Thermal Cameras Lens Selection?

There are multiple Lens options in thermal cameras from 35mm , 50mm and 100 mm.

Since it is not similar to the standard cameras having 2.8-12 or 5-50mm lenses; does 100 mm refer to telephoto visual?


100mm is telephoto, yes. It depends on the size of the sensor, as it would in visible light cameras.

If you look at FLIR's F series, for example, a 100mm lens gives you an extremely narrow 4.6° horizontal field of view in the 320x240 model or 6.2° in the VGA model.

For comparison, using a camera with 1/3" sensor, a field of view of 6° would take a focal length of ~45mm.

One other thing I forgot to mention: most thermal lenses are generally not field swappable. So you need to buy the camera with the proper lens selection. FLIR offers the ability to change out the camera module, leaving the housing and some electronics in place, but the lens itself is not changeable.

One exception is the Axis Q19 series which does let you choose from a few lenses. This is not typical, though. Those lenses are also very expensive. The 60mm model sells for over $2000 USD online. Since the camera itself may sell for $4000+, that makes swapping out a lens a very pricey proposition.

100mm is telephoto, yes. It depends on the size of the sensor, as it would in visible light cameras.

Based on the chart and your 1/3" example, am I correct in sumrising that thermal camera sensors are somewhat larger than the usual ones for visible light surveillence?

Thank you Ethan

Ethan, good job explaining.

Thermal imagers are not sized according to CCTV standards. Thermal cameras are "detectors" that provide an image. The major resolutions in security are 320x240 and640x480. A 100mm long lens on a 640x480 produces a 6 degree field-of-view where a 100mm long lens on a 320x240 produces a 4 degree field-of-view. There is a lot more to the concept but this is just a quick overview. To better understand distances, lenses and field-of-view FLIR offers a free tool www.flir.com/raven that allows you to design using various focal lengths and imagers.

Greg

A 100mm long lens on a 640x480 produces a 6 degree field-of-view where a 100mm long lens on a 320x240 produces a 4 degree field-of-view.

Isn't it necessary to know the pixel size/pitch as well, to determine the FOV angles?