Editing Image intensifier (section)

==Terminology==
There are several common terms used for Image Intensifier tubes.

===Gating===
Electronic Gating (or 'gating') is a means by which an image intensifier tube may be switched ON and OFF in a controlled manner. An electronically gated image intensifier tube functions like a camera shutter, allowing images to pass through when the electronic "gate" is enabled. The gating durations can be very short (nanoseconds or even picoseconds).  This makes gated image intensifier tubes ideal candidates for use in research environments where very short duration events must be photographed. As an example, in order to assist engineers in designing more efficient combustion chambers, gated imaging tubes have been used to record very fast events such as the wavefront of burning fuel in an internal combustion engine.

Often gating is used to synchronize imaging tubes to events whose start cannot be controlled or predicted.  In such an instance, the gating operation may be synchronized to the start of an event using 'gating electronics', e.g. high-speed digital delay generators. The gating electronics allows a user to specify when the tube will turn on and off relative to the start of an event.

There are many examples of the uses of gated imaging tubes. Because of the combination of the very high speeds at which a gated tube may operate and their light amplification capability, gated tubes can record specific portions of a beam of light. It is possible to capture only the portion of light ''reflected'' from a target, when a pulsed beam of light is fired at the target, by controlling the gating parameters. Gated-Pulsed-Active Night Vision (GPANV) devices are another example of an application that uses this technique. GPANV devices can allow a user to see objects of interest that are obscured behind vegetation, foliage, and/or mist. These devices are also useful for locating objects in deep water, where reflections of light off of nearby particles from a continuous light source, such as a high brightness underwater floodlight, would otherwise obscure the image.

===ATG (auto-gating)===
Auto-gating is a feature found in some image intensifier tubes. Auto-gated tubes rapidly shut off current to the photocathode and microchannel plate at a high frequency that is imperceptible to the user. By varying the duty cycle, it is possible to reduce the overall "ON" time of the image intensifier, while still presenting an image to the user. Auto-gating allows the tube to be operated during brighter conditions, for example when observing bright flashes on the battlefield, or in conditions of higher ambient light, while maintaining image resolution.
<ref>Characterization of domestic and foreign image intensifier tubes {{doi|10.1117/12.2015462}}</ref>

===Sensitivity===
The sensitivity of an image intensifier tube is measured in [[microampere]]s per [[lumen (unit)|lumen]] (μA/lm). It defines how many electrons are produced per quantity of light that falls on the photocathode. This measurement should be made at a specific [[color temperature]], such as "at a colour temperature of 2854&nbsp;K". The [[color temperature]] at which this test is made tends to vary slightly between manufacturers. Additional measurements at specific wavelengths are usually also specified, especially for Gen2 devices, such as at 800&nbsp;[[nanometer|nm]] and 850&nbsp;nm (infrared).

Typically, the higher the value, the more sensitive the tube is to light.

===Resolution===
More accurately known as '''limiting resolution''', tube resolution is measured in line pairs per millimeter or lp/mm. This is a measure of how many lines of varying intensity (light to dark) can be resolved within a millimeter of screen area. However the limiting resolution itself is a measure of the Modulation Transfer Function. For most tubes, the limiting resolution is defined as the point at which the modulation transfer function becomes three percent or less. The higher the value, the higher the resolution of the tube.

An important consideration, however, is that this is based on the physical screen size in millimeters and is not proportional to the screen size. As such, an 18&nbsp;mm tube with a resolution of around 64 lp/mm has a higher overall resolution than an 8&nbsp;mm tube with 72 lp/mm resolution. Resolution is usually measured at the centre and at the edge of the screen and tubes often come with figures for both. Military Specification or [[milspec]] tubes only come with a criterion such as "> 64 lp/mm" or "Greater than 64 line pairs/millimeter".

===Gain===
The gain of a tube is typically measured using one of two units. The most common (SI) unit is cd·m<sup>−2</sup>·lx<sup>−1</sup>, i.e. [[candelas]] per meter squared per [[lux]]. The older convention is Fl/Fc ([[foot-lambert]]s per [[foot-candle]]). This creates issues with comparative gain measurements since neither is a pure ratio, although both are measured as a value of output intensity over input intensity. This creates ambiguity in the marketing of night vision devices as the difference between the two measurements is effectively [[pi]] or approximately 3.142x. This means that a gain of 10,000&nbsp;cd/m<sup>2</sup>/lx is the same as 31.42 Fl/Fc.

===MTBF ([[mean time between failure]])===
This value, expressed in hours, gives an idea how long a tube typically should last. It's a reasonably common comparison point, however takes many factors into account. The first is that tubes are constantly degrading. This means that over time, the tube will slowly produce less gain than it did when it was new. When the tube gain reaches 50% of its "new" gain level, the tube is considered to have failed, so primarily this reflects this point in a tube's life.

Additional considerations for the tube lifespan are the environment that the tube is being used in and the general level of illumination present in that environment, including bright moonlight and exposure to both artificial lighting and use during dusk/dawn periods, as exposure to brighter light reduces a tube's life significantly.

Also, a MTBF only includes operational hours. It is considered that turning a tube on or off does not contribute to reducing overall lifespan, so many civilians tend to turn their night vision equipment on only when they need to, to make the most of the tube's life. Military users tend to keep equipment on for longer periods of time, typically, the entire time while it is being used with batteries being the primary concern, not tube life.

Typical examples of tube life are:

First Generation: 1000 hrs<br>
Second Generation: 2000 to 2500 hrs<br>
Third Generation: 10000 to 15000 hrs.

Many recent high-end second-generation tubes now have MTBFs approaching 15,000 operational hours.

===MTF (modulation transfer function)===
The [[modulation transfer function]] of an image intensifier is a measure of the output amplitude of dark and light lines on the display for a given level of input from lines presented to the photocathode at different resolutions. It is usually given as a percentage at a given frequency (spacing) of light and dark lines. For example, if you look at white and black lines with a MTF of 99% @ 2 lp/mm then the output of the dark and light lines is going to be 99% as dark or light as looking at a black image or a white image. This value decreases for a given increase in resolution also. On the same tube if the MTF at 16 and 32 lp/mm was 50% and 3% then at 16 lp/mm the signal would be only half as bright/dark as the lines were for 2 lp/mm and at 32 lp/mm the image of the lines would be only three percent as bright/dark as the lines were at 2 lp/mm.

Additionally, since the limiting resolution is usually defined as the point at which the MTF is three percent or less, this would also be the maximum resolution of the tube. The MTF is affected by every part of an image intensifier tube's operation and on a complete system is also affected by the quality of the optics involved. Factors that affect the MTF include transition through any fiber plate or glass, at the screen and the photocathode and also through the tube and the microchannel plate itself. The higher the MTF at a given resolution, the better.