Night-Vision Terminology

Night vision has its own language—and a fair amount of that language gets misused.

A housing is called a tube. Gain is confused with brightness. White phosphor is treated like a generation. FOM gets advertised as if it tells you everything about a system.

It does not.

This lesson gives you the working vocabulary needed for the rest of the academy. These are intentionally short definitions. Each important subject receives a dedicated lesson later.

Basic Night-Vision Terms

Night-Vision Device — NVD

A general term for equipment designed to improve vision under low-light conditions.

Depending on the context, NVD may refer to an image-intensified monocular, binocular, goggle, sight or another low-light viewing system.

Do not confuse the abbreviation NVD with Night Vision Devices, the approved manufacturer and supplier.

Night-Vision Goggle — NVG

A head- or helmet-mounted night-vision device.

People frequently use "NVG" as a catch-all term, even when discussing a monocular. The term is common, but it does not tell you whether the system contains one tube or two.

Image Intensification — I²

The process of converting incoming photons into electrons, multiplying those electrons and converting the amplified signal back into visible light.

You may see this written as:

• I²
• I2
• Image intensification
• Image-intensified night vision

Image-Intensifier Tube

The sealed electro-optical assembly that performs the actual image-intensification process.

The tube contains the photocathode, microchannel plate, phosphor screen and high-voltage power supply. It is installed inside the night-vision housing.

The tube is the expensive little heart of the system.

Housing

The external body that holds and protects the tube and optics.

The housing normally contains the battery compartment, controls, electrical contacts, mounting interface and environmental seals. A rugged housing is important, but the housing alone does not determine the image quality produced by the tube.

System

The complete working device: housing, tube, objective lens, eyepiece, power source and controls.

When someone says they purchased "a PVS-14," they are usually referring to the complete system—not merely its housing or tube.

Common Device Configurations

Monocular

A night-vision device with one optical channel viewed by one eye.

The AN/PVS-14 is the best-known example of this format.

Binocular or Dual-Tube System

A system with a separate image-intensifier tube and optical channel for each eye.

A dual-tube system does not provide true depth perception in exactly the same way as unaided human vision, but using both eyes can improve comfort, visual integration and movement for many users.

Biocular

A system using one image-intensifier tube whose output is divided between two eyepieces.

A biocular device is not the same thing as a dual-tube binocular.

Bridge

A mounting assembly that connects two monocular devices so they can be used together.

Bridged monoculars can provide flexibility, but they may differ from a purpose-built binocular in weight, balance, controls and adjustment.

Optical Components

Objective Lens

The front lens that gathers light from the environment and focuses it onto the image-intensifier tube.

The objective focus is adjusted as viewing distance changes.

Eyepiece or Ocular Lens

The rear lens through which the user views the tube's output screen.

Diopter Adjustment

An adjustment that focuses the tube's displayed image for the individual user's eye.

The diopter adjustment does not focus the outside environment. That is the objective lens's job.

Field of View — FOV

The angular width of the environment visible through the device.

A larger field of view shows more of the scene at once, but optical design always involves tradeoffs involving size, distortion, eye relief, resolution and weight.

Eye Relief

The distance between the user's eye and the eyepiece at which the full image can be viewed.

Collimation

The optical alignment of two channels in a binocular system so the images can be comfortably combined by the user's visual system.

Poor collimation can create eye strain, headaches or difficulty merging the two images. Proper collimation requires appropriate equipment and knowledge.

Inside the Image-Intensifier Tube

Photocathode

The light-sensitive input surface that converts incoming photons into electrons.

Modern Generation III tubes are characterized by a gallium-arsenide photocathode. L3Harris identifies the GaAs photocathode as a defining difference between Gen II and Gen III technology. See the L3Harris explanation of Gen III image-intensification technology.

Microchannel Plate — MCP

A thin plate containing millions of microscopic channels that multiply the electron signal.

The introduction of the MCP was the major architectural change associated with Generation II image intensifiers.

Phosphor Screen

The output surface that converts the amplified electron pattern back into visible light.

The phosphor screen produces the image color seen by the user.

Green Phosphor

A traditional phosphor output commonly associated with the familiar yellow-green night-vision image.

NVD identifies P-43 as the conventional green phosphor used in many systems. See NVD's explanation of phosphor types.

White Phosphor

A phosphor output producing a pale monochrome image that often appears white with a blue or blue-gray cast.

NVD identifies P-45 as a white-phosphor output. White phosphor is an output color—not a separate night-vision generation.

Tube Construction Terms

Filmed

A Generation III tube using an ion-barrier film to help protect the photocathode from ion feedback produced during operation.

Thin-Filmed

A Generation III design using a reduced-thickness ion barrier.

Thin-filmed remains Generation III.

Unfilmed

A Generation III design that removes the conventional ion-barrier film.

Unfilmed also remains Generation III. It is not formally "Generation IV." L3Harris states that the U.S. government has formally assigned three image-intensifier generations: Gen I through Gen III. See the L3Harris generation explanation.

Autogating

Rapid electronic regulation of the tube's operation as incoming light levels change.

Autogating improves performance in changing or mixed lighting and helps control the effects of bright sources. It does not make a tube impossible to damage.

Automatic Brightness Control — ABC

Internal control that reduces tube gain as output brightness rises beyond a designed threshold.

ABC helps keep the viewed image within a manageable brightness range.

Manual Gain

A user control that allows the apparent output brightness to be reduced.

Manual gain does not create additional environmental detail. It lets the user manage how brightly the available information is displayed.

Performance Terms

Gain

The amount of light amplification produced by the tube.

More gain can make an image brighter, but excessive visible noise or poor contrast can still limit useful performance. Brightness alone is not image quality.

Resolution

The tube's ability to distinguish fine detail, commonly reported in line pairs per millimeter.

Resolution is measured under controlled conditions. A high resolution number does not guarantee strong performance in very low light.

Signal-to-Noise Ratio — SNR

A measurement comparing useful image information to background noise.

Higher SNR generally allows the image to remain cleaner and more usable as available light decreases.

Figure of Merit — FOM

A calculated value: FOM = resolution × signal-to-noise ratio.

FOM is useful for broad classification, but it does not include every important tube characteristic. NVD provides the same FOM calculation in its image-intensifier overview.

Equivalent Background Illumination — EBI

The tube's background output when no useful light is reaching the photocathode.

Higher EBI can conceal faint scene information, particularly in warm conditions. Lower is generally desirable, but EBI must be considered with the rest of the tube's specifications.

Halo

The glow or circular area appearing around a concentrated bright light source.

A smaller halo can help preserve detail near headlights, streetlights and other high-contrast sources.

Photocathode Sensitivity

A measurement of how effectively the photocathode responds to incoming light.

This is useful technical information, but it should not be treated as a complete prediction of what the finished system will look like.

Tube Data Sheet or Spec Sheet

The individual test record supplied with an image-intensifier tube.

A legitimate tube data sheet may include SNR, resolution, FOM, EBI, halo, gain, photocathode response and cosmetic information.

The sheet describes one particular tube—not every tube from the same manufacturer or product family.

Image Characteristics and Artifacts

Scintillation

The sparkling or grain-like visual noise that becomes more obvious as available light decreases.

Scintillation is the system telling you that it is running short of useful photons.

Blooming

A broader loss of detail caused when an intense light source overwhelms part of the image.

Chicken Wire or Honeycomb

A faint fixed pattern associated with the structure or manufacturing boundaries of the microchannel plate.

L3Harris describes this as a hexagonal grid that may become visible under certain lighting and gain conditions. It is different from random scintillation.

Dark Spot

A fixed dark imperfection that remains in the same position as the device is moved.

Tube spots are normally evaluated by size and their location within inspection zones.

Zone 1, Zone 2 and Zone 3

Concentric areas used when describing the location of tube cosmetics.

• Zone 1: Center of the image
• Zone 2: Middle area
• Zone 3: Outer area

A spot near the center is generally more noticeable than an equivalent spot near the edge.

Environmental and Use Terms

Ambient Light

Light already present in the environment, including moonlight, starlight, sky glow and artificial lighting.

Passive Use

Operating an image-intensified system using existing environmental light without adding an infrared illuminator.

Active Infrared

Using an infrared illuminator, laser or beacon to add energy to the environment.

Active infrared may be difficult to see with the unaided eye, but it can be detected by other compatible night-vision equipment.

Infrared Illuminator

A device that projects infrared energy to provide additional illumination for night vision.

Think of it as a flashlight for the tube.

Infrared Aiming Laser

A laser that projects an aiming point intended to be viewed with compatible night-vision equipment.

Laser classification, output restrictions, eye safety and proper employment receive their own section later in the academy.

Photonic Barrier

A bright area that reduces the user's ability to see into or beyond a darker area.

Streetlights, headlights, illuminated windows and brightly lit doorways can become photonic barriers. They do not form a physical wall; they dominate the image and reduce contrast in the area behind or around them.

Detection, Recognition and Identification — DRI

Three different levels of visual certainty.

• Detection: Something is present.
• Recognition: Its general category can be determined.
• Identification: Its specific identity can be established with adequate confidence.

Seeing a warm shape through thermal or a moving shape through night vision does not automatically mean it has been identified.

BigRed's Short Version

The tube makes the image. The housing protects the tube. The objective focuses the outside world. The diopter focuses the display for your eye.

White phosphor is a color—not a generation.

FOM is useful—but it is not the whole report card.

Autogating helps—but it does not make the tube immortal.

And "I can see something" is not the same statement as "I know exactly what it is."