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.
Resolution
The resolution of the visible image created by the night vision device. Indicated by the number of lines (strokes) per millimetre; the higher this indicator, the more detailed the image is capable of creating the night vision device, the better small details will be visible on it. However such devices will cost accordingly.
In models with an image intensifier tube (see "How it works"), the resolution is highly dependent on the generation of the transducer.
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.
Offset of the exit pupil
The offset is the distance between the eyepiece lens and the exit pupil of an optical instrument (see "Exit Pupil Diameter"). Optimum image quality is achieved when the exit pupil is projected directly into the observer's eye; so from a practical point of view, offset is the distance from the eye to the eyepiece lens that provides the best visibility and does not darken the edges (vignetting). A large offset is especially important if the night vision device is supposed to be used simultaneously with glasses — after all, in such cases it is not possible to bring the eyepiece close to the eye. It is also relevant for devices that can be installed on a weapon: the greater the distance to the eye, the less likely it is to get injured due to recoil.
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.
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.
