Detection range
The greatest distance at which a night vision device is capable of detecting individual objects.
The methods by which manufacturers determine this parameter may vary in detail, but the general principle is the same. Usually, the distance is indicated at which, with an illumination of 0.05 lux (a quarter of the moon) and a medium-contrast background, a rather large object can be seen — for example, a human figure with a height of about 170 cm is most often taken. of this object, but only to notice the very fact of its presence. Simply put, a detection range of, say, 200 m means that “something that looks like a person” can be seen in such a device at a distance of 200 m, but individual parts (head, hands) cannot be disassembled.
It is also worth noting that in fact this parameter is highly dependent on the characteristics of the situation. For example, a dark object on a very light background will be visible further, and on a dark one it may not be noticeable even up close; a similar phenomenon is observed for thermal imagers (see "Type"), only regarding the difference in temperature, and not in colours.
Optical magnification
The degree of image magnification that a night vision device is able to provide without digital image processing, solely due to the optical system. Such an increase is considered to be preferable to digital, because. it does not impair the clarity of the visible image; and for models based on image intensifier tubes (see "How it works"), this is generally the only available option.
Theoretically, the higher the magnification, the greater the detection range (see above), since a powerful increase allows you to see smaller objects. However, it does not always make sense to chase the maximum performance. The fact is that with increasing magnification, the angular field of view decreases and the minimum focus distance increases (see both below), which can create problems at close range. It is also worth noting that a high degree of magnification adversely affects the luminosity of the entire system — as a result, the actual detection range in complete darkness may be higher for a device with a lower magnification, because. it "catches" more light. Yes, and this parameter affects the cost accordingly.
Note that night vision devices, unlike classical binoculars and monoculars, most often have a fixed magnification. Models with the possibility of smooth adjustment are almost never found, and the only option is to use additional nozzles (see "Form factor").
Now on the market are night vision devices with the following optical zoom:
1x,
2 – 3x,
3.1 – 4x,
> 4xFocal length
The focal length of a night vision device. This term means such a distance from the optical centre of the lens to the photocathode of the image intensifier tube or the matrix of a
digital device(see "Operation principle"), at which a clear image is obtained on the photocathode/matrix.
In general, long focal lengths are characteristic of optical systems with a high degree of optical magnification (see above). However, in the case of night vision devices, this dependence is not rigid — it is simply easier to ensure a high magnification with long-focus optics. In fact, this means that models with the same focal length can differ markedly in magnification. But what this indicator directly affects is light transmission: other things being equal, longer optical systems transmit less light, which negatively affects the capabilities of the device. This is also true for thermal imagers (see "Type"), because their working infrared range in this case also obeys the general laws of optics.
Receiver resolution
Resolution of the matrix installed in the thermal imager (see "Type") or digital night vision device (see "Operation principle"). Usually indicated in pixels horizontally and vertically, for example 640x480.
On the one hand, the higher the resolution, the clearer and more detailed the image will be. On the other hand, increasing the resolution without changing the size of the matrix means that less light will fall on each pixel — and this negatively affects the detection range (see above) and leads to the appearance of noise. Therefore, the resolution of receivers in modern night vision devices is small — in terms of the usual megapixels, it rarely exceeds 0.3 megapixels. And it hardly makes sense to unambiguously compare different models in terms of this parameter — after all, the actual quality of work also largely depends on the size of the receiver, signal processing features, etc.
Refresh rate
The frame rate on the screen of the thermal imager (more precisely, on the display visible to the shooter through the eyepiece). The higher this frequency — the smoother and less jerky the image is, the better the thermal imager is suitable for looking at fast moving objects. On the other hand, this indicator directly affects the price of the device and reduces its battery life, because. a high frame rate requires a powerful (and therefore expensive and high consumption) electronic hardware.
Angular field of view
The angle of view provided by a night vision device — that is, the angle between the lines connecting the observer's eye with the two extreme points of visible space. Wide viewing angles allow you to cover a large area, but the magnification factor (see above) is low; in turn, increasing the magnification leads to a decrease in the field of view.
Continuous operating time
The longest time that the device is able to work continuously from fresh batteries without replacing / recharging them. It should be taken into account that manufacturers usually indicate this time for perfect conditions: high-quality batteries, low brightness of the visible image, operation without the use of IR illumination, video output and video recorder (if available, see "Additional"), optimal operating temperature (see below) ) etc. Therefore, in fact, the time of continuous operation may well be lower than stated. Nevertheless, this parameter makes it possible to both evaluate the battery life of individual devices and compare them with each other.
Weight
The total weight of the night vision device in the working position. If a helmet-mask is included in the kit (see above), its weight, usually, is also taken into account.
The large weight of the device makes it inconvenient to use — both when working with hands and when attached to a helmet-mask. On the other hand, with similar characteristics, lighter models will either cost more or have poor quality materials.