Night Vision FAQ (Frequently Asked Questions)

Figure of Merit, also known as FOM, is the result of multiplying a tubes center resolution and signal to noise ratio values together. FOM is used as a primary benchmark for tube performance and export control.

Omni VIII contract specs indicate a 64 lp/mm minimum center resolution and a 25 signal to noise ratio minimum.  This is the lowest current Milspec requirements indicating a minimum FOM of 1600 to meet minimum Omni VIII Milspec requirements. Tubes with a FOM value lower than 1600 cannot pass as US Milspec Omni VIII intensifiers. It is important to note that there are several performance requirements other than FOM for a tube to be considered a Milspec tube, however, FOM is a great starting point for gauging tube performance.

At Send Nods. we believe that there are three major FOM performance benchmarks that end users can use to judge what system is right for them.  The three benchmarks are a 1792, 2000, and 2376 minimum FOM.  Although these benchmarks are not everything they can be used to determine what price point and performance level is a good fit.

A 1792 minimum FOM tube is going to be a solid milspec tube. Although it will not be of the performance level of a 2000 plus FOM tube, the performance level will be to the point where the customer is still getting a solid system that will perform well in most lighting conditions.  Where users are sacrificing at this level is extreme low light.  This level of tube will have more noise when light levels are extremely low and need supplemental IR.

A 2000 minimum FOM tube is going to be a high performing tube.  This level of tube is going to be the most value price wise when factoring the price jump from a 1792 minimum FOM performance level and the performance gains.  The performance gains at this level will be noticeable in low light performance and image quality.

The 2376 minimum FOM benchmark is an absolutely phenomenal tube and the highest minimum FOM offered outside of the SOCOM / MIL markets.  This level of tube is for the customer who wants the absolute best. These tubes perform incredibly well in all lighting conditions including extreme low light.  This level of tube will perform in a much wider spectrum of low light conditions without the need of supplemental IR.  Users will notice the upmost image quality with very fine details that are able to be distinguished at farther distance over other FOM levels.  Users will also notice the upmost performance in extreme low light conditions.

Resolution is the ability of an image intensifier tube or night vision systems to resolve an image.  Night vision tube resolution is measured in Line Pairs per Millimeter (lp/mm).  This is accomplished by using a bar chart on a night vision test set.  With resolution, the higher the number the better.  A higher resolution indicates a tubes ability to distinguish fine details and differences in objects extremely close together.

Center resolution is multiplied with he signal to noise ratio to determine FOM.

Before we can define signal to noise ratio it is important to understand what signal and noise are independently.  Signal is the true light that is being picked up by the photocathode.  This is what turns into the image you see as the tube amplifies this light.  Noise is the scintillation that a tube puts off.  This is seen as a sparkle effect in the tube and is being put off by the microchannel plate (MCP).  So the signal to noise ratio (SNR) is the ratio of signal to noise in your image intensifier tube.  The higher the SNR the more signal there is compared to noise in a tube.  This directly correlates to low light performance as less light is hitting the photocathode the noise is more pronounced.  A higher SNR will perform better in low light conditions.

Signal to noise ratio is multiplied with the center resolution to determine FOM.

Night Vision EBI is the amount of light in an image intensifier tube that is visible when the tube is turned on but no light is hitting the photocathode.  For an image to be visible through an image intensifier tube the light hitting the photocathode must overpower a tubes natural background illumination or the image will be washed out.  This means that the EBI determines the lowest light levels that an image is able to be seen through the tube. The lower the EBI number the better.

Halo is rings of light that form around concentrated bright light sources when looking through night vision.  Halo is typically prevalent with street lights and other similar concentrated light sources.

An image intensifier tube has three main components (or layers) that give the tube life.  The photocathode is the first of the three layers.  The photocathode absorbs light energy in the form of photons and releases electrical energy in the form of electrons.  Gen 3 image intensifier tubes use a Gallium Arsenide (GaAs) photocathode which provides improved resolution and light sensitivity.

Photocathode sensitivity is the measure of how well a photocathode in an image intensifier tube converts light energy into electrical energy. Photocathode sensitivity is measured in micro-amps/lumen (µA/lm).

The second main component (or layer) in an image intensifier tube is the microchannel plate (MCP).  The microchannel plate is a disk which consists of millions of holes or channels in the disk.  The channels in the MCP amplify the electrons produced by the photocathode. The number of channels in the microchannel plate is a major factor in determining resolution.

The final component (or layer) in an image intensifier tube is the phosphor screen.  The phosphor screen is a screen coated in phosphors.  The phosphors on the phosphor screen are stimulated by the multiplied electrons from the microchannel plate.  This causes the phosphor screen to convert the electrons back to photons, or light, which produces the image that is seen by the user.

A night vision image intensifier tube is the heart and soul of your night vision device.  The Intensifier tube is what gives you the ability to see at night.  An Image Intensifier does this by taking in photons of light, converting them into electrons, then amplifying the electrons, and converting the amplified electrons back into photons.  This process lets the user operate in very dark environments with very little light.

It is important to note that since an image intensifier tube simply amplifies light, they require some ambient light to work.  Although this can be at extremely low levels of light there has to be some light present.

The data sheet is a piece of paper that logs the individual specs on an image intensifier tube.  Data sheets show performance data, cosmetic blemish information, and contract information.

A night vision housing is the part of the night vision system that houses the image intensifier tubes.  The housing provides a power source for the tubes and protects them from debris and damage.  This is the outer portion that you see.  Housings all have different features and excel in different areas.  In general there are three types of housings: clip-on housings, binocular housings, and monocular housings. 

Clip-on housings are designed to house an image intensifier tube to be used in front of a normal day scope at magnification.  Binocular housings are designed as a system to house two image intensifier tubes so both eyes are aided. A monocular housing houses a single tube and can be helmet mounted or used as a handheld.

Helmet mounted housings have two main types fixed bridge and articulating housings.  Please see our article on the differences and pros / cons here

The objective lens is the lens on the front of the night vision system.  This lens is used to adjust distance focus for the system.  Distance focus will range from close focus to infinity.

The ocular lens, also known as the eyepiece, is the rear lens closest to the users eye.  This lens is used to adjust the focus to the correct diopter setting for the users eye.  Since this focus is adjusted to the prescription of the users eye, once focused it does not have to be refocused unless moved or the users eye sight changes.

Autogating is the process of the tubes power supply constantly providing and cutting power to the tube to help protect from high light damage and retain optimal performance of the night vision without blinding the user when exposed to various high light sources.

Manual Gain is a system feature that give the user the ability to manually control the brightness of their system.  This is accomplished by turning a gain knob on the night vision system.

Manual gain units use 11769 style image intensifier tubes which provide a pigtail off of the tube to allow for manual gain control.

Automatic brightness controls is the feature which automatically controls the brightness of the tube based on environmental conditions.  However, unlike a manual gain image intensifier tube, ABC tubes are designed to protect against extreme high light and do not provide the dynamic range of brightness control a manual gain tube has.  10160 image intensifier tubes use automatic brightness control since they do not have a gain lead to manually control brightness.