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When using night vision gear, especially thermal imaging, a certain amount of interpretation will be required when trying to identify "targets" at extended ranges. With thermal, a potential target may appear as a moderate sized "hot" spot.
Even though from a shooters view point it may not be desired or even possible to shoot at that "extended" range, it is always useful, if not essential, to know exactly what it is you are seeing so you can decide on how to react to the situation.
What is "Extended Range"?. This depends entirely on what equipment you are using as to what that might be. Naturally the higher specification models will provide better detail, and at a greater distance, therefore allowing a better assessment than those makes / models with lower specs.
Interpretation becomes easier after using any type of "NV" for some time and knowing from experience how a specific "target species" acts / moves etc. and also what might be seen in that particular area of operation.
Be aware that should a particular model of digital / image intensifying "NV" for example, state a maximum range of 300 m, ( How is that maximum range measured [ see below ] ) and you wish to shoot at around 150 m, will this be more than enough?
No it won't, this is detection range only, you need to recognize what it is you are seeing further away to start with and see what is in the background.
Remember the detection range is for detecting a man size (unless specifically noted otherwise) target, so therefore if the target target is smaller than that, the "300 m range" will be considerably less.
This applies to all types of night vision.
It is always a more difficult shooting at night with "NV" equipment regardless of its type or specifications.
When using a firearm at any time, you MUST ALWAYS correctly identify your target BEFORE shooting.
For any digital or image intensifying gear you need to look specifically at distance quoted, (maximum / detection) and under what conditions it applies.
Those conditions are especially important as it is normally requires a "1/4 moon", whether stated or not.
Be sure you understand what 1/4 moon means! (It is what most people will call 1/2 moon!)
Unless you are using some military specifications that refer to the amount of the moon face illuminated as well as providing a figure in "lumens", the detection distances normally referred to are quoted with "quarter moon", which to the average user means HALF MOON ILLUMINATED, ie first quarter or last quarter.
Any less, the shorter that distance will be, regardless any claim made about the device.
You can increase the usable range of image intensifying N.V. in lower light conditions, eg. starlight or less, with external infrared illumination to the point that the magnification of the unit, and or the lens quality becomes the limiting factor. (See comments regarding IR illumination in notes on a later page)
Infrared illumination does not help with thermal imaging.
When investigating image intensifying night vision, the image "resolution" is governed by the "line pairs per millimetre", often just quoted as "lines per millimetre". The greater the number the more defined the image will be.
This increases as the Generations go up, and unfortunately so does the price. For example, Gen 1 units range from around 25 - 45 lp / mm, Gen 2 and 2 + about 45 - 64, for Gen 3, a reputable manufacturer will in most cases, state a minimum figure, usually starting at 60, and for selected Gen 3 grades up to 68 - 72, again, with a minimum quoted, rather than "up to".
Sometimes FOM (Figure of Merit [signal to noise ratio x lp / mm]) is stated, the higher the number the better also applies here.
Don't get confused with image quality / resolution, with the ability to see greater distances in the dark.
The light magnification ability of image intensifying NV increases with the Generations. Gen 1, generally speaking, amplifies the available light around 900 - 1000 times, Gen 2, around 20,000 times, Gen 2 + 22,000 to 25,000 times and Gen 3, in all "grades", from 50,000 to around 70,000 times.
So you can see from that, it does not matter how sharp an image may be, you still need the light amplified to some degree to be able to see it.
In other words you may have a stunningly sharp image but no way to amplify the available light, it can be the same as looking with the naked eye!
Unless you are using some military specifications that refer to the amount of the moon face illuminated as well as a figure in "lumens", the detection distances referred to are quoted with "quarter moon", which to the average user means HALF MOON ILLUMINATED, ie first quarter or last quarter. Any less, the shorter the distance will be, regardless of any claims made regarding the device.
This is also where signal to noise ratio becomes important, as if the 'noise' is high then you will need a strong input signal to overcome the inherent noise level before you can actually see an image of usable clarity.
For those of you that are two way radio users, the same thing applies when a weak "received" signal is lost in the "noise".
You need a stronger wanted signal to be able to hear clearly.
Be aware, be very aware, that when you are told "this scope / viewer, has an xxxx Gen 2 high performance tube, is as good as Gen 3" it is NOT, it will not perform like Gen 3 ! There is almost one exception though, and that is the PHOTONIS XR5 tubes. These tubes are Gen 2 + but due to manufacturing methods they will actually perform to the level of Gen 3 in all areas of performance regarding resolution, signal to noise, FOM etc., but they are LOWER in light amplification by about 30%.
So while resolution provides clarity or "sharpness" of the image, the light magnification provides the detection distance and brightness, along the with inbuilt optical magnification.
Also remember, as optical magnification increases, the sensitivity decreases unless you also increase the lens diameter to retain the same F number value. This applies to all types of optical systems.
It is also worth mentioning that while thermal will, to some degree, see through smoke, fog etc. it, like all types of "night vision", its sensitivity is reduced.
Moisture (humidity) will tend to produce an image, where you seem to not be able to quite focus as sharply as normal, although you can still see "hot targets" well.
Air pressure will also affect sensitivity by a small amount as well.
Certainly rain, even very fine rain that at times you may not even realize is falling, will produce a very noticeable degradation of the overall image quality.
Even with the above, thermal will still provide a usable outcome for the user, even more so than image intensifying NV.
Gain control for image intensifying NV, if available, should be used sparingly, as too high a setting in low light will only introduce noise to the image.
Gain control for thermal will assist with varying levels of temperature.
Brightness and contrast can normally be adjusted as required to suit individual needs.
When making a decision as to what you should purchase, choose carefully, because night vision gear of any kind that does not fulfill your requirements or meet your expectations becomes an expensive exercise, regardless of how little or how much you have spent.
Unfortunately no amount of written information can cover all aspects of use.
Site updated, SEPTEMBER 22, 2021.