NVGs are only one part of an entire Night Vision Imaging System (NVIS). The system comprises an FAA-approved NVG-compatible lighting system, properly trained flight and maintenance crews, and an FAA-accepted maintenance program for the aircraft NVIS lighting system and the NVGs.

The ability to see during hours of darkness is vital to many specific areas in the aviation industry. Whether it’s HEMS, SAR, or even law enforcement / the military, a lot of it wouldn’t be possible without this remarkable feat of engineering: letting humans see at night.

But how do pilots use NVG, and why does it require training to use them properly?

The Basics of Night Vision Goggles

First, NVGs do not allow for vision in pure darkness. They are light amplifiers. They need a small amount of light to turn into something usable for our very limited naked eyes.

They cannot do their job if no light is available to amplify. Nonetheless; even the smallest amount of light can be the difference between useless and useful.

So, their function is to collect as much light as possible and then amplify it. But how do we amplify light? Any other scopes, like telescopes, microscopes, and binoculars, can help with light and focus but don’t amplify light.

So what’s the solution? Electricity!

You might wonder what light has to do with electricity. It’s not much alone, but we can link the two, which is exactly what NVGs do. Get light – turn it to electricity – amplify electricity – convert it back to (more intense) light.

The NVG Process: How Do Pilots Use NVG?

Let us keep things simple and divide the process of what NVGs do into five steps:

1. Dim light enters the NVG tube lens
2. Light enters the photocathode, which turns it into electricity
3. Electricity becomes amplified by the photomultiplier
4. Electricity hits a phosphor screen, turning it into light
5. We see the enhanced image through a fiber-optic inverter

1. Dim Light Reaches the NVG Tube Lens

First, the NVG must collect as much light as possible using an objective lens. How much light depends entirely on the conditions under which we fly.

A desert with a layer of clouds will be challenging as there is little light to work with. The flight crew is always briefed on the availability of light before startup.

This includes moonlight, starlight, cultural light, time of year, time of day, etc. There are multiple models to forecast this, indicating a certain amount of Lux (or millilux) throughout the night, representing the intensity of illumination in lumen per square meter.

2) Light (Photon) Reaches the Photocathode

This light then strikes the component known as the photocathode. Its purpose is to convert light (photons) into electrons since, as you may recall, we want to increase the electricity rather than the light.

Photocathodes are “grown” by vaporizing certain elements in a vacuum, creating certain types of crystals. Doing this is challenging and a big contributor to why NVGs are costly.

The consistency of the crystals grown on the vacuum side of the component is never the same, resulting in a visual image that will always appear slightly different between the ‘same’ sets of NVGs.

The photocathode’s sensitivity is calibrated for both the visual light spectrum and infrared, which is why infrared lights can also be employed (tactical missions) to illuminate environments.

3) Electricity Becomes Amplified by the Photomultiplier

After the photocathode generates the electrons, it enters the photomultiplier. One more latest type is called the microchannel plate or MCP.

The MCP is a thin slice of material equipped with millions of tiny glass tunnels that amplify the amount of electrons going through the tunnels.

How do they do this? Well, the surface of these tunnels is coated with a material that causes secondary electron emissions every time an electron strikes it. So, new ones get added each time the electron bounces off the walls.

The result is a big stream of electrons for every little photon that initially hits the photocathode! The tubes are tilted (about 5°) to ensure the electrons initially bounce off the wall. If not, it would go through without stopping. Since the ratio is around 1:1000, about 1000 electrons will come out for every electron that enters!

4) Electricity Hits a Phosphor Screen, Turning It Into Light

Then, the phosphor screen comes into play. It consists of a very thin phosphor layer deposited inside the optical fiber. Its function is to turn the electrons into visible light.

It attracts and accelerates the electrons because of its positive charge. Phosphor emits light once electrons strike it. The original but amplified picture will appear once the phosphor screen is hit by all these extra electrons!

The light we usually see here (green) depends on the type of phosphor used. Newer NVG types now have white phosphor, which many pilots prefer compared to green – but different individual preferences are always present.

5) We See the Enhanced Image Through a Fiber Optic Inverter

The pilots then look through the fiber optic converter (FOI) and the lens, which gives us an idea of the phosphor screen’s appearance. There we have it: being able to see in the dark! Every third generation has the FOI (and later) NVG, which functions to invert the image.

Like other optics, the image becomes mirrored twice, so to see the environment clearly, it will have to get inverted a 2nd time. To attain this, the FOI consists of millions of heated microscopic light-transmitting fibers that invert the image without requiring additional lens assembly.

It also adjusts the light, making the image look very far away so it doesn’t feel like you’re staring at a screen 2 cm away from your eyes!

What Light Can Night Vision Goggles See?

Illumination level is crucial for Night Vision Goggles (compared to FLIR). It doesn’t matter whether or not the light comes from artificial or natural sources – NVGs can benefit from flying close to cities just as much—if not more—than they could from natural light.

The first obvious one is the moon. The moon reflects around 7% of the sunlight that shines on it. How much of that light reaches Earth depends on a few factors. Let’s look at all the crazy ways light levels can be influenced at night!

Lunar cycle: 1 cycle has a 29.5-day duration – a full moon provides significantly more light than a quarter moon. The moon’s phase depends on the time of year and the global position of the aircraft.

Moon angle: the moon’s altitude compared to the horizon. The higher it is (similar to the sun), the more intense the illumination. Have a peek at the table below. The moon’s elevation is the horizontal axis, and the illumination levels (millilux) are the vertical axis. As you can see – the elevation makes a big impact. Simultaneously, a high quarter moon can be brighter than a low full moon!

Lunar Albedo: This is the moon’s amount of reflectivity, which is different since the moon’s surface is not the same everywhere. For example, the moon is 20% brighter during the 1st quarter than during the 3rd quarter because of the different exposed lunar surface.

Earth-moon distance: a closer moon indicates more light. Even on a moonless night, though, around 40% of the light is given by airglow. These are light-emitting particles in the atmosphere. Starlight is the next most important factor after that and provides around 10% of the light of a full moon, based on your location.

The sun: civil twilight (0 to 6 degrees below the horizon) is too bright for NVGs as most have automatic gain circuitry (a fancy term for brightness protection). A sun lower than 13 to 18 degrees below the horizon (astronomical twilight) would be extremely dark for NVG operations if we discount all other light sources. Ignoring all other light sources, the sweet spot is nautical twilight, which is 6–12 degrees below the horizon. Artificial illumination: Cities, torches, vehicles, etc., can all help but, at the same time, cause issues due to their intensity.

Earth’s surface albedo: earth’s reflectivity, like the moon, depends on its surface. A snowy surface, for example, will reflect a lot better than tarmac and will, hence, increase NVG performance.

Terrain contrast: it is far more difficult to fly in the desert with little to no terrain contrast than with surfaces with varying degrees of reflectivity, like a tree in front of a house.

Terrain shadowing: you can see shadows through NVGs at night! For example, trees and anything else with some height will offer a clear shadow with a low full moon. Objects within these shadows will, of course, be more difficult to spot as well (wires, etc).

Hopefully, this has shed some light on some topics you might need to become more familiar with, whether NVGs are completely alien to you or you are an experienced NVG pilot. If you want to learn more about Night Vision Devices, please reach out to us today!