On a moonless night over the desert, the world outside a fighter canopy is black. No horizon, no terrain, no other aircraft, only the glow of the instruments. Then the pilot flips two small tubes down in front of their visor, and the landscape reappears in shades of green: ridgelines, roads, the wingman's jet a mile away.

Night vision goggles, NVGs for short, turned the night from something military aviators survived into something they could exploit. Here is how the tubes work, what they cost the pilot in return, and why the cockpit had to be relit to make them usable.

Turning Starlight Into a Picture

An NVG does not see heat; that is a thermal imager. It collects whatever light there is, from the moon, the stars, a distant town, and multiplies it. Light hits a photocathode at the front of the tube and knocks out electrons. Those electrons are multiplied many times over in a microchannel plate, then strike a phosphor screen that glows. The pilot looks at that screen through an eyepiece.

The result is monochrome, and traditionally green because green was considered the easiest colour for the eye to watch for long periods. Third-generation tubes, introduced in the late 1980s, use a gallium arsenide photocathode for much better resolution, and are the standard in military aviation today.

From Ground Goggles to ANVIS

The first goggles flown by US Army aviators were adapted from equipment built for soldiers on the ground. The AN/PVS-5, the Army's mainstay for aviation from 1972 until 1990, had a limited field of view, poor light amplification and excessive weight, and pilots could not read their maps through it. Testing for a dedicated aviation replacement began in 1982, and in 1989 the Army adopted the AN/AVS-6 Aviator's Night Vision Imaging System, or ANVIS, the first goggle it used that was designed specifically for flying.

ANVIS-style goggles hang from a mount on the front of the helmet and can be flipped up out of the way. Current aviator models such as the AN/AVS-9 are widely listed with a field of view of about 40 degrees, a tunnel compared with natural vision, so NVG pilots learn to scan constantly by moving their heads.

Looking Under the Goggles

There is a trick to flying on goggles that surprises people. Pilots do not read their instruments through the tubes. The goggles sit slightly in front of the eyes, and the pilot glances underneath them at the cockpit displays with the naked eye, then back up through the tubes at the world outside.

That only works if the cockpit lighting cooperates. Ordinary cockpit lights are bright enough to swamp an image intensifier, so NVG-equipped aircraft use specially filtered lighting that the goggles barely respond to. Preparing a cockpit for NVGs is therefore not just a helmet job; every light source, from the displays to the warning lamps, has to be made compatible.

What the Goggles Take Away

NVGs give a pilot the night, but not the day. The image is grainy and monochrome, the field of view is narrow, and judging distance without colour or a wide view is harder. Bright lights, a town or a burning target, can bloom and wash out the picture. Extra weight on the front of the helmet adds strain on the neck, which matters in a fighter pulling g.

That is why NVG flying is a qualification in itself, trained first in simulators and then in the air. When Lockheed Martin added NVG capability to the USAF's F-16 Mission Training Centers in 2004, the company framed it as essential rather than optional.

Helicopters, Tiltrotors and Beyond

Helicopter crews arguably depend on goggles even more than fighter pilots, because they fly low, land in unprepared spots and hover in the dark. Medical and search-and-rescue operators, military and civilian, now fly goggle-equipped missions routinely.

The newest fighters are moving the image off separate tubes and into the helmet itself. The F-35's helmet-mounted display projects night imagery straight onto the visor, doing away with clip-on goggles. But across most of the world's air forces, the familiar pair of tubes flipped down in front of the eyes is still how pilots own the night.

Sources: Wikipedia (Night-vision device; AN/PVS-5), Lockheed Martin press release of 3 December 2004, U.S. Air Force and U.S. Navy image records, C.W. Lemoine (video).