f-number (ƒ/#) or Aperture/Iris Size
An f‑number (ƒ/#) or f‑stop refers to the ratio of a lens’s focal length to its aperture’s diameter, and indicates the the amount of light coming through the lens. Lenses are typically specified with their maximum aperture ability.
A low ƒ/# means a larger maximum aperture (also called a fast lens), which results in a higher light or infrared collecting ability while a high ƒ/# means a smaller aperture (slow lens), which has lower light or infrared collecting ability. The lower the f‑number, the better the lens in visible and NIR imaging, as it results in a system with increased light (or Lux) sensitivity and allows the camera to capture more accurate images in lower light levels and respond better to IR or white light illumination.
For thermal infrared cameras, having a lower f‑stop number increases the image contrast and clarity, which results in higher detection distances. This is particularly true for uncooled LWIR cameras where lower f‑number lenses like ƒ/1.0 to ƒ/1.6 are required. An ƒ/1.0 Ge lens allows for 2.5x more infrared thermal energy to be transferred to the infrared sensor than an ƒ/1.6 lens.
When looking at thermal imaging it is very important to look at the lens’s f‑number, as this is often as important the thermal sensor in determining the actual performance of the system. Infiniti uses mostly ƒ/1.0 and ƒ/1.1 lenses in our LWIR thermal cameras to ensure high contrast, long-range performance for detection, recognition and identification.