Image tube intensifiers are to night vision devices what engines are to cars. Without them, the device would be incomplete and, more importantly, nonfunctional. For anyone looking to buy a night vision device or understand how it works, knowing the components that make it up is essential. From a user’s perspective, understanding the internal workings of a night vision device helps you appreciate its performance and capabilities.

In this blog, we’ll explore two main angles: first, what an image intensifier physically is, and second, what its on-paper performance actually means for the user.

THE ANATOMY OF IIT’S

Image intensification is a process through which, ambient lighting is amplified by several orders of magnitude, allowing us to turn darkness into visible images.
An Image Intensifier Tube (IIT), is generally made up of three main components that work in unison to achieve an amplified image; each of these performs a specific task that is consequently necessary in order for the next component in line to work as intended.

THE PHOTOCATHODE

The first component is the photocathode, a membrane that when charged negatively with electric current, is able to transform light (photons), into electrically charged particles (electrons), thanks to a phenomenon called “Photoelectric Effect”.

MICRO CHANNEL PLATE

The MCP is the second component in line, and arguably the greatest breakthrough in night vision tech, as it allows to multiply electrons, thus amplifying light in the end.
Once the Photocathode has transformed Photons into Electron, the stream of flowing electrons is now multiplied through a chain reaction that takes place in the Micro Channel Plate, essentially a membrane peppered with millions of microscopic channels, in which electrons pass through impacting the walls of each of said channels.
As the electrons bounce off, they create a chemical reaction which produces more electrons, virtually multiplying the total amount by thousands of times.

PHOSPHOR SCREEN

The human eye is not able to detect electrons, as such our brain is not capable of processing them as visual information, thus a component capable of turning electrons back into photons (visible by our eyes), becomes necessary; Because of this, beyond the MCP, a Phosphor Screen is installed containing phosphorescent chemicals, which, when struck by flowing electrons, emit visible light, thus creating the final image.

The color of said image is directly dependent on the type of phosphor used, and is the reason why night vision devices are mostly either green (P-43 green phosphor) or white (P-45 white phosphor).

TUBE PROTECTION MECHANISMS

When too much light enters the image intensifier, components such as the MCP or PS, may get damaged if precautions aren’t taken to limit the amount of oncoming light.

Damage can show in the form of blemishes, darker spots or shadows, which may be either temporary or in many cases permanent and unrepairable. It is important to note that a blem in it self is not a defect, it is a part of the manufacturing process and is common in tubes when they are unused, but . Because of this, embedded protection systems, are crucial to safeguard both the equipment, and the safety and efficiency of the operator.

AUTOMATIC BRIGHTNESS CONTROL [ABC]

Automatic Brightness Control or ABC, is a safety mechanism that will automatically lower voltage (thus lowering Gain) to the MCP, in order to reduce the number of flowing electrons, which in turn can prevent damage to the phosphor screen; a simple yet effective way to keep the device running, in conditions where it’s not dark enough for the device to use maximum brightness safely.

ABC uses Gain Control, but can be considered a sequential step to normal Automatic Gain Control, as only kicks in when light levels become dangerous for the tube itself, while AGC regulates tube gain constantly to keep brightness at a comfortable level for the wearer.

AGC is not present on Manual Gain Systems which give the user the possibility to dial in gain levels manually, while ABC is still present as a safety mechanism.

BRIGHT SOURCE PROTECTION [BSP]

Arguably the most effective safety feature, like ABC is present in all modern IIT’s (with the exception of Auto-gated tubes, which do not feature BSP).

BSP only intervenes in the event that light levels are too high even after ABC sets gain to the minimum level, and it works by cutting down voltage directly to the Photocathode, making for a very robust protection mechanism; The downside to this is that resolution will drop immediately when this is into effect.

A common misconception is that image intensifier tubes without auto-gating will get damaged more easily when exposed to high light, but this is untrue as BSP is extremely effective at protecting the IIT.

AUTO-GATING [AG]

A mechanism featured in more high-end IIT’s, that works by turning the tube on and off thousands of times per second by inverting the polarity directly from its internal PSU.
Unlike BSP, Auto-Gating protects the intensifier tube from intense light sources while still maintaining high resolution, while also prolonging its life cycle as it virtually halves the tube’s runtime.

UNDERSTANDING SPECSHEETS

Questions about NV equipment performance, are some of the most frequently asked in the night vision world, most specifically related to tube specifications, or “specs”, so in this post we are going to briefly go over what specs are and how they affect the performance and use and use of night vision devices.

EVERY TUBE IS UNIQUE

Every Image intensifier tube is built with unique characteristics, which can be measured, quantified, and reported on a Spec Sheet or Data Record, that will give us an idea of how well our device will perform in various conditions.

In the following paragraphs you will learn how to read and understand said parameters, and ultimately how to choose your night vision device based on your performance needs.

RESOLUTION

Resolution is a metric that helps us understand the extent to which our tube is able to resolve details. It is measured using “line pairs per millimeters” or Lp/mm.

Generally speaking a resolution of 64 Lp/mm or more is considered a very good value, but from our experience testing various devices, a resolution of 57 Lp/mm is also more than enough for headborne use, as only a trained eye will be able to appreciate the difference.
On the other hand for NV clip-on applications, or generally anything that involves magnification greater than 1x, having more resolution is most often a noticeable improvement.

Signal to noise ratio (SNR)

Generally considered a very, if not the most important factor when choosing our device.
It’s the metric that tells us how effective an image intensifier will be at processing light and creating a usable image to our eyes. Higher SNR values will result in a less noisy / grainy image.

The nature of SNR is determined by the mechanism of operation of every analog amplification system, including image intensifier tubes: When amplifying light, additional lighting will be generated, which can be of two types: signal (creates a useable image) and noise (creates an unusable image, scintillation).

As such, higher SNR values will result in cleaner and sharper images, but it is important to note that as amplification (technically referred to as “Gain”) gets higher, more noise will be created, so higher SNR values may be required to produce satisfying images as gain increases.

Since Gain is often associated with generations, our experience tells us that generally speaking Gen 2+ tubes, require a minimum SNR of 21 to operate in most conditions, while Gen 3 tubes will typically require 25+.

FIGURE OF MERIT (FOM)

The fastest and most convenient way to judge a tube’s performance without checking the whole spec sheet, mostly important for retailers when purchasing large quantities of image intensifier tubes.

FOM is obtained by multiplying resolution by SNR; So for example an image intensifier with a resolution of 64 Lp/mm and a SNR value of 22, will have a Figure of Merit of 1408.

FOM = Res x SNR

LUMINANCE GAIN

One of the most important factors, which is often overlooked, when it comes to choosing one device over another, as it quantifies its ability to amplify light.

Higher gain will result in brighter images, but the downside to this, will be increased noise; This is very important, as SNR plays a big role here, as low SNR will result in less usable gain before the image turns into static noise.

Manual gain devices have the ability to allow the user to dial in the brightness and subsequently the amount of noise in their image.

EQUIVALENT BACKGROUND ILLUMINATION (EBI)

Helps us understand how good an intensifier tube will perform in extremely dark environments.
Lower values are preferable to higher ones, as EBI is directly inversely proportional to sharper contrast in very low light environments.

A maximum value of 0.25 ulx is widely considered enough to navigate and perform tasks with ease.
For perspective, most military issued tubes, both for ground and aviation use, have a 0.25 ulx threshold.

HALO

Halo is in our opinion the most underrepresented spec in the whole book, as it’s neither always present in data records, nor is it not very discussed by most NV content creators or high profile individuals in the industry. Halo helps us measure the size of hues created around light sources, and just like EBI, lowervalues are always preferable. It’s important to note that this factor is more important for urban applications than for rural exploration