Learn — HuntGrade

Inside a thermal scope

How a thermal scope makes a picture.

A thermal scope turns heat differences into a picture, so you can see animals even in the dark. Here’s how its main parts work together to make that happen.

Lens + sensor

Capture the scene

Processing

Create the image

Display + eyepiece

Present it to your eye

Bring the scene into focus.

The front lens collects infrared light, which we can’t see with our eyes, and directs it onto the sensor. You turn the front focus ring to sharpen the picture.

See the lens + sensor comparison

Focal length: Measured in millimeters. With the same sensor, a longer focal length captures a narrower field of view and makes the animal’s image larger on the sensor.
F-number: Focal length divided by the effective aperture diameter. A lower f-number admits more infrared energy, all else equal. It describes a different property from focal length.
Focus: The front focus control sharpens the scene on the sensor for the distance you’re viewing. The eyepiece adjustment separately brings the screen into focus for your eye.

Sources: Focal length · F-number · Focus controls

Pick up heat differences.

The sensor picks up heat differences between an animal and its surroundings. It sends that information to the scope’s electronics to turn into a picture.

See the lens + sensor comparison

Resolution: 640×512 means 640 pixels across and 512 down. More pixels can cover more of the scene or sample the same scene more finely, depending on the lens and pixel pitch.
Pixel pitch: The distance between neighboring pixel centers, in micrometers (µm). Count × pitch gives the sensor’s active dimensions: 640 pixels at 12 µm span 7.68 mm.
Thermal sensitivity: NETD means noise-equivalent temperature difference, measured in millikelvin (mK). Lower values indicate finer temperature discrimination under comparable conditions. Check whether the figure describes the sensor or the whole device.

Sources: Sensor specifications · NETD explained

Turn the signal into a picture.

A small computer inside the scope takes information from the sensor and builds the picture. It controls how light or dark things appear and which colors you see.

Compare capture and presentation

Contrast and palettes: Contrast settings change separation within the image. Palettes assign brightness or color to the signal, such as showing warmer areas in white-hot or black-hot.
Calibration: Calibration corrects uneven sensor response as conditions change. On shutter-based systems, you may notice a brief pause while the scope takes its reference.
Digital zoom: Ordinary digital zoom crops and enlarges the captured picture. It makes existing detail bigger without putting additional sensor pixels on the animal.

Sources: Image controls · Calibration · Digital zoom

Show the picture on a screen.

A tiny screen inside the scope shows the picture, aiming crosshairs and menus. This is the screen you’re looking at when you look through a thermal scope.

See the display + eyepiece comparison

Screen size: Usually stated as a diagonal in inches. The area used for the picture and the eyepiece optics work together to set how large the view appears.
Display resolution: The screen’s pixel grid draws the image and overlays. A higher-resolution screen can present the capture more finely, but it does not increase the sensor’s captured pixel count.
Refresh rate: Hertz (Hz) describes updates per second. The sensor, processing and display all participate in the moving image; a screen’s specification alone does not describe the whole system.

Sources: Sensor and display pixels · Display and frame specifications · Apparent FOV

Make the screen easy to see.

The eyepiece works like a magnifying glass for the tiny screen. It makes the picture look larger, and its focus adjustment helps you see it clearly.

See the display + eyepiece comparison

Apparent field of view: How wide the displayed picture appears to your eye. It differs from scene FOV, which describes how much of the surroundings the scope captures.
Eye relief: The distance behind the eyepiece where you can see the full view. Eye position affects whether the image is visible edge to edge.
Diopter adjustment: Focuses the display for your eyesight. Use the reticle or menu text to judge sharpness, then adjust the front lens to focus the scene.

Sources: Apparent FOV · Eye relief · Focus controls

Lens + sensor

A wider view and more detail are different upgrades.

When people say a 640-class scope looks clearer, they may be describing a wider view, a longer lens, or both. Those upgrades change different parts of the picture.

Sensor upgrade

384 vs 640: where do the extra pixels go?

Keep the 35 mm lens and pixel pitch the same. The larger sensor records a wider crop, with the same number of pixels on the hog.

Compact 35 mm lens illustration — **Wider view**
640×51212.5° FOV

More scene. Same detail on the hog.

The wider picture can look finer because the hog and each sensor pixel appear smaller on the same-sized display. Crop back to the 384 view and enlarge it equally, and the hog has the same captured detail.

Lens upgrade

What changes when the lens gets longer?

Keep the 640×512 sensor, pixel pitch and distance the same. At 100 yards, the longer lens projects a larger image of the hog onto the sensor.

How much scene you seeThe full scene captured by each lens.How much detail you captureThe same hog, enlarged equally.

35 mm lens

Wider view

640×51212.5° FOV

The same hog, enlarged equally

50 mm lens

Narrower view

640×5128.8° FOV

The same hog, enlarged equally

43% more pixelsacross the hog

Grid spacing enlarged for illustration.

More pixels on the hog. Less scene around it.

A longer lens spreads the animal across more sensor pixels. When a 640-class scope also has a longer lens, it is easy to credit the sensor for a difference the lens made.

Display + eyepiece

Why do some scopes feel like looking at a bigger screen?

Apparent FOV is how big the image looks to your eye. Think of watching the same football game on an 80″ TV instead of a 55″ TV, with your couch the same distance away.

Same lens + sensor

50 mm · 640×512 · 12 µm

Same captured picture

8.8° scene FOV

2.5×base mag8.8° FOV

22.0°apparent FOV

Smaller view

AGM Rattler V2 50-640

0.39″ display

3.5×base mag8.8° FOV

30.8°apparent FOV

Larger view

AGM Adder V2 LRF 50-640

1.03″ display

The hog looks bigger. The captured detail stays the same.

The eyepiece also affects apparent size. Apparent FOV is calculated from published FOV × base magnification, not measured through the eyepiece.

Magnification + zoom

What changes when you zoom?

Base magnification is your starting view. What happens next depends on where the enlargement takes place.

Base magnification

The lens, sensor, display and eyepiece set how big the scene looks before added zoom.

See the starting view

After capture

Digital zoom

A crop of the captured picture fills the screen. The hog looks bigger, but uses the same captured pixels.

Full captured picture

320×256 pixels fill the screen

After capture

Ocular zoom

An adjustable eyepiece magnifies the display. Existing detail looks larger to your eye; the sensor captures no extra detail.

The display stays the same

Eyepiece

The same display, magnified

Before capture

Dual focal length

Switching the front optics changes the capture. The longer setting puts more sensor pixels on the hog and shows less surrounding scene.

Two front-lens settings

35 mm · wider view

60 mm · more pixels on the hog

Illustrative views · 640×512 sensor · 12 µm pitch · 100 yards. Ocular enlargement is schematic; the visible display area depends on the eyepiece.

Sensitivity · NETD

See smaller heat differences.

NETD measures thermal sensitivity in millikelvins (mK). The lower the number, the smaller the heat differences a scope can distinguish.

40 mK

15 mK

Reading the specs

How NETD informs the score

Some brands publish sensor NETD, others system NETD. We identify which figure is listed before choosing what goes into the score.

Sensor vs. system

A scope can have two NETD figures: one for the sensor, one for the complete system.

Sensor NETD

The detector’s sensitivity before image processing.

System NETD

Sensitivity of the complete scope, including noise reduction from image processing.

Why we use system NETD

We score complete scopes, so we prefer the manufacturer’s system NETD when available.

Example manufacturer specs

System NETD

Used in the score

20 mK

Sensor NETD

35 mK

How we choose the figure

We review specs and conflicts. Unlabeled NETD is treated as system NETD.

System NETD published
Use the system figure.
Only sensor NETD published
Use it as the scoring fallback.
Neither figure published
Research an estimate.

ID Range

Spotting heat is easier than identifying the animal.

Detection asks whether something is there. Identification asks whether you can tell what it is. The distance depends on how much detail the scope captures.

Identification

It’s a hog.

Distinguish identifying features.

Recognition

It’s an animal.

Distinguish its general form.

Detection

Something is there.

Spot a heat source.

Same scope. Farther hog. Fewer pixels.

The lens and viewing scale stay fixed in all three enlarged details.

35 mm lens · 640×512 · 12 µm

100 yardsCloser

Loading scene…

41.5 pixels across the reference target

Gold mark: 13-pixel ID threshold

150 yardsNear the calculated ID limit

Loading scene…

27.6 pixels across the reference target

Gold mark: 13-pixel ID threshold

400 yardsFarther away

Loading scene…

10.4 pixels across the reference target

Gold mark: 13-pixel ID threshold

One consistent comparison

Our ID range uses the same target and threshold for every scope.

We calculate where a 1.3 m dimension on the reference hog spans 13 sensor pixels. The lens and pixel pitch determine that distance.

AGM Rattler V2 35-640

319 yards

Calculated ID range

Manufacturer detection range answers a different question. Check the maker’s target size and conditions before comparing it with identification of our hog target.

Our range is a calculated comparison, not a guaranteed field result. Weather, contrast, processing and the observer also matter. These illustrative views do not validate the ID threshold.

Put it together

Use the wider sensor to make room for a longer lens.

First the sensor adds surrounding scene. Then the longer lens trades some of that width for more pixels on the hog. Follow one change at a time.

Same hog · 100 yards · 12 µm pitch

Start here

384×288 sensor · 35 mm lens

7.5° field of view

Same hog area, enlarged equally

Starting detail

Add sensor width

640×512 sensor · same 35 mm lens

12.5° field of view

Same hog area, enlarged equally

Same captured detail

Then add lens length

Same 640×512 sensor · 50 mm lens

8.8° field of view

Same hog area, enlarged equally

+43% pixels across the hog

The final combination gives you both.

Compared with the starting scope: 8.8° instead of 7.5° FOV, with 43% more pixels across the hog. The combination matters more than either spec alone.

The lens and sensor set the capture. Sensitivity, processing and the display shape the finished picture.

Explore the Buying Guide How we score scopes

Understand what changes the view.

Bring the scene into focus.

More scene. Same detail on the hog.

35 mm lens

50 mm lens

More pixels on the hog. Less scene around it.

Smaller view

Larger view

The hog looks bigger. The captured detail stays the same.

Base magnification

Digital zoom

Ocular zoom

Dual focal length

Sensor vs. system

Sensor NETD

System NETD

Why we use system NETD

System NETD

Sensor NETD

How we choose the figure

System NETD published

Only sensor NETD published

Neither figure published

It’s a hog.

It’s an animal.

Something is there.

Same scope. Farther hog. Fewer pixels.

Our ID range uses the same target and threshold for every scope.

The final combination gives you both.