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An examination of the history and evolution of the night vision goggle is important before discussing any specific night vision device. In the mid 1960s, U.S. forces began looking for ways to overcome darkness on the battlefield not so much to optimise it toward their advantage as to deny its use to the North Vietnamese. U.S. forces began with searchlights, simple and effective, but hardly designed for rapid manoeuvre and (worst of all) quite conspicuous. The need for enhanced night vision capability became evident as U.S. forces pinpointed their position to the enemy when trying to illuminate for themselves. An intermediate answer came in the form of near-infra-red searchlights and viewers. These were relatively simple and provided U.S. forces with a means of viewing the battlefield without visible light. Unfortunately, the simplicity of the viewers' image converter tubes soon made them common place, for both sides, and U.S. forces once again found themselves illuminating their own positions. What they really needed was a night sighting device that did not emit a traceable signature. That device was found in the image intensification tube.
FIGURE 1: Image Intensifier
An image intensifier is an electro-optical device used to detect and intensify optical images in the visible and near infrared region of the electromagnetic spectrum for the purpose of providing visible images. Light enters the image intensifier tube and is focused, by the objective lens, onto a photo cathode receptive to both visible and near IR fight (See Figure 1). Photons (particles of light) striking the photo cathode cause a release of electrons proportionate in number to the number of photons that strike the photo cathode. In turn, an electrical field produced by the device's battery source accelerates the released electrons away from the photo cathode surface. The accelerated electrons are directed toward and strike a phosphor screen. This screen emits an amount of light proportional to the number and velocity of electrons that strike it. The electrical field applied between the photo cathode and the phosphor screen also accelerates the electrons, which serves to brighten the projected scene. Thus, the picture delivered to the user is converted from a small amount of lights to accelerated electrons and back to light. The amount of light amplification produced in an image intensifier tube is referred to as the tube gain. The gain of an NVG is the ratio of the brightness of the output, compared to the illumination of the input. Image intensifier technology is generally spoken in terms of first, second or third generation systems. The next few sections will be dedicated to a generic description of intensifier tube components and the different generation systems technologies.
Night Vision Goggle Components
The first component of an NVG is the objective lens. The lens is actually a combination of optical elements, which function to focus the incoming photons of light on the photo cathode. The objective lens may require refocussing when the viewed object moves toward or away from the viewer. Some second generation and all third generation tubes have incorporated a special coating on their objective lenses to reduce interference from modern cockpit lighting schemes. These "minus blue" filters are designed to reject visible light and have 50% transmission points between 625 and 655nm for ANVIS (Class A) type minus blue filters. These filters are fitted to the NL-94-AU NVGs used at Becker Helicopter Services.
The photo cathode is responsible for converting the incoming light energy into electrical energy in the form of electrons. The photo cathodes utilised in Image Intensifier (II) tubes are processed, or 'grown,' by vaporising several elements in a vacuum chamber and depositing them on the vacuum side of the transparent input window before the vacuum seal is made. Photo cathode growing is a very delicate process in which the quality and type of the crystals grown directly affects the gain and sensitivity of the II tubes. First and second generation tubes from the USA use an S-20 multi-alkali photo cathode which provides for a sensitivity of between 400-850nm with a broad peak from 500 to 600nm (See Figure 2). Since the sensitivity of the photo cathode extends into the near infrared, they are able to detect energy in that region which is invisible to the human eye. The gallium arsenide photo cathode in the third generation tubes surpasses the photosensitivity of the S-20 multi-alkali photo cathode beyond 550nm and has a photo response from approximately 600-900 nanometres. This is significant since the S-2 night sky spectral irradiance photon rate is 5-7 times greater in the region of 800-900nm than in the visible region in the neighbourhood of 50Onm. In engineering terms, the sensitivity of the third generation photo cathode is more than 1 000 microamps lumen, as compared to the 250-550 microamps lumen range of the second-generation photo cathode. It is easy to become entangled in these numbers, but the end result is that the third generation tube is far more sensitive in the region where near infrared light from the night sky is plentiful.
The NL-94-AU follow a similar path however with increased technological advances they tend to use a different phosphor resulting a slightly differing image colour, contrast, and intensity.
FIGURE 2: Comparison of Second Generation and Third Generation ITT Sensitivities
Since the output of the phosphor screen is determined by both velocity and number of electrons, second and third generation image intensifiers use a device known as a microchannel plate (MCP). The MCP (See Figure 3 & 4) is a very thin (1 mm) wafer comprised of millions of tiny glass tubes. The MCP is located next to the photo cathode. Electrons exiting the photo cathode are channelled first through the MCP. The inside passages of the MCP tubes are coated with a material that causes secondary electron emissions. The tiny glass tubes are tilted, typically at a 5 degree bias angle, to ensure a first electron impact near the channel entrance. As each electron strikes a wall, more electrons are emitted from the wall, each of which will in turn strike the wall again, emitting more electrons, etc. As a result of this process, for each electron that enters the MCP, 1 000 or more will exit. These electrons are in turn accelerated forward, maintaining their relative spatial position, until they strike and excite the phosphor screen.
The NL-94-AU NVGs also utilise a Micro Channel Plate in the same way as the US NVGs. The real difference however is that the US NVGs require the assistance of an Ion barrier filter to assist in the production of photoelectrons. A major problem for the GEN III tubes is that many of the created photoelectrons are stopped in the ion barrier film on top of the MCP and do not contribute to the output brightness (See Figure 3). This is a fundamental problem for filmed MCP tubes, as the film prevents part of the signal photo-electrons from reaching the MCP holes and hence they don't get multiplied and don't participate in the output brightness. The decrease in Detected Quantum Efficiency caused by the film can be as high as 40-50%. These trapped electrons will not be amplified.
TYPICAL ITT GEN 3
FIGURE 3: NL-94-AU vs TYPICAL ITT Gen III Micro-channel Plate
FIGURE 4: Generic Micro-channel Plate
The phosphor screen is a very thin layer of phosphor applied to the output fibre optic system. Phosphors emit light when electrons strike them. The output light wavelength is a function of the type of phosphor used. Most current image intensifier tubes use P-20 phosphor, which emits a yellow-green light at 560 nm and matches the peak sensitivity of the photopic human eye. Other phosphors, including P-43 and P-22, have been used in recent tube designs; the function of the phosphor screen is to convert electrical energy back into a visible image for the eye to use.
In first generation devices, sufficient spacing exists between the photo cathode and phosphor screen to allow for image inversion to occur as electrons actually crossover in the middle of the tube (See Figure 5). This spacing, however, is not available in second and third generation devices where the photo cathode, MCP, and phosphor screen are in "proximity focus." That is, they are located very close to each other, providing the miniaturisation necessary for a helmet mounted system. With these systems, image inversion is accomplished by attaching the phosphor screen to what is referred to as a fibre optic inverter. This inverter is actually a bundle of millions of microscopic light transmitting fibres. In second and third generation systems, the fibre optic bundle is heated and given a 180 degree twist providing the needed inversion to produce an upright image without requiring a second eyepiece lens. The fibre optic inverter also collimates the image, making the image at the eyepiece lens appear to be at the appropriate distance from the viewer. Without collimation, the eye's focus would be set for the distance to the eyepiece lens; a distance which would place severe strain on the eye and lead to significant human factor problems.
FIGURE 5: First Generation Image Intensifier
The eyepiece lens is the final optical component of the image intensifier tube. As with the objective lens, the eyepiece lens is a series of optical components. The function of the eyepiece lens is to focus the light from the phosphor screen and fibre optic inverter onto the eye. The dioptre adjustment for this lens allows for some corrective lens wearers to use NVGs without the aid of their spectacles. However, the majority of personnel who use corrective lenses, such as for astigmatism, will still need to use them when operating with NVGs.
Constant exposure of the image intensifier tube to bright light sources may result in damage to the photo cathode, the MCP, and the eye. The power supply to second and third generation tubes, therefore, has been designed with two automatic protection features designed to control the gain of the intensifier tube. The first, an automatic brightness control (ABC), automatically adjusts MCP voltage to hold eyepiece brightness to a preset level for a full range of ambient illumination levels by controlling the number of electrons which exit the MCP. This protects the viewer from bright flashes and provides sufficient light to the viewer under low ambient light conditions. The second protection feature, the bright source protection (BSP) circuit, limits the number of electrons leaving the photo cathode by reducing the voltage between the photo cathode and the input side of the MCP. This feature automatically activates when high input light levels cause excessive photo cathode current to flow. The BSP circuit is extremely important because the service life of a second or third generation tube is largely a function of the photo cathode service life. Photo cathode end of life is primarily caused by ion contamination from the MCP. The higher the light input, the more ions are generated and the shorter the life expectancy of the tube.
First Generation Image Intensifiers
First generation technology was made common by the starlight scope used by snipers in Vietnam and attained high light level gains by means of a three stage configuration of the simple image intensification tube (see Figure 6). The three-stage coupling provides gains in the range of 40,000 to 60,000 (compared to gains of 40-60 attained in a single stage tube).
FIGURE 6: First Generation Image Intensifier
First generation tubes are very durable and have a very long life, well into the ten thousand-hour range. However, they possess significant deficiencies. First, they are extremely susceptible to "blooming". Blooming is a tendency for the tube to de-gain if a bright light source appears anywhere in the devices' field of view. As a result, the overall contrast of the intensified image is greatly reduced. Secondly, first generation tubes require relatively high voltages to attain the necessary energy levels for light amplification. The weakest parts of the first generation system are the 6.75 volt battery and the oscillator module that converts the battery energy to an alternating current source for the intensifiers power supply. But the most significant drawback encountered with the first generation device is its size. The starlight scope, for instance, is about a foot long. This is fine for snipers but useless for pilots.
Second Generation Image Intensifiers
Second generation image intensifiers were the first system small enough to be worn on headgear. Much of the size invested in the first generation tube was necessary due to the voltage required for sufficient electron velocity to produce the desired gain. But remember the amount of light produced by the II tube is proportional to both the number and velocity of electrons that strike the phosphor screen. Therefore, by increasing the number of electrons, the required velocity of those electrons may be decreased to achieve the same gains.
Essentially, this is the concept achieved by means of the MCP and proximity focus in the construction of the second generation in tube. Furthermore, by reducing the distances between these components, there is little or no image distortion caused by the influence of stray magnetic fields acting on the electrons, which occurs occasionally in first generation devices. Second generation tubes are still somewhat susceptible to "blooming" when exposed to a bright light source. However, this tendency is minimised with the introduction of the MCP. Whereas first generation devices suffered saturation of the entire field of view when exposed, the MCP allows the saturation to be confined to individual channels; thus, contrast degradation is localised. This localised saturation sometimes appears on the tube as a halo effect around the image of the bright light source. This halo effect will degrade the contrast of adjacent portions of the intensified images. Generally, at medium light levels second generation tubes can be operated for 2,000 to 4,000 hours. This is a marked reduction in durability as compared to the 10,000 hour service life with first generation tubes. Furthermore, the gain associated with second-generation tubes is somewhat lower than that of the first generation systems; 10,000 vice 40,000 to 60,000. It is important to point out that there are no inherent qualities in second generation II technology that would lead to improved resolution. Nevertheless, the improvements in second generation technology associated with miniaturisation and its accompanying adaptability for aviation use clearly offset its comparable shortcomings.
Third Generation Image Intensifiers
There are only two major changes that mark the difference between second and third generation image intensifiers. First, a gallium arsenide (GaAs) photo cathode has replaced the S-20 multi-alkali photo cathode, and secondly an aluminium oxide film has been applied to the MCP. The results of these changes are most noteworthy. Performance is significantly improved under low illumination and service life extension is equal to first generation levels of greater than 10,000 hours. The addition of the aluminium oxide film to the MCP of the third generation tube directly offsets shortened service life of the photo cathode due to ion bombardment.
The film is transparent to electrons. Therefore, the electrons pass, from the photo cathode to the MCP just as before. However, the film is not transparent to ions. They are trapped in the aluminium oxide film and prevented from contaminating the photo cathode. The only repercussion encountered with the introduction of the aluminium oxide film is a voltage increase between the photo cathode and the MCP. This, in turn, requires increased spacing between the two components to prevent arcing. This spacing causes an increased halo size when viewing bright light sources.
The AN/AVS-9 and NL-94-AU NVGs
The AN/AVS-9 and NL-94-AU NVGs are system designed from the outset for aviation. Both are lightweight, self-contained, helmet mounted systems with the AN/AVS-9 being Gen III NVGs and the NL-94-AU are considered as GEN IV (Europe) NVGs. Both sets of NVGs are shown below in Figures 7 and 8 respectively. The binocular consists of two identical monocular assemblies mounted on a pivot adjustment shelf (PAS). A helmet mount assembly is provided to attach the binocular to the aviators' helmet.
FIGURE 7: AN/AVS-9, ANVIS
FIGURE 8: NL-94-AU NVGs
NVG Power Sources
There are presently only one type of battery that may be used in the AN/AVS-9; the 2 x 1.5 volt AA alkaline battery. The NL-94-AU can use the same batteries as the AN/AVS-9 but they can also use Lithium batteries. The service life of each battery will be affected by environmental temperature. Users should consult with the relevant NVG User Manual before using the NVGs.
There is one type of battery pack, which is compatible with the AN/AVS-9 and NL-94-AU when they are used on a flight helmet. The battery pack consist of a housing that contains a primary and secondary battery power source and a three-position switch (Figure 9). The battery pack is known as the slimline. The battery pack for the AN/AVS-9 enables counterweights to be mounted on it. The NL-94-AU uses a similar battery pack but it allows for a flat counterweight to be mount between the helmet and battery pack. Depending on which set of NVGs are used there is a circuit card inside that will make the low battery indicator blink instead of giving a steady red light when the battery voltage drops to 2.4 vdc. This signals the user to switch the ON/OFF/ON switch on the power pack to the secondary battery compartment. Again, users should consult the relevant manual before using the NVGs.
FIGURE 9: Power Pack
Do not use mercury or rechargeable NiCad batteries. Using these batteries could result in a system failure.
The ANVIS 9 and NL-94-AU mount incorporates a red LED at the base of the mount that comes on when active battery voltage drops to 2.4V DC, signalling the user that remaining battery life is approximately 30 minutes. As mentioned earlier, depending on the type of battery pack, the red LED may be steady or blink. Both mounts also allows the binocular assembly to "break away" from the helmet during a crash load of 10g to 15g.
AN/AVS-9 and NL-94-AU Improvements
The improved tubes in the An/AVS-9 have resulted in an increase in range performance at all light levels compared to previous GEN Ill tubes, as well as increasing effectiveness at lower light levels. Improvements include higher photo cathode sensitivity, higher tube and overall system resolution, smaller halo around point sources of light, reduced tube "dark spots," higher signal-to-noise (S/N) ratio, and longer tube life. Both a reduction in the MCP channel spacing and a reduction in component spacing have achieved this, ie. the distances between the MCP and phosphor screen and the MCP and photo cathode. Smaller spacing give better electron proximity focus and hence higher resolution. The reduction in MCP channel spacing has resulted in an increase in the number of MCP channels to over 6 million. Since the diameter of a halo from a point light source is a function of the photo cathode to MCP spacing and since this is reduced in the higher resolution tubes, the halo diameter is now smaller.
The specified life of Omnibus III tubes is 10,000 hours, five times as much as GEN 11 under the same operating conditions, and 2,500 hours more than the previous GEN Ill specification. The tube may now outlive the system, and performance will not be degraded over the service life of the system.
The AN/AVS-9 NVGs also now incorporate an auto-gating control [AGC] function (often referred to as an auto-gain function). The details of the implementation of the AGC function are not generally disclosed by the manufacturer; therefore, the AGC is considered as a black box of the NVGs. There are many possible methods of implementing an AGC, such as gating the photocathode proximity focusing voltage or controlling the microchannel plate current or voltage. Each method of implementation may affect other aspects of the NVG image such as halo and channel noise.
When the power supply is "auto-gated," it generally means the system is turning itself on and off at a very rapid rate. This, combined with a thin film attached to the microchannel plate (an ion barrier) reduces blooming. While "blooming" can be noticeably less on systems with a thin film layer, systems with thicker film layers can be perfectly acceptable depending on the end user's application. Deciding which night vision goggle is better should not be based solely on blooming.
European Gen 4 NVGs
Other nations, outside of the US, have continued to develop NVG technology. Recently a new NVG called the NL-94-AU was produced which does follow the original US NVG intent of doing away with the ion barrier film. Subsequent testing and evaluation of the new NVGs has revealed that these NVGs do produce an acceptable NVG image and are deemed as being suitably reliable, albeit not under the testing and approval of the US Army. The NL-94-AU NVGs are deemed, by European aviation agencies, as matching the Generation 4 description in terms of technology, reliability and performance. The US still do not recognise these NVGs as Generation 4 equivalent.
The other feature of the NL-94-AU NVGs is they too have an auto-gating function. The NVG image intensifier enables the user to see even more during a full 24-hour day/night operation. This is done by the use of a fully integrated Auto-Gating unit, which controls the image not only during day-night-day transitions but also during dynamic lighting conditions, e.g. night operations in urban areas. An integrated unit has automatic control over the gain and gating of this tube type when it becomes active at the higher light levels. In practice this means no blooming to hinder your mission but dependable imagery throughout. The NL-94-AU terminology often refers to this technology as IRIS which sees a system that control light through a controlled aperture system.
The NL-94-AU has a specified tube life of 15,000 hours.
Other Recent NVG Improvements
Other recent NVG improvements include incorporation of individual Inter-Pupulary Device (IPD) adjustments on each side of the pivot adjustment shelf (PAS), and 25mm eyepiece lenses. The 25mm eyepiece lenses will provide a 40 degree FOV over a greater range of eye reliefs as compared to current 18mm eyepiece lenses. The individual IPD adjustment capability should aid in centring the eyepieces directly in front of the user's eyes, thereby reducing accommodative eye fatigue and improving visual performance. Both the 25mm eyepiece lens and dual IPD adjustment PAS are currently being received via the supply system.
Lasers and NVGs
Laser Effects on Night Vision Devices
The proliferation of lasers, whether used as rangefinders, designators, flash blinders, or designated weaponry, poses a significant optical electro-optical threat to our night operations. Our reliance on high technology night systems that are sensitive to very low levels of reflected visible and near infrared light is jeopardised by a weapon, which can direct visible or infrared energy against us. Use of FLIR is also threatened by the introduction of directed energy weapons. Meeting the demands of hardening our night systems and protecting our aircrews with appropriate counter measures begins with a basic understanding of the laser, its strengths and limitations, and the depth of its potential threat.
The laser is an electro-optical device, which produces ultraviolet, visible, or infrared radiation by the process of controlled, stimulated emission. The acronym LASER stands for Light Amplification by the Stimulated Emission of Radiation. Lasers generate a collimated beam (ie. parallel rays) of intense, coherent (ie. the photons are in-phase), monochromatic light. The emitted beam can be either continuous wave (CW) or pulsed. A pulsed laser is usually more powerful and damaging than a CW because the energy is condensed into rapid, bullet-like pulses. These pulses are commonly in nanoseconds (10-9 seconds). The wavelength of energy emitted is determined by the lasing medium (eg. Ruby, Neodymium, etc.) which in turn determines its application, its effectiveness as a threat, and the protection required to counter its effects. Figure 10 shows the spectral location of the sensors we use at night, and the laser lines that pose an in-band threat to those sensors. Those lasers that emit energy in the same spectral region as the sensitivity of the sensor are in-band to the sensor, and require very little power or exposure duration to produce effects. Those lasers outside the sensitivity of the sensor are out-of-band, and usually require more power, or longer periods of exposure, to adversely affect the sensor.
FIGURE 10: Sensors and Lasers in the Electromagnetic Spectrum
Lasers and Night Vision
The human eye is sensitive to visible wavelengths between 0,40 and 0.70 microns. Common lasers within this band include the continuous wave green Argon (0.514 microns), the pulsed green Doubled Neodymium (0.532 microns), and the pulsed red Ruby (0.694 microns). All laser energy between 0.40 and 1.4 microns is absorbed by the retina and could produce retinal damage, including Neodymium: YAG (invisible at 1.064 microns). Out-of-band lasers (wavelengths shorter than 0.40 microns and longer than 1.4 microns) are absorbed by the front structures of the eye. Out-of-band lasers with sufficient power can produce thermal eye injuries. Laser eye protection must be specifically designed for the threat wavelength, and must have a high scotopic transmissivity; that is, sufficient light must be able to pass through the protection to allow safe night operations.
Lasers and Night Vision Goggles
ANVIS intensifier tubes are sensitive from 0.60 to 0.90 microns. Common lasers, which are in-band to the ANVIS, are Ruby and Gallium Arsenide (GaAS). Continuous wave laser in-band effects on ANVIS approximate that of conventional light. Tubes will de-gain, bloom, shutdown, and, in extreme cases, incur immediate damage. ANVIS does afford protection to the aircrew from the laser in that they will protect the sensitive central field of view of the eye. Since the ANVIS is not a direct view optic laser energy is absorbed mechanically within the intensifier tube and is not transmitted to the eye. Pulsed laser in-band effects on ANVIS may include rapid damage to the device without the pilot perceiving any warning (bloom, etc.). Out-of-band CW effects could be expected to produce effects similar to in-band effects, but will require much higher power levels and extended exposure times to do so. Pulsed lasers, with their associated power levels, can be expected to produce greater out-of band effects than continuous wave. There are, in fact, two types of protection with which we must concern ourselves:
Protection as it pertains to the user's eyes
Protection as it pertains to the NVG device
As previously mentioned, ANVIS provides an automatic degree of protection to the user in that his central field of view is protected. This protection is provided by the internal structure of the intensifier tube acting as a mechanical stop against the laser. There is no automatic provision however, to protect the aircrewman's peripheral vision. Laser spectacles that may fit behind the NVG and will provide this type of peripheral protection are currently being devised, but certain compatibility difficulties must be worked out before these find widespread use among aviators. Laser protection for the NVGs themselves is now available with the introduction of the Light Interference Filter (LIF).
Light Intensity Filters
The LIF is an out-of-band optical filter designed to protect ANVI S intensification tubes from laser damage. The LIF does not provide any additional protection for the human eye. By design, LIFs reduce the amount of light entering the NVG tubes. Tests of the LIF concluded that ANVIS performance may be reduced during low light level and low target/terrain contrast conditions (featureless desert, snow, etc.). During conditions of high target/terrain contrast, regardless of light level, the LIFs did not degrade ANVIS performance. The LIF will only mount to those ANVIS modified with a LIF sleeve (adaptor) over the objective lens assembly. The LIF sleeve, along with mounting instructions, is part of the LIF package when received. All ANVIS delivered under the Omnibus III contract include the new LIF. Existing ANVIS can be modified with either the original or new LIF.
Identifying NVG Faults
NVG's can suffer from unserviceabilities that must be recorded in the Operators Log Book. These include Shading, Edge Glow, Bright Spots, Emission Points and Flashing, Flickering or Intermittent Operation. These are described in the ANVIS AV/AVS-9 Operator's Manual or the NL-94-AU Users Manual as appropriate.
Dangers and Actions in the Event of Breakage of the Monocular Tubes
DO NOT inhale or swallow phosphor screen material. Induce vomiting and see doctor immediately. DO NOT allow phosphor material to come into contact with skin, mouth or open wound - wash immediately with soap and water.
NVG AF / BF servicing
The full explanation of NVG servicings can be found in the ANVIS AV/AVS-9 Operator's Manual or the NL-94-AU Users Manual as appropriate.
Mount and Adjust NVG for operation
The full explanation of how to mount and adjust the battery pack and NVG can be found in the ANVIS AV/AVS-9 Operator's Manual or the NL-94-AU Users Manual as appropriate.
As a brief review, the following comments are important to remember:
A thorough understanding of the principles of the operation of NVG's is important so that you as the operator can more effectively use this device in the field environment.
Use the ANVIS AV/AVS-9 Operator's Manual or the NL-94-AU Users Manual, as appropriate, as the reference when setting up you NVG, when conducting AF / BF and when identifying possible unservicabilities.