Infrared thermal imagers are capable of body‑temperature screening because they convert temperature readings into visual images. Each pixel in a thermal image represents a temperature point, and the quantity of temperature points depends on the infrared pixel count of the device itself. What factors influence the measuring distance of an infrared thermal imager? Below are insights shared by the editorial team of Kaimorui.

Lens Focal Length

Lens focal length is a key factor determining the detection distance of a thermal imager. It directly defines the image size of a target, namely how many pixels the target occupies on the focal plane. This is generally expressed as spatial resolution (Instantaneous Field of View, IFOV). IFOV refers to the angular subtension of each pixel in object space, i.e. the minimum resolvable angle of the system. It is calculated as the ratio of pixel size (d) to focal length (f): IFOV = d/f.

The pixel count occupied by a target’s image on the focal plane can be calculated from target dimension, distance between target and thermal imager, and spatial resolution (IFOV). The ratio of target size (D) to target‑imager distance (L) gives the target angular subtension. Dividing this value by IFOV yields the number of pixels occupied by the target image: n=(D/L)/IFOV=(D·f)/(L·d).

It can be concluded that a longer focal length allows the target image to occupy more pixels and delivers longer detection distance. On the other hand, a longer focal length reduces the field‑of‑view and comes with higher costs.

Detector Performance

Lens focal length theoretically defines detection distance, yet detector performance plays a critical role in practical deployment. While focal length governs image dimension and pixel occupancy, detector performance determines image quality including blurring level and signal‑to‑noise ratio. Detector performance can be analyzed in terms of pixel size, thermal sensitivity and signal processing.

Smaller pixel size produces lower IFOV and achieves longer detection distance. Thermal sensitivity and signal processing of the detector govern image sharpness. Where the detector delivers insufficient thermal sensitivity, increasing lens aperture can be adopted to improve imaging performance.

Atmospheric Conditions

Though thermal radiation penetrates the atmosphere better than visible light, atmospheric absorption and scattering still exert impacts on thermal imager imaging. Heavy fog and heavy rain will notably degrade the detection range of thermal imagers.

Above is the content shared by our team. We hope it helps you better understand infrared thermal imagers. Functionally, infrared thermal imagers fall into two categories: hand‑held thermal imagers and telescopic long‑range thermal imagers, which differ in their application scenarios.