Sensitivity of thermal imaging sensors of the first generation was limited by background radiation. This problem was overcome in second-generation thermal imaging sensors by using modified front-end optics that reduced unwanted flux. However, this resulted in a fixed f-number for all fields-of-view.
Second-generation thermal imagers are vector detectors, usually containing 64 or more elements. The two-dimensional scanner was somehow simplified in the vertical direction to include only the interlace motion.
Third-generation thermal imagers contain dual-band, two-dimensional arrays with several columns of elements and dual/variable f-number optical system. These thermal imagers still scan in one direction and perform time-delay integration of the signal in the scanning direction to improve signal-to-noise ratio.
Fourth-generation thermal imagers contain two-dimensional array detectors (160×120, 320×240, 680×480) called focal plane arrays that do not require any scanning mechanism for acquiring two-dimensional pictures.
Applications of night vision equipment
Night vision equipment finds extensive usage in a wide range of applications in military, law enforcement, surveillance, security, navigation, hunting, wild life observation and so on. These devices are used not only by the military, police forces and law enforcement agencies for navigation, surveillance and targeting, these are also used by private detectives, fire-fighters, hunters and nature enthusiasts. Both image enhancement and thermal imaging night vision equipment have preferred areas of application to which these are better suited than the other.
Image enhancement night vision devices
Image intensification night vision needs at least a small amount of light to operate. This ambient light, in some cases, may be provided by the moon and stars. In the case of total darkness, the night vision device can see no better than the naked eye. This shortcoming is overcome with the use of an IR illuminator, which works like a flashlight for night vision while staying invisible to the naked eye.
Use of an IR illuminator has a drawback. It makes the device highly prone to be detected by enemy’s night vision device.
Night vision technology is used extensively by the military and police due to the instant tactical advantage they get through the ability to see in the dark, and precisely detect and even identify the target even from a distance. AN/PVS-14 (Fig. 7) is one such versatile night vision scope extensively used by armed forces and special operations units. It is a monocular night vision device designed for use by the individual soldier in a variety of ground-based night operations. While the dark-adapted unaided eye provides situational awareness and vision of close-range objects, the night vision device provides long-range vision of potential threats and targets, thus, making it a force multiplier in night time warfare.
Night vision cameras are extensively used for surveillance and security applications, especially for around-the-clock surveillance, indoors and in controlled environments. Such cameras provide enhanced security by improving video images in low-light conditions. With the help of IR illumination, these can be effectively used in near total dark conditions, too. Perhaps, the greatest feature of night vision cameras is that these are true 24×7 cameras, as these are the only cameras with the ability to record any event in a protected area around the clock when connected to a digital video recorder.
Night vision cameras are available in two basic styles: dome and bullet. The resolution typically varies from 400 TV lines to 700 TV lines. Security cameras with long-range night vision find extensive usage in monitoring large parking lots, huge and dark warehouses, apartment complexes and other similar assets on 24×7 basis. Traffic monitoring and monitoring of suspicious activity in public places, security installations and critical national assets are other important application areas of security cameras with night vision capability.
Thermal imaging night vision devices
Thermal imaging devices are used on land-based armoured fighting vehicles, naval vessels and aerial platforms such as aircraft, helicopters and missiles for surveillance, target acquisition and tracking applications. Some common non-military applications of these systems include surveillance of living things, search and rescue operations during fire-fighting, detection of gas leaks, monitoring of volcanoes and detection of heat in faulty electrical joints.
In military applications, thermal imaging devices offer several distinct advantages. The first is their immunity to detection by the adversary. These are passive sensors that do not emit any radiation for generation of images.
Second, it is extremely hard to camouflage the target from the sensor as these sense heat radiation.
Third, thermal imagers can see through smoke, fog, haze and other atmospheric obscurants better than sensors operating in the visible spectrum.
One limitation of thermal imaging sensors is that these cannot discriminate between a friend and a foe. Friendly forces can use heat beacons to overcome this shortcoming.