops, smartphones, and the chips inside them. Fewer companies make the internal parts for a scope than a computer, so a limited supply drives the price up.

How Far Do Thermal Scopes Work?

How far thermal scopes work depends on the scope itself. A top-end thermal imager will catch heat signatures 2,000 yards away with a clear field of view. 

…All you will see is a blob. 

However, you can identify larger animals, like deer, humans, and bears, using the same scope at about half that distance. Get under 500 yards, and you’ll see individual features on the target. 

On cheaper scopes, you need to be under 200 yards to clearly see features in the thermal image.

What Should I Look For in a IR Scope?

Source: An IR illuminator on a scope

IR stands for infrared, and what thermal imaging does is take infrared light and convert it to an image the naked eye can see. In that respect, a thermal infrared device is an IR device. (2)

However, when it comes to hunting scopes, IR has a different meaning. In scopes, IR refers to the IR light source used with night vision scopes.

Here’s a quick rundown on NV

Night vision devices require some light source, so if it’s too dark, the scope won’t work. However, you can make the NV scope work with an IR light source. The IR illumination, as it’s called, may be included with an NV scope but sometimes needs to be bought separately. 

In other words, IR is not the scope but the light source for the night vision scope.

Now that’s cleared up, a better question is:

What Should I Look For in a Thermal Scope?

Here are three categories of what you should look for in a good thermal scope:

1. Refresh rate – Refresh rate refers to how fast the electronics process the thermal image. A high rate means that what you see through the scope is in or very close to real-time. This matters because if you’re tracking a running critter, the internal computer has to process the image and present it to you. 

Low rates will cause the screen to freeze. So, what you’ll see on the screen is not what is actually in front of you. 

The rate is measured in Hertz (Hz). 60 Hz is the highest rate as of this writing, but as technology improves, so will the rate. High refresh also comes with high image quality. (3)

2. Waterproof – Waterproof rating is measured as “IPX-number,” and higher numbers mean better protection. IPX-7 means the device is waterproof for at least 30 minutes when held under a meter of water. Some scopes say “water-resistant” without giving you an IPX rating, so these will withstand mist or light rain but are not waterproof.

3. Battery – All these devices run off batteries, and battery life ranges from a few hours to 10+ hours. Evidently, long battery life is best, but the more you do, such as scanning and recording video, the faster the battery will run down. 

You can take a portable charging unit into the field with you, but that’s more stuff to haul around. Thermals with a replaceable battery mean you can easily swap a dead battery for a live one while hunting.

Where Did Thermal Imaging Get Started? 

Thermal infrared imaging started with a researcher using the world’s first thermal camera. 

It took many years for the thermal camera to move outside a lab and medical research fields into practical applications. And primarily, it split into two thermal image channels – civilian and military use. (4)

The thermal imaging cameras we use today come from the military side. The military needed a way to see through objects that obstruct visible light and a better way to see at night. The devices also help to easily track heat-emitting sources.

What Are the Common Uses for Thermal Imaging Technology? 

The practical uses for thermal infrared imaging technology today are roughly broken into these categories:

  1. Medical – Doctors use thermal imaging tech to diagnose and treat patients. These thermal cameras are much bigger than scopes and rely on near-infrared to provide much more precise images than a hunting scope.
  2. Commercial – Handheld devices are used to find hotspots in buildings and on the ground. Many fire departments use a thermal unit to sweep buildings to find and extinguish hot spots. This is the second use for thermal after it was originally developed.
  3. Military – The military uses thermal imaging cameras to see through camo and other obstructions to find a hiding enemy. Law enforcement uses it for the same purpose and also for finding people lost in the woods.
  4. Hunting – We hunt at night with thermal imagers. No spotlight is needed because the target emits heat that infrared cameras can pick up. The shooting world is a latecomer to the thermal market because it wasn’t originally developed for hunters.

What is the Best Thermal Scope?

In our opinion, the best thermal imaging scope out there is the ATN Thor 4 640. Make sure to read the full article for a review of the Thor 4 and many other industry leading brands.

Frequently Asked Questions

How well do thermal scopes work for varmint hunting?

With a good one, thermal scopes work astonishingly well for varmint hunting. You can see the varmint on a cloudy, moonless night and through light brush and grass. 

Infrared scopes let you see critters much farther away than night vision, and you won’t have a spotlight to spook it. Although image quality will worsen with distance and more obstructions between you and the target, the true benefit is thermals work equally well in low-light to no-light conditions.

How do thermals work in the mountains?

Thermals work very well in the mountains, especially in nighttime conditions. A thermal scope let’s you scan mountainous terrain without a spotlight. 

How long do thermal scopes last?

A quality scope with proper use will last thousands of hours in the field. You will notice a drop in high image quality as the scope’s internals begin to wear out. 

You’ll likely wear out the battery before wearing the internal thermal imaging electronics that provide high image quality (another reason to get a scope with a replaceable battery).

The thermal imaging internals should last well beyond the factory warranty, as 10 years of use (at most) from a scope is common with top manufacturers. The electronics inside will eventually degrade, so you’ll need to replace them or get a new scope. Mid-infrared devices will wear out faster.

High Quality Scopes Like The Pulsar Trail Will Last For Years

Do thermal scopes work through glass?

In general, thermal scopes won’t work through glass, as glass is excellent at thermal insulation. 

Think about storm windows in a house. They keep the heat and cold out of the house. The same principle applies to thermal imaging devices. 

How do thermal optics work?

Thermal optics work by picking up heat, not light, and converting that into an image you can see. This is why thermals can work in complete darkness and during the day and also why insulation will defeat a thermal device. 

If you watched the movie Predator, Arnold hid from the alien by covering himself with insulating mud. The alien had thermal vision on and therefore couldn’t detect him. 

Will thermal scope work in fog?

Yes, thermal scopes will work in fog to some degree. A thermal imaging device looks for hot and cold spots; a light fog will not interfere with that. 

However, a heavy fog will insulate the heat signature as the target gets farther away, so the scope will not work. Looking through a fog at long distances will have the same insulating effect.


  1. DHS, Thermal Imaging Technology, retrieved from https://www.dhs.gov/sites/default/files/publications/ThermalImagingTech_TN_0305-508.pdf
  2. NASA Science, Infrared Waves, retrieved from https://science.nasa.gov/ems/07_infraredwaves
  3. Insights Samsung, How does refresh rate work for monitors, retrieved from https://insights.samsung.com/2022/03/07/how-does-refresh-rate-work-for-monitors/
  4. BBC, The man who makes you see the invisible, retrieved from https://www.bbc.com/future/article/20170614-thermal-imaging-reveals-the-hidden-heat-lost-from-your-home

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