Image intensifier

An image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low light conditions such as at night, to facilitate visual imaging of low-light processes such as fluorescence of materials to X-rays or gamma rays, or for conversion of non-visible light sources such as near-infrared or short wave infrared to visible.

Introduction

Image intensifier tubes (IITs) are an electro-optical device that allows many devices such as night vision devices and medical imaging devices to function. They convert low levels of light from various wavelengths into visible quantities of light at a single wavelength.

History

Development of image intensifier tubes began during the 20th Century and has led to continuous development since inception.

Pioneering work

The idea of an image tube was first proposed by G. Holst and H. De Boer (Netherlands) in 1928[1] but early attempts to create one were not successful. It was not until 1934 that Holst, working for Philips, created the first successful infrared converter tube. This tube consisted of a photocathode in close proximity to a fluorescent screen. Using a simple lens, an image was focused on the photocathode and a potential difference of several thousand volts was maintained across the tube, causing electrons dislodged from the photocathode by photons to strike the fluorescent screen. This caused the screen to light up with the image of the object focused onto the screen, however the image was non-inverting. With this image converter type tube, it was possible to view infrared light in real time, for the first time.

Generation 0 - Early infrared electro-optical image converters

Development continued in the US as well during the 1930s and mid-1930, the first inverting image intensifier was developed at RCA. This tube used an electrostatic inverter to focus an image from a spherical cathode onto a spherical screen. (The choice of spheres was to reduce off-axial aberrations.) Subsequent development of this technology led directly to the first Generation 0 image intensifiers which were used by the military during World War Two to allow vision at night with infrared lighting for both shooting and personal night vision. Early night vision devices based on these technologies were used by both sides and used to great effect in Okinawa, to target Japanese soldiers coming out of caves during the night. However the downside of active night vision (When infrared light is used) is that it is quite obvious to anyone else using the technology.)

Unlike later technologies, early Generation 0 night vision devices were unable to significantly amplify the available ambient light and so required the infra-red source to be useful. These devices used a S1 photocathode or "silver-oxygen-cesium" photocathode, discovered in 1930 which had a sensitivity of around 60 µA/lm and a quantum efficiency of around 1% in the ultraviolet region and around 0.5% in the infrared region. Of note, the S1 photocathode had sensitivity peaks in both the infrared and ultraviolet spectrum and with sensitivity over 950 nm was the only photocathode material that could be used to view infrared light above 950 nm.

Solar blind converters

Solar blind photocathodes were not of direct military use and are not covered by "generations". Discovered in 1953 by Taft and Apker[2], they were originally made from cesium telluride. The characteristics of "solar blind" type photocathodes are a response below 280 nm in the ultraviolet spectrum, which is below the wavelength of light that the atmosphere passes through from the sun.

Generation 1 - Significant amplification

With the discovery of more effective photocathode materials, which increased in both sensitivity and quantum efficiency, it became possible to achieve significant levels of gain over Generation 0 devices. In 1936, the S-11 cathode (cesium-antimony) was discovered by Gorlich, which provided sensitivity of approximately 80 µA/lm with a quantum efficiency of around 20%; this only included sensitivity in the visible region with a threshold wavelength of approximately 650 nm.

It was not until the development of the bialkali antimonide photocathodes (potassium-cesium-antimony and sodium-potassium-antimony) discovered by A.H. Sommer and his later multialkali photocathode (sodium-potassium-antimony-cesium) S20 photocathode discovered in 1956 by accident, that the tubes had both suitable infra-red sensitivity and visible spectrum amplification to be useful militarily. The S20 photocathode has a sensitivity or around 150 to 200 µA/lm. The additional sensitivity made these tubes usable with limited light such as moonlight, while still being suitable for use with low-level infrared illumination.

Cascade (passive) image intensifier tubes

A photographic comparison between a first generation cascade tube and a second generation wafer tube, both using electrostatic inversion, a 25mm photocathode of the same material and the same F2.2 55mm lens. The first generation cascade tube exhibits pincushion distortion while the second generation tube is distortion corrected. All inverter type tubes, including third generation versions, suffer some distortion.

Although originally experimented with by the Germans in World War Two, it wasn't until the 1950s the U.S. began conducting early experiments using multiple tubes in a "cascade", by coupling the output of an inverting tube to the input of another tube, which allowed for increased amplification of the object light being viewed. These experiments worked far better than expected and night vision devices based on these tubes were able to pick up faint starlight and produce a usable image. However, the size of these tubes, at 17 in (43 cm) long and 3.5 in (8.9 cm) in diameter, were too large to be suitable for military use. Known as "cascade" tubes, they provided the capability to produce the first truly passive night vision scopes. With the advent of fiber optic bundles in the 1960s, it was possible to connect smaller tubes together which allowed for the first true Starlight 'scopes to be developed in 1964. Many of these tubes were used in the AN/PVS-2 rifle 'scope which saw use in Vietnam.

An alternative to the cascade tube explored in the mid 20th century involves optical feedback, with the output of the tube fed back into the input. This scheme has not been used in rifle scopes, but it has been used successfully in lab applicaitons where larger image intensifier assemblies are acceptable.

Generation 2 - The micro-channel plate

Second generation image intensifiers use the same multialkali photocathode that the first generation tubes used, however by using thicker layers of the same materials, the S25 photocathode was developed, which provides extended red response and reduced blue response, making it more suitable for military applications. It has a typical sensitivity of around 230 µA/lm and a higher quantum efficiency than S20 photocathode material. Reduction of the cesium to cesium oxide in later versions improved the sensitivity in a similar way to third generation photocathodes. The same technology that produced the fiber optic bundles that allowed the creation of cascade tubes, with a slight change in manufacturing, allowed the production of micro-channel plates, or MCPs. The micro-channel plate is a thin glass wafer with a Nichrome electrode on either side across which a large potential difference of up to 1000 volts is applied.

The wafer itself is manufactured from many thousands of individual hollow glass fibers, aligned at a "bias" angle to the axis of the tube. The micro-channel plate fits between the photocathode and screen and electrons that strike the side of the "micro-channel" as they pass through it elicit secondary electrons, which in turn elicit additional electrons as they too strike the walls, amplifying the signal. By using the MCP with a proximity focused tube, amplifications of up to 30,000 times with a single MCP layer were possible.By increasing the number of layers of MCP, additional amplification to well over 1,000,000 times could be achieved.

Inversion of Generation 2 devices was achieved through one of two different ways. The Inverter tube uses electrostatic inversion, in the same manner as the first generation tubes did, with a MCP included. Proximity focused second generation tubes could also be inverted by using a fiber bundle with a 180 degree twist in it.

Generation 3 - High sensitivity and improved frequency response

A third generation Image Intensifier tube with overlaid detail

The third generation of tubes were fundamentally the same as the second generation, however they possessed two significant differences. Firstly, they used a GaAs/CsO/AlGaAs photocathode which is more sensitive in the 800 nm-900 nm range than second generation photocathodes. Also, to protect the photocathode from positive ions and gases produced by the MCP, they feature a thin film of aluminium oxide attached to the MCP. The high sensitivity of this photocathode, greater than 900 µA/lm, allows more effective low light response. This is offset by the thin film, which typically blocks up to 50% of electrons.

Super Second Generation

Although not formally recognized under the U.S. generation categories, Super Second Generation or SuperGen was developed in 1989 by Jacques Dupuy and Gerald Wolzak. This technology improved the tri-alkali photocathodes to more than double their sensitivity while also improving the microchannel plate by increasing the open-area ratio to 70% while reducing the noise level. This allowed second generation tubes, which are more economical to manufacture, to achieve comparable results to third generation image intensifier tubes. With sensitivities of the photocathodes approaching 700 uA/lm and extended frequency response to 950 nm, this technology continued to be developed outside of the U.S., notably by DEP and now forms the basis for most non-US night vision equipment.

Generation 4

Presently, there have been only one successful fourth generation tubes as the tube life (Mean Time To Failure) of prototypes did not meet military requirements; however, several notable manufacturers such as L-3 Electro-Optical Systems have produced "Gen 4" tubes. These tubes employ the same photocathode as Generation 3, but no longer use an ion-barrier film. Additionally, these tubes are autogated and can be used in high light environments such as daylight and when exposed to high levels of artificial light.

The autogating function works by gating the tube when too much light is present. Gating in a second, third or fourth generation tube is achieved by reversing the polarity of the photocathode/MCP segment and effectively stops all electron transition from the photocathode to the MCP.

Terminology

There are several common terms used for Image Intensifier tubes.

Sensitivity

The sensitivity of an image intensifier tube is measured in Micro-Amperes per 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 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 nm and 850 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 mm tube with a resolution of around 64 lp/mm has a higher overall resolution than an 8 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 criteria such as "> 64 lp/mm" or "Greater than 64 line pairs/millimeter".

Gain

The gain of a tube is measured in one of two possible ways. The most common way is cd/m²/lx or candles per meter squared per lux. The other way is to measure gain as Fl/Fc (Foot-lamberts over Foot-candles). This creates issues with comparative gain measurements since neither is a pure ratio, although both are measured as a value of input intensity over output intensity. This creates ambiguity in the marketing of night vision devices as the difference between the two measurements is effectively pi or approximately 3.14159 times. This means that a gain of 10,000 cd/m²/lx is the same as 31.4159 Fl/Fc. With a lack of convention on this item, if the units for gain are not specified, Fl/Fc should typically be assumed.

MTTF (Mean Time To 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 that 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 MTTF 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
Second Generation: 2000 to 2500 hrs
Third Generation: 10000 to 15000 hrs.

It should be noted that many recent high-end second-generation tubes now have MTTFs approaching 15,000 operational hours.

MTF (Modulation Transfer Function)

The Modulation Transfer Function on 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% @ 2lp/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.

See also

• Night vision device

References

• Historical information on IIT development and inception [3]
• Discovery of other photocathode materials [4]
• Several references are made to historical data noted in "Image Tubes" by Illes P Csorba ISBN 0-672-22023-7
• Selected Papers on Image tubes ISBN 0-8194-0476-4