The Human Eye and Night Vision Goggles

Introduction

One of the most common causes of fatal helicopter accidents is Controlled Flight Into Terrain (CFIT) at night. (FlightWeb). The major contributor to such accidents is the pilot's level of situational awareness, or rather - not being able to see where they are going, or what might be in the way of unobstructed flight. As helicopters are routinely used in low-altitude missions - they are frequently required to fly in close proximity to trees, power lines, telephone poles, etc. As such, it is vitally important for the pilot to be able to see and avoid these hazards to navigation. During daylight hours, these hazards pose a minimal risk to flight. However, as the sun sets and the amount of visible light wanes, these same hazards become life threatening.

The reliance on night vision technology has become invaluable in assisting aviators in flying at night. By magnifying the ambient light, Night Vision Goggles (NVG's) and devices reduce the problems associated with unaided vision in the dark.

The Human Eye and Night Vision Goggles

Vision is a physical sense that uses light to provide information to the brain in order to make conclusions about the surroundings. The human eye is an intricate organ that works best when there is an adequate supply of light. Our eyes have been often compared to cameras in that a camera filters light through the lens, controls the intensity with an aperture and focuses the image onto a film. Light coming into an eye is filtered through the cornea, the iris regulates how much light comes through, and then it is focused by the lens onto the retina.

During the day, we are able to detect color, depth perception, distance and visual acuity. At night, all of those senses are greatly diminished. Visual clarity can be reduced to 20/200 or greater at night. The sensitivity of the retina (where images are focused) is greatly hindered when the light levels drop.

On a clear night (with a full moon and no clouds), the unaided eye can make out details of the surrounding terrain from approximately 100 feet in the air. With NVG's, everything on the ground can be clearly seen from an altitude of 1,500 feet above ground, and up.

In using the same camera analogy in describing the eyes, we can use that of a video camera to describe how Night Vision Goggles (NVG's) operate. When we use a video camera, we are not looking at an image directly, but are viewing an electronic image displayed on a screen. The NVG senses ambient light (sometimes as faint as starlight or low-level moonlight), processes it through an image intensifier called a photocathode, magnifies the intensity of the light some 1,000 to 3,000 times, and then projects the image onto a phosphor screen that in turn creates a monochrome, video-like image, which the user sees through the eyepieces. The end results of the processing is a concentrated view into the dark.

NVG equipment comes in mainly two different forms - monocular or binocular. Their shapes and sizes vary greatly. However, a helmet-mounted binocular form is almost exclusively used for aviation applications.

Limitations and Challenges

Despite the success of increasing the ability to see clearly at night, NVG devices do have their limitations. While we have made great strides in improving the technology, it is still quite bulky and adds weight to a pilot's helmet. A typical set of NVG binoculars and mounting assembly weigh close to one pound--which does not include the weight of the helmet and battery pack. This extra weight can cause pain and fatigue with pilots that have any prior spine or vertebrae injuries.

There are also problems associated with visual fatigue due to the restricted field of view, problems with focusing the eye, difficulties with binocular rivalry (how the brain reacts when a different image is simultaneously presented to each eye), difficulty with depth perception and estimating distances and slopes of terrain, and the effects of ambient illumination levels.

A particularly troublesome challenge with utilizing NVG's is that the aircraft and cockpit lights must be NVG compatible,