1 Types of image intensifier tubes
Image intensifier system IIS (night vision device NVD) are
the imaging systems built using an image intensifier tube consisting of a
photocathode, an anode in form of a phosphor screen, and other optional
components. The tube intensifies a low-luminance image of the observed objects
created on the photocatode into a brighter image created on the anode. In other
words, the photocathode due to the radiation impinging onto its surface emits
electrons that are focused onto a phosphor screen that emits brighter light.
Spectral sensitivity of the IIS depends on design but generally IIS used for
surveillance applications are sensitive to radiation from about 0.4
m m to 0.8 m m and
sometimes up to 0.9 m m.
Typical image intensifier system IIS consists of an optical
objective, an image intensifier tube, an ocular, a chassis, and an optional
illuminator. The image intensifier tube is a heart of the image intensifier
system. Therefore the term "image intensifier" is often used as synonym of the
image intensifier system. Next, the image intensifier systems IIS are commonly
called night vision devices (NVD) although also other infrared imaging systems
like thermal imagers or LLLTV cameras enable imaging at night conditions.
The technology of image intensifier tubes has progressed
steadily over the years. There are so far five generations of image intensifier
tubes: 0, 1, 2, 3 and possibly 4.
Generation 0 refers to the technology of World War II,
employing fragile, vacuum-enveloped image converters with poor sensitivity and
little gain. These are single stage tubes that achieve image intensification due
to acceleration by high voltage of electrons emitted by the photocathode and
striking the phosphor screen. Typically S-1 photocathode (sensitivity up to 60m
A/lm), electrostatic inversion and electron acceleration were used to achieve
gain. Gen 0 tubes are characterised by significant distortion and necessity for
active illumination (large searchlights with infrared (IR) filters). These tubes
were in past used in active night vision systems cooperating with an
illuminator. High power tungsten bulbs covered with an IR filter suppressing
visible radiation were used as illuminators. Active character of use of first
generation image intensifier systems was their significant disadvantage.
Generation 1 - First generation tubes are improved Gen 0
tubes. Typically S-10 or S-20 photocathode (photo sensitivity up to 200m
A/lm), electrostatic inversion and electron acceleration are used to achieve
gain. Because of higher sensitivity Gen 1 NVD were the first passive night
Focusing is usually achieved by using an electron lens to
focus electrons originating from the photocathode onto the screen (Fig. 2).
In the inverter diode tube presented in Fig. 2 an
electrostatic field directs the photoelectrons and focuses an inverted image on
the phosphor screen. Electron lens can be achieved by combining an
electrostatical field with an axial magnetic field provided by either a solenoid
or permanent magnet. An uniform magnetic field enables to achieve good
resolution over the entire screen and at the same time keeps distortion low.
Fibre optics windows are used in Gen 1+ tubes to minimize degradation of the
image resolution towards the edge of the tube. The fibre optics enables also
efficient coupling to another image tube, to an imaging detector or to
Fig. 2. a)Diagram of a NVD built using a Gen1 tube
b)photo of Gen 1 tube
First generation image intensifier tubes are characterised by
good image resolution (25-30 lp/mm), a wide dynamic range (the ability to
reproduce the ratio between the bright and dark parts of an image), and low
noise. However, they are also characterised only by a moderate gain. Luminance
gains in the single stage tubes are usually in the order of 100 to 500 (up to
1500 in Gen 1+ tubes) in situation when luminance gain in the order of 105
is necessary to achieve ability to see at overcast starlight night conditions.
Due to low production costs Gen 1 NVDs still dominate in commercial market but
they are rarely used for military applications.
Generation 2 image intensifier tubes represent a significant
breakthrough in night vision technology. Gen 2 tube are single stage imaging
tubes built using an S-25(extended red) photocathode (with sensitivity of
240-450 m A/lm) and a microchannel plate (MCP) to
increase significantly luminance gain up the level about 18 000-30 000 cd/m2/lx.
They are also typically equipped with automatic gain control and bright spot
The microchannel plate is an array of miniature electron
multipliers oriented parallel to one another; typical channel diameters are in
the range 10-100 m m and have length to diameter
ratios between 40 and 100. Channel axes are typically normal to, or biased at a
small angle (» 8 ° ) to
the MCP input surface. The channel matrix is usually fabricated from a lead
glass, treated in such a way as to optimise the secondary emission
characteristic of each microchannel and to render the channel walls
semiconducting so as to allow charge replenishment from an external voltage
source. Parallel electrical contact to each channel is provided by the
deposition of a metallic coating on the front and rear surfaces of the MCP,
which then serve as input and output electrodes, respectively. The total
resistance between electrodes is in the order of 109 W
. Such microchannel plates allow electron multiplication factors of 103
– 106. Spatial resolution is limited only by channel
dimensions and spacing; 12 m m diameter channels with
15 m m center-to-center spacings are typical.
Fig. 3. Principle of work of a microchannel plate
Fig. 4. Gen 2 image intensifier
tube (inverter MCT tube) a)diagram b)photo
Fig. 5. Gen 2+ tubes (proximity
focus MCT tube) a)tube diagram b)photo
First Gen 2 tubes were manufactured similarly to the Gen 1
tubes using the inverter diode tubes (Fig. 4). Newer Gen 2 tubes are usually
proximity diode tubes shown in Fig. 5. They are characterised by lower size and
higher gain in comparison to typical Gen 2 tubes.
Gen 3 tubes are similar to the Gen 2 tubes in design. The
primary difference is the material used for the photocathodes. Second generation
image intensifiers use photocathodes with a multialkali coating whereas third
generation image intensifiers use photocatodes with a GaAs/AlGaAs coating. This
latter material is characterised by quantum efficiency in the near infrared over
10 times better in comparison to photocathodes with a multialkali coating.
However, the advantage of higher sensitivity of photocathode is significantly
reduced by necessity to use a protective ion barrier film to increase tube life
that cause some signal losses. Therefore luminance gain of Gen 3 tubes (range
20000-90 000 cd/m2/lx) is usually no more than about 1.2-3 times
higher that luminance gain of typical Gen 2 tubes (range 18000-30 000 cd/m2/lx).
Gen 4 tubes are modified Gen 3 tubes. The main difference is
the protective ion barrier that was removed in Gen 4 tubes. Gated power supplies
are also used to pulse on and off the Gen 4 tubes. Using these quick, controlled
voltage pulses, Generation 4 tubes are able to adjust automatically to lighting
conditions - less power is consumed when ample light is available, and more in
darker situations. However, it must be emphasised that the existence of Gen 4
tubes is not officially confirmed by the US authorities.
All these types of intensifier tubes are also differentiated
by the nominal useful diameter of the photocathode. Typical diameter values are
18, 25, and 40 mm.
There is a common view that higher generation number means
better tube. It is usually true if we compare Gen 0, Gen 1 or Gen 2 tubes but do
not have to be true if we compare Gen 2, Gen 3 or Gen 4. Gen 3 and Gen 4 tubes
are manufactured exclusively by USA manufactures. European and Asian
manufactures instead of changing technology concentrated on improving Gen 2
technology and offer different equivalents to Gen 3 tubes (Supergen, SHD-3,
XD-4, XR5). These equivalents can be sometimes better than Gen 3 tubes. Gen 2
tubes are also generally more tolerant to urban night conditions.
Parameters of image intensifier tubes
Testing modern image intensifier tubes is not an easy task.
MIL series standards propose measurement of 35 parameters to characterize fully
these tubes. The parameters can be generally divided into four basic groups:
Image quality parameters,
Tab. 1. Parameters of image intensifier tubes
Image quality parameters
Photocathode Luminous Sensitivity
Photocathode Radiant Sensitivity
Equivalent Background Input (EBI)
1. Limiting Resolution
(center, peripheral, high level)
2. Modulation Transfer Function (MTF)
3. Signal To Noise Ratio (S/N)
4. Blemishes (dark spots, white spots, chicken wire)
5. Output Brightness Uniformity
7. Useful cathode diameter
8. Image Alignment
9. Shear Distortion
10. Gross Distortion
11. Image Rotation
13. Temporal magnification
14. Fixed pattern noise
15. Ion barier film defects
|1. Bright Source Protection
Burns In (ESS)
3. Reliability test
4. Accelerated reliability test
5. Extreme temperature test
6. Extreme humidity tests
|1. Rise Time
2. Decay Time
3. Phosphor decay time (luminance persistence)
- Power voltage
- Input current
- Reversed polarity
Photometric parameters describe tube sensitivity to incoming light. During
photometric tests tube is uniformly illuminated. No image is created on the tube
photocathode. We are not interested in tube ability to create an image but in
tube ability to magnify input illumination.
Image quality parameters describe tube ability to create high
quality copy of the image generated at the photocathode plane. Images of some
standard targets are created at the photocathode plane and quality of the image
generated at the tube screen is analyzed during image quality tests.
Reliability tests describe tube resistance to harsh work
conditions like extreme illumination levels, bright spots, extreme temperature,
humidity, vibration etc. The tests helps us to select tubes of long life time.
Temporal parameters describe tube temporal reactions to light
or voltage impulses. These parameters, especially phosphor decay time enable
determination of tubes not suitable for surveillance of high speed targets.
Electrical parameters describe tube electrical reactions to
light or voltage changes.
Definitions of photometric parameters
Image quality parameters
Photocathode luminous sensitivity
The ratio of the photocathode current generated by the
input illumination to the input luminous flux [m
Photocathode radiant sensitivity
The ratio of the photocathode current generated by the
input illumination at specified wavelength to the input radiant flux [m
The ratio of the output screen luminance to the
input illuminance [cd/ m2 lux]
The luminance of screen of the tube in saturated state
(high input illumination) [cd/m^2]
Equivalent background input (EBI):
The additional input illuminance required to provide an
output luminance increase equal to the mean background screen luminance due
only to the tube’s dark current cd/ m2
Temporal fluctuations and drifts of average brightness of
the screen of the II tube
Photocathode luminous sensitivity gives information about
sensitivity of tube photocathode to polychromatic light of 2850K color
temperature (the spectrum covers both visible and near infrared range).
Generally higher photocathode luminous sensitivity the better the II tube.
However, this rule is valid only when two tubes manufactured using the same
technology are compared.
Photocathode radiant sensitivity gives information about
sensitivity of the photocathode to monochromatic 880 nm infrared light.
Generally higher photocathode radiant sensitivity the better performance of the
II tube in near infrared range.
Photocathode luminous sensitivity is usually proportional to
photocathode radiant sensitivity. Higher photocathode luminous sensitivity means
typically also higher photocathode radiant sensitivity.
Both photocathode luminous sensitivity and photocathode
radiant sensitivity cannot be measured when the tube is potted because there is
no direct access there to photocathode. These parameters can be measured only
for bare tubes not having mechanical case and electronic module. Practically
this means that these parameters can be measured by manufactures of II tubes or
at some repairing depots. Typical users of II tubes cannot measure these
parameters without partial destroying tested tube. The parameters listed below
can be easily measured by both manufacturers and typical users of II tubes.
Luminance gain gives information about how many times
brightness of the image of the observed scenery on the II tube screen is higher
than brightness of the original scenery. Image intensifier tubes of too low
luminance gain cannot be used for observation when illuminance is low. However,
we must remember that higher luminance gain means higher intensity of the noise.
Increase of the luminance gain will not improve tube performance when the gain
is above a certain level.
The EBI phenomenon is caused by thermal emission of the
photocathode. This means that even when there is no incoming light, the
photocathode emits some electrons that are later multiplied and finally generate
some brightness of the screen of the tube. During observation of targets at low
illuminance conditions the EBI effect creates visible haze in the image. In
other words we can say that higher EBI means lower contrast of the image at low
Bad operational stability does not means that tube
sensitivity is low, image is noisy or image quality is low. Bad operational
stability means practically significant fluctuations of screen brightness. These
fluctuations can be very disturbing for users of night vision devices. The users
will be quickly tired when use NVD with tube of bad operational stability is
Tab. 3. Definitions of image
Image quality parameters
Limiting Resolution (center, peripheral, high level)
Parameter defined as the smallest resolution pattern
which the observer can see and distinguish between the black lines and the
clear area between the black lines.
Modulation Transfer Function (MTF)
Parameter defined as an output signal modulation to input
signal modulation when the signal is sinusoidal wave. It is a measure of the
degradation of an image as it appears at the output screen of the tube as
correlated to the input pattern which is normalized to 100 percent contrast
at a spatial frequency equal to or less than 0.2 lp/mm.
Signal To Noise Ratio (S/N)
Parameter defined as ratio of the average signal at the
output of an intensifier tube to the true root-mean-square (rms) value of
the signal fluctuations about the average at a specified electronic
measurement system bandwidth.
Parameter defined as a circular area of brightness
evidenced on the assembly output imaging screen occurring as a result of a
small bright source input and concentric with the input.
Useful cathode diameter
Parameter defined as diameter of part of photocathode
that reacts to incoming radiation.
a)The opaque or dark spots which exceed a contrast of 30
percent of their surrounding area. It is typically required that the dark
spots shall not exceed the size and quantities specified in special tables.
b) The bright spots or a pattern that may flicker or
appear intermittently on the image screen in one general area
c) The phenomenon defined as a predominant pattern of
dead fibers which has a diameter equal to or less than about 22.5m
m – 2 single fibers and whose light transmission is so degraded that with
light projected through the optic, single fibers in the area of question
cannot be distinguished or identified as single fibers with the use of 50
Measure of displacement of the spot image that should to
be created at the point of passing the optical axis through the screen.
Parameter that describes phenomenon that causes the image
of a strait line to have a discrete, localized, lateral displacement (i.e.,
Parameters that describe phenomenon that causes the image
of a straight line to curve. Gross distortion is caused by a long-range
deformation or flow of fibers during fabrication.
Output Brightness Uniformity
The ratio of the maximum to minimum brightness variation
over the useful screen area when the photocathode is uniformly
Ion barrier film quality
Hole defects in ion barrier film
Fixed Pattern Noise
The phenomenon defined as discernible spatial gain
variation seen on the screen of the tube. There two components: Multi-Multi
Pattern Noise and Multi-Boundary Pattern Noise. MMPN is a phenomenon defined
as discernible spatial gain variation between individual multi-patterns or
groups of multi-patterns. MBPN is the phenomenon defined as
discernible spatial gain variation between peripheral and interior channels
of a multi-pattern or group of channels.
The phenomenon of angular rotation of output image in
comparison to input image different than desired 180 degrees. .
The radio of size of output image to size of the input
The phenomenon of temporal variation of tube
magnification. This means that size of output image vary with time.
Limited resolution gives information about tube ability to
create recognizable image of two small targets located close each other. Higher
value of limiting resolution means that the tube can generate recognizable
images of smaller targets separated by smaller distance. USAF1951 standard
target is typically used for measurement of the limited resolution. The task of
observer is to determine the the smallest pattern that he is able to resolve.
Limited resolution is measured as center resolution or peripheral resolution
depending on location of input image, at low or at high illumination level.
Fig. 1. Image of USAF 1951 target during resolution test
Limited resolution is a good measure of tube ability to
create image of small details. The parameter has a long history in optics and
was a traditional measure of many optical systems. However limiting resolution
can be poor measure of overall image quality. There are tubes of the same
limiting resolution that according to human perception differ significantly in
the image quality. The reason is that limiting resolution describes tube
performance only in high spatial frequency range but not in middle or low
frequency range that have more significant influence on perceived image quality.
Because of this situation nowadays optical objectives and imaging systems are
evaluated not by limiting resolution but by their MTF function. Additional
advantage of MTF over limiting resolution is the fact that MTF is an objective
parameter measured automatically when limiting resolution is a subjective
parameter measured by humans.
Fig. 2. MTF of two tubes of the
same limiting resolution
Fig. 3. Image generated by two tubes of the same limiting
resolution but different MTF a) MTF at 30lp/mm equals to 0.4 b) MTF at 30lp/mm
Signal To Noise Ratio gives information how noisy is the
image generated by the tube; more precisely about temporal variation of
brightness of very small portions of the screen image. If we capture a sequence
of images of tube screen using a video camera then temporal noise generate
differences between frames. Low SNR can significantly degrade image quality,
makes more difficult to recognize small details and also makes more difficult
long work with such tubes.
Fig. 4. Image generated by an II tube during SNR test
Halo gives information influence of high brightness spot
targets on image of neighbour targets. High values of halo means that the tube
cannot create recognizable image of targets located close to high brightness
sources. This parameter is particularly important if the tube is to be used at
Size of photocathodes are standardized: 12 mm, 18 mm, 25 mm
etc. There are however some differences in size of photocathodes even offered by
the same manufacturer. Useful photocathode diameter should be measured and known
because it determines later important parameter of night vision devices or LLLTV
cameras: the field of view. It can have also indirect influence on image
Tube blemishes can be met in every image intensifier tube.
Tubes are never flawless, and every tube will have blemishes to some degree. The
basic question is if they can be treated as only cosmetic defects or as
important defects that significantly degrade tube performance.
From five types of blemishes presented in Tab. 2, the dark
spots are usually most common and most easily noticeable. If size and number of
dark spots are too high then this defect can limit significantly tube ability to
detect small target. The same is valid in case of bright spots.
Fig. 5. Photos of a few dark spots of different sizes and
Chicken wire is less frequently noticeable and rarely can
significantly degrade tube performance.
Image alignment effect creates situation when image of spot
generated exactly at the center of the photocathode is not created exactly at
the center of the tube screen but is displaced slightly from the center (up to
one millimeter). It is typically measured as the distance be image of the test
reticle projected on the photocathode of the assembly concentric with the
optical axis and the optical axis of the assembly at screen plate. This effect
has no big importance at applications when user want only to get an image of the
observed targets. However, this effect is critical at military applications when
the tube is a component of a sighting system.
Fig. 6. Displaced image of spot target due to image alignment
effect (the bright spot should be in the center)
Gross distortion is usually caused by a long-range
deformation or flow of fibers during fabrication. This defect can be tolerated
up to high levels in most surveillance applications when image is only evaluated
subjectively. However, it can be a big hindrance in applications that require
precise determination of position and size of observed targets.
Fig. 7. a)Gross distortion target b)Image of gross distortion
target distorted due to gross distortion effect (attention: the effect was
Shear distortion is due to localized misalignment errors in
the assembly of fibers or multifibers. Shear distortion in fiber bundles is
sometimes referred as incoherency. It causes that images of lines have a
discrete, localized displacement (Fig. 8). Shear distortion up to some level is
acceptable but if occurs at some critical areas (the centre) then it can be a
serious problem in boresigting applications.
Fig. 8. Influence of shear
distortion effect on image of a single circle
Image inversion is a phenomenon of angular rotation of output
image in comparison to input image different than 180 degrees. For single
channel nigh vision devices image rotation even at the level of several angular
degrees difference than desirable inversion is often acceptable and this
parameter is not important. However, image inversion is a very important
parameter for dual channel night vision devices when the manufacturer must build
the device using two tubes of almost identical image rotation. If not then the
observer can get headache and perceived image quality can drastically
Fig. 9. Image inversion phenomenon a)original input image b)
output image seen on the tube screen
Ion barrier film defects are holes in ion barrier film that
is to protect the photocathode against bombarding ions. From final image quality
point of view ion barrier film defects can be treated as a kind of blemishes.
Fixed Pattern Noise appears as hexagonal pattern is in almost every tube, the
question is only at what illumination level. As we can see in Fig. 10 there are
some brighter and darker hexagonal multies and there are brightness differences
between peripherial and interior channels of hexagonal multies (webbing).
Multi-Multi Fixed Pattern Noise creates brightness differences between
different hexagonal multies.
Multi Boundary Pattern Noise creates brightness differences between
peripherial and interior channels of hexagonal multies (webbing).
Although Fixed Pattern Noise is easily noticeable it can rarely significantly
decrease image quality and degrade tube performance. It is usually only a
cosmetic effect but over a certain limit it can be problem for the observer.
Fig. 10. Screen of II tube with
noticeable both Multi-Multi Pattern Noise and Multi-Boundary Pattern Noise
Association of Nigh Vision
Manufacturers, Annual meeting – report, 1999.
MIL-PRF-49052G "Image intensifier assembly, 18 millimiter
microchannel wafer, MX-9916/UV, 1999
MIL-STD-1858, Image intensifier assemblies, performance
parameters of; 1981
DEP 2283A3 "Image intensifier - 18
mm microchannel wafer XX1940