Light & Dark
Light & 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 come 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 artificial illumination, this can often be achieved by using good quality low light binoculars, such as 7x50 or 8x56, which simply magnify objects so they become more clearly visible.
Beyond this, the use of a night scope becomes necessary.
When an Image Intensifier is used in really dark conditions the quality of the picture on the phosphor screen drops dramatically.
Its 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 vis¬ible 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 (IR) light.
In starlight, some 70% of light is in the infrared spectrum.
Infrared & Night Vision
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.
Lamps with infrared (IR) filters offer good range and are eye safe, but they are bulky, can consume a lot of power, get hot and can exhibit a very visible red glow.
Infrared illuminators using LED’s are eyesafe with many devices having a short-range one built-in.
With many of these the useful range is restricted to ?50 metres, but more powerful add-on variants are available.
Most Gen 1/Super Gen 1+ and Gen 2 systems are most sensitive in the semi-covert spectrum of IR (800-850nm) and IR illuminators cover¬ing this may exhibit a red glow from the LED.
Only Gen 3 and digital systems are sensitive enough to be used with truly covert (900nm+) IR illuminators.
Laser illuminators are the best compromise of size and power, but most are not eye safe - they can burn the retina at the back of the eye even at substantial range.
There is, therefore, a major liability in using such a product.
The only high power Class 1 certified eyesafe laser currently known to be available is from LaserLuchs. They are compact, give very good range capability and have a very ‘clean’ - no blemishes - circular output.
The use of IR illuminators is the most cost effective way of getting big increases in Intensifier performance.
The downside would be if someone else was also using a night scope in the same area - they would potentially see your IR illuminator very clearly.