---

5. Low Light, Electromagnetic Radiation

Whether radio, microwaves, light, x-ray or gamma radiation - these are all manifestations of electromagnetic radiation. They only differ by the wavelength (frequency). This is also the key to the different characteristics concerning their diffraction and reflection. For example long radio waves follow the Earth's curvature, while short microwaves spread straight-lined (e.g. communication earth-satellite-earth). The more highly the frequency of a wave, the more highly also the energy is.

The human eye can detect only a small part of the electromagnetic spectrum. Between 380 and 780 nanometers wavelength (1 nm = 1.000.000.000ths part of a meter) we see this radiation as light of different colouring. Below 400 nm is the ultraviolet range, above 750 nm is the invisible infrared (IR) range. This portion (above 750 nm) of the spectrum is particularly of interest to NV-manufacturers, because a large part of all at night existing EM-radiation is in the upper IR-range. Important low light-sources are:

• moon - full moon approx. 0.01 lux, new moon approx. 0.001 lux

• stars - with cloudless sky approx. 0.0001 lux

• artificial light emissions and their reflection at the clouds, e.g. cities or motorways

In order to be able to use still more existing radiation the development of more efficient night vision devices aimes at shifting the working range into the deeper IR spectrum. Due to the lower frequency of infrared radiation, in relation to the visible light, a (slightly) different reflection behavior can be noted. Indeed some materials and objects appear differently contrasting with a night vision device than watched with the naked eye under daylight (more reflection or absorption of IR radiation). For example some in daylight black and dark looking surfaces are all of a sudden even more brightly than other normally light surfaces in the monochromatic picture of an image intensifier.

Thermal imaging devices (e.g. FLIR, Forward Looking Infra Red) work the very deep IR spectrum. In opposition to classical night vision devices these imagers use the distribution of all radiant heat availible to generate an image of the surrounding environment. Generally radiant heat is always present, as long as an object possesses energy over the absolute zero (-273.15 °C or 0° K). In practice a detectable object must have a different temperature as the background, in order to be visible with a thermal scan. Therefore this technology is in the best way suitable for detection of all radiating objects (e.g. locating individuals, recognition of fire nests, overheating mechanical parts or military target acquisition). As generating an image only from temperature differences thermal imaging devices represent a very abstract night vision. Up to now their benefits are rather for detection than for orientation because in case of same temperatured surfaces of a different kind the imager can not display details or only at low-contrast.

6. Objektive Lens

Of course, first there must be any information input before the image intensifier can amplify the low light. Therefore a good tube can not perform to its limit when the objective lens reveals to be an elimination filter for infrared light or has an inappropriate projection characteristics. Since the tube can strengthen only the low light passed and collected by the objective lens, it is obvious that an appropriate tuning between the optical and electronic part should take place (night vision devices are dependent on every little light information availible!). The task of the objective lens to collect as much as possible light and to focus an image on the image intensifier tube's input window is accompanied with the demand for a good transmission level for IR radiation, which must pass the lens system. Because the main working range of NVD objective lens is in the invisible IR spectrum - and IR-radiation shows slightly other EM-characteristics (cf. 5. Low Light, Electromagnetic Radiation) - usually recompensed, coated lenses with different focal lengths are used (small values with eyeglasses, larger focal lengths with night vision sights).

Left: Losses: Filtering of IR-light & reflections at every single lens surface
Right: AN/PVS-7B with inter-changeable (Camera-) objective lens

Also popular in the civilian range are night vision devices with inter-changeable objective lenses, which were originally developed for photo cameras. However there are also some difficulties: In connection with a modern night vision device an objective lens suitable for photo cameras can even reduce the potential system performance, since e.g. a part of the IR radiation can be reflected or swallowed by the lens. While working with common photography equipment in more or less 'bright' light conditions the user of night vision devices must realise that these devices have to work nearly all the time within the 'range of the lack'. This explains, why optically complex lens systems, for example zoom lenses or objective lenses with large focal lengths (huge magnification), are usually less suitable as an objective lenses for a night vision device. But if there is a special demand for huge magnification one way to meet the inevitable loss of light arising at each lens is to increase the objective lens diameter (which makes a night vision device somewhat large and heavy, e.g. optical arrays of the astronomy). In despite of using inter-changeable objective lenses from commercial photography equipment there are special clip-on afocal objective lenses for some NVDs availible. Not at least because of some devices being filled with dry nitrogen (prevents condensation and moisture on the optics from the inside, guarantees also a long service life of the tube), the danger of pollution from outside and the units sealment, modern night vision devices are considered as closed systems. However the use of changeable objective lenses, which are popular because of availability, zoom shot characteristics and versatility, with NVDs of the 1st Generation can still be justified, since their disadvantage is limited here (working range of Gen1 is in essential portions within the visible spectrum)

Especially NVDs of the 1st Generation are taking benefits from objective lenses with an adjustable diaphragm (iris), because depending upon current light conditions (dawn, full moon, new moon, overcasted sky, artificial light) the tube can be protected by a manual choice of the iris. If an objective lens does not have an adjustable diaphragm, then it certainly has an aperture (simply the size of the opening diameter), which decides how much light passes through the objective lens. The relative aperture or f/stop is a ratio of focal length to the lens diameter. The smaller the numerical f/stop value is, the more faster the lens is - more light passes through to the image intensifier tube. For example a f/stop of approx. 1.4 should not be exceeded with NVDs. Finding a zoom or a fixed focus objective (large focal length) that nearly reaches such an f/stop is possible when the disadvantage of large diameter and weight is accepted (e.g. objective lenses of sport photographers). It showed up that with night vision devices focal lengths substantially over 100 mm (magnification about 3-4 power) are appropriate only with special applications (e.g. marine monitoring at sea, distant area surveilance - complex optics needed for fast lenses with high magnification).

Huge parabolic reflector objective lens in size comparison to a AN/PVS-7B goggle

 

Apart from light losses out of ex-filtration of certain wavelengths and a bad f/stop value, also larger light portions can get lost simply by an oversized objective plane. The incoming light is focused beyond the edge of the image intensifier's input window. This portion of light information is not at the disposal of the tube any longer. Especially when using commercial photo objective lenses (which are mostly designed for 35 mm) far over half of the light (!) is lost this way, because the photocathode input window of modern Gen2 & Gen3 image intensifier tubes has only a diameter of 25 or 18 mm.

Meanwhile the use of particularly fast, highly infrared permeable lenses (recompensed optics) with a minimum of built-in lenses (because of light losses at every transition from gaseous to solid media) is obvious with modern military night vision devices. For example current American night vision goggles of the AN/PVS-7B(D) series are delivered factory outfitted with a small, but very fast (f/1.17, focal length: 26 mm) objective lens at a high level of IR permeability. For this unit there are special objectives lenses (so-called afocal) with higher focal length as accessories availible, so that magnifications of 3-5 power are possible by simply clipping-on to the standard objective lens. It is clearly to see by the conus-shaped objective lens body that in order to compensate the light losses larger and therefore heavy lenses are used. Furthermore only since the introduction of Gen3 tubes an useful employment of the afocal lenses with this night vision goggles is possible (in exceptions there are still some Gen2 tubes on the commercial market for this device first deployed with the US armed forces in the late 80's). However the AN/PVS-7 with mounted afocal objective lens is somewhat bulky and heavy that its rather a night vision goggle (large lever arm) than a relatively location-bound observation device. The same afocal objective lens can be used also with the AN/PVS-14 monocular.

With unity magnification and a large field of view (40°-50°) the objective lenses of modern night vision goggles can be kept quite small. The focussing is done by hand and reaches generally from approx. 25-50 cm to infinity. Already starting from short distances there is no more further focussing needed (setting is infinity). In close range (e.g. map reading) the sharpness depth is limited and therefore one has to adjust the focus repeatedly. Particularly with military night vision devices from Eastern Europe the objective lens is often designed as so-called fixed focus objective lens. Without a possibility for manual focusing given anything more far away than a certain minimum distance is displayed always sharply. Actually only a limited range is seen perfectly sharp with a fixed focus objective lens. But in practice this is insignificant due to the observation range being limited anyway (mostly it is under 200 meters). The big disadvantage of this design usually shows up in a minimum distance to be kept of approx. 5-15 meters, within everything is seen only blurry. If this circumstance may not be yet of importance with magnifying observation devices, a sharpness range starting from 10 meters (despite unity power) is problematic with some Eastern European night vision goggles for example. Together with strong IR-illuminators (vehicles headlights with attached IR-filters) most of these goggles were used for tank driving. If there is a need to see the ground beneath your feet, or to take a closer look at the surroundings, these (older) night vision goggles are not appropriate. The military advantages with fixed focus objective lenses are ih the rugged design and insensitivity in field deployment. With some objective lenses of this kind a different sharpness range can be adjusted factory-side with special spaceing adapters (screw-on rings).
In any case attention should be paid on the glass quality of the objective lens and on fully recompensed optics of a night vision device. In the past some western manufacturers also put very expensive state of the art tubes into cheap housings with plastic lenses (scratch-proof?) to be sold on the commercial market. It is very doubtful whether thereby the potential of an image intensifier tube is fully used.

The visible light is only a small cutout of the entire electromagnetic spectrum. The curve shows the sensitivity of the human eye within the different wavelengths. Compared to red approx. twice as many gradations of green can be seen by the human eye (our biological heritage). This is why most night vision devices have a greenish picture. Depending upon the utilized phosphor mixture also different colors for other applications (cockpit illumination) are possible.

Recompensed AN/PVS-7B objective lens (1x)

night vision weapon sight AN/TVS-5 (6,5x, range 1.2 km): A strong magnification means inevitably light losses. They become balanced by large lenses (parabolic reflector objective).

In comparison two night vision devices of the Russian Filin-series (both Gen1, on the left 2 power, on the right 7 power): the accompanying disadvantages of strong magnifications are already obvious from the outside.

AN/PVS-7D with 3x clip-on lens (f/1.5, 75 mm)

Due to its compactness the GN2 has a complex optical system. Focussing is by a lever next to the lens.

---

changing light conditions