If you went to your AME and said “I have lost my colour vision, I can’t see depth, and my vision is reduced to 2 small circles in the middle of my original field of view”, you’d probably not walk out with a class 1 medical. Somehow though, pilots use these exact conditions to fly to and from sometimes very hostile environments or congested areas using Night Vision Goggles (NVG’s). It is quite impressive just how much we can suddenly see and use to our advantage after flipping down the goggles in an otherwise dark environment.
The ability to see during hours of darkness is of vital importance to a lot of specific areas in the aviation industry. Whether it is HEMS, SAR or even law enforcement / the military, a lot of it would not be possible without this remarkable feat of engineering: allowing humans to see at night. But how exactly does it work, and why does it require training to be able to use them properly? And why the hell can one set of goggles easily cost more than £10.000? Let’s find out!
First of all, NVG’s do not allow for vision in pure darkness. They are basically light amplifiers. They need a small amount of light to turn into something that is usable for our very limited naked eyes. If there is no light available to amplify, they simply cannot do their job. No need to get grumpy though, even the smallest amount of light can actually be the difference between no use at all and actually offering a lot of benefits.
So their function is to collect as much light as they can, and then amplify it. But how do we amplify light? Any other scopes such as microscopes, telescopes and binoculars can help with light and focus, but they don’t amplify light itself. Is light even amplify-able? Turns out – light is easy to dim (sunglasses, filters, layers etc) but amplifying it is not something we can achieve without some mental gymnastics. To a lot of people, (and for a long time for scientists as well), this is how the NVG’s working principle looks like:
So what’s the solution? Well, there is something that we amplify on a regular basis and is relatively easy to do for us puny mortals: electricity! What does electricity have to do with light you ask? Well not much by itself, but we can make a link between the two, which is exactly what NVG’s do. Get light – convert it to electricity – amplify electricity – convert back to (more intense) light – tadaa! Let’s dive in.
To stay away from diving into crazy technical depths at once, let’s keep things simple and divide the process of what NVG’s do into 5 steps:
- Dim light enters the NVG tube lens
- Light enters the photocathode which converts it to electricity
- Electricity gets amplified by the photomultiplier
- Electricity hits a phosphor screen, converting it into light
- We see the enhanced image through a fibre optic inverter
1) Dim light enters the NVG tube lens
So first we need to collect as much light as we possibly can using an objective lense. How much light depends completely on the circumstances we fly in. A desert with a layer of cloud is going to be really challenging as there is not much light to work with. For flying across western Europe however, there is a lot of cultural light to work with, even in poor weather. Flight crew always brief how much light there is available before startup. This includes starlight, moonlight, cultural light, time of year, time of day etc. There are multiple models to forecast this, which will indicate a certain amount of Lux (or millilux) throughout the night, which is the unit representing intensity of illumination in lumen per square meter.
2) Light (photon) enters the photocathode which converts it to electrons
This light then strikes the component called the photocathode. It’s function is to convert light (photons) into electrons, as we want to be able to amplify the electricity, not the light itself – remember? Photocathodes are “grown” by vaporising certain elements in a vacuum – which creates a certain type of crystals. Doing this is not an easy task and this is a big contributor to why NVG’s are not cheap to manufacture. The consistency of the crystals that are grown on the vacuum side of the component are never exactly the same, and this results in a visual image that will always look slightly different between the ‘same’ sets of NVG’s.
The sensitivity of the photocathode is calibrated for both the visual light spectrum as well as infrared, which is why infrared lights can be used (tactical missions) to illuminate environments as well. A rather strange feeling to have your steerable landing light set on IR, not seeing anything with the naked eye, but then through the goggles, you can see this beacon of bright light!
3) Electricity gets amplified by the photomultiplier
After the electrons are generated by the photocathode, it enters the photomultiplier. One more recent type is called the microchannel plate or MCP. The MCP is basically a very thin slice of material filled with millions of tiny glass tunnels that are all amplifying the amount of electrons going through the tunnels.
How do they do this? Well, the surface of these tunnels are coated with a material that causes secondary electrons emissions every time an electron strikes it. So each time the electron bounces off the walls, new ones get added:
The result is a big stream of electrons for each little photon that initially hit the photocathode! The tubes are slightly tilted (about 5°) to make sure the electrons initially actually bounce off the wall, otherwise it would pass straight through. The ratio is roughly 1:1000, so for each initial input electron, around 1000 will come out!
4) Electricity hits a phosphor screen, converting it into light
Then, the phosphor screen comes into play. It consists of a very thin layer of phosphor deposited on the inside of the fibre optic. Its function is to convert the electrons into visible light.
It achieves this because it attracts and accelerates the electrons due to its positive charge. Phosphor emits light when electrons strike it. When all these extra electrons hit the phosphor screen, the original picture – but amplified – will show up! The type of light we usually see here (green) is dependent on the type of phosphor used. Newer types of NVG now have white phosphor, which a lot of pilots prefer compared to green – but there are always different individual preferences.
5) We see the enhanced image through a fibre optic inverter
The pilots then look through the lens and fibre optic converter (FOI), which shows us what the phosphor screen looks like and there we have it: seeing in the dark! The FOI is present in all 3rd generation (and later) NVG’s and its function is to invert the image. Just like with other optics, the image gets mirrored twice, so to see the environment upright, it will need to get inverted a 2nd time. To achieve this, the FOI consists of a bundle of millions heated microscopic light-transmitting fibres that invert the image, without the need for an additional lens assembly. It also collimates the light, making the image appear very far away so it does not feel like you are staring at a screen 2 cm away from your eyes! Neat huh?
LIGHT LEVEL VARIABLES
So how much light is enough? First of all let’s have a quick look at the electromagnetic spectrum that is relevant to us here. Left represents a small wavelength, all the way to the right represents larger wavelengths of light.
I deliberately did not include any units to simplify the diagram. But the spectrum ranges roughly from all visible light to near infrared, all the way to mid and far infrared. For visible light, all we need is the naked eye. If we put on NVG’s we can now see the visible spectrum + near infrared light. In order to see anything with even bigger wavelengths, you will need FLIR – which is a different system not to be covered in this article.
So how do we determine whether our NVG’s will perform on any given night? Obviously the more light available to work with, the better. The 3 main variables are: illumination levels, terrain contrast and atmospheric conditions.
For NVG (unlike FLIR) illumination level is a critical factor. It does not matter whether or not the light comes from natural or artificial sources – flying near cities can give NVG’s just as much of a boost (sometimes too much) as natural light could. The first obvious one is the moon. The moon reflects about 7% of the sunlight that shines on it. How much of that light reaches us depends on a few factors. Let’s have a 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 gives significantly more light than a quarter moon. The phase of the moon depends on the time of year and global position of the aircraft.
- Moon angle: its altitude compared to the horizon. The higher it is (just like the sun), the more intense the illumination. Have a peek a the table below. The vertical axis is the illumination levels (millilux) and the horizontal axis is the moon’s elevation. As you can see – the elevation makes a big impact. At the same time though a high quarter moon can be brighter than a low full moon!
- Lunar Albedo: this the moon’s amount of reflectivity and is different due to the fact that the surface of the moon is not the same everywhere. For instance, the moon is 20% brighter during the first quarter than it is during the 3rd quarter due to the different exposed lunar surface.
- Earth-moon distance: a closer moon means more light
- Even on a moonless night though, about 40% of the light is provided by airglow. These are particles in the atmosphere emitting light. Starlight is the biggest influencer after that and provides about 10% of the light of a full moon, depending on your location.
- The sun: civil twilight (0-6 degrees below the horizon) is too bright for NVG’s as most have something valled automatic gain circuitry (a fancy term for brightness protection). If we ignore all other light sources, a sun lower than 13-18 degrees below the horizon (astronomical twilight) would be too dark for NVG operations. The sweet spot is nautical twilight (6-12 degrees below the horizon) if we ignore all other light sources.
- Artificial illumination: cities, vehicles, torches etc can all help but st the same time cause issues as well due to their intensity.
- Earth’s surface albedo: earth’s refleftivity, just like the moon, depends on its surface. A snowy surface for instance will reflect a lot better than tarmac and will therefore increase NVG performance.
- Terrain contrast: flying in the desert with little to no terrain contrast is a lot more challenging than having surfaces with different amount of reflectivity such as a tree in front of a house.
- Terrain shadowing: you can actually see shadows through NVG’s at night! With a low full moon for instance, trees and anything else with some height will give a clear shadow. Objects inside these shadows will of course be harder to spot as well (wires etc).
NVG HUMAN FACTORS
The introduction of NVG’s to the aerospace industry have introduced a number of aeromedical concerns that we as pilots should respect in order to use them safely. It is important to remember that NVG’s do provide direct viewing of an object. You are looking at a screen that has ‘recorded’ the environment using different tricks explained earlier. This means that while they are reliable and accurate, they should be treated like any other instrument with limitations and a need for cross checking with other crew to get an accurate perspective of what is really going on outside the aircraft. Let’s have a look at the main challenges that come with NVG operations from a human factors perspective.
• NVG spatial disorientation
Correctly interpreting the image being shown by the phosphor screen is one of the most important limits to overcome. As discussed in the intro earlier, flying with NVG’s basically result in no more colour vision, reduced depth perception, poor field of view and poorer clarity compared to our normal eyesight during day VFR operations. Being aware of these limitations, and treating them as such, is important for safe use of NVG’s.
• Field of view (FOV)
During hovering or vertical take-offs, a lot of our visual cues are processed via the peripheral vision, which during NVG’s is almost impossible to use in the normal fashion. This can lead to spatial disorientation. The average field of view for a set of goggles is around 40°. This is a significant reduction compared to our usual (almost) 180°. The main way to counteract this limitation is by using head movements to scan the environment. This must be managed though as increasing head movements in a dark environment with a reduced FOV can also result in disorientation and (neck) fatigue. If your goggles are setup wrongly (i.e too far away in this case), the FOV can reduce a lot, making this problem even worse. The recommended distance across different sets is around 25 mm (eye relief). But this also depends on your individual preferences and fit. To give the pilots not using NVG’s a rough idea of what it looks like, have a look at this image to get a rough idea of the FOV reduction (not to scale).
• Visual Acuity and Resolution
But then even within this FOV, the resolution and visual acuity within this circle is still less than otherwise used to with the naked eye. A person with 20/20 visual acuity can sharply see an object with 1 minute of arc on their retina at 20’ distance. If you have 20/40 vision, that means you need to get twice as close (10’) or double the objects size (2 minutes of arc) to see the same object clearly. Obviously any of these visual tests are conducted in well illuminated environments with maximum contrast (black on white). The higher the contrast, the easier things are to see. With the NVG display, contrast is naturally lower due to the nature of its working principle (phosphor) and has monotone colours, whether green or grey tones. Objects will be harder to distinguish, as well as slopes, details and other sometimes critical cues in the environment.
• Interpupillary Distance Adjustment
When setting up goggles, the tubes themselves should line up with your pupil, where the centre of each pupil lines up with the centre of each tube. This can be adjusted, but not properly adjusted tubes can result in other risks such as eye strain and fatigue and can even cause near sightedness of the pilots compensation for the misaligned image is to bring the goggles closer. An error of even a few millimeters can already reduce visual acuity and cause problems such as a lower observable resolution per tube.
• Diopter (eyepiece) Adjustment
The other setting that should be carefully selected is the eyepiece adjustment. While the goggles could compensate for less than perfect eyesight, they cannot compensate for astigmatisms. Studies have found that new NVG pilots tend to put too much negative strength on the lens focus. This is usually compensated for by your eyes, but as we get older this becomes harder.
• Objective Lens Focus Adjustment
The average focal range of a tube is around 10 inches to optical infinity (basically 20 feet +). The reason NVG’s present objects that are very far away sharply is because the system collimates to infinity – which is why everything that is far away looks in focus even though you are looking at a TV screen. This is accomplished by the FOI discussed earlier. If you don’t adjust this correctly though, the focus could be too close to the pilot and therefore display everything further away slightly out of focus.
• Tube Alignment and Binocular Fusion
Our eyes and brain are wired in such a way that two of our vision systems (eyes) combine 1 single image. One eye is usually dominant and therefore provides most of the information, the brain will process both signals to get 1 clear image. The same is achieved while looking through goggles and they should be adjusted until the system becomes a fused circle, rather than 2 slightly overlapping circles. Incorrect alignment, again, can lead to eye strain, double vision, fatigue and disorientation.
• Reduced depth perception
As we are looking through a lens to see a screen, the depth perception we are used to with the naked eye is reduced. We can still rely on estimation of distances by using monocular cues (learned from experience) such as a small tree and a big tree where the big tree is only bigger because it is closer. This is of course not always the case, so a reliance on this type of depth perception carries its own risks. Distance estimation becomes trickier for us this way and objects tend to appear further away than they actually are.
Circadian disruption is usually a given when flying on NVG’s. Long shifts or missions combined with operating outside your normal rhythm can lead to multiple risks: complacency, computational errors, communication errors, irrational decision making and irritability are the most common ones. Luckily there are loads of ways to mitigate these such as FTL schemes, time off, NASA naps on base, and training for managing workload properly in flight.
There are quite a few NVG related regulations for both the FAA and EASA world. We will only briefly go into the EASA regulations as we want to stay awake 🙂
First of all, you won’t see the term NVG very much in general regulations, usually the term used is NVIS (night vision imaging system) as a group term for things like NVG’s, FLIR and others. Under EASA, we will need to look in Part-OPS – Subpart H: SPA.NVIS. It’s attached here if you would like to brush up on it yourself (Oct 2019 version, page 1028):
The equipment any NVG capable aircraft needs by law is:
- Airworthiness approval
- Radio altimeter
- NVIS compatible lighting
- Helmet with NVG attachment + backup power source
- All onboard NVG’s shall be of the same type and model
- Continuing airworthiness covering windscreens, NVIS lighting, NVG’s and any other relevant equipment
The most critical NVIS requirements EASA describes are (GM1 SPA.NVIS.140 SECTION 3.1.1):
• aircraft internal lighting has been modified or initially designed to be compatible
• environmental conditions are adequate for the use of NVIS (e.g. enough illumination is present, weather conditions are favourable, etc.);
• the NVIS has been properly maintained in accordance with the minimum operational performance standards;
• a proper pre-flight has been performed on the NVIS confirming operation in accordance with the continued airworthiness standards and training guidelines; and
• the pilot(s) has been properly trained and meets recency of experience requirements.
EASA also prescribes the 7 minimum items to cover during the crew briefing before any NVG flight takes place. Save it for your own reference and check if there are things your company or base is currently missing (if you are flying under EASA regulations):
OPERATING ENVIRONMENT CONSIDERATIONS
The tricky thing with weather when flying on goggles is the fact that a deterioration of cloudbase or visibility can be hard to detect due to the fact that goggles can make it feel like there is no cloud. They are very good at picking up any light, so if there are lightsources in the distance, but there is a cloud between those and the aircraft, you might have difficulty seeing the cloud, as the NVG’s might do an excellent job showing you what is behind the cloud. This increases the risk of inadvertently entering IMC conditions. EASA has this to say on NVIS operating minima (SPA.NVIS.130):
(a) Operations shall not be conducted below the VFR weather minima for the type of night operations being conducted.
(b) The operator shall establish the minimum transition height from where a change to/from aided flight may be continued.
To help with this as pilots, we should periodically look underneath the goggles to double check you senses and to make sure are still flying under acceptable VFR conditions, or whatever your company OPS manual dictates. The same counts for light mist, snow, or rain. The NVG image will change slightly, usually halos become prominent (light rings around light sources) due to diffraction of the light. In addition to this, you might see the light become more dim and scintillation in the image could increase (light flashes / sparking).
Another threat is the LED lighting on top obstacles. Some of them will not be detected by NVG’s and the only way to spot them is by periodically looking underneath your goggles.
If you want to read more or know what I have based this article on, this is a list of amazing references:
Hopefully this has shed some light on some topics you might be less familiar with, whether NVG’s are completely alien to you or you are an experienced NVG pilot.
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