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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.
L:
Gen1 PC-PS/24-11 mm (GUS), R: Gen3 18 mm (US)
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°.
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.
The USAF Resolving Power Test Target (1951) is used for the determination
of line pairs per millimeter.
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
