Glossary of Night Vision Terms

Automatic Brightness Control (ABC)

An electronic feature which automatically reduces voltages to the microchannel plate to keep the image intensifier’s brightness within optimal limits, and protects the tube. The effect of this can be seen when rapidly changing from low-light to high-light conditions; the image gets brighter and then, after a momentary delay, suddenly dims to a constant level.

Black Spots

Cosmetic blemishes in the image intensifier or dirt/debris between the lenses. Black spots in the image intensifier do not affect the performance or reliability of the device and are inherent in the manufacturing processes.

C-Mount

A standard still and video camera lens thread size for mounting to the body of a camera. Usually 1/2" or 3/4" in diameter.

Diopter

The unit of measure used to define eye correction or the refractive power of a lens. Adjustments to an optical eyepiece accommodate for differences in individual eyesight. Most systems provide a +2 to -6 diopter range.

Eye Relief

The focus-range distance between the user’s eye and the eyepiece lens of an optical device at which the viewed scene can clearly be seen.

Field-of-View

The diameter of the imaged area when viewed through an optic.

Gain

Also called brightness gain or luminance gain. This is the number of times a night vision device amplifies light input. It is usually measured as tube gain and system gain. In any night vision system, the tube gain is reduced by the system’s lenses and is affected by the quality of the optics or any filters; therefore, system gain is a more important measurement to the user.

Gallium Arsenide (GaAs)

The semiconductor material used in manufacturing the GEN III photocathode.

Generations of Image Intensifiers

Image intensifier tubes are classed by generation (GEN) numbers assigned by the US Army’s Night Vision Laboratory. Each GEN denotes a new technology or manufacturing process that improved since the previous generation.

• GEN I - Vietnam Era technology. GEN I has the most distortion and the least tube life. GEN I image tubes are most commonly found in imported night vision devices.
• GEN II - 1960s development of microchannel plate (MCP) amplifier component provided the needed breakthrough for high gain and small size.
• GEN II+ - 1970s development increased image tube bias voltage to improve gain. Additionally, a glass faceplate was added to improve resolution.
• GEN III - 1980s development provided substantially improved gain and a bandwidth which reached farther into the near IR region. Improvements in the microchannel plate (MCP) and a gallium-arsenide photocathode greatly enhanced tube life and performance.
• GEN III+ - Generally referred to as GEN III Enhanced, 1990’s improvements in the photocathode and MCP resulted in increased gain and resolution. Not a generally accepted term, but used herein for brevity.

IR Illuminator

Many night vision devices incorporate an infrared (IR) diode which emits invisible light. IR light cannot be seen by the unaided eye; therefore, a night vision device is necessary to see this light.

IR Laser

High-power devices providing long-range illumination capability. Ranges of several thousand meters are common. Most are not eye-safe and are restricted in use. Each IR laser should be marked with a warning label like the one shown here. Consult FDA CFR Title 21 for specific details and restrictions.

Image Intensifier Tube

A high-vacuum device which collects photons and amplifies these as electrons. This amplification of photons allows the viewer to see more light than without a night vision device.

Line Pairs per Millimeter (lp/mm)

Units used to measure image intensifier resolution.

Magnification

The magnifying power of the lens. Four power (4X) indicates that the image will appear four times larger than if viewed with a 1X lens.

Microchannel Plate (MCP)

A metal-coated glass disk which multiplies the electrons produced by the photocathode. Found only in GEN II and GEN III systems, the MCP can have between 2 and 6 million holes (or channels) in it.

MILSPEC

Term for Military Specification. It is the minimum acceptable requirements for products procured by the Department of Defense. Use of the term MILSPEC indicates that the product meets applicable military specifications.

NATO-STANAG

Term for the North Atlantic Treaty Organization STANdard AGreement. This can be described as an international MILSPEC. Photoresponse - See Photosensitivity.

Photosensitivity (Photoresponse)

Also called photocathode sensitivity. The ability of the photocathode material to produce an electrical response when subjected to light waves (photons). The higher the value, the better the ability to produce a visible image under darker conditions.

Resolution

The ability of an image intensifier or night vision system to distinguish between objects close together. Measured in line pairs per millimeter (lp/mm).

Signal-to-Noise Ratio (SNR)

A measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. A tube’s SNR determines the low-light-resolution of the image tube; therefore, the higher the SNR, the better the ability of the tube to resolve objects with good contrast under low-light conditions. Because SNR is directly related to the photocathode’s sensitivity and also accounts for phosphor efficiency and MCP operating voltage, it is the best single indicator of an image intensifier’s performance.

System Gain

Equal to tube gain minus losses induced by system components such as lenses, beam splitters and filters.