Richard Sanders explains the ins and outs of night vision optics.
Every night vision device works by amplifying the visible (ambient) and near-infrared light it can detect. Available, ambient, light reflects from the viewed object, passes through the objective lens (the front lens) and creates an image of that object on the photo cathode screen, located on the front of the electronic intensifier tube. Inside this EIT the light is electronically amplified and projected in a green glow on a phosphorescent screen, located on the back of the EIT. Then the illuminated image is picked up by the ocular (eyepiece) and brought to the eye.
In absolute and total darkness, there is no ambient light available for the device to amplify. All night vision optical devices are fitted with an infrared illuminator that emits an infrared beam of light on to the object. This infrared light falls outside the visible spectrum of the human and animal eye and is invisible to them. Only the IR illuminator diode glows red which will be visible at short distance (up to 30 metres).
The magnification of all night vision devices is generally much lower than that of their daylight counterparts or spotting scopes. It is important to consider the fact that when employing higher magnification, the objective lens elements are thicker and more complex, thereby transmitting less light than thinner and less complex lens elements. The magnification of most night vision devices is never higher than seven-times at this stage of the technology, preserving the best possible balance between image magnification and the light transmission.
First generation or second generation or third generation?
The two most important characteristics of a night vision device are the qualities of the EIT and the optical system of the unit. Internationally accepted terminology classifies the EIT as one of the following generations: 1, 2 and 3, with an unknown possibility of a fourth now that we have digital technology.
A Gen-1 night vision device employs a vacuum tube with a photo cathode sensitivity of 120 to 280 microamperes per lumen. These tubes amplify the ambient light by a factor from 120 to 900 times with a resolution in the centre of the image at 20 to 38 line-pairs per millimetre. Gen-1 units are most popular at the consumer level due to their relatively inexpensive price compared to higher generation levels. One of the specific characteristics of Gen-1 units is that the image at the centre of the field of view is sharp and clear, while being somewhat distorted and less clear at the edges. Even within Gen-1, tubes may be quite different in their performance.
During the manufacturing process all tubes are evaluated and graded according to their quality and performance and are sold at significantly different prices depending on their light amplification and image clarity. This of course is reflected in the final price of the Night Vision device.
A Gen-1+ tube is a further improvement of the Gen-1 tube with a fibre-optic plate installed at the front or the back of the tube. This allows for a dramatic improvement of the image resolution and it prevents distortion especially at the edges. Such a tube has a light amplification of 1000 times and the sensitivity increases to nearly 300 microamperes per lumen. Resolution in the centre is 50 line-pairs per millimetre. A Gen-2 tube differs from Gen-1 and 1+ by employing a special electron amplifier – the micro channel plate (MCP). The performance characteristics of such a tube is much greater light amplification of 20 000 times the ambient light; increased sensitivity of 300-320 microamperes per lumen and increased lifetime of the tube (4000-5000 hours, compared to 2000-2500 hours of Gen-1 and 1+). Two types of the intensifier tube with MCP are available: the 25 mm and 18 mm. From the user perspective, the larger the diameter, the more efficient the tube is, however it would also require a larger night vision unit when assembled.
The Gen-2+ tube is an improvement of the Gen-2 tube with the light amplification of 22 000-25 000 times and sensitivity of 300-600 microamperes per lumen (please note that all Gen-2+ tubes are Russian-made, as the US technology switched from Gen-2 to Gen-3, bypassing the Gen-2+. Today, due to Russian legislation, the maximum sensitivity allowed for export outside of Russia is 350 microamperes per lumen).
Resolution in the centre is 39-45 line-pairs per millimetre. Lifetime is 4000-5000 hours. Gen-2+ units are equipped with automatic gain control, which regulates the brightness of the image depending on the outside light condition. The units also have a flash (daylight damage) protection and virtually distortion free image even at the edges. Until recently, such units were used mainly by the military, but now they have become more widely available.
A Gen-3 tube differs from a Gen-2+ by the use of a photo cathode based on Gallium Arsenate with even greater sensitivity, especially in the Extended-IR spectrum (900-1600 microamperes per lumen), centre resolution of 50-64 line-pairs per millimetre and an expected tube life of 10 000 hours. Such tubes are utilised in the military night vision and the commercial availability is limited and mostly left to the tubes, which did not pass the military specifications during the initial testing, or to the used (refurbished) tubes.
Buying a night vision instrument
Anyone buying a night vision device should consider two factors. Firstly, the purpose for such a device will determine the generation level; secondly, the ruggedness and quality of the device overall.
For general viewing at night, eg, wildlife and general security observation, without the need for specific identification of objects, first generation will be more than adequate. Gen-2 instruments come at a much higher cost; therefore it stands to reason that justifying that spend will demand a valid reason for such higher quality.
The ruggedness may also play a major role in the choice of devices and brand names on the market. Where this becomes vital is, for example when a night vision rifle scope is needed.
Magnification of the device offers little choice as explained earlier, but for ease of comparison, 1x magnification can be likened to the naked eye’s ability to see over distance at low light level, which could be around 100 to 150 metres. A five-times magnification night vision device will allow visibility up to ±300 metres, perhaps up to 400 metres with a full moon gracing our skies.
A five-times magnification night vision device, which has that 300-metre capability may also require an accessory to allow better visibility in total darkness. As mentioned earlier, some ambient light is required to allow the EIT to do its job, which, in total darkness will require the use of an IR Illuminator. The built-in IR Illuminators are capable of reaching a distance of about 100 metres, after which they are incapable of supplying the ambient light needed by the device. One very useful accessory now would be the Extended Range IR Illuminator which can reach roughly 200 metres, and the Extended Range LASER IR Illuminator, capable of 1000 metre range. The efficiency and effectiveness of the latter is clearly and markedly visible.
The digital age
Traditional night vision devices employ electron bombardment vacuum tube technology to amplify ambient light. New techniques use image processing, special optics and low-light sensing technology to bring night vision into the digital age.
Here are the highlights and the specific differences between the digital versus the older technologies.
* Better image quality with high, 30-line-pairs per millimetre, edge-to-edge resolution.
* Better image quality with no image distortion from photocathode or phosphorescent screen blemishes.
* Easier dioptre focusing.
* Twice the sensitivity to IR illumination.
* Ambient light amplification capability higher than Gen-1 technology at default exposure.
* Video capture connectivity.
* Image capture capability onto memory card.
* Programmable for light sensitivity, display characteristics, exposure control, video format, date and time stamp and power save.
* Bright light see-through capability. No bright-spot blooming or halo-effect typical with conventional night vision.
* More robust. No risk of damage from bright light exposures. No fragile vacuum tubes.
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