Night Vision: 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 10 lux. At this level colours start to lose their brilliance. The brightest moonlight produces about 0.1 lux and our eyes can then see little colour at all. Starlight levels are less than 0.001 lux. It then becomes difficult to move around with any confidence because we are only vaguely aware of the largest shapes and objects that move. Light levels below this are now uncommon in the developed world, except in the remotest rural areas.

Light consists of small particles of energy called photons. Lower light levels mean fewer photons. If a picture contains insufficient photons it will lack information and be incomplete. We do not usually notice this effect because our eyes and 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 to the brain. Rather like setting a slower shutter speed on a camera, the eye slows 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 comes into use at the rear of the eye. These can only see in black and white, which is why we lose our sense of colour in the dark. This whole process is called '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.

As darkness falls, there comes a point when we can no longer see properly and our eyes need help. In developed areas, where there is usually some scattered artificial illumination, this can often be achieved by using good quality law light binoculars, such as 7 x 50 or 8 x 56, which simply magnify objects so they become more clearly visible. Beyond this, the use of an Image Intensifier be-comes necessary.

When an Image Intensifier is used in really dark conditions the quality of the picture on the phosphor screen drops dramatically. It’s overall brightness level falls, and individual scintillations (tiny flashes of light) become more clearly visible. Such a picture is said to have become 'noisy' and we describe this situation as being 'information-limited'. The only way to improve any image that shows signs of information limiting is to increase the number of photons used to create it.

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 only ever be overcome by the use of active illumination, be it visible or covert. All generations of Image Intensifiers need light to work - they cannot provide a useful image in complete darkness. They are, however, sensitive to infrared light. At night, some 70°jo 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.

Semi-covert/covert illumination can be produced in three ways. Infrared lamps offer the greatest range and they are more eye-safe, although they are bulky and can consume a lot of power. Laser illuminators are generally less eye safe - they stay cool and use very little power, but are generally expensive. The only illuminators that are truly discreet use LED's. Unfortunately, with many of these the useful range is usually restricted to <25 metres, although there are a few exceptions. Only high grade Gen 2 or Gen 3 systems are sensitive enough to be used with truly covert IR illuminators. Due to the tube sensitivity, IR illuminators produced for Gen 1/1 + and standard Gen 2 systems produce a dull red glow from the LED and are described as semi-covert.

The use of IR illuminators is the most cost effective way of getting big increases in Intensifier performance, and the only time you would hit a serious problem would be if someone else was using an Image Intensifier - they would potentially see your illuminator very clearly.