What is F-stop and why is F Numbers 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. Lenses are typically specified with their maximum aperture and max focal length/zoom power.
The Lower the ƒ/ number the more thermal IR energy is reaching the infrared sensor which increases the distance, contrast, and overall performance of the thermal infrared cameras especially for surveillance where long range detection recognition and identification is required in even in low contrast scenes. A thermal cameras sensitivity is measure by its NEdT Noise Equivalent Temperature Difference often described in milli-Kelvins (thousandths of a degree) is determined by the thermal cameras 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 more important than sensor as it relates to cameras overall cameras NEdT sensitivity particularly for uncooled LWIR cameras which are the most common type for thermal imaging.