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Page 1 (General) Page 2 (Generations) Page 3 (Low Light, Objektive Lens) Page 4 (Image Intensifier Tube) Page 5 (Eyepiece Lens, IR Light Sources)
Page 1 (General) Page 2 (Generations) Page 3 (Low Light, Objektive Lens) Page 4 (Image Intensifier Tube) Page 5 (Eyepiece Lens, IR Light Sources)

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8. Eyepiece Lens

The task of an eyepiece lens is to magnify the photocathode's image of the scenery produced by the IIT of a night vision device. The usual magnification, which fits to the human eye, is by a factor of 8 or 10 times.

Just like it is the case with a camera the human eye has an adjustable diaphragm. An human eye that is adapted to the darkness has a maximum pupil diameter of approx. 7 mm. The image projected at a given distance to the eye (eye relief) by the eyepiece lens should be larger than 7 millimeters, so that a movement of the eye is possible within the projection. If it would be smaller than the pupil diameter the generated image would be considered as being too dark. In case of being equal in size one would be forced to keep the eye straight. Only by moving the head the scenery would be registered. By extended viewing time not only a fatigue of the neck musculature can occur.

The right dimensioning and adjustment of the eyepiece lens to fit the night vision device are also important, if regarding the different output window sizes of image intensifier tubes. There are curved or flat (glass fiber twist) output windows, which display the image upside-down or upright. For example a strong blurring of the image's edges can occur if the eyepiece is not adapted to the respective screen (however, with most devices of the 1st Generation still the IIT is mainly responsible for the notorious blurry edges).
Apart from the task of turning the image around by 180° an eyepiece lens should also be capable of adjusting to users with a corrected vision by a diopter adjustment. Using night vision equipment without wearing eyeglasses is a matter of course even with occasional employment. With so-called 'panoramic eyepiece lenses' this is unfortunately not possible. Nevertheless they offer a comfortable night vision in a usually good quality. But they still represent a compromise between the requirement of an observation with two eyes and the need for keeping the original costs relatively low.
For NVDs of Gen0 and Gen1 it is a matter of precaution having embedded special x-ray blocking additives in the eyepiece material (e.g. lead glass).

If there is a choice to make in terms of material of an eyepiece lens (glass or plastic) generally glass (despite it's higher weight) should be preferred. Unter the same conditions plastic lenses become also foggy but glass lenses are mostly of better optical quality. Besides that, often plastic is more susceptible to temperature-dependent material expansions and scratches. To protect from mechanical damage modern NVDs are equipped with so-called 'sacrificational windows' at objective and eyepiece lens. At the eyepiece lens the also called 'demist shields' serve a further purpose: In damp and cold weather they prevent to a large extent fogging of the (cold) optics due to the isolating air layer between the window and eyepiece. Fogging of the optics from the inside is often prevented by a purging the complete night vision device with dry nitrogen.

9. Infrared illuminator

Contrary to civilian applications of night vision devices military users and law enforcement units mostly refuse the use of additional infrared illumination (IR illuminator) - of course with exception of IR designators. Their experiences with systems of the Generation 0 (active night vision devices) accelerated the development of image intensifier tubes, which did not need traitorous IR illumination any more. If at all, a source of infrared light can only be seen with the naked eye on short distances in complete darkness by directly looking into. But to the opponent equipped with night vision devices it represents almost a brightly 'position light'. In addition for long observation distances efficient illuminators were needed, which were mostly very large in size and because of their power supply very heavy.
Therefore many military night vision devices have only a small built-in infrared LED, which can be used for map reading or for orientation within short distances in areas completely without any low light (whereby within buildings night vision devices are not used mostly at all because of the reduced field of view and the blinding effect of strong white flashlights on the opponent).

However, the civilian user can quite improve the performance of a night vision device by using infrared illuminators e.g. within security surveillance or wildlife observation. Since different generations of night vision devices have also different working ranges within the electromagnetic spectrum, additional IR-light should always correspond to the respective wavelength of the used generation of night vision device.
An IR illuminator designed for Gen2 and Gen3 would be less effective together with 1st Generation image intensifier tubes, which have a main working range next to the visible range (around 780 nm). Because of this, a suitable IR light source for Gen 1 tubes is still visible with the naked eye by a deeply red glowing. With large ranges (e.g. over 100 meters) there must also be considered that power demand and consumption behave to a linear increase of illuminating exponentially. Under certain circumstances reflections by obstacles at short ranges can overload the image intensifier tube and can decrease the image quality by over-radiating the background. Lucky, if the IR light source has focussable optics. Unfortunately only a few infrared illuminators are equipped with a regulated power switch, which not only saves battery power, but also provide more flexibility.

Basically there are three categories of IR illuminators: infrared LED illuminators, infrared laser illuminators and any other light sources with infrared filter.

AN/PVS-5 eyepiece lens

optical system AN/PVS-7B

Exception: in the AN/PVS-7B a so-called 'Non Inverting' tube is utilized. This IIT does not turn the image coming upside-down from the objective lens, because the following optics (collimator) flip & split the image upright again. Both eyepiece lenses have a diopter adjustment of their own and are adjustable in interpupilary distance for individual use.

head-mounted monocular & NV scope
eyepiece NVG BM 8028
US GIs at night operations

different IR-Illuminators

From left to the right: LED illuminator for Gen1, IR-laser illuminator (also focussable for IR designating purposes), flip-open IR filter for flashlights, single IR LEDs for building up own IR illuminators


chemical IR-Snaplights

Beside IR Beacons Western armed forces also use chemical infrared snaplights for marking purposes (their brightness is also enough for close orientation in the surrounding environment).

M2, cal. 50 - high intensity IR-laser designator