Night vision implies the ability to see at night. It is a vital ingredient of battlefield transparency. The side that possesses this capability will have a distinct advantage over their opponents if the latter are not similarly equipped. Hence this capability is considered an essential ingredient of modern warfare and developments in science and technology have made it possible to develop such devices which enable humans to see in the dark as well as under inclement weather conditions such as fog, rain, and snow, and even through smoke and dust. The side that can see better by night will have greater advantage on a battlefield and other issues being equal it may turn out to be a war-winning factor. In urban environment of counter-insurgency and counter-terrorist operations its vital importance for security forces involved cannot be downplayed.

The term night vision device (NVD) usually refers to a complete unit, including an image intensifier tube, a protective and generally water-resistant housing, and some type of mounting system. Many NVDs also include sacrificial lenses, IR illuminators and telescopic lenses. Research and development (R&D) is being undertaken globally to enhance the reach, improve the resolution and reduce the weight of night vision devices in order to provide a better edge to own side.

The Indian Army too has hand-held NVDs on its inventory in various categories and quantities albeit equipping both in terms of quantity and quality on its weapon systems is still not satisfactory. For example, its assault rifles are not fitted with night scopes. The concept and philosophy for night vision accessories too needs refining if we are to learn from the past mistakes. For example, when the HHTIs were first imported from Israel and France only one charger per four HHTIs were procured. This created major problems with widely dispersed deployments in J&K and forced the infantry to improvise chargers, which may have caused inadvertent damage to the equipment. Another example was of artillery which went in for numerous laser target designators but only one charger that was kept centrally at the School of Artillery and every time charging was needed, individual designators had to be flown in and out. And there is no gainsaying that our Defence Research and Development Organisation (DRDO) and public sector undertakings (PSUs) are way behind in the field of NVDs compared to their foreign counterparts. Our night vision products are bulkier and with lesser resolution.

Types of NVD

Night vision devices used for military purposes are of two types – Image Intensifiers and Thermal Imagers. Both have their advantages and disadvantages and advent of new technologies is resulting in more sophistication and better products. Some basic characteristics of both types of NVDs are given in succeeding paragraphs.

Image Intensifiers: Night Vision Goggles and Night Scopes
Today, the most popular and well-known method of performing night vision is based on the use of image intensifiers. Image intensifiers are commonly used in night vision goggles and night scopes. More recently, on-chip gain multiplication CCD cameras have become popularised for performing low-light security, surveillance and astronomical observation.

The working of the night vision device (image intensifiers) involves the amplification of the available light to achieve better vision. Image Intensifiers are more common as their light amplification technology uses the small amount of ambient light like moon / stars light and converts this light energy (photons) into electrical energy (electrons). An objective lens focuses available light (photons) on the photocathode of an image intensifier. The light energy causes electrons to be released from the cathode which are accelerated by an electric field to increase their speed (energy level). These electrons enter holes in a microchannel plate and bounce off the internal specially-coated walls which generate more electrons as the electrons bounce through. This creates a denser ‘cloud’ of electrons representing an intensified version of the original image.

The final stage of the image intensifier involves electrons hitting a phosphor screen. The energy of the electrons makes the phosphor glow. The visual light shows the desired view to the user or to an attached photographic camera or video device. A green phosphor is used in these applications because the human eye can differentiate more shades of green than any other colour, allowing for greater differentiation of objects in the picture.

The extensive use of night vision goggles in the military and law enforcement segments is a major factor that will result in the steady growth of this market segment during the next four years

All image intensifiers operate in the above fashion. Technological differences over the past four to five decades have resulted in substantial improvement to the performance of these devices. The different paradigms of technology have been commonly identified by distinct generations of image intensifiers. Intensified camera systems usually incorporate an image intensifier to create a brighter image of the low-light scene which is then viewed by a traditional camera.

The advantages and disadvantages of Image Intensifier devices are listed below.

Advantages

• Excellent low-light level sensitivity.
• Enhanced visible imaging yields the best possible recognition and identification performance.
• High resolution.
• Low power and cost.
• Ability to identify people.

Disadvantages

• Since they are based on amplification methods, some light is required. This method is not useful when there is essentially no light.
• Inferior daytime performance when compared to thermal imagers.
• Possibility of blooming and damage when observing bright sources under low-light conditions.

Thermal Imaging

Thermal imaging is a method of improving visibility of objects in a dark environment by detecting the objects’ infrared radiation and creating an image based on that information. Thermal imaging, nearinfrared illumination, low-light imaging are the three most commonly used night vision technologies. Unlike the other two methods, thermal imaging works in environments without any ambient light. Like near-infrared illumination, thermal imaging can penetrate obscurants such as smoke, fog and haze.

The next question is how does thermal imaging work? An easy way to understand is that all objects emit infrared energy (heat) as a function of their temperature. The infrared energy emitted by an object is known as its heat signature. In general, the hotter an object is, the more radiation it emits. A thermal imager (also known as a thermal camera) is essentially a heat sensor that is capable of detecting tiny differences in temperature. The device collects the infrared radiation from objects in the scene and creates an electronic image based on information about the temperature differences. Because objects are rarely precisely the same temperature as other objects around them, a thermal camera can detect them and they will appear as distinct in a thermal image. Thermal images are normally grayscale in nature: black objects are cold, white objects are hot and the depth of gray indicates variations between the two. Some thermal cameras, however, add colour to images to help users identify objects at different temperatures.

Uncooled and Cryogenically Cooled Devices
Thermal imaging devices are generally ‘Uncooled’ or ‘Cryogenically Cooled’. The uncooled ones are more common wherein the IR detector elements are contained in a unit that operates at room temperature. These devices are noiseless, activate immediately and have in-built batteries. Cryogenically cooled devices have the elements sealed inside a container that cools them to below 0 degree Celsius. The advantage of such a system is the incredible resolution and sensitivity that result from cooling the elements. Though more expensive and more susceptible to damage from rugged use, these systems enable a soldier to see whether a person is holding a gun more than 300 metres away. Unlike traditional most night-vision equipment which uses image enhancement technology, thermal imaging is great for detecting people or working in near-absolute darkness with little or no ambient light.

Uses of Thermal Imaging

• First developed for military purposes, thermal imaging has since been adopted by law enforcement, fire and rescue teams and security professionals. For law enforcement and security staff, thermal imaging detects suspicious activity over long distances in total darkness and through fog, smoke, dust, foliage, and many other obscurants. This allows officers to approach in stealth mode and make better informed decisions more quickly. Cameras may be hand-held, vehicle-mounted, tripod-mounted, or weapon-mounted.
• For security and surveillance systems, thermal imaging cameras complement CCTV cameras to provide comprehensive threat detection and integrate seamlessly with larger networks.
• For predictive maintenance, thermal imaging reveals ‘hot spots’ where failure may be imminent in many electrical and industrial facilities and installations.

Night Vision Devices — Generations of Image Intensifiers

A night vision device can be either a first, second, third or fourth-generation unit. What this stands for is what type of image intensifier tube is used for that particular device; the image intensifier tube is the heart and soul of an NVD.

First-generation is currently the most popular type of night vision in the world. Utilising the basic principles described earlier, a first-generation unit will amplify the existing light several thousand times letting you clearly see in the dark. These units provide a bright and sharp image at a low cost, which is perfect, whether you are boating, observing wildlife, or providing security for your home. You may notice the following when you are looking through a first-generation unit.

• A slight high-pitched whine when the unit is on.
• The image you see may be slightly blurry around the edges. This is known as geometric distortion.
• When you turn a first-generation unit off it may glow green for some time.
• These are inherent characteristics of a first-generation unit which are normal.

Second-generation is primarily used by law enforcement or for professional applications. This is because the cost of a secondgeneration unit is approximately $500 to $1,000 more than a first-generation unit. The main difference between a first- and a second-generation unit is the addition of a micro-channel plate, commonly referred to as a MCP. The MCP works as an electron amplifier and is placed directly behind the photocathode. The MCP consists of millions of short parallel glass tubes. When the electrons pass through these short tubes, thousands more electrons are released. This extra process allows second-generation units to amplify the light many more times than first-generation giving you a brighter and sharper image. There are various categories of upgrades in second-generation tubes. Each is an upgrade on the former with slightly better resolution, better signal to noise ratio and a longer tube life.

Third-generation. By adding a sensitive chemical, gallium arsenide to the photocathode, a brighter and sharper image has been achieved over second-generation units. An ion barrier film was also added to increase tube life. Third-generation provides the user with good to excellent low light performance. Similarly third-generation devices have a number of upgrades with better features.

Fourth-generation. Gated/filmless technology represents the biggest technological breakthrough in image intensification of the past 10 years. By removing the ion barrier film and ‘Gating’ the fourth-generation system demonstrates substantial increases in target detection range and resolution, particularly at extremely low light levels. This generation of devices also has a number of upgrades improving the performance slightly with each upgrade. The use of film less technology and auto-gated power supply in fourth-generation image intensifiers result in:

• Up to 100 per cent improvement in photo response.
• Superb performance in extremely low light level (better S/N and EBI).
• At least triple high light level resolution (a minimum of 36 lp/mm compared to 12 lp/mm).

Choice of NVDs

While choosing NVDs, three important performance parameters that need to be born in mind:

• Signal-to-noise ratio (SNR)
• Resolution and modular transfer function (MTF)
• Lifetime of the tube.

Signal-to-noise ratio is by far the most important parameter for an Image Intensifier. It is a measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. The value of the SNR determines the resolution at very low light-levels. Therefore, the higher the SNR the better the ability to resolve image details under low light-level conditions. The SNR is related to the specific design of the tubes.

MTF is the maximum line density on a target that can be resolved by a human eye and is expressed in line pairs per mm (lp/mm). A more objective performance indicator is given by the modulation transfer function. High MTF values at low spatial frequencies provide sharp images with a good contrast.

Lifetime of an Image Intensifier is an extremely important parameter for night vision applications. A number of different definitions are used depending on the manufacturer.

All Image Intensifier tubes provide a green illuminated picture and no night vision tube is similar to another. All tubes have different cosmetics in terms of small spots or specs, photocathode colouring, or a chicken wire effect from the micro channel plate. Most cosmetics are only noticed during viewing in high light situations such as viewing with the daylight filter on in a lit room. Most commercial and military systems are thoroughly tested by manufacturers to ensure reliability.

Market Trends

According to the analysts upcoming market trend that will positively affect the growth prospects of the night vision devices market is the augmented utilisation of graphene. Graphene is an ultra-thin, ultra-light, ultrastrong, and ultra-flexible material that can be used in any product. The new graphenebased thermal sensor is just one atom thick, and it uses a cryogenic cooling system to identify heat patterns from a long distance. The incorporation of graphene not only helps to reduce the weight of night vision devices but also helps to lower the price of the device.

Product-based segmentation of the night vision devices market is as under:

• Night vision goggles
• Night vision cameras
• Night vision scopes

The market research analysts have estimated the night vision goggles product segment to account for approximately 47 per cent of the total market share by 2020. The extensive use of night vision goggles in the military and law enforcement segments is a major factor that will result in the steady growth of this market segment during the next four years. Extensive market research carried out by the analysts has shown that the military market segment will post an impressive market value of nearly $4 billion by 2020.