An image intensifier or 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 in X-rays or gamma rays (X-ray image intensifier), or for conversion of non-visible light sources, such as near-infrared or short wave infrared to visible. They operate by converting photons of light into electrons, amplifying the electrons (usually with a microchannel plate), and then converting the amplified electrons back into photons for viewing. They are used in devices such as night-vision goggles.
Introduction Image intensifier tubes (IITs) are optoelectronic devices that allow 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.
Operation
Image intensifiers convert low levels of light photons into electrons, amplify those electrons, and then convert the electrons back into photons of light. Photons from a low-light source enter an objective lens which focuses an image into a photocathode. The photocathode releases electrons via the photoelectric effect as the incoming photons hit it. The electrons are accelerated through a high-voltage potential into a microchannel plate (MCP). Each high-energy electron that strikes the MCP causes the release of many electrons from the MCP in a process called secondary cascaded emission. The MCP is made up of thousands of tiny conductive channels, tilted at an angle away from normal to encourage more electron collisions and thus enhance the emission of secondary electrons in a controlled Electron avalanche.
All the electrons move in a straight line due to the high-voltage difference across the plates, which preserves collimation, and where one or two electrons entered, thousands may emerge. A separate (lower) charge differential accelerates the secondary electrons from the MCP until they hit a phosphor screen at the other end of the intensifier, which releases a photon for every electron. The image on the phosphor screen is focused by an eyepiece lens. The amplification occurs at the microchannel plate stage via its secondary cascaded emission. The phosphor is usually green because the human eye is more sensitive to green than other colors and because historically the original material used to produce phosphor screens produced green light (hence the soldiers' nickname 'green TV' for image intensification devices).
History The development of image intensifier tubes began during the 20th century, with continuous development since inception.
Pioneering work The idea of an image tube was first proposed by G. Holst and H. De Boer in 1928, in the Netherlands , 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 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 II to allow vision at night with infrared lighting for both shooting and personal night vision. The first military night vision device was introduced by the German army as early as 1939, developed since 1935. Early night vision devices based on these technologies were used by both sides in World War II.
Unlike later technologies, early Generation 0 night vision devices were unable to significantly amplify the available ambient light and so, to be useful, required an infrared source. These devices used an S1 photocathode or "silver-oxygen-caesium" photocathode, discovered in 1930, which had a sensitivity of around 60 μA/lm (Microampere per Lumen) 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 converters, also known as solar blind photocathodes, are specialized devices that detect ultraviolet (UV) light below 280 nanometers (nm) in wavelength. This UV range is termed "solar blind" because it is shorter than the wavelengths of sunlight that typically penetrate the Earth's atmosphere. Discovered in 1953 by Taft and Apker , solar blind photocathodes were initially developed using cesium telluride. Unlike night-vision technologies that are classified into "generations" based on their military applications, solar blind photocathodes do not fit into this categorization because their utility is not primarily military. Their ability to detect UV light in the solar blind range makes them useful for applications that require sensitivity to UV radiation without interference from visible sunlight.
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 infrared sensitivity and visible spectrum amplification to be useful militarily. The S20 photocathode has a sensitivity of 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
Although originally experimented with by the Germans in World War Two, it was not until the 1950s that 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 applications where larger image intensifier assemblies are acceptable.Martin L. Perl and Lawrence W. Jones, Optical Feedback Image Intensifying System, , Oct. 27, 1964.
Generation 2: 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. Oxidation 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, allowed, with a slight change in manufacturing, 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 1,000 volts is applied.
The wafer 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. 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
While the third generation of tubes were fundamentally the same as the second generation, 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. Secondly, the photocathode exhibits negative electron affinity (NEA), which provides photoelectrons that are excited to the conduction band a free ride to the vacuum band as the Cesium Oxide layer at the edge of the photocathode causes sufficient band-bending. This makes the photocathode very efficient at creating photoelectrons from photons. The Achilles heel of third generation photocathodes, however, is that they are seriously degraded by positive ion poisoning. Due to the high electrostatic field stresses in the tube, and the operation of the MicroChannel Plate, this led to the failure of the photocathode within a short period - as little as 100 hours before photocathode sensitivity dropped below Gen2 levels. To protect the photocathode from positive ions and gases produced by the MCP, they introduced a thin film of sintered aluminium oxide attached to the MCP. The high sensitivity of this photocathode, greater than 900 μA/lm, allows more effective low light response, though this was offset by the thin film, which typically blocked up to 50% of electrons.
Super second generation