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.
Lens diameter
The diameter of the entrance lens that the lens of the night vision device is equipped with.
This parameter is one of the most important for any optical device, including night vision devices: the
larger the lens, the more light (or infrared radiation) enters it and the more sensitive the optics are, all other things being equal. The downside of this is an increase in the size, weight and cost of the device. In addition, do not forget that various tricks and additional technologies can be used in the design; therefore, by itself, a large lens is far from always an unambiguous indicator of a high class.
Focal 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.
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.
Min. focus distance
The smallest distance to the observed object, at which it will be clearly visible through the night vision device. For normal use of night vision devices, it is necessary that this distance does not exceed the minimum expected distance to the objects in question; thus, it must be borne in mind that the higher the magnification factor (see above), the greater the focus distance, usually.
Exit pupil diameter
The diameter of the exit pupil created by the optical system of a night vision device. The exit pupil is called the projection of the front lens of the lens, built by optics and electronics in the region of the eyepiece; this image can be observed in the form of a characteristic light circle, if you look into the eyepiece not close, but from a distance of 30 – 40 cm.
The practical significance of this parameter is that for normal visibility it must be no less than the size of the pupil of a person looking into the eyepiece. The diameter of the human pupil can vary from 2-3 mm in bright light to 7-8 mm in the dark. Therefore, the larger the size of the exit pupil of the night vision device, the better the visibility, usually; this is especially true with a minimum amount of light, when the brightness of the image is low even when viewed through the device. On the other hand, this feature significantly affects the cost of the device.
Power source
The type of batteries used by the night vision device for operation. Technically, these can be both replaceable elements of a standard size and built-in batteries. However, in fact, the second option is practically not found, because. it does not make it possible to quickly replace a dead battery with a fresh one — and this possibility is critical for most NVD applications.
As for specific types of batteries, the most common options are:
— CR123. The element that has received the greatest distribution among modern night vision devices. These batteries are similar in shape to the popular AA batteries (see below), but their diameter is larger, and their length is noticeably smaller: 17 and 35 mm, respectively. The operating voltage is 3.7 V, which provides good power, sufficient for normal NVD operation. This led to the popularity of CR123.
— AA. Classic 1.5 V finger batteries. The power of such a power supply is lower than that of CR123, which requires more cells; on the other hand, such batteries are easier to find commercially.
— AAA. "Mini finger" or "little finger" batteries, a smaller version of AA cells. Due to their small size, such elements do not differ in power and capacity, and therefore are used only in relatively simple devices for which compact dimensions are important.
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.
Operating temperature range
Ambient air temperature range at which the night vision device will be guaranteed to maintain normal performance. This indicator is very important, given that such devices are most often used outdoors, where the temperature variation can be very large. Of course, when choosing, you should take into account the conditions in which you plan to work with the device — for example, it hardly makes sense to specifically look for a frost-resistant model for hunting in the summer. But in general, the wider the operating range, the higher the overall reliability of the device, the less sensitive it is to adverse climates.
It is worth noting that going beyond operating temperatures does not necessarily lead to instant failure; however, even with the preservation of operability, the probability of failure increases many times, and such cases are not covered by the manufacturer's warranty.
