How Night Vision Works
People have numerous requirements to be able to see at night, and powerful illumination systems have been available since the first lighthouse went into service at Eddystone Rock, near Plymouth, Devon in November 1698. However, the drawback with all systems, until just prior to the Second World War, was that they were simply methods of illumination, with the obvious, and frequently very dangerous drawback, that everyone could see the source and origin of the light. For this reason it became imperative during World War ll that a solution be found for battlefield use, if a decisive advantage was to be achieved by one side gaining the ability to operate at night.
This breakthrough came about in 1936, when the first active Infra Red system was developed using a silver photocathode. These systems were very bulky and extremely primitive by today’s standards, but at the time, they represented a major military advantage. Active infrared systems continued in use until the late 1970’s in some countries, but NATO forces were phasing them out by the late 1960’s to be replaced by image intensifiers. The main drawback of Active Infra Red systems was that to operate they required powerful Infra Red Lamps, which meant that the range was restricted by the performance of the lamp. In addition, although Infra Red light is not visible to the naked eye (other than a very dull red glow if you are close to the lamp), a major problem could arise during military use should both sides utilize Active Infra Red systems, in that each side can see the light emitted by the others Infra Red Lamp. Hence, you are back to square one. With the source of the light easily identifiable the system is rendered virtually useless, and probably lethal, as you would only have to shoot at the lamp to take out the person holding it. It was at this point that new technology was urgently required, and Image Intensifiers or Starlight Scopes were developed.
The advantage of these systems was that Infra Red Lamps were no longer required, and for this reason they are referred to as Passive Night Vision devices. The principle of operation, is that they pick up whatever ambient light is available from the moon and stars, and amplify it so that the signal is strong enough to energize a sensitive screen. In the case of the earlier systems known as First Generation, or GEN1 (large numbers of which are still manufactured today), the principle of operation involved a light amplifier consisting of three elements enclosed in a vacuum tube.
The elements are: 1. The Photocathode 2. A Microchannel Plate 3. A Phosphor screen.
The photocathode receives particles of light, known as photons, through the front lens of the device which it then converts into electrons.
The microchannel plate consists of a cluster of Micro Channels. The actual number of microchannels varies, but runs into several millions, and are sometimes referred to as Rods. As the electrons travel through the plate, bouncing several times against the walls of the channel, they are accelerated and more electrons are created. This means that if one electron enters the plate, thousands will exit the channel and hit the phosphor screen, they then exit the screen as photons producing the green image, familiar to all night vision equipment users, which is visible through the eyepiece.
It is this process which brings us to the important subject that divides the good units from the bad, and that is the degree of Gain provided by the tube. Although the following explanation is simplified to avoid baffling you with unnecessary technical details, we hope that it will help you to understand the difficulties encountered by the manufacturers of the tubes. These problems centre not just on the continual need for development to increase the gain, but also on image definition, which, to a great extent depends on the number of channels that can be packed into the Microchannel Plate. It is for this reason that the manufacturers produce several types of microchannel plate, the main two being 18mm & 25mm, with the 25mm plate being capable of carrying considerably more channels. The main difference between Generation II and Generation III systems, (we will cover GEN. II + later) lies in the tubes, and the front end or third element, the Photocathode. It is this component that determines how many of the photons will be converted into electrons. In the systems employing the very latest technology (GEN.III), sodium potassium cesium antimonide (tri-alkali), is replaced by gallium arsenide, which is sensitive to infrared radiation (with wavelengths of up to 9 microns). In addition, it is important to bear in mind that with all night vision systems, each electron impact in the microchannels, generates gas ions which move back to the photocathode and impair its efficiency. However, in the case of GEN. III systems, the gallium arsenide photocathode can sustain the loss resulting from the build up of an ion barrier film. The less advanced GEN. II systems, using sodium potassium cesium antimonide tubes (tri-alkali), cannot deal with this build up and the performance is impaired.
SECOND GENERATION PLUS (GEN. II+).Second Generation Plus is an enhanced version of the earlier second Generation devices, but capable of operating further into the infrared spectrum than GEN.II. However, as the designation implies, it is less sophisticated than GEN. III, in that it evolved just prior to the change from Tri-alkali tubes to gallium arsenide, which took place with the development of Third Generation technology. GEN.II+ does offer a useful performance increase over GEN. II systems, but is becoming less common since the development of third generation equipment. The reason being, that in terms of price, they are not much cheaper than third generation alternatives, but still suffer with most of the drawbacks of the older technology.
IMAGE INTENSIFIERS AND INFRA RED LAMPS.
Although image intensifiers, regardless of generation, all utilize available light, they cannot see in certain situations. The following are just a few examples: in shadow, underneath leaf covered trees, in barns and out buildings, between piles of materials in a yard, inside cars and lorries, underground car parks etc. To summarize, if an area is pitch black with no ambient light at all no intensifier will be able to intensify light that doesn’t exist. It is rather like playing a blank CD, you can turn up the volume as much as you like, but the amplifier can’t amplify something that is not there. The principle with Night Vision Devices is exactly the same, and for this reason infrared lamps still have a vital role. Firstly, they are comparatively cheap, bearing in mind that American Police type lamps cost around £50, and even 1,000.000 candle power lamps, which give incredible performance, are under £200. They are therefore the cheapest way to get massive increases in performance. For example, if you paid £1000 for a system and £200 for a 1,000.000 candle power lamp, you would have the performance of a system costing several thousand pounds more. The only time you would hit a serious problem would be if someone else was using a scope, and the lamp advertised your position. Secondly, there is the problem of no go areas. An Infra Red lamp opens up these areas effectively, and for this reason you should consider having some sort of infra red illuminator to hand in case it’s needed, or to assist the scope on a night when thick cloud restricts moon and starlight.
We now come to the various types of illuminator which basically consist of two options. Laser Illuminators and Infrared Lamps. Both are highly effective, but our personal recommendation is that you opt wherever possible for a lamp. As a rule, the range is greater, and in addition it will not generally damage the eye if you look directly at it, whereas some lasers can actually cause retina damage if you look straight at them. These lasers are what is simply termed in the industry as non-eye safe. A reputable dealer will generally know what is, and what is not eye safe on the market. For obvious reasons it is an area where certainty is crucial, especially if children are involved, as due to the fact that no pain is suffered, people can be unaware that they have damaged their eyes until it is too late.
THERMAL IMAGERS (TI).
Thermal imaging is the latest type of equipment, and whilst it performs a similar function to an image intensifier, in other ways it has certain very distinct advantages. Firstly, it solves the problem mentioned earlier, that when in zero light situations an image intensifier cannot operate without the help of infra red light Although this is acceptable in civilian circumstances, it poses great difficulties on the battlefield. Infra red lamps are completely out of the question in this area, and thermal imaging wins hands down, not only because it can operate in Zero light conditions, but it can also operate in fog, smoke, snow and even when camouflage nets have been used to conceal men and tanks.
HOW IT WORKS.
Thermal imaging works in the visible band of 0.4 to 0.7 Microns, and from 0.7 to 12 Microns in the infra red spectrum. Although thermal imagers have been around for some time, it is only recently that they have become a potential rival for the latest image intensifiers. This is because until the current solid state models were developed, all thermal imageries required a cooling system which was usually in the form of a nitrogen or compressed air cooling bottle. Therefore they were extremely bulky, and due to this restriction caused by size and weight, they were only suitable for tripod mounting, fixed surveillance positions or reconnaissance. The way in which the new models score is that they are uncooled, and having dispensed with the bulky gas bottles that are no longer required, they have become portable. For example, the Pilkington Lite weighs only 3.5kg. Also, with the new systems now being solid state technology, they are easier to manufacture in large numbers, and much less fragile than their predecessors.
For certain applications a thermal imager is vastly superior to an image intensifier, in that it can find a person under snow, and troops and vehicles hiding under trees. Methods do exist to defeat it, but it is difficult. The best battlefield defence is a special smokescreen created by firing salvos from 66mm projectors fitted to AFV hulls. These projectors use M76 grenades which produce a smokescreen of hot fragments that descend slowly and can be topped up with more salvos. Special camouflage nets that dissipate heat slowly can also be used, these are sold by Barracuda in Sweden, and Bridport in the UK.
The drawback at the current time of thermal imaging is cost. With a basic system at around £10,000, thermal imaging is expensive, especially when you consider an excellent image intensifier is only a fraction of this cost. It must also be considered, that as with any system, it is only as good as the operator behind it. As range increases people and cars simply become a hot blob. For this reason, we believe it will be some time before thermal imaging takes over from a good image intensifier in civilian applications, but it definitely has its place, especially for tracking fugitives in open country, or finding adults or children lost in remote areas.
THE FUTURE.
It has become clear that third generation image intensifiers are probably the final generation, and from here on we will just see technical improvements, rather than the emergence of third plus or fourth generation systems. The most likely future developments will go down the route of combining thermal imagers and image intensifiers into one unit. This is probably out of the question at the moment, as the system would be too bulky and we don’t yet have the technology to get it right. However, the ability to alternate from one technology to the other at the flick of a switch would be a huge breakthrough, and the logical path to follow. For this reason it seems a virtual certainty that light weight combined systems will be in production within 10 years, or perhaps sooner if circumstances demand it.