What is F‑Stop and Why is the F‑Number of the Lens So Important for Thermal Infrared Cameras?

An f‑number (ƒ/#) or f‑stop refers to the ratio of a lens’s focal length to its aperture’s diameter (lens opening vs focal length) and indicates the amount of IR energy (heat) coming through the lens to the infrared sensor. An f/1.0 lens means the aperture diameter is equal to the focal length, whereas an f/2.0 lens would mean the aperture diameter is half of the focal length. Lenses are typically specified with their maximum aperture (some lens apertures are internally adjustable) and max focal length/zoom power.

The lower the f‑number the larger the aperture diameter, meaning more thermal IR energy is reaching the infrared sensor. This increases the detail, contrast, and overall performance of the thermal infrared cameras, especially for surveillance where long-range detection, recognition and identification is desired in even in low contrast scenes. A thermal camera’s sensitivity is measured by its NETD (Noise Equivalent Temperature Difference). Often described in millikelvins (mK, thousandths of a degree), NETD is determined by the thermal camera’s sensor sensitivity and the f‑stop of the lens.

An ƒ/1.0 Ge lens allows for 250% more infrared thermal energy to be transferred to the infrared sensor than an ƒ/1.6 lens. This means the f‑number can be even more important than the sensor as it relates to a camera’s overall NETD sensitivity, particularly for uncooled LWIR cameras which are the most common type of thermal imaging. See the following image for a visual example of how much the area of the aperture changes with different f‑stop values.