Night Vision, how does it work?

A brief history of night vision devices.

The devices we sell (except for devices using thermal technology) mainly use light intensification technology.

This principle was first put into action during the closing days of WWII, but really picked up in the 60s with the creation of first generation light intensification tubes (which we'll refer to as "tubes" in this article).

Generation 2 devices appeared in the 70s and the 80s will see the appareance of generation 3 tubes, mainly in the United States.

These tubes were mostly used for military applications, mostly mounted on vehicles such as tanks, AFVs or helicopters.

The different generations:

The term "generation" is a transition based system used by the military to classify the differences between the various technologies used in light amplification tubes. Each generation was recognized via treaty between the AUSCANNZUKUS countries. Other technologies and trademarks exist, such as +,-, filmless, thinfilm etc. however such names aren't officially recognized and are considered trademarks, regardless of their performance.

Gen0 (unofficial name) : This generation stands out and can't be compared to the others because of its limited historical use and because it doesn't work on a light intensification principle. So called Gen 0 devices simply were scopes with near infrared light sensitivity, and as such required constant infrared illumination to work. The very first passive, autonomous light amplification device appeared with Gen1. Gen0 was developed during WWII and required cumbersome hardware to operate (a large power pack, and respectively big infrared light and scope) as pictured below. Gen0 is considered a common term and never was officially defined.

Gen1 (tube lifespan of ~1,000 hours) : This is the first generation to use the principle of light amplification to operate, and as such don't require constant IR illumination . Devices of that generation are passive and are in most cases characterized by a noticeable fish eye effect. Still available today, this generation significantly improved over the years through many modernizations most notably thanks to Gen1 Core technology (Armasight trademark), which allows a better resolution and a reduced fisheye effect. They nevertheless lag behind Gen2 and 3 in terms of performance, as can be seen on the picture below.

Gen2 (tube lifespan of 2,000 to 5,000 hours) : Gen2 represents a considerable step forward in terms of technology. It is the first generation to use an MCP (Micro Channel PLate) to significantly boost the gain of the tube. (the role of the MCP is detailed below). In addition, modern Gen 2 does away with the optical distortion present on Gen1 and produces a perfect image. Performances reached by Gen 2 nowadays are astonishing.

Gen2+ designation: This commercial name usually defines tubes with an S20 or S25 "extended red" photocathode which allows for better performance in a starlit environment.

In the absence of this name on the tube Specsheet, said tube won't have the expected performance of a Gen2+ unit. Only the factory Specsheet can be trusted to determine the performance of a tube.

Super second gen SSG (tube lifespan 15000 hours) : Also known as SHP hypergen supergen, designates second generation tubes with an extra high sensitivity photocathode. Those tubes are top of the line devices, and in direct competition with the latest third generation tubes. The French military currently operates such tubes, among other users.

The picture below shows the view through an  ECHO from Photonis. One can see how performant one such tube can be. (A Specsheet from an ECHO tube can be found further in this article.)

Gen3 (tube lifespan of ~15,000 hours): Gen3 differentiates itself from the previous generations via the use of a gallium arsenide (GaAs) photocathode (the physical light receptor).However it retains both the method of operation and the architecture of the previous Gen2 tubes. This photocathode allowed the first gen3 devices to outclass gen2 ones by delivering higher sensitivity and gain. Unfortunately the nature of a gen3 tube will not guarantee its performance or quality, only the Specsheet can be trusted to assess the performance of a specific tube. 

As you may have understood by now, saying that Gen2 is inferior to Gen3 makes no practical sense, as the only thing that sets them apart is the use of a material. Each of these two generations has its pros and cons. FOM (Concept developed below) is a useful tool to compare tubes, however it is unreliable compared to studying the specsheets.

Gen4 : It is a defined generation. These tubes are standard Gen3 tubes that lack positive ion barrier filming. L3 (formerly Litton) have pioneered this concept, however their questionable durability has led to doubt in the tubes' ability to fulfill Gen4 requirements.

Principle of image intensification

Now that we went through the different generations, we'll focus on the physical principles of the tubes. We'll try to describe it as simply as possible. As its name suggests light intensification uses residual light and amplifies it to the point where this invisible source can be turned into an exploitable image for the human eye. We will now put a long-lasting myth to rest. Indeed a lot of people getting into night vision tend to believe that "one can see in the dark".

It is possible to see in the dark using a night vision device, but not in absolute darkness. This is simply due to the fact that even the best tube in the world will need a little bit of residual light for it to amplify it and render an image. The overall quality of a tube is defined by its ability to recreate a "legible" image with as little residual light as possible, but we'll touch on that later. This aspect is offset by the fact that these tubes are infrared (IR) sensitive allowing them to be effective in any condition while remaining invisible to those not equipped with similar devices.

Now let's go back to the basic elements that make a tube work. The following picture shows the three main elements of a tube: The photocathode, the Micro Channel Plate (MCP) and the phosphor screen. (note that this schematic drawing doesn't show the electrical power unit).

Image intensification happens in three steps:

Step1: Residual light under the form of photons (light particles) is collected by the photocathode which turns those photons in electrons (electrical particles).

Step2: Electrons travel through the Micro Channel Plate and are multiplied (by a given factor depending on the tube) by going through micro-channels that compose the MCP.

Step3: Multiplied electrons hit the phosphor screen (which can be green or white) and will generate photons(light) and form an exploitable image for the human eye.

Green vs White Phosphore

As we just saw, the final image we see is generated by the phosphor screen. Initially Gen2 and Gen3 units featured P22 phosphor screens, a rather aggressive shade of green to the eyes, which was eventually replaced by P43 phosphor. While still green it's a lot more comfortable to the eye, especially over longer periods of use. Then came P45 "white phosphor" which quickly became the reference thanks to its even greater comfort compared to green P43.

No phosphor can be considered better than the other. It's a matter of personal preference, certain users will prefer P43 over P45 and vice versa. From a purely physiological standpoint green offers more contrast over white, however it all comes down to each individual and how their brain manages to process and get used to the image projected on the screen.

The following picture shows the different shades of phosphor.

Advice and tips:

If you're new to night vision there's a few things you should know about the sensitivity of these highly technological devices. Indeed, certain factors need to be taken into account when putting them in use, as well as a few useful precautions. Here's a list of our advice:

A tube, no matter what make or model it is should never be exposed to direct sunlight unless fitted with daycaps (lens covers) on the objective lens. Such an exposure will damage the tube permanently even with the device turned off. Protecting your objective is your responsibility. The picture below shows sunlight damage to a tube.

Also, avoid prolonged exposure to an intense light source, like a flashlight and above all: lasers. Be they red, green or IR.

Depending on the use you're going to put your NVGs to, we advise you to constantly protect the objective lenses of your devices using sacrificial lenses.

Prolonged storage in damp or wet conditions should be avoided even if your device is waterproof. It may deteriorate it over time.

Finally should you experience problems with your device, we strongly recommend not to attempt to repair it or to disassemble it yourself. In such an event you should contact us directly so we can take care of your device.

Glossary and specific abbreviations.

NVG: Acronym for "Night Vision Device".

IR : Stands for infrared. It refers to a wavelength that is invisible to the naked eye but to which tubes are sensitive. Simply put the tube "sees" the IR but our eyes can't see the IR. Conversely the tube allows us to see the IR. 

Gain : The ability of a tube to multiply electrons. A good image being worth a thousand words, here's the difference between a low and high gain tube (respectively left and right).

Autogating : The ability of a tube to self regulate its gain depending on the intensity of the light it is subjected to. In other words the tube constantly protects itself from sources of light and reacts accordingly. The main advantage of autogating is it allows a tube to function in a dynamically lit environment and allows the user to see an exploitable image in such conditions. Autogating will however not protect the tube 100% if exposed to prolonged bright light.

Autogating is particularly important for military and law enforcement use when they're using unsuppressed firearms and are exposed to explosions.

Photonis currently is the leader when it comes to autogating technology, mostly thanks to their "Ultra Fast Autogated" technology present on their Echo / Echo+ / 4G / 4G+ series of tubes.

The following images show the different reactions of two tubes, one autogated(top), one non-autogated(bottom).

Resolution : The resolution of a tube describes its ability to precisely render an object in given light conditions. The unit for measuring resolution is lp/mm, aka Line pairs per millimeter. Simply put, it measures the ability of a tube to distinguish a number of black and white line pairs printed over 1mm. The higher the value, the more accurate the image rendered by the tube will be.

The picture below illustrates this line pair notion by showing a tube (to the left) with lower resolution than the one on the right.

SNR (Signal to Noise Ratio) : The SNR is a ratio that determines the performance of a tube and its capacity to render a clear and precise image depending on the luminous intensity the tube receives. All tubes behave the same way which means that as luminosity decreases the rendered image will degrade and become grainy until the tube reaches its limits and only grain will clutter the image. It is at that point that turning on an IR light source will be required to see a clear and exploitable image again. Even the best tube in the world will be subjected to such limits, the difference being the higher performing the tube will be, the less residual light it will need to render a clear image. As such SNR measures the tube's ability to maintain a clean image depending on the amount of residual light. The higher the SNR, the more performant the tube will be considered. An SNR of 20 is considered a good SNR while an SNR of 30 is rated excellent. 

The following picture shows two different tubes with two different SNRs. The notion of grainy image is apparent here with the tube on the left having a lower SNR than the one on the right.

FOM (Figure Of Merit) : FOM is the most commonly used way of determining a tube's performance. It is calculated multiplying Resolution by SNR. The higher the FOM, the higher the performance of the tube.

FOV (Field Of View) : The FOV of an NVG is its angle of vision. Generally speaking the FOV of a device is of 40° but certain models offer a wider FOV, thus allowing to see more at the same given distance.

Halo : This effect describes the luminous halo which surrounds an intense light source. It is the consequence of an optical reflexion of electrons inside the tube. As explained earlier, Gen2 tubes are better at mitigating halos than Gen3, although the latter have improved in that field. Photonis is the undisputed leader in halo mitigation technology. The following picture shows the difference in halos between a Photonis tube and a Gen3 tube.

Black Spots or Blems : Blems, also known as black spots are fixed, visible spots located in the field of vision of a tube. They are immobile and won't evolve over time. They are a side effect of standard tube manufacturing process and are in no way related to the tube's performance or operation. Milspec tubes have a strict set of requirements for blems and only tolerate very few or none at all. Commercial tubes available on the civilian market are subjected to a less stringent set of requirements, thus tubes that failed to pass the milspec qualification generally end up on the commercial market. However, although commercial tubes tend to have more blems (we've never witnessed commercial tubes with over three blems) their performance remains very high. These spots are barely visible under normal conditions and generally reveal themselves when used in high brightness environments, conditions under which NVGs become generally unnecessary.

Data Sheet : The Data Sheet is a document stating the minimal and sometimes maximal specification for a model of tube. The Data Sheet does not represent the exact performance of a given tube. This can only be shown by the Specsheet, as detailed further in the article.

The following picture is an extract of a Photonis Echo tube datasheet.

Specsheet : A tube Specsheet represents the exact measure of its performance. It compiles the three datum previously exposed, including FOM. The Specsheet indicates (among other things) the tube's serial number and its date of manufacture. The Specsheet is printed by the manufacturer after the tube's completion and quality control.

Over here at NODS we commit to only selling units whose tubes come with their respective Specsheets. That way you know precisely what tube you have and its calculated performance. 

We aim for transparency. You know exactly what you're purchasing.

The following Specsheet (the serial number of which was redacted) comes from a Photonis white phosphor ECHO tube. 

L’OMNI : OMNI represents neither the performance or the technical data of a tube. It only states the year the United States military signed the sales contract for a specific tube. In order to be eligible for sale the tubes must at least qualify for the technical specifications defined by OMNI. As repeated multiple times above, it is very complicated to compare tube performances without their respective Specsheets.

Tube format or Drawing :Many different tube formats exist, and all of these tubes are made to fit inside a specific housing and have different MSRPs. We have standardized on 37mm MX10160 or MX11769 with manual gain. That way, our tubes are compatible with a majority of devices.

If after all you still have questions, do not heistate to contact us.