Thermal Optics Glossary of Terms and Definitions

By bmatheus

Thermal imaging riflescopes, binoculars and monoculars are some of the most advanced optics available to civilians. Many sportsmen and women may not be as familiar with the internal mechanics of thermal imaging as they are with traditional magnified hunting scopes. Thermal imaging isn’t cheap, so those looking to purchase thermal should become familiar with the specifications and features to make sure they purchase a unit that meets all their needs. To help you understand what thermal is and what we mean when we use certain terms when describing Pulsar thermal devices, we’ve provided this easy-to-understand glossary of thermal imaging terminology and definitions.

Black and white thermal image, hogs are shown in bright white contrast to a dark, colder background.
Thermal imaging detects the different heat signatures between objects, so you can quickly identify your target.

What is Thermal Imaging?

Thermal imaging technology reads heat signatures using infrared thermography to detect and “see” objects in any light environment—even total darkness.

How Does Thermal Imaging Work?

Thermal imagers use a microbolometer sensor consisting of resistive vanadium oxide or amorphous silicon film and a special Germanium lens to focus infrared light onto the microbolometer sensor. The vanadium oxide and amorphous silicon detect electrical resistance changes related to temperature.  These changes in the temperature are converted into a very detailed temperature pattern of electrical signals called a thermogram. The thermogram is then sent to a signal processing unit that translates the information so you can see the image.

What is thermal imaging? Click here for the answer.


A microbolometer is a specific type of uncooled bolometer (a type of instrument that measures radiant energy) used in thermal imaging devices. The microbolometer is made of a range of pixels.

Germanium Lens

Germanium is a silver/gray/white metallic chemical element member of the carbon group. Infrared waves can pass through it and it has a high refractive index and low optical dispersion.


A pixel is the smallest unit that makes up a digital image, they are either a dot or a square on your display screen.

Pixel pitch is the distance from the center of one pixel to the next. The number of pixels affects resolution and viewing distance.
Pixel pitch is the distance from the center of one pixel to the next. The number of pixels affects resolution and viewing distance.

Pixel Pitch

The pixel pitch refers to the amount of space between two pixels. It is measured in micrometers or microns from the center of one pixel to the next.


A micron is the unit of measurement of infrared radiation wavelengths, pixels are also measured in microns.


The visible light spectrum, light we can detect with our eyes, ranges from 380nm to 750nm (violet to red) is a small portion of the entire electromagnetic spectrum. Infrared spectrum is just next to visible spectrum at a lower energy level.
The visible light spectrum, light we can detect with our eyes, ranges from 380nm to 750nm (violet to red) is a small portion of the entire electromagnetic spectrum. The infrared spectrum is just next to the visible spectrum at a lower energy level.

Infrared light or infrared radiation (IR) is a form of electromagnetic radiation produced when atoms absorb and then release energy. IR has longer wavelengths than visible light, so we typically can’t see it.


Heat is the transfer of internal energy from electromagnetic waves from one area to another.

Digital Zoom

View of hog targets through a thermal riflescope
Even in open fields, identification from a distance can be challenging. In dense vegetation cover or fog, you will be surprised what is visible through a good, thermal weapon sight!

Digital zoom increases the perceived magnification of the thermal or digital device. As digital zoom is applied, the central image is cropped and increased to match the same aspect ratio as the original image. The result is a zoomed-in view of the image of the target, but with a sacrifice in image quality.  Higher resolution sensors will be able to achieve higher digital zoom ranges without significant loss of image quality. A low pixel pitch specification will also help maintain image quality when digital zoom is applied.

Frame Rate

Frame rate is the frequency at which an imaging device produces consecutive images. Frame rate is depicted in hertz (Hz) which is one cycle per second. The higher the frame rate, the more motion is captured so images appear smoother. Generally, the human eye will not detect a significant difference in frame rate above 30 Hz. Higher frame rates, however, will enhance the details and clarity of fast-moving targets.

This picture shows the difference in Hertz.
This picture shows the difference in Hertz.


Electromagnetic radiation is described by its frequency, which we refer to as Hertz. A Hertz is one cycle per second.


The F/number of an optical system is the ratio of the focal length of the lens to the diameter of the front lens and determines the light gathering power of the lens which affects the sensitivity of the system. To have quality sensitivity, it must run a low f/number, typically 1.4 to 2. With a low f/number and a long focal length, thermal devices maintain high sensitivity with increased viewing range.

Focal Length

Focal length is the distance between the entrance lens and the area in the lens where gathered light comes to a point, designated in millimetres. The focal length directly effects how far you can see through your thermal device. Longer focal lengths mean a smaller field of view, but a longer detection range.

AMOLED Display

The Active-Matrix Organic Light-Emitting Diode (AMOLED) is a type of display made of LED and organic compounds used to light up pixels.

Defective pixel in a Pulsar Trail xp50 thermal
Pulsar units have an automatic pixel repair feature.

NUC (Non-Uniform Correction)

Pixel-to-pixel responsivity produces distortion called FPN (fixed-pattern noise.) Non-uniformity correction is an algorithm that fixes these distorted pixels.

Field of View

The field of view is the amount of space you see through your unit.

Are there terms not listed here that you would like defined? Tell us in the comment section and our product engineers will do their very best to answer you.

For more advanced explanations of specifications and features, click here and here.