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Understanding Image Intensified (I2) Night Vision.

Understanding Night Vision Technology

 Image intensifier tube technology has evolved over the past 50 years through a series of “generations.” In order to differentiate night vision products and determine which is best for your application, you should understand the generations.Each generation (Gen) has been defined by specific technology advancements. To date, there have been four generations of image intensifier devices produced: Gen 0 through Gen 3.

Night vision device (NVD), also known as a night optical/observation device (NOD), is an optoelectronic device that allows images to be produced in levels of light approaching total darkness. The image may be a conversion to visible light of both visible light and near-infrared, while by convention detection of thermal infrared is denoted thermal imaging. The image produced is typically monochrome, e.g. shades of green. NVDs are most often used by the military and law enforcement agencies, but are available to civilian users. The term 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 optical components such as a sacrificial lens, or telescopic lenses or mirrors. An NVD may have an IR illuminator, making it an active as opposed to passive night vision device.

Night vision devices were first used in World War II and came into wide use during the Vietnam War. The technology has evolved greatly since their introduction, leading to several "generations" of night vision equipment with performance increasing and price decreasing. Consequently, they are available for a wide range of applications, e.g. for gunners, drivers and aviators. (Wikipedia)


Gen 0— In 1929, Hungarian physicist Kálmán Tihanyi invented the infrared-sensitive (night vision) electronic television camera for anti-aircraft defense in Britain.

The first military night vision devices were introduced by the German army as early as 1939. The first devices were being developed by AEG starting in 1935. In mid-1943, first tests with infrared night-vision devices and telescopic rangefinders mounted on Panther started. Two different arrangements / solutions were created and used on Panther tanks. Solution A - Sperber FG 1250 (Sparrow Hawk), with range up to 600m, was made up of one 30 cm infrared searchlight and image converter operated by the commander. This was matched by an earlier experimental Russian version dubbed the PAU-2 and was field tested in 1942. From late 1944 to March 1945, some Panzerkampfwagen V Panther Ausf G (and other variants) mounted with FG 1250, were successfully tested. By the end of World War II, the German Reich had equipped approximately 50 (or 63) Panther tanks, which saw combat on both the Eastern and Western Fronts. The "Vampir" man-portable system for infantrymen was being used with Sturmgewehr 44 assault rifles. Parallel development of night vision systems occurred in the USA. The M1 and M3 infrared night sighting devices, also known as the "sniperscope" or "snooperscope", were introduced by the US Army in World War II, and also used in the Korean War, to assist snipers. They were active devices, using a large infrared light source to illuminate targets. Their image intensifier tubes function using an anode and an S-1 photocathode, made primarily of silvercaesium, and oxygen and an electrostatic inversion with electron acceleration were used to achieve gain.

After the WW2, the first practical commercial night vision device offered on the market was developed by Dr. Vladimir K. Zworykin working for the Radio Corporation of America, it was intended for civilian use. Zworykin's idea came from a former radio guided-missile. At that time infra-red was commonly called black light, a term later restricted to ultraviolet. It was not a success due to its size and cost. (Wikipedia)


Gen 1—The “starlight scopes” of the 1960s (Vietnam era) had three image intensifier tubes connected in a series. These systems were heavy and bulky. The Gen 1 image was clear at the center but distorted around the edges. First generation passive devices, introduced during the Vietnam War, were an adaptation of earlier active GEN 0 technology, and rely on ambient light instead of an infrared light source. Using an S-20 photocathode, their image intensifiers produce a light amplification of around 1,000×, but are quite bulky and require moonlight to function properly. (Wikipedia, various)



Gen 2—Second generation devices feature an improved image-intensifier tube utilizing micro-channel plate (MCP) with an S-25 photocathode, resulting in a much brighter image, especially around the edges of the lens. This leads to increased illumination in low ambient light environments, such as moonless nights. Light amplification is around 20,000×. Also improved were image resolution and reliability.

Later advancements in GEN II technology brought the tactical characteristics of "GEN II+" devices (equipped with better optics, SUPERGEN tubes, improved resolution and better signal-to-noise ratios) into the range of GEN III devices, which has complicated comparisons. (Wikipedia)



Gen 3— Third generation night vision systems maintain the MCP from Gen II, but now use a photocathode made with gallium arsenide, which further improves image resolution. In addition, the MCP is coated with an ion barrier film for increased tube life. However, the ion barrier causes fewer electrons to pass through, diminishing the improvement expected from the Gallium arsenide photocathode. Because of the ion barrier, the "halo" effect around bright spots or light sources is larger too. The light amplification is also improved to around 30,000–50,000×.Power consumption is higher than GEN II tubes.

Generation 3+ (GEN III OMNI IV - VII)

Generation II, III and IV devices use a microchannel plate for amplification. Photons from a dimly lit source enter the objective lens (on the left) and strike the photocathode (gray plate). The photocathode (which is negatively biased) releases electrons which are accelerated to the higher-voltage microchannel plate (red). Each electron causes multiple electrons to be released from the microchannel plate. The electrons are drawn to the higher-voltage phosphor screen (green). Electrons that strike the phosphor screen cause the phosphor to produce photons of light viewable through the eyepiece lenses.

U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) is part of the governing body that dictates the name of the generation of night vision technologies. Although the recent increased performance associated with the GEN-III OMNI-VI - VII components is impressive, the U.S. Army has not yet authorized the use of the name GEN-IV for these components.

GEN-III OMNI-V - VII devices can differ from standard Generation 3 in one or both of two important ways. First, an automatic gated power supply system regulates the photocathode voltage, allowing the NVD to instantaneously adapt to changing light conditions. The second is a removed or greatly thinned ion barrier, which decreases the number of electrons that are usually rejected by the Standard GEN III MCP, hence resulting in less image noise and the ability to operate with a luminous sensitivity at 2,850 K of only 700, compared to operating with a luminous sensitivity of at least 1,800 for GEN III image intensifiers. The disadvantage to a thin or removed ion barrier is the overall decrease in tube life from a theoretical 20,000 hrs mean time to failure (MTTF) for Gen III type, to 15,000 hrs MTTF for GEN IV type. However, this is largely negated by the low number of image intensifier tubes that reach 15,000 hrs of operation before replacement.

While the consumer market classifies this type of system as Generation 4, the United States military describes these systems as Generation 3 Autogated tubes (GEN-III OMNI-VII). Moreover, as autogating power supplies can now be added to any previous generation of night vision, "autogating" capability does not automatically class the devices as a GEN-III OMNI-VII. Any postnominals appearing after a Generation type (i.e., Gen II +, Gen III +) do not change the generation type of the device, but instead indicates an advancement(s) over the original specification's requirements. (Wikipedia)



Night Vision Terminology

Automatic Brightness Control (ABC): An electronic feature that automatically reduces voltages to the microchannel plate to keep the image intensifier’s brightness within optimal limits and protect the tube. The effect of this can be seen when rapidly changing from low-light to high-light conditions; the image gets brighter and then, after a momentary delay, suddenly dims to a constant level.

Auto-gating: The ATG function was designed to improve the BSP feature to be faster and to keep the best resolution and contrast at all times. It is particularly suitable for Aviator’s Night Vision goggles, operations in urban areas or for special operations. ATG is a unique feature that operates constantly, electronically reducing the “duty cycle” of the photocathode voltage by very rapidly switching the voltage on and off. This maintains the optimum performance of the I² tube, continuously revealing mission critical details, safeguarding the I² tube from additional damage and protecting the user from temporary blindness.

The benefits of ATG can easily be seen not only during day-night-day transitions, but also under dynamic lighting conditions when rapidly changing from low light to high light conditions (above 1 lx), such as sudden illumination of dark room. A typical advantage of ATG is best felt when using a weapon sight which experiences a flame burst during shooting (see figures below showing pictures taken at the impact zone of a dropped bomb). ATG would reduce the temporary blindness that a standard BSP tube would introduce, allowing them to continuously maintain “eyes on target”.

ATG provides added safety for pilots when flying at low altitudes, and especially during takeoffs and landings. Pilots operating with night vision goggles are constantly subjected to dynamic light conditions when artificial light sources, such as from cities, interfere with their navigation by producing large halos that obstruct their field of view. (Wikipedia)



Black Spots: These are cosmetic blemishes in the image intensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the performance or reliability of a night vision device and are inherent in the manufacturing processes.


Blooming: Momentary loss of the night vision image due to intensifier tube overloading by a bright light source. When such a bright light source comes into the night vision device’s view, the entire night vision scene becomes much brighter, “whiting out” objects within the field of view. Blooming is common in Generation 0 and 1 devices.


Bright-Source Protection (BSP): An electronic function that reduces the voltage to the photocathode when the night vision device is exposed to bright light sources such as room lights or car lights. BSP protects the image tube from damage and enhances its life; however, it also has the effect of lowering resolution when functioning.


Diopter: The unit of measure used to define eye correction or the refractive power of a lens. Usually, adjustments to an optical eyepiece accommodate for differences in individual eyesight. Most ITT systems provide a +2 to -6 diopter range.


Distortion: There are two types of distortion found in night vision systems. One type is caused by the design of the optics, or image intensifier tube, and is classical optical distortion. The other type is associated with manufacturing flaws in the fiber optics used in the image intensifier tube.


Classical Optical Distortion: Classical optical distortion occurs when the design of the optics or image intensifier tube causes straight lines at the edge of the field of view to curve inward or outward. This curving of straight lines at the edge will cause a square grid pattern to start to look like a pincushion or barrel. This distortion is the same for all systems with the same model number. Good optical design normally makes this distortion so low that the typical user will not see the curving of the lines.


Equivalent Background Illumination (EBI): This is the amount of light you see through a night vision device when an image tube is turned on but no light is on the photocathode. EBI is affected by temperature; the warmer the night vision device, the brighter the background illumination. EBI is measured in lumens per square centimeter (lm/cm2). The lower the value, the better. The EBI level determines the lowest light level at which an image can be detected. Below this light level, objects will be masked by the EBI.


Emission Point: A steady or fluctuating pinpoint of bright light in the image area that does not go away when all light is blocked from the objective lens. The position of an emission point within the field of view will not move. If an emission point disappears or is only faintly visible when viewing under brighter nighttime conditions, it is not indicative of a problem. If the emission point remains bright under all lighting conditions, the system needs to be repaired. Do not confuse an emission point with a point light source in the scene being viewed.


Eye Relief: The distance a person’s eyes must be from the last element of an eyepiece in order to achieve the optimal image area.


Fiber Optics Manufacturing Distortions: Two types of fiber optic distortions are most significant to night vision devices: S-distortion and shear distortion.

- S-Distortion Results from the twisting operation in manufacturing fiber-optic inverters. Usually S-distortion is very small and is difficult to detect with the unaided eye.

- Shear Distortion can occur in any image tube that uses fiber-optic bundles for the phosphor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area, as though the line were “sheared.”


Figure of Merit (FOM): Image intensification tube specification used to qualify exportability. Calculated on resolution (line pairs per millimeter) x signal-to-noise.


Fixed-Pattern Noise (FPN): A faint hexagonal (honeycomb) pattern throughout the image area that most often occurs under high-light conditions. This pattern is inherent in the structure of the microchannel plate and can be seen in virtually all Gen 2 and Gen 3 systems if the light level is high enough.


Footlambert (fL): A unit of brightness equal to one footcandle at a distance of one foot.


Gain: Also called brightness gain or luminance gain. This is the number of times a night vision device amplifies light input. It is usually measured as tube gain and system gain. Tube gain is measured as the light output (in fL) divided by the light input (in fc). This figure is usually expressed in values of tens of thousands. If tube gain is pushed too high, the tube will be “noisier” and the signal-to-noise ratio may go down. U.S. military Gen 3 image tubes operate at gains of between 40,000 and 70,000. On the other hand, system gain is measured as the light output (fL) divided by the light input (also fL) and is what the user actually sees. System gain is usually seen in the thousands. U.S. military systems operate at 2,000 to 3,000. In any night vision system, the tube gain is reduced by the system’s lenses and is affected by the quality of the optics or any filters. Therefore, system gain is a more important measurement to the user.


Gallium Arsenide (GaAs): The semiconductor material used in manufacturing the Gen 3 photocathode. GaAs photocathodes have a very high photosensitivity in the spectral region of about 450 to 950 nanometers (visible and near-infrared region).


I² (Image Intensification): Collects and intensifies the available light in the visible and near-infrared spectrum. Offers a clear, distinguishable image underlow-light conditions.


IR (Infrared): Area outside the visible spectrum that cannot be seen by the human eye (between 700 nanometers and 1 millimeter). The visible spectrum is between 400 and 700 nanometers.


IR Illuminator: Provides a light source (invisible to the unaided human eye) for the night vision system to amplify. Operates at approximately 880 nanometers.


lp/mm (Line Pairs per Millimeter): Unit used to measure image intensifier resolution. Usually determined from a 1951 U.S. Air Force Resolving Power Test Target. The target is a series of different-sized patterns composed of three horizontal and three vertical lines. A user must be able to distinguish all the horizontal and vertical lines and the spaces between them.


Lumen: A measure of the perceived power of light. The lumen can be thought of casually as a measure of the total “amount” of visible light emitted.


mA/W(Milliamps per Watt): The measure of electrical current (mA) produced by a photocathode when exposed to a specified wavelength of light at a given radiant power (watt).


MCP (Microchannel Plate): A metal-coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in Gen 2 and Gen 3 systems. MCPs eliminate the distortion characteristic of Gen 0 and Gen 1 systems. The number of holes (channels) in an MCP is a major factor in determining resolution. ITT’s MCPs have 10.6 million holes or channels compared to the previous standard of 3.14 million.


Modulation Transfer Function (MTF): A measurement of the ability of an optical system to reproduce (transfer) various levels of detail from the object to the image, as shown by the degree of contrast (modulation) in the image.


Near-Infrared: The shortest wavelengths of the infrared region, nominally 750 to 2,500 nanometers. Also see IR (infrared).


Photocathode: The input surface of an image intensifier tube that absorbs light energy (photons) and in turn releases electrical energy (electrons) in the form of an image. The type of material used is a distinguishing characteristic of the different generations.


Photocathode Sensitivity: Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The units of photocathode sensitivity are micro-amps/lumen (μA/lm). A lumen is a scientific unit that measures light at wavelengths the human eye sees (violet through red). Since image intensifier tubes see light that the eye does not, it is important to know the spectral (color) content of the light used in testing photocathode sensitivity. Photocathode sensitivity is measured using a light source with a color spectrum similar to a theoretical black body operating at 2,856°K (2,856 degrees Kelvin). This light source was chosen because it has a color spectrum similar to the color of a night sky illuminated only by stars. Photocathode sensitivity measured with a different color spectrum light source will yield different readings.


Resolution: The ability of an image intensifier or night vision system to distinguish between objects close together. Image intensifier tube resolution is measured in line pairs per millimeter (lp/mm) while system resolution is measured in cycles per milliradian. For any particular night vision system, the tube resolution will remain constant while the system resolution can be affected by altering the objective or eyepiece optics and by adding magnification filters or relay lenses. Often the resolution in the same night vision device is very different when measured at the center of the image and at the periphery of the image. This is especially important for devices selected for photography or video where the resolution of the entire image is important.


Scintillation: A faint, random, sparkling effect throughout the image area. Scintillation, sometimes called “video noise,” is a normal characteristic of microchannel plate image intensifiers and is more pronounced under low-light conditions. Do not confuse scintillation with emission points.


Signal-to-Noise Ratio (SNR): SNR is a ratio of the magnitude of the signal to the magnitude of the noise. If the noise in the scene (see “scintillation” definition) is as bright and as large as the intensified image, you cannot see the image. SNR changes with light level because the noise remains constant but the signal increases (higher light levels). The higher the SNR ratio, the darker the scene can be and the device still performs. The effect of SNR ratio in I2 devices is similar to that of a television far away from the TV station. At long distances from the station, the TV picture becomes noisy, and the ”snow“ blocks the picture.


Spectrum: The range of electromagnetic energy from cosmic rays to extra-low frequency.


Thermal Imaging: Senses radiation and temperature differentiation from the 7.5 to 13.5 micron range and creates a thermal picture (image of emitted heat energy). Better for detection than recognition.


μA/lm (Microamps per Lumen): The measure of electrical current (μA) produced by a photocathode when it is exposed to a measured amount of light (lumens).