Glossary Definition

f-number (ƒ/#) or Aperture/Iris Size how much light or heat a Camera Lens provides to thermal or visible sensor

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 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  is for 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, long range cameras like our 135x optical zoom (15.5-2075mm) due to their high power zoom focal lengths have much higher F stop which is why there is no long range low light cameras. This why long range cameras are either thermal or require our ZLID (Zoom Laser IR Diode) Illumination ir order to have night vision. 

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

Infiniti optics provides a wide variety of EO/IR camera solutions all the way from a component level, open frame modules (lens and sensor attached) or as complete integrated PTZ cameras with Visible and SWIR sensors for ultra long range imaging  required by a host of  military and defense applications. 

For more information about thermal infrared imaging for night vision and surveillance see our Thermal Imaging Explained page


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Full Glossary