March - April 2005
Night Vision Maintenance and Support
NIGHT VISION GOGGLES
Modifying Your Cockpit Lighting
The Future 0f Night Vision Technology
Night Vision Maintenance and Support
By Lee A. Stephens
Hoffman Engineering Corporation
Though originally developed for ground use by the U.S. Army, night vision technology has advanced so rapidly over the last 20 years that today it is used in almost every facet of military, police, EMS, SAR and even commercial aviation operations. Today, missions that were once thought impossible are now nightly occurrences. This wide acceptance and use, along with diminishing budgets, has emphasized the need for proper maintenance and testing of night vision devices and even more thorough training.
The earliest Night Vision Devices (NVDs) were created in the 1940s and 1950s, but did not come into mainstream military use until the 1970s. Successful use of these devices by foot soldiers soon led to use by aviators. (Note: This article presumes the reader has a basic knowledge of Image Intensification (aka: I2). A good beginner’s guide and explanation video can be found at: www.ittnv.com/itt/Active/MilTopMenu/HowNVWorks
Using NVDs requires special training so that the user recognizes that even though his night sight is greatly improved, his depth perception and field of view are reduced. Mission safety depends on accurate "situational awareness," therefore proper training is critical.
If you have never viewed through night vision goggles, you can simulate their limitations for yourself. This evening, go into your backyard and look through two toilet paper tubes. Then try walking with them "on" for a short distance. Even areas familiar to you can become difficult to navigate.
Actual in-flight NVD training is optimal but expensive. As an alternative, on the ground training can be performed through a variety of methods including classroom, video and simulation. Most common is the use of a "terrain board" with special lighting to simulate different visual effects on common urban and geographical scenes. Terrain boards were originally used by the Marine Corps for night vision training but now are commonly used throughout the military and by private training schools with night vision curriculums.
The terrain board is one of the most economical NVD training tools in use today. With the introduction of computerized "Night Sky Simulation Projectors," instruction can be custom tailored to the course and the student’s level of experience. Further, accurately created nighttime scenes can be saved and repeated later for more consistent training and instruction.
Budget reductions and safety issues have mandated that equipment last longer and perform more reliably. Test equipment has been developed that allows for in the field testing of NVDs, including their electrical parameters and optical performance. Additionally, the ability to properly focus the NVD is a critical safety issue.
Within the scope of NVD training and use, the single most important issue is focus. Lack of proper NVD focus from the start compromises every aspect of your night vision mission. Let’s presume you have 20/20 vision (with or without correction). When wearing a pair of Aviator Class Goggles (AN/AVS-6, AN/AVS-9), your visual acuity will drop to 20/30 or less in most cases. Newer NVD types have shown better performance but this has been within controlled situations.
NVD focus historically has been conducted through the use of an "Eye-Lane," which is basically a darkened room the user enters. He or she then focuses his or her goggles on a resolution chart mounted on a wall at a distance of 20 feet. The drawbacks of using this method include: availability of a dedicated room, proper lighting upon the chart or "target" and focusing at 20 feet vs. the infinite night sky. Another method is having the user focus on a star or some geographical feature such as a sharp ridgeline off into the distance. Again, limitations occur, as the simulation cannot replicate weather or atmospheric conditions. And, as with the "Eye-Lane," it can be difficult to achieve the proper lighting.
Today, the most accurate method of NVD focus in the field is through the use of a device called the ANV-20/20 Infinity Focusing System. Originally designed for the USAF, this device allows the user to view into a specially designed system that presents a target to the user that is optically equivalent to infinity regarding focus. The ANV-20/20 is battery operated, contains a calibrated light source that emits a prescribed light level for optimum focus and is lightweight for field portability.
When the user looks into the device, a custom resolution target is displayed (as shown above) under very precise lighting conditions. The numbers within each box are correlated to the Snellen Visual Acuity System (ie: 20/70, 20/20, etc.). The procedure involves having the user adjust his goggle’s focus until he can determine visual separation in both the horizontal and vertical bars within the box defined as "70." Then, working from "70" to the lowest numbered box he can focus to, the procedure is repeated. The last value the user is able to focus to is his absolute visual acuity with those particular goggles.
The USAF has done extensive research on proper focusing techniques for particular NVD types. Contact me for resources in obtaining these documents.
NVDs, just like any other piece of equipment you use, require routine inspection and maintenance to ensure proper operation and mission safety. The most current piece of test equipment available for NVD maintenance is the ANV-126A Test Set. This device allows the user to perform all of the required electrical and optical inspections necessary to properly maintain and adjust all ground and aviator type goggles, including those that have filmless tubes and gated power supplies. A high brightness test is also incorporated to evaluate NVDs designed for use in urban environments.
In order for the NVD to work properly in a cockpit, the interior lighting must be made to be appropriate for use with night vision devices. The topic of aircraft Night Vision Lighting itself is larger than the scope of this article permits. A separate article within this issue addresses lighting and its importance to a successful night vision program.
Crew Station//Aircraft Lighting Inspection
Once lighting requirements for the aircraft have been determined and the lighting system installed, inspection of the cockpit and/or crew station is highly recommended and, in some cases, required. Keep in mind that the lighting products and NVIS (Night Vision Imaging System) compatible instruments, from component level to aircraft integration, have passed through many hands along the way. The simple act of over-tightening a bezel can crack an NVIS lens and render the instrument unsafe for flight.
Though never directly viewed by the goggle in flight, any lighting within a night vision compatible cockpit should appear to be off when viewed by a night vision device and visible clearly to the unaided eye. One might think they can simply use their existing NVD to perform this type of inspection, but this is a risky assumption. Many factors come into play such as NVD type used, Image Intensifier Class within the NVD and any associated filtering such as Laser Interference Filters( LIFs). A particular NVD may be highly desirable for the mission but a poor candidate for crew station inspection.
To the opposite extreme (but certainly the most accurate) is the use of a Laboratory Grade Radiometer, but these radiometers are extremely expensive and require the extraction of the instrument or display from the cockpit for testing.
The use of an NVIS Inspection Scope allows quick assessment as to the compatibility of the light with regards to the goggle. These devices are battery operated and are designed for ease of use within the crew station. Further, an internal reference source should be incorporated to provide the inspector with a variable comparative reference for appropriate light levels for given instrument and display types. Additional filters can also be utilized to inspect different lighting classes and even covert sources for exterior applications.
When an inspector views through the scope, he is able to see the instrument lighting in direct comparison to the calibrated reference source (at the left side of image.) Items appearing brighter than the reference indicate a failure with regards to prescribed light levels.
Cockpit and crew station design engineers who need to evaluate NVIS illuminated crew stations benefit greatly from the ability to test the lighting under different night sky conditions. A specially designed test system that utilizes custom lighting and resolution targets now allows the engineer to evaluate the crew station while on the ground in a darkened hangar.
These types of systems can be used by those who retrofit or upgrade helicopters for night vision compatibility in measuring a pilot’s resolution ability prior to and after the modification, thus verifying increased mission capability.
I have attempted to address the majority of the key components related to a comprehensive night vision program and by no means imply that these are the only ones. Each of the topics within this article could easily be an article on its own. Further, with new Electro-Optic/Thermal technologies under development (ENVG, ANVG, etc.), testing, maintenance and training will have to evolve as well.
As you can see, a successful night vision program does not begin with, nor end with, the acquisition of a set of goggles. Proper training, device maintenance and thorough overall system inspections are key elements in ensuring pilot, crewmember and public safety.
NOTE: The author wishes to thank the following organizations for their assistance and photos for this article: USAF Research Laboratory (Armstrong Labs), Mesa, AZ; Naval Air Warfare Center (NAWC-AD), Patuxent River, MD; USAF 56th OSS/OSKL – Wing Life Support – Luke AFB, AZ.
NIGHT VISION GOGGLES
Are They For You?
By Kevin P. Means
San Diego Police Department
In the last 15 years, I’ve seen the world of nighttime operations change dramatically. Forward Looking Infrared (FLIR) revolutionized our ability to search in dark environments, but FLIR wasn’t an answer to the risks associated with flying in dark environments. In fact, glare from the FLIR’s display made the pilot’s job even more difficult. Some of those flights were just downright scary.
The San Diego Police Air Support Unit started using Night Vision Goggles (NVGs) several years ago. There was no particular incident that moved us in that direction, the opportunity to acquire them just presented itself. We originally didn’t think they’d be of much use. Someone even thought that they were nothing more than a poor man’s FLIR, only to be used by the Tactical Flight Officer. We knew nothing about incompatible lighting, NVG limitations, training, etc.
Like FLIR, NVGs are military trickle-down technology. Originally designed for military applications, they have clear parallel benefits for airborne law enforcement operators. Most of the lessons learned about NVG operations were learned by Army aviators decades ago. Early generation NVGs (Generation 0 and 1) were contributing factors in several nighttime accidents. Their performance was relatively low, and their displays distorted the image, sometimes significantly.
Generation 2 goggles, like the early-model PVS-5s had better performance and much less optical distortion. New technology, a coin-sized microchannel plate, multiplies the number of electrons that strike it and thereby increasing the amount of optical information that can be processed. But the PVS-5s were bulky and reminiscent of one of those face-huggers on "Alien." Their fully enclosed design prevented a pilot from being able to clearly see the instrument panel. One former Army aviator told me that the only way he could interpret his instruments while wearing the PVS-5s was to memorize where the numbers were on the gauges and look for the fuzzy needle’s clock position. The gauges were so out of focus that there was no way to actually read them. Another attempt at solving this problem was to focus one of the tubes outside the cockpit at infinity, and the other on the instrument panel. I can only imagine the intensity of the headache at the end of the flight.
In years since, we’ve seen the introduction of Generation 3 goggles like the ANVIS (Aviators Night Vision Imaging Systems) 6s and 9s (F-4949s). Their performance is significantly better than the older technology, and their open design enables the pilot to see around the goggles to view the instruments. The manufacturers continue to get more performance out of the microchannel plate and the internal electronics of the goggle.
NVG Limitations and Misconceptions
An occasional problem associated with NVGs is a phenomenon known as "blooming." Blooming appears to the aviator as a halo around visible light sources. This can cause the goggles to gain-down, which means they have less performance. The internal circuitry of the goggle does this automatically to protect the intensifier tubes. When this happens, the surrounding terrain and obstructions that were barely visible may no longer be visible at all. And the halo around the offending light source can block the pilot’s view of obstructions near the light source. This is usually only a problem when flying low-level in very dark environments (such as when landing or taking off from an off-airport site). When operating low-level, the pilot’s view of the outside world is more horizontal, and with the exception of other aircraft, that’s where all the obstructions are.
Newer technology has significantly reduced blooming. The power supply (the battery pack mounted on the back of the pilot’s helmet) is "auto-gated," which means it‘s turning itself on and off at a very rapid rate. This, combined with a change in the thickness of a thin film attached to the microchannel plate (an ion barrier) reduces blooming. Some goggles have no film layer at all. I recently performed a side-by-side comparison of a new set of goggles with a thin film layer, and a new set of goggles with no film layer. Blooming was noticeably less on the goggles with no film layer. But in my opinion, the other goggles were perfectly acceptable. Deciding which goggle is better should not be based solely on blooming. Some say that by removing the film layer completely, the life of the intensifier tubes is shortened.
Another limitation of NVGs is their field of view. The Generation 3 goggles available today are limited to 40 degrees. Stare through a set of toilet paper rolls for a few minutes and you’ll get a pretty good idea of what 40 degrees is. It’s not quite that bad, but you’ll get the point. It’s important to understand that when you’re using NVGs you’re not looking through them, you’re looking at them. After the image is intensified, it’s displayed on two phosphorus screens – essentially TV screens right in front of your eyes. Some pilots initially find this view of the outside world somewhat claustrophobic, but training and good scanning procedures usually alleviate this.
I’ve also heard some say that flying with NVGs is comparable to instrument flight on the scale of VFR and IMC conditions. They argue that pilots should have an instrument rating to fly with NVGs because they could find themselves in environments that are so dark that NVGs cannot provide them with enough visual cues to maintain control of the aircraft. Or, they’d have to resort to instrument flight if the goggles failed while flying in very dark environments. I think an instrument rating is a good thing to have, but that argument already exists for pilots flying without NVGs.
I acknowledge (and in no way minimize) the fact that inadvertent flight into IMC conditions is a killer, whether those conditions were caused by clouds or darkness. But you address this issue in the same manner that you address it for pilots who fly at night without NVGs, through training. You teach them to recognize the conditions that might be too dark or too hazardous for NVG operations. Try flying over a large, calm body of water or over the desert on an overcast night, when there is no moon or cultural lighting (reflected lighting) and you’ll see what I mean. (Actually, don’t try it, just take my word for it.) NVGs have limitations, and pilots need to learn about them before they fly with goggles.
Some people believe that there is no depth perception when using NVGs. That’s simply not true. There is depth perception – but there is no three-dimensional viewing. There is a difference. Your brain interprets the two-dimensional images displayed by the goggles and perceives depth based on visual cues. But there are some optical illusions that can throw you a curve. For instance, it can be extremely difficult, and sometimes impossible to judge your distance from certain light sources. The intensity of the light, its frequency (color) and its focal direction can make some light sources look farther away, or closer than they actually are. It’s very deceiving and can be a problem when trying to judge your distance from other aircraft, towers etc. There are other examples but the point is, pilots must learn about these things in training.
Training and Recurrent Training
Like all other flight technology, you need to know how to use your equipment to be safe and effective. If you’re contemplating the use of NVGs in your operation, you should consider initial and recurrent training mandatory, and you should get this training from a reputable, experienced entity.
Just because someone flew with goggles in the military many years ago, doesn’t mean that they’re automatically qualified to teach NVG operations. You should adopt strict policies and procedures for NVG operations as well as minimum flight requirements to ensure that pilots are proficient with NVGs, not just current.
Cockpit lighting has been a source of headaches for NVG operators from day-one. Incompatible lighting generated from inside the cockpit (backlit gauges, radio displays, master-caution lights etc.) can cause the goggles to gain-down and perform even worse than external light sources can.
Nothing gets your attention more than a red, radar-altimeter altitude-warning light going off in your face just as you’re about to land in a very dark environment. Until recently, the lighting for most radios and instruments was not designed to be compatible with NVGs. But the market is responding to our needs and you can get just about everything you need installed with NVG compatible lighting.
The Legalities of NVG Operations
Part 135 operators can apply for an STC to use NVGs – many EMS operators have. But the FAA has not authorized NVGs for Part 91. As a member of the Radio Technical Commission for Aeronautics committee (RTCA Committee) I have been involved in the process of creating the initial draft of rules and regulations for NVG operations for Parts 91 and 135. So I have some insight into the FAA’s mentality. I’ll tell you right now that the FAA simply does not want Joe Blow flying around in his Cessna 150 with NVGs under Part 91. But the problem is – most airborne law enforcement operators also operate under Part 91 – even though we’re Public Aircraft. So what’s the bottom line?
Technically, the Pilot in Command can only fly with NVGs on Public Aircraft missions. That is – you can use NVGs but you cannot have any passengers on board whose presence is not required or associated with the mission. So if your friend is in the backseat on a ride-along, you’re not supposed to use NVGs because you lose your Public Aircraft status when passengers are on board. If your friend is a paramedic, and you’re responding to an injured hiker call, you can use NVGs if your paramedic-friend is going to provide medical assistance. Search and Rescue missions are Public Aircraft missions and your friend’s presence is now associated with the mission. I spoke with my local FSDO and they told me that they consider law enforcement personnel in the back seat to be non-flying crew members who are there for a reason – a familiarization flight. Under that premise, you could legally use NVGs. But do yourself a favor and check with your FSDO first. We all know that there is a long history of FSDOs interpreting Public Aircraft missions differently.
I could go on and on about NVG operations, but there’s far too much information than can be addressed in one article. I want to emphasize this – I have worked nights for many, many years because that’s where most of the activity is, and it’s still fun. I put my NVGs on as soon as I see the glow from the setting sun disappear and I don’t take them off until I’m on the ground. I wear them over a well lit city and in very dark mountains, and I’ve become very comfortable with them. If I have to land at an unfamiliar or familiar off-airport site, or if my engine quits at night – I have a lot more options available to me simply because I can see better.
I still hear some people use the early accident history of NVGs as a reason for not acquiring them. In 1910, I’m sure that most people felt the same way about airplanes too. But the world has moved on and the night vision technology available today is far superior to what was used in the 70’s and 80’s. Are they a panacea for nighttime operations? No – but they’re a big step in that direction.
Modifying Your Cockpit Lighting for Night Vision
By Richard Borkowski
The introduction of night vision goggles into civilian aviation imposes additional demands not only on the pilot but on the aircraft itself. In addition to the expense of procuring goggles, the pilot is required to obtain specialized training in goggle utilization. The goggles, although greatly beneficial to the pilot in enhancing his night vision capabilities, do require that the cockpit instruments and displays be modified in order to be compatible with goggle usage.
During the past few years, the Federal Aviation Agency (FAA) has been acutely aware of the need for regulations governing the use of goggles and the peripherals, including cockpit illumination. In order to ensure goggles safety by qualified users, the FAA has issued a policy statement in Change 16 to Order 8300.10, Safety Inspector’s Handbook, and in Volume 2, Chapter 1 of the Advisory Circular. In essence, the FAA requires a Supplementary Type Certificate (STC) for "flight deck lighting changes to support night vision goggle use or any approval related to night vision goggles."
In order for the cockpit lighting to be compatible with goggles, the illumination of the cockpit’s instruments and displays will require modification. The modification dictates that the inherent infrared associated with the illumination be eliminated, leaving only visible light available to the pilot or flight crew. This elimination of the infrared permits the goggles to acquire miniscule amounts of infrared available outside of the cockpit. The infrared available from the night sky added by the goggles’ gain control amplification allows for maximum visibility in a nighttime environment.
The elimination of the infrared within the cockpit can be accomplished in a number of ways.
Current methods of modification include the use of "open ring" bezels, and the addition of adjunctive lighting systems that may include flood lighting, postlight modification and the modification of the instrument or display itself. Some of these methods require that the cockpit’s integral illumination be eliminated so these secondary lighting alternatives can be effective. Most of these secondary lighting alternatives filter the light source with an NVIS Green filter that not only retards the infrared but changes the lighting color to green. Where postlights are utilized, instruments traditionally are without inherent lighting, so the postlights are thus utilized to provide reflective illumination. In this instance, the postlight can be modified with a filter that retards the infrared but also permits the choice of green or white lighting.
The method of choice is to filter the lighting of the instrument. Currently, there are filters, such as SHADOWS™, that permit modification of the instrument's lighting by removing the instrument's cover glass and replacing it with an infrared retardant filter. This modification process is gradually being accepted by the FAA through the use of "process specifications". There is such a vast assortment of instruments of varying designs and manufacturers that the acquisition of "process specifications" for all instruments becomes a long and arduous task.
An alternative used with great success during the past year in achieving a number of STCs for law enforcement and EMS operators is the use of external filters (SHADOWS SCREENS™) for both instruments and displays. For flight and engine instruments, infrared retardant filters constructed of glass or poly carbon can be employed. The filter selection is predicated on the light source, whether incandescent, florescent, LCD or LED. The filters can be mounted in an aluminum frame and affixed over the instrument’s bezel utilizing the existing mounting screws. The frame for the filter is approximately 0.100" thick, and when placed in position, minimizes any reduction in the cone of visibility. Mechanical or electrical modification to the cockpit or airframe is not required, thus eliminating costly installation charges. In addition, the external filter approach allows the instrument to remain generic, thereby eliminating the need for a special maintenance program and retaining the advantages of local repair and/or overhaul.
A distinct advantage of the infrared retardant filter is the utilization of the instrument's inherent white lighting. Retaining the white lighting eliminates the need for repainting dials or warning flags that are generally affected by the saturated green illumination of the NVIS Green filters. In addition, the transmission of visible lighting with the infrared filters is two and half times greater than transmission with green filtered lighting. This advantage is particularly noticeable when a pilot or crew comes off of the goggles to view the instruments. The transmission advantage of the infrared retardant filter permits immediate recognition of the instrument's function without any adjustment of pupil dilation which often occurs with the lower transmission of green lighted instruments.
Those considering the use of infrared retardant filters as the modification method should be forewarned that there are a number of infrared filters available for instrument modification and all of them exhibit some degree of a light blue tint. Some of these filters have a deeper blue coloring than others. The deeper the color, the less transmission is available and the day time visibility can be greatly diminished.
Today, many cockpits include not only electro-mechanical instruments but also solid-state displays. Regardless of the light source of these displays - CRT or active matrix LCDs - a filter solution is available. In numerous instances, the filter is again mounted in a frame and placed directly over the display’s surface. The lighting contractor should provide modification options for consideration. It is imperative that the fit of the frame is such that light leaks from the display are eliminated.
Consulting with your lighting contractor is an important aspect of any modification project. He may offer suggestions based on his previous installation experience so that other options can be discussed and decided upon. These options may include placing and sealing the filter directly to the display’s surface with optical adhesive or by possibly replacing the LRUs existing Polaroid filter with one specifically designed as a compatible replacement. Many of the NAV/COMM displays are modified by replacing the original filter with an infrared retardant filter. It has been found that double filtering (infrared filter over an existing filter provided by the OEM) does not meet the sunlight readability criteria required by the FAA. Replacing the existing filter allows for the modified LRU to be night vision goggle compatibile as well as sunlight readable.
If new or updated accessories are being added to your cockpit, check with your lighting contractor or with the manufacturer of the equipment/system in question to determine if an off-the- shelf version is already available NVG compatible. Many system manufacturers are considering NVG compatible lighting versions of their equipment due to the increasing demands of the military, law enforcement, and EMS operators. An example of a LRU incorporating NVG compatibility is the Avalex monitor. The Avalex monitor incorporates NVG compatibility off the shelf, saving the aircraft user the added expense of having the modification accomplished after the fact.
An area of the modification process that has yet to be clarified for the civilian aviation market is how military specifications for NVG/ANVIS compatibility come into play. If we believe that truth is a moving thing, we must consider that the mil specs that dominate our NVG thinking are good guides, but only that, good guides. In a number of instances, the mil specs provide modification latitude that is acceptable criteria for military programs but are somewhat restrictive for civilian lighting upgrades. Today’s filter and goggle technology has advanced to a point where we are now able to accomplish upgrades that were unheard of only a few years ago or after the mil specs were made public.
The largest area of contention between the dedicated use of the mil specs and today’s technology deals with the filtering of illuminated pushbutton switches and master caution/warn displays. The mil specs have recommended chromaticity coordinates that permit diluted color intensities for the aviation red and amber (yellow) colors. The mil spec color coordinates provides a peach or watermelon color for red and orange/lime color for yellow. Today, there are available various modification methods that allow the maintenance of the original red/red and amber/amber colors that are the mainstay for universal caution and warning alert. In addition, the traditional colors provide superior sunlight readability over the mil spec colors.
Many of the latest modifications have included the need for illuminated panels. Panel upgrades are generally determined by their function and location, with a final determination being made by the FAA for that particular airframe model. Should the panels require NVG compatibility, modification is available in green and white lighting. Consult with your lighting contractor to determine if new panels are required or if the existing panels can be modified. Cost and lead time become an important factors.
It should be remembered that modifications to NVG compatible lighting can be an expensive proposition. To control the costs and to establish exactly what you are contracting for, it is important that you consider the following:
Is there an STC currently available for my particular model airframe? If not, what are the ramifications?
Where is the modification accomplished? Can it be done locally?
How long will it take to accomplish the modification?
Who and where are the various lighting contractors located?
Are examples of their work available and who are their customers?
Is specialized aftermarket support required?
When soliciting a quotation for the NVG lighting modification, the lighting contractor should be totally aware of your cockpit configuration. A survey visit by the lighting contractor often pays dividends in the long run. Digital photos are a great aid in cockpit asset accountability and work towards eliminating any cost increase surprises. In return, the lighting contractor should provide you with an itemized listing of the requirements for modification, not just a total lump sum cost, so that you will be aware of exactly what you are paying for. Remember, it’s your money and your airframe. Be an educated consumer, and don’t be afraid to ask questions.
The Future 0f Night Vision Technology
By Joseph C. (Chuck) Antonio, MD, Aerospace Medicine
Night Vision Imaging Systems Pilot Instructor
National Test Pilot School
The FAA will soon provide a new certification process for approving the use of NVGs in civil aircraft. In the meantime, those that have received approval through the Supplemental Type Certificate (STC) process can continue to appreciate the advantages NVGs provide during night operations.
Probably the first technological advancement for civil applications will be upgrades to current NVGs. Upgrades will likely address improvements in the following areas: 1) resolution (image quality), particularly during low illumination conditions, 2) reduced size of halos surrounding intensified light sources, improving image quality when viewing urban areas, and 3) increased field-of-view (FOV) of the image. Additionally, newly designed image intensifiers may result in smaller and lighter NVG binocular systems, which may be necessary to support an increase in the FOV.
The next major upgrade in technology will likely be the introduction of helmet-mounted displays (HMDs). At a minimum, for nighttime operations, these will likely include image intensifiers integrated into the helmet. To get the night vision image to the user’s eyes, the intensifiers will be coupled to optics that transmit the image to a projector, which then projects the image onto either the visor or combiners. This is called an "optically coupled" system. A properly designed and constructed HMD should help reduce the adverse forward center-of-gravity (CG) present when wearing NVGs, thus reducing physical fatigue.
Another technology of interest is the Night Vision Camera (NVC), which can be integrated into an HMD instead of an image intensifier. An NVC uses a photosensitive charged couple device (CCD) as the sensor and a miniature display for presenting the image. When light strikes a CCD, it is changed into electrons that are then used to form an image (as in digital cameras). However, current CCDs are limited in sensitivity and the resulting image at night is not nearly as good as that provided by image intensifiers. To help improve the nighttime performance of CCDs, image intensifiers can be coupled with them, thus increasing the amount of light striking the CCD and improving image performance. Since CCDs result in a flow of electrons, this is called an "electronically coupled" system. Another method used to help improve image quality in electronically coupled systems entails the development of digital algorithms that will help offset, but not completely eliminate, some of the losses in image quality noted with these systems.
Nevertheless, even with digital enhancements, electronically coupled NVC systems cannot provide images as good as those provided by current NVGs. However, research continues in this area.
Another benefit of using NVCs in an HMD is the ability to digitally integrate heads-up flight symbology with the image. To accomplish this most effectively, HMDs may also incorporate a technology known as head tracking. Head tracking can be accomplished via a number of different methods, but the general purpose is to provide the user’s head position information to a computer.
The computer can then take current aircraft position and flight information and project it heads-up onto the HMD visor or combiners. Knowing the head position will allow correct presentation of the artificial horizon (i.e., correctly aligning with the real horizon) relative to the line of sight, for example when viewing to the left or right of the aircraft’s direction of flight. For this to function best, an aircraft databus is required so the information can be used in digital format (necessary when using a projection system). There are other potential advantages with a head-tracked system that are "operation specific," but they will not be addressed in this article. It should be noted that either analog or digital flight information such as altitude or airspeed could be presented in an HMD or even an NVG system without head tracking. However, the incorporation of an artificial horizon should not be included unless it correctly aligns with the actual horizon due to the real potential of spatial disorientation when viewing away from the direction of flight. It is also important to understand that flight information could also be presented heads-up during the daytime.
Forward-looking infrared (FLIR) devices are currently being used extensively at nighttime as an aid to viewing the outside scene. Since FLIR systems operate in a different spectrum of energy than do NVGs (3-5 or 8-12 microns for FLIR systems versus 0.6 to 0.9 microns for NVGs), they present imagery that is different than, but complementary to, intensified imagery. This results in imagery that is useful over a broader range of environmental conditions. As of now, these systems are not used specifically for pilotage (i.e., using the imagery to fly and navigate) because the imagery does not correctly overlay the real world and is not presented to the user heads-up – as is the NVG image. However, since the visual information is transformed to a digital format, FLIR imagery could in the future be projected onto an HMD visor or combiner. In order to make this information suitable for heads-up presentation, it will be necessary to accomplish two very complicated tasks: 1) modify the image so that it represents the real world from the pilot’s viewpoint (necessary since the FLIR is located away from the pilots eye position) and 2) determine the pilot’s head position via a very accurate head-tracking system. Effectively accomplishing these tasks requires the derivation of complicated digital algorithms and computers capable of crunching very large databases extremely rapidly. Consequently, the cost of this capability may far exceed the budgets of many civilian operators.
Should the incorporation of FLIR imagery in an HMD be desirable, there are a few other things to consider. If the HMD includes intensified imagery, there needs to be a smooth and simple way of selecting between the two image sources. In the future, this may be accomplished by digital fusion of the FLIR and intensified images. Fusion of digital imagery from various sources has been researched for a number of years, but it is an extremely complicated task to accomplish – from a technical perspective as well as from the perspective of the user. Technically, it will be necessary to fuse imagery from various sources that are not co-located and to accomplish it as close to real time as possible. Time is required to accomplish all of the computer manipulations, and time is required for the head tracking system to determine head position, both of which can lead to the image being presented to the user at a time-difference (or lag) from the real world (the time lag is also called latency). From the standpoint of the user, the image presented must be useable (i.e., recognizable, easily decipherable and accurate). Determining what aspects of each sensor should be presented at what time and how to present the information are subjects of ongoing research. All of this requires even more complicated digital algorithms and much faster computers than are required for standard head-tracked HMD systems using imagery from only one sensor.
Regardless of the level of sophistication of night vision systems introduced in the future, it will be vitally important to keep in mind how to effectively and safely train operators in their use. For example, it is not a good assumption to believe that an HMD can simply be inserted into an existing NVG training profile. The visual imagery presented via an HMD will have additional constraints and differences (e.g., viewing an image through a see-through visor), particularly if flight symbology is added (head-tracked or not). Also, if heads-up symbology is presented, training on its use will need to be added to the curriculum.
It will likely be years before many of the previously described technologies make there way into the civilian market, but the descriptions are not inclusive of all technologies, and variations could be introduced sooner via other methods such as synthetic vision or eye-tracking systems.
Additionally, there is ongoing research to develop symbology designs that will help reduce spatial disorientation and improve situational awareness (such as "highway in the sky"), and some of these could be included in future HMD symbology sets.
In order to help pave the way for the introduction of advanced night vision systems, it is always useful to know the difference between the "nice-to-haves" and what is really needed to safely and effectively accomplish nighttime operations. Keeping this in mind when defining requirements will help orient future research and development in the right direction.