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Page 1 (General) Page 2 (Generations) Page 3 (Low Light, Objektive Lens) Page 4 (Image Intensifier Tube) Page 5 (Eyepiece Lens, IR Light Sources)
Page 1 (General) Page 2 (Generations) Page 3 (Low Light, Objektive Lens) Page 4 (Image Intensifier Tube) Page 5 (Eyepiece Lens, IR Light Sources)

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7. Image Intensifier Tube (IIT)

The first generations of image intensifier tubes (Gen0, Gen1) were individually shaped. Sometimes their design was adapted to the need of the night vision device and therefore a large number of different tubes appeared. Approximately starting from the 2nd Generation on a modular concept became generally accepted with many night vision devices. The tube module with a miniaturized power supply was combined to one single unit. This way night vision devices could be designed around a standardized IIT-module size. In case of a tube failure or an upgrade to a state of the art tube the whole expensive optical device can be used furthermore. Easier and faster maintenance is another advantage.
Apart from a few special applications of image intensifier tubes today there are essentially two designs: Modern image intensifier tubes have a working plane of 18 mm or 25 mm in diameter. Both the input window (the photocathode) and the output window (the phosphor screen) are about the same size. Due to further efforts in reducing the weight and size of night vision devices, suitable for law enforcement and military applications, the 18 mm image intensifier tube became accepted as a standard particularly for NV-monoculars and night vision goggles. The goal is to reduce the dimensions of the IIT without unnecessarily limiting its efficiency (the smaller the working plane, the less quantities of light can be processed). An area of 18 mm in diameter is about 250 square milimeters of working plane, which was considered to be a limit for head-mountable units. Tubes utilized in sniperscopes or oberservation units often have a working plane diameter of 25 mm. This is almost twice the size (approx. 500 square milimeters) of an 18 mm tube. Here its less the compactness which counts but more the overall performance in system light gain. Up to a certain point a 25 mm tube can compensate light losses from a complex optic because it simply processes twice as much light than an 18 mm tube.
For obvious reasons the smallest but most efficient tubes are used within aviation applications: In flight possible high g-accelerations and fast head movements must still be accomplished with night vision goggles mounted. Because of latest developments (miniaturization) in night vision technology it appears to be realistic that next Generations of IITs will have a working plane diameter of 16 mm or even 12 mm. Besides this there still will be special types of IITs, e.g. the (outdated) design of a three-staged 1st Generation tube (3 stacked up Gen1-tubes, tube-on-tube design), or for night vision goggles with increased field of view.

left:
sectional view Gen1, single-stage NVD, low gain, in center of image resolution is acceptable


right:

sectional view Gen1,
3-stage NVD, generally high gain at low contrast and resolution

Meanwhile most night vision devices use the 18 mm proximity image Intensifier tube. Although with night vision goggles only the use of two independent tubes provides a genuine depth perception, this configuration is rather avoided out of reasons of cost. The production of an image intensifier tube is still very complex and sensitive (by the way: dual tube design provides additional safty in case of tube failure).

Applying the sensitive coating of the photocathode and the phosphor screen is one thing, an other one is the production of the main unit, the micro channel plate (MCP). It must be arranged from up to 12 million fibers, with a out-corrodable core, bundled in a hexagonal structure. After an etching procedure the manufacturer receives a plate out of hollow fibers (micro channels). This plate will afterwards be coated with a film of electrically leading material (thin film technology). In the end photocathode, MCP and phosphor screen (if also necessary glass fiber twist) must be manufactured without any blemishes as far as possible and put together in a vacuum-sealed housing.


As already described in the working principle, the actual amplification (the multiplication of the electrons driven out from the photocathode by particles of low light) is achieved within the micro channels by applying a high voltage to both sides of the MCP. Via built-in circiuts of the power supply (attached around the tube module) it is possible to control the image intensification by constantly measuring and regulating the output current. Practically this is an adjustment to actual light conditions of the surrounding environment for protection of the tube from high wear (ABC function, Automatic Brightness Control). Another function of modern tubes against cross fade (e.g. by direct light sources) within the image is BSP (Bright Source Protection). This function prevents that a spot-like light source over-radiates the background or large parts of the image (without BSP: gain would be strongly reduced, large areas of the image would not be visible any more). Technically seen this function is connected with the 'natural saturation point' of the MCP (i.e. maximum emission degree of electrons from the photochemical coating of the micro channel's inner wall).

The described functions do not only bring image quality improvements, but ensure also a substantially longer life span of modern image intensifiers compared with the tubes of the 1st Generation. Gen1 tubes were 'loaded' in fact like a capacitor by switching on (generally no continuous operation of the power supply was required, short switching on on was enough to see for a few minutes). This was practised on one hand in order to save battery and on the other hand to avoid a damage from strong light sources. Because Generation 1 IITs can get seriously damaged even by switching on for a few seconds in bright light conditions, some manufacturers later equipped selected top of the line Gen1 tubes with a power supply featuring an ABC function (caution: Most Gen1 tubes are WITHOUT ABC! Motionless sources of light and steady images can burn in with all generations of IITs - it is only a question of time!). Under ideal conditions, i.e. no direct observation of very bright sources of light, keeping protected from light when not used (for example also with switched off equipment direct sunlight can cause a lasting damage of the photocathode) and no operation within fields of strong electromagnetic radiation the service life of 1st Generation tubes is about 1000-2000 working hours. By definition the end of an image intensifiers 'life' is reached when the tube performs only at 50 percent of the original light amplification. Within professional application it is considered as 'used up' and will be replaced by a new one. While the service life of the 2nd Generation has been increased to approx. 2500-5000 hours, current 3rd Generation tubes can operate approx. 10,000-15,000 hours before they are used up. In reality is not a total of working hours indicated by the manufacturer what limits the service life of an image intensifier mostly. Under military conditions a tube is either prematurely used up because of increased wear or replaced by a current one due to the technological progress.

In some cases a reprocessing of used Gen2 & Gen3 image intensifier tubes for the commercial market is worthwhile. This procedure is called 'Reconditioning' and it includes adjusting of the power supply and a new sealment of the tube. However it should not remain unmentioned that a reliable statement over the further life span of a reconditioned tube is very delicate. Generally an image intensifier tube with the usual manufacturer guarantee, which does not suffer a failure right at the beginning of its employment has a good chance of operating throughout the full life span.

Because of the difficult production each new tube is an unique example in the context of certified performance characteristics (specifications). Fresh from the factory every image intensifier tube comes with a data sheet of the manufacturer, on which the actually measured characteristics are registered. Even certain inevitable blemishes, e.g. acceptable spots, are noted on this document (by the way: as a further reason for high prices and evidence of the difficult production process an inevitably quantity of sub-standard tubes per quality grade tube, which meets military specifications, is produced along). A variety of measured performance parameters of an IIT is specified on the data sheet.

In order to be able to make a statement about the quality of a tube, first reference is provided by the data of the photocathode's sensitivity. Under standardized conditions (an emitter with defined color temperature of 2856° K) the amount of emitted electrons stimulated by this irradiation of the photocathode is measured - i.e. the produced level of electrical current. Indicated in microamperes per lumen (µA/L) meanwhile modern 3rd Generation tubes can reach values over 2100 µA/L (in comparison: Gen1 max. 150 µA/L, Gen2 max. 600 µA/L, Gen3 starting from approx. 1000 µA/L). The high performance of a gallium-arsenide (GaAs) coating of the photocathode becomes particularly clear at values for sensitivity within the infrared range. This parameter is determined in milliamperes per watt (mA/W) at a wavelength of 830 Nm. Due to the high infrared portion in the nocturnal low light this parameter describes the utilization of low light resources very precisely. In this measuring range the values from older photocathodes are generally very low, so that they are even hard to detect from tubes of 2nd Generation. On the other hand the most modern tubes of the 3rd Generation reach values up to approx. 200 mA/W.

Due to the instability of the GaAs coating of the photocathode a protective film (ion barrier) must be attached, which unfortunately is also reducing the performance of the Gen3 IIT. So-called new 'filmless' or 'autogated' tubes do not have an ion barrier (or at most a very thin one), but shutter the power supply in intervals of a few milliseconds that the GaAs coating cannot get damaged any more by misguided free ions. Whether this technology will officially be designated as 'Generation 4' is determined on another level.

A further interesting parameter describes the resolving power of a image intensifier tube. Two far away objects, which are standing next to each other, can merge optically into a single object. Therefore the optical resolution of a tube is an important quality criterion, which is measured by pairs of lines per millimeter (lp/mm). The maximum number of clearly distinguishable pairs of lines, displayed on a standard board at in a certain distance, is counted within one millimeter of the phosphor screen. night vision devices of the American armed forces have a minimum resolution of 64 lp/mm at present (in comparison: older Gen1 and Gen2 devices are only around 25-40 lp/mm). However, higer standards are on the brink of breakthrough (for example some European manufacturers already guarantee 78 lp/mm).

During the transformation of light into electrons the natural background noise of a tube becomes apparent to the user as a noisy or grainy image. Under very low light conditions (e.g. leafy forest at new moon) this effect can dominate the image and outweigh small differences in brightness. The ratio of light information to background noise is indicated as 'signal to noise ratio (S/N)'. With this parameter given on the data sheet it is possible to assess the low light performance. The first Gen2 IITs had a S/N of 4.5 while nowadays tubes of the 3rd Generation have about 20 (with peak values over 30). With the values of luminance sensivity, resolution and signal to noise ratio, image intensifier tubes can be assessed already quite exactly in their overall-performance. Further measured values on the data sheet (e.g. the ratio of input to output information, the so-called 'modular transfer function, MTF'), provide additional information for a more exactly determination of the tube's quality.

Often a certain multiple of (total) light amplification is unfortunately stated as an apparent criterion for night vision devices. The light amplification (luminance gain) is also registered on the data sheet, but however refers only to the image intensifier tube and does not represent a multiple in overall light gain (system light gain). It is only the ratio of incoming to outgoing density of light (measured in 'footlambert by footcandela, fL/fc'). A tube of Generation 1, which in first place has a 'light information loss' from over two thirds at the photocathode - in order to increase in second place the small remainder by ten times, has of course a high internal ratio of 'light multiplication', but nevertheless probably only a detail-poor image. On the one hand 3-staged tubes of the 1st Generation are very bright, but on the other hand also 'information losses' unfortunately multiply by serial stacking (low-contrasting image, like a 'too brightly adjusted television').

A statement on light amplification makes only sense as a value of the entire system light gain of a night vision device. A subjectively assessed actual light multiplication of the single-stage 1st Generation tube by about 50 times faces the system light multiplication of the 2nd and 3rd Generation tubes by approx. 1000 and 2000 times (reference is hereby environmental brightness to output brightness of the night vision device).

The often heared assertion about a certain observation range of a night vision device, with appropriate image intensifier tube, is also mostly not reliable. Beside the optical lens system even the smallest changes in the entire local conditions (clouds, moon, temperature, air humidity, artificial light emissions, etc.) can drastically intervene in the maximum observation range, so that the stated range is undercut not rarely around far more than half.

Since image intensifier tubes always amplify small amounts of 'light information' to useful images, also the smallest variation from the starting-point can be multiplied to unwanted results at the end. The outstanding meaning of a tube also shows up in the price of purchase: Approx. 2/3 of the costs of a modern night vision device is associated with the image intensifier tube.

some 18 mm European IIT

different sizes & types of IIT
L: Gen1 PC-PS/24-11 mm (GUS), R: Gen3 18 mm (US)

sectional view 1-stage NVD Gen1
sectional view 3-stage NVD Gen1
approx. 6 mio. micro channels
primary electron produces secondary electrons

tube module & power supply
An opened 18 mm tube of the 3rd Generation:
The power supply is arranged around the actual tube module. Over two screws at the lower surface adjustments of the PS can be made. The MCP is in the upper third of the module. Almost half of the tube module consists of the glass fiber bundle, which turns the upside-down image with most image intensifer tubes by 180°.

Northrop Grumman (formerly LEOS) data sheet
Litton Electro Optical Systems (now NG) data sheet for an image intensifier tube of 3rd Generation: possible blemishes are marked in the upper left figure (zones 1, 2 & 3), on the right table measured values are noted next to the minima.

pair of lines clearly visible?
The USAF Resolving Power Test Target (1951) is used for the determination of line pairs per millimeter.

Gen1: light losses at the photocathode
Gen1 (3s): big internal gain of the remaining light
Gen3: smaller internal gain, but less initial losses of 'light information', better result in the end