Modernising defence - DEFENCE MANAGEMENT JOURNAL, June 2004
Accuracy in the dead of night*
Jim Hardman, Survival & Airborne Delivery IPT, outlines the history and innovative technology used in airborne night vision goggles.
Traditionally, the night has provided succour to armies, giving them respite from combat, and allowing them to regroup and recover. It also allowed them to manoeuvre into position for an attack at dawn. In order to prevent these manoeuvres from happening, and to exploit the darkness themselves, commanders have tried to exploit technology to enable their forces to operate effectively at night.
During the First World War, bombing raids conducted by Zeppelins over London had a disproportionate effect on the morale of the populace compared to the damage they caused; this was primarily due to the lack of defences. All the Royal Flying Corp could do was to fly aircraft over London and hope to 'bump' into the enemy, but in order to destroy him, you had to see him!
Little changed before the advent of the Second World War. Rapid advances in technology during WW2 meant that, with the advent of airborne radar, aircraft flying at night could be detected, but they still had to be seen and positively identified before they could be attacked. Furthermore, the very act of flying at night was much more demanding due to the lack of visual references and cues to aid situational awareness. Bombing targets was much less precise and so carpet bombing evolved, and in order to improve accuracy, pathfinder forces developed. Even so, they relied upon bomb aimers being able to see target markers. Once again, the watchword is see. To counteract troop movements at night, interdictor aircraft relied on targets being illuminated by flares dropped either by themselves or accompanying aircraft.
Later conflicts, such as the Korean War and Vietnam, still relied on these techniques of illuminating targets before attacking them. Because the Vietcong operated extensively at night, the US invested heavily in the development of night vision devices to the extent that, now, the darkness does not provide a hiding place. It still remains true, however, that, in order to attack a target, it must first be seen and positively identified, especially in today's political climate.
Development of night vision devices began during the Second World War when infrared (IR) sights were developed. These devices needed active IR searchlights in order to work, the problem with them being that, if the enemy fielded similar technology, then the IR searchlight gave away the position. Development concentrated on passive devices that wouldn’t give away the user's position.
Image Intensifiers, which are a key element of modern NVGs, work by capturing ambient or existing light, and intensifying it. Generation 1 used anodes and cathodes to accelerate electrons, but the drawback was that the image was distorted and life was relatively short. The next major improvement was Generation 2, which introduced a microchannel plate. Incoming light strikes a photocathode, where the photons are converted to electrons; these electrons pass through millions of microchannels (holes) in the plate, where they cause thousands of other electrons to be released. These multiplied (intensified) electrons strike a phosphor screen, where they are converted back to light; a green phosphor is chosen because the eye can discriminate more shades of green than any other colour. The final major development is Generation 3, which increases the intensification even further through the use of Gallium Arsenide as the photocathode. Each successive Generation offered increased sensitivity, life and operated more effectively in lower light levels.
Image Intensifiers cannot work without any light, although current models work at very low light levels (starlight). Conversely, too much light can overpower the Intensifier, and cause it to fail or shut down, effectively blinding the pilot.
The main advantage of NVG to aviators is an increase in night-time situational awareness, significantly increasing flight safety. Aircrew can fly manoeuvres almost the equal of those they can fly in daylight, and they can see enemy targets in the sky or on the ground whilst remaining nearly invisible to their opposition. Because most natural backgrounds reflect infrared light more readily than visible light, potential terrain hazards and targets can be far more discernible.
There is a price to pay to be able to see at night. NVGs have a number of disadvantages that mean that aircrew cannot just fit them to their helmet and take off. They have a 40° field of view, producing tunnel vision. Peripheral vision is used for scanning instruments but effective scanning techniques are more important than ever in order to maintain situational awareness. Visual Acuity (quality of vision – the finest detail that can be resolved) is reduced, as is resolution and contrast. Hazard detection is more difficult (although better than not having NVGs at all). There is a loss of depth perception such that different sized objects side by side can appear to be closer or further away than they actually are. Some objects can appear to be further away than they actually are because loss of sharpness is associated with distance.
Because a green phosphor is used, there is a loss of colour discrimination – everything appears in shades of green and objects of different colours may not be seen, as they may appear to be the same shade. Bright light sources can cause halo effects that may mask objects; they may even cause the Image Intensifier tube to shut down, leaving the aircrew momentarily blind. Operating using NVGs is very tiring and neck strain is a common complaint due to carrying the extra mass of the devices on the head. This strain is exacerbated by the goggle being mounted on the forehead and pulling the head forward. In aircraft that do not use an ejection seat, counterbalance weights are used on the back of the helmet to prevent the head rotating forward but they increase the mass being carried on the head.
However, these disadvantages can all be overcome by training aircrew in the use of NVGs. The main solution is to practise and practise, so that aircrew know what to expect, and can adjust their perceptions and procedures accordingly. Aircrew are also provided with access to physical training instructors, who can provide a series of exercises aimed at strengthening the neck and shoulder muscles.
Where we are now
As a result of procurement strategies in the past, there are a number of different NVG types in service with UK Armed Forces. The different services had different philosophies regarding the use and benefits of NVGs, and units were equipped accordingly. Although the UK led the world in the development of Image Intensifier Tubes (IIT), production became uneconomical compared with the economy of scale advantages gained by American companies. The UK does not have an organic IIT capability and must rely on importing from foreign suppliers. There is a European dimension to the provision of IIT but, until recently, their tubes could not match the performance of those available from the USA. Recent trials and evaluation of the latest European tubes by QinetiQ Farnborough show that performance is now reaching that of IITs that can be obtained from the US; however, tube life still needs to be evaluated.
Systems are becoming more sophisticated, with the introduction of Display Night Vision Goggles (DNVG). DNVG have a small display fitted to one of the monoculars so that the operator can see flight symbology superimposed over the night scene. This reduces the need for aircrew to monitor instruments outside the field of view of the goggles. Original systems utilised a small CRT mounted on the goggle with beam splitters to inject the imagery into the eyepiece.
Unfortunately, these systems were heavy and carried very high voltages. The latest systems use LCD flat panels mounted in front of the objective lens and viewed through the monocular; these have the advantage of being lighter in weight and using more friendly 'driver' voltages. The night scene is viewed through the LCD with symbology superimposed on it.
A further development is panoramic NVGs. These have two additional monoculars fitted to provide a 100° field of view; hence, the operator has peripheral vision, along with the ability to look 'under' the goggle to scan instruments. Thus, situational awareness is enhanced and head movement reduced. The major drawback with these goggles is the extra weight due to the extra monoculars. This makes the goggle uncomfortable and significantly increases neck strain for all but the shortest missions. The added bulk may also cause problems in some aircraft cockpits by impacting the structure, canopy or instruments.
To overcome the problems of mass, CG and ejection/crash risks involved with NVG operations, work is currently being carried out to develop integrated helmet display solutions. Here, either optical mixing of IIT with the display image may be used, or electronic mixing of the output of an image intensified camera with the display image is an alternative. Using optical mixing limits the position that IITs can take on the helmet because of the need to relay the output of the tube to the user's eye. Careful matching of the image brightness from the IIT and the symbology generator is required, but there is no loss of resolution of the IIT image; however, the brightness is reduced. Electronic mixing removes brightness problems but at much reduced resolution. There is no need, however, for a complicated optical path to present the image to the eye.
All integrated helmet solutions suffer from the effects of viewing the world through widely spaced IITs. One major problem is that depth perception is exaggerated, an effect known as hyperstereopsis. The major effect for aircrew is that the ground appears much closer than it actually is; when on the ground, the crew feels as if they are sat in a hole. There is also an area of blindness in front of the helmet, due to the tubes being mounted away from the eyeline, and the view of the outside world may be obstructed by the canopy/windscreen structure, since that view is no longer from the cockpit design eye position.
Due to the problems with integrated helmet designs, it is likely to be some years before they are a viable alternative to NVGs. Challenges remain to provide a suitably robust design that is lightweight. There are many variations of NVG on the market today, but all are compromised to an extent by the need to be as light as possible whilst being strong enough to meet the demands of the operating environment. The other major challenge is obtaining the most up-to-date and effective Image Intensifier Technology. European manufacturers are reaching or exceeding the capability of the technology that can be supplied by the US. However, formations need to be equipped with the same standard, so that all members of that formation can see the same thing.
Night vision devices are here to stay. As has been demonstrated in recent large-scale conflicts, Forces equipped with NVG can operate effectively round the clock whilst denying the enemy the opportunity for respite. In an environment where positive identification is required, the ability to see without being seen is an undoubted advantage. NVGs do not turn night into day; they have their limitations and are physically demanding to use, but with training, these limitations can be effectively overcome.