The first question that people interested in buying a thermal imaging monocular usually ask is “how far can I see with it?” This is a very reasonable question to ask.However, the complexity of this question is the difference between the “detection range”listed on the specification sheet and the actual detection distance of various sizes of targets. We cannot simply conclude how far we can “see" a target all based on the “detection range”.

Thermal imaging is a technology that enables detection of objects in total darkness and in very diverse weather conditions.The distance you can see a given target with a thermal imaging monocular is called the “range” in the thermal imaging industry. To correctly determine the range of a thermal imaging monocular requires some sophisticated modeling. There are many variables to consider including the type of thermal imaging  monocularyou are using, the type of lens you are using, the nature and size of the object you want to detect, the atmospheric conditions and the very definition of what it means to “see” a target. There is  a distinction needs to be made between degrees of “seeing” a target:

Detection: In order to detect if an object is present or not, its critical dimension needs to be covered by 1.5 or more pixels. 1.5 pixels in a staring array is equivalent to 0.75 “cycles”, which is the unit of system resolution originally used in Johnson’s definition.

Recognition: Recognizing an object is defined as seeing what type of object it is. It means being able to make the distinction between a person, a car, a truck or any other object. In order to recognize an object it needs to be subtended by at least 6 pixels across its critical dimension.

Identification: This term is often used in the military sense of the word, which means seeing if someone is “friend or foe”. In order to do this, the critical dimension of the object in question needs to be subtended by at least 12 pixels.

Range performances for Cyclops series Thermal Imaging Monocular

What critical factors of a thermal monocular affect detection/recognition/identification range?

1. Lens: a critical parameter

Focal length and f-number are the two principal parameter descriptions of the lens.

The focal length is easy to find on the specification sheet. The larger the focal length, the smaller the angle of view and the farther you can see. The larger aperture (the lower F value) of the lens, the stronger ability to receive infrared radiation, and the better the final image quality of the device will have.For example, a 75mm lens is always better than a 50mm lens for long-range shooting. An F/1.0 lens is always better than an F/1.2 lens for better image quality.

The focal length determines the instantaneous field of view (IFoV) of a monocular. This is the angular field of view of a single pixel – the smallest angle that can be resolved by the monocular, provided that there is sufficient thermal contrast.The IFoV then determines the distance at which a target’s critical dimension subtends the required number of pixels to achieve detection, recognition or identification. The longer the focal length of the lens, the smaller the IFoV becomes, which translates into more pixels across a target at a fixed range.

2. Cooled versus uncooled thermal imaging cameras

There is also a difference between how far you can see with a cooled and with an uncooled thermal imaging camera. Cooled camera systems are more expensive, but generally have a longer range than uncooled systems under many conditions.

3. Thermal Detector

The larger the area of the thermal radiation projected by the target onto the Focal Plane Array (FPA), the more temperature data it could absorb, the more detailed picture the system could get, and the easier it to distinguish the target. As the target gets further, the area of the target’s thermal radiation projected onto the FPA will become smaller, and the number of pixels covered will decrease. When the amount of the pixel covered by targets on the FPA is equal to the amount of the DRI range, the target distance is equal to the DRI distance.  

Under the premise that other factors of the device remain unchanged, the detector’s smaller pixel pitch makes more pixels fit in the FPA. Take a human as an example. The recognition range requires at least 6 pixels, when there are only 6 pixels on the FPA of a 17-micron device, the number of pixels covered on the FPA of a 12-micron device can be more than 8 pixels, so the 12-micron device has a longer recognition distance than the 17-micron device.

4. Algorithm, OLED, Eyepiece

After being projected onto the FPA, the energy will be converted into digital signals by the FPGA, and then it comes to a series of signal processing. Then, the display shows the image. The algorithm, the resolution and the size of the display affect the magnification of the image. Finally, the image on the screen is further magnified through the eyepiece and greeted by the eye.

5. Atmospheric conditions

Although thermal imaging monocular can see through total darkness, light fog, light rain and snow, the distance they can see is affected by these atmospheric conditions.

Rain and fog can severely limit the range of thermal imaging systems due to scattering of light off of droplets of water. There are different types of fog, and some fogbanks are denser than others because the water droplets have grown bigger through accretion. A thermal imaging monocular will have more difficulty seeing through these dense types of fog, and its range will be reduced. The same goes for heavy rainfall and snow.Additionally, rain can reduce contrast because it will cool the surfaces of targets.

Despite degraded performance in fog, rain and snow, thermal imaging monocular will still allow operators to see targets further than is possible with visible light imaging systems.

Range is affected by many variables

In summary, there is no easy answer to the question “how far can I see with a thermal imaging monocular?” It depends on a large number of environmental and system variables, including the nature of the target (parked vehicle versus running vehicle), the background (hot desert versus coldsnow), and atmospheric conditions (clear skies versus fog). It also depends on the specific monocular and lens combination you choose. The engineers at ThermTec can help you to determine the ranges at which you can detect various targets in various conditions with thermal imaging systems.

About ThermTec

ThermTec is a global leading manufacturer of products concerning infrared thermal imaging technologies, providing the latest and best thermal technologies and solutions that improves the way people perceive the world, such constructs a safer and more efficient living and working condition for humankinds.