Using a thermal imager for hunting has great benefits for finding game, target identification and tracking wounded animals. However, success depends on the device chosen, environmental conditions and user experience.
Recent tragic fatal shootings by unlicensed gun owners using thermals highlights the importance of safety. The greatest danger is inexperience, followed by incorrect thermal imaging choice.
I will begin with experience.
A thermal imager, in the hands of an inexperienced user can lead to accidents, especially at night. The biggest problem is misidentification caused by lack of image clarity and not taking the time to observe movement. For instance, a possum in a tree at a near distance can look the same size as a person in the background at a longer distance. Both look as bright as each other, unlike what would be expected using a spotlight.
Image clarity and Spatial Resolution
The ability to positively identify an animal with a thermal varies by animal size and distance, weather conditions and immediate environment. Image resolution is determined by Spatial Resolution, which varies by device. The following calculation is purely theoretical, but it is a good reference in determining the current device.
Animal size and distance.
Positive identification with a thermal imager requires a sufficient number of pixels to form a clear image. A high magnification and a sensor with more pixels has higher Spatial Resolution. Here is an example:
Thermal #1 has a 17μm 640×480 array and a 35mm lens that has a horizontal field-of-view of 31.3m @ 100m (17.8˚)
Thermal #2 has a 17μm 640×480 array with a 50mm lens that has a horizontal field-of-view of 21.8m @ 100m (12.4˚)
Spatial Resolution is calculated by dividing the field-of-view of the thermal by the number of horizontal pixels in the sensor array. Firstly, convert the field-of-view in metres to millimetres for easier understanding.
Thermal #1 (31.3m x 1000) 31,300mm ÷ 640 = 48.9mm @100m
Thermal #2 (21.8m x 1000) 21,800mm ÷ 640 = 34mm @100m
Therefore, each pixel in a 640×480 array with a 35mm f/l lens will resolve a space of 48.9mm2, while a 50mm lens will resolve 34mm2 @ 100m. The further the distance, the lower the resolving power relative to object size. Fundamentally, it is not pixel size that determines Spatial Resolution, but the number of pixels that cover a particular object. However, 12μm sensors are smaller than 17μm sensors and therefore have a higher magnification and narrower field-of-view when fitted with a lens of the same focal length.
Johnson’s Criteria
According to Johnson’s Criteria (developed by the US Department of Defence), the minimum number of pixels required to identify an object with a probability of 50% is 14 vertical pixels. For hunting safety 50% probability is unacceptable, so if we doubled this figure, then added a margin to account for differences in the imaging performance of the thermal imager, we get a practical starting point to begin evaluating device capability. Remember that these figures are based on viewing in optimal conditions.
printing onto newsprint, these images clearly display the required
level of detail for positive identification. The deer’s movement will
further add certainty, especially at a longer distance.
Weather conditions and immediate environment
Image clarity is affected by humidity that blocks and disperses the image entering the lens, while high temperatures reduce the thermal contrast between the object and its surroundings. What is feasible on paper may be completely inaccurate in the field, for instance, a cool evening after a warm day often means that the air is heavy with moisture (which eventually forms dew), reducing device performance.
Larger pixels are more sensitive than smaller pixels because they have a greater surface area. For instance, a 17μm pixel has twice the surface area of a 12μm, making the larger sensor inherently better at penetrating moisture. Sensitivity is another factor that contributes to image clarity but is not included in Johnson’s Criteria calculations.
Immediate environment, such as grasses and other foliage further affect image clarity. The lay of land is another consideration especially when part of an animal or person is hidden. For instance, a person standing on a track that is lower than the observer may present only a view of a head, which could be mistaken for a rabbit at closer range or a larger animal at a longer range.
My advice
Never shoot at a blob, no matter how sure you are that it is a target species. Take your time, observe movement and move close enough for positive identification. If an animal is spooked and runs off, don’t beat yourself up over it, you did the right thing.
Experience is the key to safe and successful thermal hunting. Never buy a thermal and take it hunting until you gain experience with its functions and distance limitations. This can only be achieved by observing different animals in a variety of conditions and distances; and always be sure that your background is safe. These are the principles of safe shooting regardless of what technology is used. The New Zealand Deerstalkers Association is currently setting up hunter safety information and courses that include thermal, which is a very welcome move indeed.
Finally
Choose your thermal imager based on the distance that you will be hunting at most often. Having a wide field-of-view is very important if you are hunting forestry, bush and other close quarter environments. A higher magnification and narrower field-of-view is more important for shooting at longer distances in open country. The quality of the device is important, but all thermals look good in a shop or when looked through for the first time. Pulsar thermals are widely used by professionals because they are the best.
See the Pulsar range at www.pulsarthermal.co.nz
