Before an image intensifier tube is able to enhance low light, this must first be “captured” and be collected. A tube then turns out to be insufficient when the lens with a blocking filter for infrared light can be equated or has a false projection. Since the tube can only reinforce the collected and transmitted by the lens low light, an appropriate coordination between the optical and electronic part to be held.
An objective has to collect as much light and focus an image of the environment on the image intensifier. This requires constant demands for an appropriate transmittance for IR – radiation, which has the lens system happen. Since the main work area of lenses for night vision devices in the invisible infrared spectrum and whose radiation slightly different properties, are usually specially coated lenses having different focal lengths.
In the private sector are particularly night vision goggles with lenses which were originally developed for still photography, very popular. But come here to light some problems, because a suitable lens for still cameras in combination with a modern night vision device can reduce its performance potential. Thus, a large part of the IR radiation is reflected or swallowed from the lenses.
While working at the pictures with much more available light to users move from night vision devices in “poor” areas of the defect “. This is intended to illustrate the presence of visually elaborate lens systems, which are usually not required for a night vision device, except a user can utilize at any lens occurring loss of light, for example, encounter due to reflections, with appropriate lens diameter.
Contrary to interchangeable lenses known from the photograph exist for some night vision devices attachable lenses for magnification. Not least because of a possible nitrogen gas, which is to prevent the failure due to condensation and fogging of the optics inside, the risk of contamination and the sealing offered modern night vision devices as closed systems and handled. But the popular due to availability, zoom features and versatility use of photo and interchangeable lenses for night vision devices of the first generation is still possible, as they often still do not have a too big disadvantage.
Especially in the first generation devices are lenses with physical aperture is a great advantage, because the tube can be protected from too much light, depending on the situation, such as dusk, full or new moon or artificial lighting using manual iris in this way. Has an objective not a physical aperture to, at least one relative aperture, available as a measure of the size of the aperture of the lens.
This will decide case by case on the light intensity. The relative aperture is stated as “f value” of the lens. The lower the value of the number, the greater the intensity and more light reaches the image intensifier. A value of about 1.4 should not be exceeded, for example at night vision goggles. However, a zoom or a lens with a fixed, high focal length achieved correspondingly large diameter and weight, as can be seen in the context of sports photography. A use of powerful, infrared-permeable lenses with as few lenses is now taken for granted in modern military night vision goggles.
In any case, give special attention to the quality of the glass of a lens and the lenses of night vision goggles. Even Western manufacturers have already built in the past for the private sector high-quality tubes in cheap housing with plastic lenses. It is believed that in this field the possibilities are not exhausted entirely.