Understanding Maximum Range with Night Vision Optics

Choosing the Right Range for Thermal Night Vision

One of the opening questions when considering thermal night vision is always: "how far can I see?"

Your desired range will be a key factor in deciding what FLIR unit is right for you: are you in dense forest looking 25-75 yards away? Or are you out on a hilltop glassing at 500 yards?

In order to evaluate what unit will work best we have to answer: What does maximum range really mean?

FLIR Ex Series Resolution Comparison

The Johnson Criteria: if the simulated heat signature of a 6 foot tall man occupies 1.5 or more pixels on the screen, it can be "detected."

Maximum range is actually a very tightly controlled measurement that is part of FLIR's military origins. It is calculated using what is known as the Johnson Criteria: if the simulated heat signature of a 6 foot tall man occupies 1.5 or more pixels on the screen, it can be "detected." This produces the furthest distance at which a user can say, "I can see there is something there." That is why the TS32r with its large lens and detector will have a much further detection range than the pocket sized PS24.

Detection is very different from identification. A blob of pixels might give you an indication of a hot spot, but probably will not tell you whether it's a man, a truck, or a cow.

Your recognition or identification distance will always be less than your detection range, and will vary depending on target and operator experience. For example, a skilled thermal user might be able to tell the difference between a coyote and a dog at 200 yards through their PS32, while an inexperienced user might struggle to tell the difference between a deer and a cow at the same distance.

We try to use lots of images and screenshots of targets at different ranges to give you a better idea of what you'll actually see through your device.

FLIR Scout BTS with 100mm lens at 100 yards

The FLIR BTS-XR at 100 Yards Range

FLIR Scout TS32r at 100 yards

The FLIR TS32r at 100 Yards Range

FLIR Scout PS24 at 100 yards

The FLIR PS24 at 100 Yards Range

FLIR LS64 at 100 yards

The FLIR LS64 at 100 Yards Range

Images like the ones above are what you would see on the screen, and are generated by the interaction between the thermal imager's germanium lens and its microbolometer. If you have a high magnification lens, your entire view will be closer. If you have a higher resolution detector, your entire image will be more detailed.

So while the official maximum range of a given lens and sensor combo might be 1400 yards, you could find your effective distance is closer to 700 or a thousand yards. We always encourage you to inspect the range tab of any FLIR unit you're considering to see the actual view through the device, and to really approach the research phase with a good idea of what distances you're likely to use your imager at.


Questions or Comments? We'd love to hear from you below.

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