Chickenwire -

The hexagonal shape of some MCP fibre optic bundles may be seen as thin black lines, hence the name ‘chickenwire’. Could also be confused with Shear Distortion. See also ‘Fixed Pattern Noise’.
Cycles per Milliradian (cy/mr) -

Units used to measure system resolution. A milliradian is the angle created by 1 yard at a distance
of 1,000 yards. This means that a device that can detect two 1/2-yard objects separated by 1/2 yard at 1,000 yards has a resolution of 1.0 cy/mr. Do not confuse cy/mr with line pairs per millimeter (lp/mm). For example, a system can have a 3x lens attached and increase the system resolution by a factor of 3, but the image intensifier's resolution (measured in lp/mm) will not have increased.
Dioptre -

A unit of measure used to define eye correction or the refractive power of a lens. Allows adjustment of an optical eyepiece to accommodate for differences in individual eyesight.
Distortion -

Three types of distortion are most significant to night vision devices: geometric, "S", and shear.

• Geometric distortion is inherent in all Gen O and Gen 1 image intensifiers that use electrostatic rather than fibre-optic inversion of the image. Geometric distortion is eliminated in image tubes that use a microchannel plate and fibre optics for image inversion; however, some S-distortion can occur in these tubes.
• S-distortion results from the twisting operation in manufacturing fibre-optic inverters. S-distortion is usually very small and it is
  difficult to detect with the naked eye. Gen III image tubes manufactured to military standards since 1988 have virtually no perceptible S-distortion.
• Shear distortion can occur in any image tube that uses fibre-optic bundles (MCP’s). It appears as a cleavage or dislocation in a straight line within the viewable area.
• Non-inverting image intensifier tubes (mainly used in goggles) that use an MCPs and clear glass for the optics are free of distortion.

Emission Point -

A steady or fluctuating pinpoint of bright light in the image area that remains even when all light is blocked from the objective lens. An emission point will remain in a fixed position within the viewable area. If an emission point disappears or is only faintly visible when viewing under brighter nighttime conditions, it is not indicative of a problem. If the emission point remains bright under all lighting conditions this may indicate damage to the intensifier tube.

Equivalent Background Illumination (EBI) -

EBI is measured in lumens per square centimeter (lm/cm2), a lower value being better. The EBI level determines the lowest light level at which you can detect something. Below this level, objects will be masked by the EBI.

Field Of View (FOV) -

The measure of the angle (in degrees) defining the field visible through a night vision device.

Figure of Merit (FOM) -

Standard measurement now being used for defining the performance of a tube. FOM=SNR x Resolution.
Fixed-Pattern Noise (FPN) -

A faint hexagonal (honeycomb) pattern throughout the image. Most often occurs under high light
conditions. Inherent in the structure of the microchannel plate, this can be seen in virtually all Gen 2/Gen 3 systems .
Footcandle (fc) -

A unit of illuminance equal to one lumen per square foot.
Footlambert (fL) -

A unit of brightness equal to one footcandle at a distance of one foot.
Gain -

Also called brightness gain or luminance gain. The number of times a night vision device amplifies light input. Usually measured as tube gain or system gain. Tube gain is measured as the light output (in fL) divided by the light input (in fc). Usually seen in values of tens of thousands and more a laboratory figure that does not necessarily indicate a devices performance. If tube gain is too high, the tube will be "noisier" and the signal-to-noise ratio may go down. System gain is measured as the light output (fL) divided by the light input (also fL) and is based on the total system (ie tube, optics, power supply etc). System gain is usually seen in the low to mid-hundreds for Gen 1/Super Gen 1+ and several thousand for Gen 2/2+/SuperGen/HyperGen and Gen 3. System gain is the figure that potential night vision purchasers should compare, but be aware that different companies may use different test criteria. Beware of claims, especially for Gen 1 units, in the tens of thousands - this can only be tube gain and not system gain.

This figure is usually seen in values of tens of thousands. If tube gain is pushed too high, the tube will be "noisier" and the signal-to-noise ratio may go down.  military Gen II and Gen III image tubes operate at gains of between 20,000 and 37,000.
On the other hand, system gain is measured as the light output (fL) divided by the light input (also fL) and is what the user actually sees. System gain is usually seen in the thousands. military systems operate at 2,000 to 3,000. 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) -

A semiconductor material used in Gen 3 photocathodes. GaAs photocathodes have very high photosensitivity in the 450 to 950 nanometer spectral region.
Generation - See ‘4… THE GENERATION GAME’
Halo -

Measurement of blooming around a bright spot of light visible by an image intensifier tube.
Infrared (IR) Illuminator -

A powered light source, much like a torch, but in the near infrared range (generally 750-1000nm) that provides light for an image intensifier. Examples include light emitting diodes (LED’s), incandescent bulbs and lasers (non-eyesafe). Infrared light is normally invisible to the human eye, but a red glow may be visible from the illuminator’s bulb. Fully covert IR illuminators are available, but can only be ‘seen’ by top range Gen 2/Gen 3 systems. Many budget-priced commercial night scopes come supplied with a short-range LED IR Illuminator to enhance performance in low light/total darkness. All IR illuminators are ‘active’ and can clearly be seen by anyone else equipped with a night scope.
Line Pairs per Millimeter (lp/mm) -

A unit used to measure image intensifier resolution.Units used to measure image intensifier resolution. Usually determined from a 1951 Air Force Resolving Power Test Target. The target is a series of different sized patterns composed of three horizontal and three vertical lines. You must be able to distinguish all the horizontal and vertical lines and the spaces between them to qualify for that pattern
Lumen -

A unit of measurement denoting the photons perceivable by the human eye in one second.
Maximum Relative Viewing Range -

To express any sort of range performance, you must delineate the criteria that is used for the measurement - are you trying to see a fox, a man or an elephant and in what lighting conditions ? As expressed by Cobra Optics, and purely for comparison of their own devices, this is based on a man-sized object in good moonlight conditions. If there is less than good moonlight or the object being viewed is smaller than a man, maximum range should be reduced accordingly. Performance will drop off most noticeably with budget Gen 1 systems as these factors reduce. Note that some suppliers use different criteria or do not quote the criteria their range claim is based upon.
Microamperes per Lumen (μA/lm) -

The measure of electrical current (μA) produced by a photocathode when it is exposed to a measured amount of light (lumen).
Microchannel Plate (MCP) -

A thin metal-coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in Gen 2/2+/SuperGen/HyperGen and Gen 3 systems. MCPs consist of an array of glass (fibre optic) channels (anywhere from 2 to 6 million). In the best quality MCP’s the diameter of each channel can be 4-6μm. Electrons entering a channel strike the wall and knock off secondary electrons which in turn knock off even more electrons producing a cascading effect - this can result in the production of several thousand times the input number. MCP's eliminate the distortion characteristic of Gen O/Gen I systems, giving edge-to-edge definition and making devices using them most suitable for photographic/video applications.
Milliamps per Watt (mA/W) -

The measure of electrical current (mA) produced by a photocathode when exposed to a specified wavelength of light at a given radiant power (watt). As with microamperes per lumen, generally, the higher the value, the better the performance; however, it is important to know where in the spectrum the wavelength falls. Because light at night is primarily in the near-infrared spectrum (some 70%), a high value in the blue regions (400-750nm) is not as good as a moderate value in the nearinfrared
(750-1000nm).
Monocular -

A night vision device for use with one eye.
MTTF -

Mean Time To Failure - the expected lifespsn of a tube based on use in optimum conditions.
Multi-Alkali -

A semiconductive coating for Gen 1 and Gen 2 photocathodes.
Nanometer (nm) -

Unit of measurement for electromagnetic energy.
Near-Infrared -

This is the amount of light you see in an image tube that is turned on but there is no light at all on the photocathode; it is affected by temperature where the warmer the night vision device, the brighter the background illumination. EBI is measured in lumens per square centimeter (Im/cm2) wherein the lower the value the better. The EBI level determines the lowest light level at which you can detect something and, below this light level, objects will be masked by the EBI.

The shortest wavelengths of the infrared region, nominally 750 nanometers to 1μm . Gen 1 operates from around 300 to 830nm. Gen 2 operates from around 400 to 900nm. Gen 3 operates from around 450 to 950nm.
Noise -

A faint sparkling effect throughout the night vision image. Also called video noise or scintillation. It’s presence can indicate that the system is struggling to amplify what little light is available.

Phosphor Screen -

Positioned at the rear of the intensifier tube, the phosphor screen flouresces when hit by the flow of electrons within the tube and gives us a visible image. Because it is flat, it gives little or no depth perception. Green phosphor is used because the human eye can detect more contrast in green than any other phosphor colour.
Photocathode -

The input surface of an image intensifier that absorbs light energy (photons) and converts it to electrical energy (electrons). Different combinations of photocathode and input window materials can affect spectral response characteristics.
Photonic Spectrum -

 The range of electromagnetic energy from 0.1nm (x-ray) to 100μm (far-infrared). Within this range is the visible band from 400-750nm and near-infrared from 750nm-3μm.
Photoresponse (PR) -

See Photosensitivity.
Photosensitivity -

The ability of the photocathode material to produce an electrical response when subjected to light waves (photons). Usually measured in microamperes per lumen (μA/lm). The higher the value, the higher the capability of producing a visible image in dark conditions.
Resolution -

The ability of an image intensifier or night vision system to distinguish between objects close together. Image intensifier resolution is measured in line pairs per millimeter (lp/mm) while system resolution is measured in cycles per milliradian. For any particular night vision system, the image intensifier resolution will remain constant while the system resolution can be affected by
altering the objective or eyepiece optics or by adding magnification, relay lenses, filters etc. The resolution can be noticeably different when measured at the centre of the image and at the periphery. This is very important for devices selected for photography or video where the edge-to-edge image resolution is important.
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. Normally only quoted on mil spec devices.
Scintillation -

 A faint, random, sparkling effect throughout the image area. This is a normal characteristic of image intensifiers fitted with a microchannel plate (MCP). Most noticeble under the lowest light levels, it is sometimes called "video noise". Do not confuse scintillation with emission points.
Spectrum -

 See Photonic Spectrum.The range of electromagnetic energy from cosmic rays to extra-low frequency used in submersed submarine communication. Electromagnetic frequency is measured in cycle per second and wavelength in microns or nanometers. The ultraviolet region extends from 100 to 400 nm with the near-ultraviolet nominally 300 to 400 nm. The visible portion of the spectrum extends from 400 to 750nm. The infrared region extends from 750 to 2xlO5 nm with the near-infrared nominally 750 to 2,500 nm.
Wavelength -

Electromagnetic energy is transmitted in the form of a sinusoidal wave. Wavelength is the physical distance covered by one cycle of the wave. In the photonic spectrum, wavelength is measured in nanometers (nm) and micrometers (μm).

Diopter -

The unit of measure used to define eye correction or the refractive power of a lens. Usually adjustments to an optical eyepiece accommodates for differences in individual eyesight. Many military system provide a +2 to -6 diopter range.

Eye Relief -

The distance your eyes must be from the last element of an eyepiece in order to achieve the optimal image area.

Evaluation
Characteristics of night vision equipment fall into four major categories that help in evaluation: performance, human factors, suitability to its application, and the overall cost of ownership.

Performance
The very need for a night vision capability necessarily focuses on performance as the most important - Can you see a clear image when it is dark and you cannot see the scene or objects with your unaided eye? Most night vision equipment available today will provide an adequate image under higher night light conditions such as a full moon. Evaluate the following parameters to determine how well a system will perform when you need to see under truly dark conditions such as starlight.

• Photosensitivity

The ability of a night vision system to detect light energy and convert it to an electron image is reflected in the image intensifier's photosensitivity. Usually, the higher the value, the better the ability to "see" under darker and darker conditions. However, be aware that at night there is more light energy available in the near-infrared region than in the visible region. Therefore, if a device claims a high photosensitivity, make sure to find out where in the spectrum this is measured. A high photosensitivity in the blue or visible region may not perform as well as another system with a lower overall photosensitivity, but a higher value in the near-infrared region.

• Signal-to-Noise Ratio (SNR)

This is probably the single most significant factor in determining a system's ability to see when it gets dark. Be aware that SNR can be computed many ways to get desired results. Be sure to find out how SNR was computed. When measured according to mil specs, the SNR takes into account the photosensitivity, as well as the efficiency of the phosphor screen in reconverting the electron image to visible light and the "noise" contribution of the microchannel plate. Because the, SNR determines an image intensifier's low-light-resolution, the higher the ratio, the clearer will be the signal compared to the background noise, hence, the better the ability to see under increasingly darker conditions.

• Gain

This tends to be a confusing parameter when evaluating night vision devices. The most important gain measurement is the system gain. Very high gain values for an image tube are not especially significant - the military procures devices with the tube gain ranging from 20,000 to 37,000. Look for the system gain. military systems operate at 2,000 to 3,000. The higher the value the better the ability of the device to amplify the light it detects.

A word of caution; gain is only part of the story. If a system does not possess a good photosensitivity and SNR, a very high gain value simply means that you will make a poor image brighter, not better. Also, very high gain values could mean the tube is driven very hard and the life of the tube will be reduced. The very best test is field evaluation under very dark conditions.

• Resolution

Usually this is measured as tube resolution (lp/mm) or system resolution (cy/mr). The more significant measurement is system resolution as this is what the viewer will actually experience and takes into account the quality of the system's optics. If you are evaluating systems with similar optical quality and filters, the tube resolution is an important criteria. Resolution is often measured at high and low-light conditions. Most systems produce an optimal resolution at some point between very high light and very low light conditions.

As long as resolution is measured the same way using the same magnification and the same conditions (i.e., per mil specs) the higher the value, the better the ability to present a sharp picture. However, be aware that many devices will produce a sharp image in the center of the viewing area, but less sharp as you look toward the periphery. The lack of a sharp image, except at the center of the viewing area, can be due to the presence of a Gen 0 image tube or to the system's optics. Again, remember that many night vision systems will produce adequate results under higher night-light conditions, but perform poorly under darker conditions.

• Human Factors

Here, such issues as weight, size, safe equipment, and the ease of operation should be considered. Remember that the ease of operation should be determined under dark conditions where the user cannot see the device being used. What may appear to be an acceptable level of operating ease under room lights may not be "user friendly" at all when it is dark. Protracted use should also be considered when evaluating weight. What may seem an acceptable weight when using a device for a short time, may not be so when viewing for long periods of time.

Additionally, consider such functions as the on/power switch. Will you need to continually hold down the switch? - even light pressure for one finger for a long time can produce fatigue. Do you need to repeatedly press the switch to recharge the image tube? - such devices usually produce an initially bright image which gradually fades, reducing the ability to see and then shuts off unless you repress the switch. This characteristic could cause you to lose an image at a crucial moment.

• Suitability to its Application

Within this category, characteristics such as field of view (FOV), magnification, versatility, weather resistance, and image distortion affect the ability of a night vision device to perform as needed.

• Magnification and FOV

Regarding magnification and FOV, consider the distance you will need and the overall area you are observing or searching. For most surveillance or search applications, the higher the magnification or narrower the FOV, the greater the number of times you need to scan an area to avoid missing important objects or events. Usually a 1:1 lens with a 40 ° FOV provides optimal performance.

For long range observation or weaponsight applications, the amount of magnification needed will vary; however, be sure to consider the other performance characteristics of the device; as the magnification increases, FOV decreases and the F number increases, all reducing the amount of light captured. Consequently, you will need an image tube with excellent performance at very low-light levels and/or high-performance lenses.
Another factor involves the versatility of a device if it is used in situations that may require different magnification. How easily and quickly can the magnification be changed? Is it necessary to open the system to install the optics? In some cases, this may be inescapable, and the susceptibility of internal components to damage should be considered.

• Distortion

Gen 0, Gen I, and 25-mm Gen II electrostatically inverted image tubes produce a certain amount of geometric distortion in the image. In Gen III and 18-mm Gen II systems, geometric distortion is eliminated although it is possible to encounter some perceptible "S" and sheer distortion. The degree of any distortion and its interference with the application should be considered. When the application involves photography, video work, or weaponsights, the distortion and peripheral resolution are critical.

• Weather Resistance

The ability of a night vision system to operate under adverse environmental conditions is another important factor. Any system built to mil specs for environmental factors will perform suitably under almost any condition encountered. The major concern is internal fogging that destroys the ability to see an image, hence, the ability to resist humidity and moisture is vital.

In addition, when a night vision system is used on or around rivers or bodies of water, floatability can be a determining factor.