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A Basic Guide to Night Vision Contents A BASIC GUIDE TO NIGHT VISION CONTENTS PAGE 1… NIGHT VISION - PAST PRESENT AND FUTURE 3 2… LIGHT AND DARK 3 3… THE IMAGE INTENSIFIER 4 4… THE GENERATION GAME 4 5… EVALUATING THE PERFORMANCE OF A NIGHT VISION SCOPE 6-7 6… APPLICATIONS 7 7… GLOSSARY & TERMINOLOGY 8-10 8… PRECAUTIONS 10 2 1 NIGHT VISION - PAST, PRESENT AND FUTURE People have many requirements to see at night and powerful illumination systems have been around since the first lighthouse went into service at Eddystone Rock in 1698. However, these systems were simply methods of illumination with the obvious, and sometimes very dangerous drawback, that everyone could see the source of the light. For this reason it became vital that a solution be found for military use - a decisive advantage was to be achieved by gaining the ability to see at night without being seen. In 1887, Heinrich Hertz first discovered photoemission, on which night vision technology is based. In 1905, no lesser person than Albert Einstein theoretically explained the principle - the release of electrons from a solid material due to energy put in to it from radiation and light. The breakthrough came about in 1936 when the first Active Infrared system was developed using a silver photocathode. These systems were very bulky and primitive by todays 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. The main drawback of Active Infrared systems was that to operate they required powerful infrared lamps, which meant the range was restricted by the performance of the lamp. In addition, although infrared is not visible to the naked eye (other than a very dull red glow if you are close to the lamp), a major problem would arise during covert operations where both sides were using Active Infrared systems, in that each side could see the light emitted by the others infrared lamp. Therefore, you are back to square one. With the source of the light easily identifiable the system was rendered virtually useless, and probably lethal, as you would only have to shoot at the lamp to take out the person using it. It was at this point that new technology was urgently required and modern Image Intensifiers were developed. The advantage of these systems was that infrared lamps were no longer vital, and for this reason they are referred to as Passive Night Vision devices. However, all Image Intensifiers need some light to work because by their very nature they amplify the avail- able light to give a visible image. If an area is pitch black with no ambient light at all, no Image Intensifier will be able to function. For this reason infrared illuminators still play an important role and will always enhance Image Intensifier performance, but at the expense of losing the covert capability. Night vision ability can also be realised through the use of Thermal Imagers, which perform a similar function to Image Intensifiers, but with the big advantage of being able to operate in zero light conditions as well as fog, smoke, snow, etc., which is not possible with night vision devices. The latest, most sophisticated thermal imaging equipment will locate people or vehicles in trees, build- ings and through a variety of media. Although there are methods to defeat them, doing so can be difficult and expensive. Thermal Imagers obviously detect heat and not light energy and the images they produce are generally less well defined than those of an Image Intensifier. As range increases the subject simply becomes a hot ‘spot’. Although Thermal Imagers have been around for some time, the early devices required a cooling system, which was usually com- pressed gas, and their size and weight restricted them to static observation and surveillance work only. With the development of uncooled solid state models, they have become portable and a potential rival to the Image Intensifier. However, a major drawback to Thermal Imaging is cost, with basic systems starting at around £10,000 and the military/police equipment at many times this. There is one other area of development concerning night vision ability and this centres around the use of computers. Increas- ingly, developments in CCD technology are producing low light images of impressive quality. It is already possible to integrate and ‘build-up’ a visible picture but the problem here is that you do not get a real time image, giving a blurred and distorted picture if your equipment or subject is moving. The technology also already exists where the system gathers whatever information it can on the subject being viewed and in low light conditions where it is information limited, will ‘fill in the holes’ to produce a full, if not completely accurate, image. As for the future, it is certain that technological developments will allow the improvement of all the systems already existing, but it is also likely that systems will be combined and integrated into smaller, more efficient devices where two or more technologies can be alternated at the touch of a button. 2 LIGHT AND DARK We measure the level of light falling on a scene in units called lux. A sunny day can be 10,000 lux, and a bright office about 500 lux. At sunset under a clear sky and in an open area, the level will have fallen to 100 lux. At night, artificial lighting maintains the centres of towns at roughly NATURAL LIGHTING LUMENS/METRE2 (LUX) 10 lux. At this level colours start to lose their brilliance. The brightest moonlight DIRECT SUNLIGHT 1.0 t0 1.3 x 106 (100,000) produces about 0.1 lux and our eyes can then see little colour at all. Starlight 4 levels are less than 0.001 lux. It then becomes difficult to move around with FULL DAYLIGHT 1.0 to 2.0 x 10 (10,000) any confidence because we are only vaguely aware of the largest shapes and OVERCAST DAY 103 (1,000) objects that move. Light levels below this are now uncommon in the developed VERY DARK DAY 102 (100) world, except in the remotest rural areas. TWILIGHT 10 Light consists of small particles of energy called photons. Lower light levels mean DEEP TWILIGHT 1 fewer photons. If a picture contains insufficient photons it will lack information FULL MOON 10-1 (0.1) and be incomplete. We do not usually notice this effect because our eyes and QUARTER MOON 10-2 (0.01) brain have evolved to overcome this. There is an increase in the period over which photons are collected by the eye before the picture information is passed STARLIGHT 10-3 (0.001) to the brain. Rather like setting a slower shutter speed on a camera, the eye slows OVERCAST STARLIGHT 10-4 (0.0001) down to about 1/5th of a second. In addition, the pupil opens wider (fully dilated at 7mm) to let in more light, and a more sensitive set of detectors come APPROXIMATE LEVELS OF NATURAL into use at the rear of the eye. These can only see in black and white, which LIGHTING AT GROUND LEVEL is why we lose our sense of colour in the dark. This whole process is called 3 ‘dark-adaptation’ and takes tens of minutes to acquire fully (generally about 40 minutes), but is lost again by the briefest exposure to any bright light source - even a match. The widespread use of artificial lighting can prevent our eyes from becoming dark-adapted. Our workplaces are lit to regulated levels, we walk along well-lit streets and drive behind powerful headlights. It is unlikely that the inhabitants of towns and cities ever become truly dark-adapted. MOONLIGHT STARLIGHT -3 NO CLOUD (10-1 LUX) NO CLOUD (10 LUX) As darkness falls, there comes a point when we can no longer see 500 L properly and our eyes need help. In developed areas, where there is usually some artificial illumination, this can often be achieved 450 by using good quality low light binoculars, such as 7x50 or 8x56, 400 GROUND LEVE GROUND which simply magnify objects so they become more clearly visible. AT Beyond this, the use of a night scope becomes necessary. 350 When an Image Intensifier is used in really dark conditions the 300 quality of the picture on the phosphor screen drops dramatically. TIVE IRRADIANCE IRRADIANCE TIVE 250 Its overall brightness level falls and individual scintillations, tiny RELA flashes of light, become more clearly visible. Such a picture is said 200 to have become ‘noisy’ and we describe this situation as being ‘information-limited’. The only way to improve any image that 150 shows signs of information-limiting is to increase the number of 100 photons used to create it. 50 A few of the basic problems of seeing at very low light levels are so fundamental to the physical state of darkness that they can 0 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 only ever be overcome by the use of active illumination, be it vis- WAVELENGTH (nm) ible or covert. All generations of Image Intensifiers need light to REPRESENTATIVE SPECTRAL DISTRIBUTION work - they cannot provide a useful image in complete darkness. OF MOONLIGHT/STARLIGHT They are, however, sensitive to infrared (IR) light. In starlight, some 70% of light is in the infrared spectrum. The introduction of semi-covert/covert illumination overcomes this problem and users should consider having some sort of IR illuminator available to assist the Intensifier when climatic or lighting conditions restrict its performance.
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