Night Vision Goggles in Civilian Aviation

by G. J. Salazar, M.D. and Van B. Nakagawara, O.D.
Article reprinted with permission of FAA Aviation News

Night vision goggles? Aren't they for the military and police? Not anymore! On January 29, 1999, the FAA issued the first Supplemental Type Certificate (STC) to permit use of night vision goggles by a civilian helicopter EMS (emergency medical service) operator. Since then several more have been issued to other commercial operators. In addition, rulemaking was initiated (but at the time of this writing is temporarily on hold) for changes to FAR Part 91 that would permit use of this technology by general aviation pilots. With this in mind, it will only be a matter of time before pilots start hearing more and more about these significant aids to night flying. Therefore, it is important for pilots to become aware of this technology and understand some of the basic operational issues.

NIGHT VISION GOGGLES

Night vision devices include a variety of different technologies, such as forward-looking infrared radar (FLIR) and night vision goggles. The focus of this article will be on night vision goggles, more commonly known by the acronym NVG. The simplest analogy to explain how NVG's work is a video camera. The basic principle is the same in that the user is not directly seeing what they look at, but rather is viewing an electronic image of the scene.

NVG equipment may be monocular or binocular. However, in aviation, binocular, helmet-mounted equipment is almost exclusively used. Like a video camera, an NVG is an electro-optical device. Electromagnetic energy, both visible and infrared, reflected from the terrain at night enters the NVG through the objective lens. These photons of light energy are directed to an electronic processing unit called the image intensifier, which contains several components. The photocathode element in the image intensifier converts the light photons to electrons and moves them to the microchannel plate (MCP) which accelerates and multiplies them several thousand times. The electrons then strike the phosphor screen, which is ultimately responsible for emitting the visible light the user will see through the eyepiece lens as a focused image.

Unlike the video camera, the NVG does not require much light to produce an image. Light as faint as a starlight or low-level moonlight will suffice. However, the efficiency of the equipment will be degraded in total darkness or with too much light. The image intensifier will increase what little light energy there is on average several thousand times. State-of-the-art NVG's are capable of intensification on the order of 35,000 times or more. That amplified or intensified energy is projected onto the phosphor screen, which creates the visible image the user-sees through the eyepieces. The NVG image is monochrome, i.e., in one color, typically either green or amber depending on the type of phosphor used. NVG equipment lacks the ability to produce a multi-color representation of a scene.

Aviation NVG models are helmet-mounted with electrical power supplied by a battery pack attached to the back of the helmet. As with any optical device, the user has a variety of ways of adjusting fit and focus. The NVG binoculars and mounting assembly are cumbersome, weighing approximately one pound. In addition, one must factor in the weight of the helmet and battery pack.

ADVANTAGE OF NVG's

The advantages of this night vision aid technology in aviation can be summed up as an increase in nighttime situational awareness for pilots. This technology does not turn night into day, but it does permit the user to see objects that normally would not be seen by the unaided eye. This would markedly decrease the possibility of collisions with terrain or man-made obstructions. Many other benefits exist, but the bottom line is that this technology, when properly used, has the potential to significantly increase nighttime flying safety.

DISADVANTGAGES OF NVG's

Unfortunately, this increase in safety comes with a significant price. Some of the disadvantages of NVG's include:

• decreased field of aided view
  
• decreased visual acuity
  
• loss of depth perception
  
• lack of color discrimination
  
• neck strain and fatigue
  
• high initial cost to purchase
  
• require on-going maintenance
  
• need for recurrent training
  
• requires modification of aircraft lighting
  

Current NVG's provide approximately 40 to 60 degrees of aided nighttime circular field of vision, although the user retains some unaided vision by being able to look peripherally around or under the goggles. With a reduced field of vision, effective scanning techniques are even more important than with unaided vision alone. Because one is looking at an electronic image, depth perception is lost. The user must learn to recognize terrain contrast and shadowing to replace some of the lost depth perception cues. Thus, the ability of the pilot to determine precise closure on terrain or other aircraft when these are first detected is limited.

Low-light level operations inherently produce decreased visual resolution, acuity, and contrast, thereby making hazard detection more difficult. Visual acuity from NVG devices provides a vast improvement over unaided human night vision, which can be 20/200 or worse. With properly focused goggles at starlight or quarter moon, one can have nighttime visual acuity equivalent to 20/40 or 20/30. The latest generation of goggles can achieve 20/25; however, this is difficult to accomplish in an operational setting. Enhanced vision with NVG's is proportional to altitude and airspeed. With NVG's, "lower and slower" improves visual acuity. Therefore, a helicopter pilot would have some advantage over his or her fixed-wing counterpart in determining terrain features in low light conditions. In addition, newer generation equipment provides greater contrast detection, thereby improving situational awareness. It is important to note that NVG-aided acuity of 20/30 or 20/40 assumes proper cockpit lighting, properly focused and well-maintained goggles, and ideal environmental conditions.

As mentioned previously NVG's produce monochrome images. Because the eye can differentiate more shades of green than other phosphor colors, the night vision phosphor screen is typically green. This allows the user to see more detail, but with an inability to detect differences in color. Changing illumination can affect visual acuity. External incompatible light from the ambient environment could result in "washout" or halo effects, when using NVG's. This could result in glare, flash blindness, and afterimage for the pilot. Particularly troublesome is ensuring aircraft and cockpit lights are NVG-compatible. Incompatible lights make the outside scene less visible with NVG's. Changing cockpit lights to be NVG compatible is very complicated and expensive. NVG's are sensitive to light ranging from yellow-green to near-infrared wavelengths. FAA required aircraft position and anti-collision lights could cause problems for goggle wearers. NVG's are also subject to interference by environmental factors, such as rain, clouds, snow, mist, dust, smoke, and fog. In anything more than very small amounts, any of these will tend to severely degrade the performance of the equipment.

During prolonged use of helmet-mounted NVG devices, the potential for neck discomfort and other problems, such as increased general fatigue, exists because of the weight of the helmet, battery pack, and NVG device.

CONCLUSION

In summary, while NVG and other night vision technology are potentially great safety enhancements for select nighttime flight operations, they are an expensive and sophisticated pieces of equipment requiring considerable effort to implement and maintain. Night vision goggles do not turn night into day and if not properly used, rather than preventing accidents they could be the cause of one. Operational use of these devices should be accomplished only after pilots have received extensive, supervised ground and in-flight training with the equipment. Once trained pilots must strive to maintain proficiency by ongoing use and recurrent training.

G. J. Salazar, M.D. is the Regional Flight Surgeon in FAA Southwest Region, Fort Worth, Texas. Van B. Nakagawara, O.D., is a Research Optometrist at FAA's Civil Aeromedical Institute in Oklahoma City, Oklahoma.