Maintaining the night vision advantage.

Press photographs of US forces in action during the recent invasion of Iraq show the unprecented scale on which the US ground forces have deployed night vision (NV) systems. Despite US claims that NV hardware had reached Iraq via Syria shortly before the conflict, the US lead in NV was unchallenged. But as more armed forces adopt modern NV systems, the US and its allies will need an improved form of imager whose high performance can maintain this traditional tactical edge in night combat.

Improved forms of image intensification and thermal imaging have been developed, a trend which is likely to continue. Research is also under way to create multi-spectral systems which will yield better imagery than today's single-band hardware.

Although some manufacturers use the term Generation IV (Gen IV) to describe what are currently the latest types of image intensification tube, this designation does not enjoy US Department of Defense approval. The Pentagon prefers to label such devices as 'filmless and gated' tubes, a title which describes the improvements incorporated in these new-generation sensors.

On the Inside

Photons of visible light which reach the photocathode at the front end of a Gen II or Gen III image intensifying tube are absorbed in the active layer, and the resulting electrons emitted by the photo cathode are accelerated toward the input surface of the microchannel plate (MCP) by a potential of approximately 800 Volts applied between the two.

When these electrons enter the channels of the MCP, a cascade of secondary electrons is produced from the channel wall by secondary emission. This phenomenon is primarily controlled by applying a potential difference of around 900 Volts across the input and output surfaces of the MCP, which results in an electron gain of several hundred.

These secondary electrons emerge from the output surface of the MCP, and are accelerated across the gap toward the phosphor screen by a potential difference of about 6000 Volts. On arrival at the phosphor screen, each electron releases many photons, creating an output image.

The electrostatic fields used to move electrons from the photocathode to the screen electrode are also moving any positive ions within the image intensifier tube toward the photocathode. These positive ions may include the nucleus of gas atoms whose impact upon the photocathode can cause physical and chemical damage. An even larger number of gas atoms present within the tube may be electrically neutral, but are able to chemically combine with and poison the photocathode. The resulting deterioration limited the life of early-generation tubes.

The traditional solution to these problems is to use an ion-barrier film on the inlet side of the MCR However, this barrier film creates other problems. For a start, the electron voltage that must be provided between the PC and the MCP must be increased by about 600 to 700 volts to compensate for the presence of the barrier.

Although the material of the ion barrier film acts as a secondary emitter of electrons, this effect requires electrons of sufficient energy. Electrons of lower energy may be absorbed by the ion barrier film, which prevents them from reaching the MCP.

Since about 50 per cent of the input face of an MCP is made up of microchannels, around half the photoelectrons impact on the solid portion or web of the microchannel plate, creating low-energy secondary emission electrons which lack the energy to penetrate the barrier film, or to cause the film to liberate secondary electrons. As a result, up to 50 per cent of the electrons are blocked or absorbed by the barrier film, so do not reach the microchannels. (Experiments with lithographic manufacturing techniques have suggested that open area ratios of 90 per cent or more may be attainable in future tubes.)

Instead of entering the microchannels, these electrons may bounce off the barrier film, re-impacting at another location, where the process may be repeated by those electrons which fail to enter a microchannel. The result is an unwanted halo or emission of light around locations of the image. This reduces the quality and contrast of the image.

The spacing between the photocathode and the MCP is an important factor affecting the extent or degree of halo effect. Tubes generally have a gap of no less than about 250 mu metres (0.000250 metre).

In November 2002, Litton Systems was granted a patent covering a tube design in, which the photocathode-to-MCP gap was largely isolated from dimensional variabilities of the housing, and could be established and maintained precisely during manufacturing of the tube despite stack up of tolerances for the housing and its components.

In this design, the high-voltage power takes the form of an annulus which is axially aligned and stacked with the tube body, rather than surrounding it, so that the envelope diameter of the tube can be smaller than that of conventional tubes.

This form of construction allows the gap between the PC and MCP to be reduced to as little as about 20 mu metres, reducing the image halo effect, and allowing the tube to operate with voltages no higher than those on conventional tubes.

This design eliminates the ion barrier between the MCP and photocathode, so the photoelectrons have essentially no restriction on entering the MCP.

Another patent awarded to the company a month earlier describes how an insulative spacing structure in physical contact with the photocathode and MCP can be used to establish a minimum spacing distance between the two.

The key to producing a tube with an unfilmed MCP is the removal of a sufficient number of the potentially-damaging gas ions. Manufacturing techniques used by Litton Systems to achieve this include:

* the use of cladding glass which is made electrically conductive, and can be scrubbed to substantially reduce the amount of ions

* vacuum baking of the MCP to drive off ions

* scrubbing of the phosphorus screen to remove unwanted gas impurities such as carbon dioxide, carbon monoxide, hydrogen gas and other impurities

* electron beam scrubbing of the tube assembly to drive out gas impurities

* heat-cleaning of the cathode

* use of a Ti/Ta final gas getter to remove any last impurities.

Modern image intensifier tubes incorporate an automatic brightness control (ABC) which maintains a relatively constant level of brightness in the output image despite fluctuating levels of brightness in the scene being viewed, plus a bright source protection (BSP) system which prevents the tube from being damaged by high levels of current that would otherwise be generated in response to an extremely bright source.

Both measures are provided by adjusting the voltage level of the microchannel plate. However, the tube loses resolution as the voltage to the microchannel plate is reduced in response to a bright scene.

An ideal system would provide both ABC and BSP without affecting resolution. In a gated tube, the power supply has both a positive and a negative photocathode multiplier, and the photocathode is alternately connected to these via a switching system. This turns the photocathode on and off at specified intervals determined by the current or voltage levels of suitable components of the tube.

At the Right Angle

The human eyes have a visual field of around 165 degrees in the horizontal plane, and 150 degrees in the vertical, but a typical NVG can offer only 40 to 50 degrees. This reduces the situational awareness of the soldier, who must resort to repeated head movements in order to cover a wider region of the terrain ahead.

While some manufacturers offer slightly wider angles, designers face the problem that as field of view increases, the visual acuity (resolution) of an NVG declines. For the moment, the only practical way of providing a wider range is to divide the azimuth coverage between a greater number of tubes.

This approach is being taken by the US Air Force Research Laboratory, which is developing NVGs with a coverage of 100 degree horizontal and 40 degree vertical for use by the US Army and US Air Force. By using lightweight tubes of 16 mm diameter rather than the traditional 18 mm, designers have been able to devise goggles which carry a horizontal array of four tubes. The two centre tubes cover the central part of the FOV, and are observed with both eyes, while the tubes at either end of the array are seen by only one eye.

The most common detector material currently used for the focal plane array (FPA) of thermal night-vision systems is cadmium mercury telluride. For long-wave (8 to 14 microns) imagers, this must be cooled to around 80 degrees K (-190 degrees C), while mid-band (three to five microns) operation requires cooling to 193 degrees K (-80 degrees C). This cooling can be done using compressed gas or some form of cooling engine.

Development work on uncooled imagers started in the early 1980s using silicon microbolometer arrays. Materials now being used for uncooled detectors include barium strontium titanate, lead titanium oxide and various vanadium oxide compounds.

The reduction in size made possible by the elimination of cooling allows thermal imagers to be made small and light enough to be used as driver night vision systems, small observation devices and even as weapon sights. However they only operate in the eight to twelve-micron band.

Driver vision systems normally have a sensor module which produces a processed and corrected video image in analogue form(such as RS 170) which can then be presented on a display unit, sometimes referred to as display and control module Most use prisms, mirrors and similar optical elements in the path between the outside scene and the sensor.

To meet the demand for smaller and less costly thermal imaging systems whose sensor modules are as small as possible to allow maximum flexibility in mounting the system military vehicles, Thomson-CSF Optronics Canada has devised a system architecture which involves complete separation between the sensor and the digital image processing subsystem (which is housed in the display unit). The radiation path between the scene and the sensor is straight, and uses only refractive optical elements, resulting in a small size and improved performance.

The link between the sensor (which is based on an f0.8 aperture lens and an uncooled micro bolometer detector array) is digital. This eliminates the need to convert the digital signals to RS 170 (CCIR) form for transmission to the display unit, where it must be reconverted to digital format. This makes the system more immune to noise pickup, says the company, and provides better overall picture quality.

The performance of uncooled IR imagers is gradually increasing as a result of development work in areas such as pixel design, novel material processing and low noise read-out electronics. It will, however, eventually be constrained by the background radiation limit.

Improvements in FPA technology were originally focussed on detector technology. The Read-Out Integrated Circuit (ROIC) performed little if any signal processing on the detector output. The term 'smart FPAs' is used to describe FPAs that contain embedded signal-processing functions which were previously provided by separate electronics. These functions could include non-uniformity correction, spatial and temporal filtering, transient-event suppression, motion detection and analogue-to-digital (A/D) conversion.

The growing maturity of deep-submicron fabrication processes helps the integration of more circuitry onto the array's ROICs. On-chip A/D conversion offers good noise immunity, says Raytheon Infrared Operations, but existing off-the-shelf A/D converters are cheap and less risky to implement. However, the company predicts that advantages of on-chip integration will increase with time, offering designers the ability to reduce system cost and weight.

Where to Wear?

An NVG must be placed in front of a user's eyes, either by the user holding the device by hand or wearing some form of head or helmet mounting scheme. In a head-mounted arrangement, the securing straps may have to be tightened to the point of causing significant discomfort, while the fact that the outer shell of a helmet 'floats' on the skull in order to protect the user's head from injury makes it hard to maintain exact alignment of the NVG with the user's eyes.

Many NVG optical systems require that the exit pupil of the optical system be aligned with the user's eye, but normal movement such as walking or running causes the helmet and NGV to move, disrupting this alignment and failing to provide full imagery to the user.

ITT has studied indirect-view NV systems which could overcome these problems. These would use a head-mounted image intensified video camera to create an electronic image presented to the user by a flat panel microdisplay and an eyepiece forward of the user's eyes.

The flat panel display would measure less than 3.81 cm diagonally, and be positioned above the user's line of sight. A folded light path would present the screen image to the user via a prismatic eyepiece. The latter could be made from optical plastic having one-half to one-third the density of optical glass. The system would be lighter in weight than a conventional NVG, while the screen and eyepiece would not extend far in front of the viewer's eyes. These qualities would give the user better mobility and tactical freedom while the device is being worn.

In its basic form, this scheme would display the raw video signal from the camera, but the company already envisages enhancements to this basic configuration. An image processor (which the user could activate or deactivate as required) could performing real-time enhancement of the video signal, providing facilities such as contrast stretching, edge detection/ enhancement, and the mixing/overlaying of intensified video with other sources of video or alphanumeric data.

Fusion

A further improvement would be to add a thermal imaging camera (probably based on an uncooled focal plane array), and to allow the image processor to overlay the 12 and IR images. The brightness of the two images could be independently controlled, allowing the user to select full 12 or IR reformation, or any combination of the two.

Algorithm for the real-time fusion of low-light visible images with infrared images have als0 been developed at the TNO Human Factors Institute in the Netherlands. Thee have been tested by researchers at McGill University, Montreal, and the Canadian company CAE Electronics, under a project, to create an Enhanced and Synthetic Vision System for Sar helicopter crews.

ITT Industries Night 0Vision and Raytheon have teamed to develop two techniques for fusing imagery. The simplest approach involves superimposing the image from one channel with the image from the other by using some form of electronic or optical combine. Matching the brightness and angular coverage of the two images is not easy in hardware which must face to stress of daily use. Any disturbance or change in the alignment of either of the optical paths will result in the two images being misaligned, with disastrous effects on the final image.

By digitising the outputs from the two channels, then combining these using some form of summing algorithm, a system can be created which is much more tolerant of mechanical changes or imprecision.

The first potential production NVG to use dual-mode sensors will probably be the US Army's planned Enhanced NVG (ENVG). Intended to replace the widely-deployed PVS-7 NVG, the ENVG is expected to combine an image intensification channel with a thermal channel based on an uncooled sensor. In the first demonstrator hardware built by the ITT Industries/Raytheon team, the two channels were linked by an optical combiner.

Most night vision systems are monochromatic, amplifying white light and creating an output image which is usually green due to the phosphors used. Trials have shown that NVGs fitted with white phosphor displays provide better object recognition, and are preferred by users. By observing the scene in two wavelengths or frequency bands, and using the difference between the two channels, it is possible to generate pseudo colours. This should provide faster target recognition and fewer recognition errors.

Beyond the Battle

By the late 1990s, techniques had been devised for creating multi-spectral focal plane arrays in which the individual detector elements corresponding to the different colours lie in the same plane, being created by selective deposition and diffusion of material.

The US Army Research Laboratory's Advanced Sensors Collaborative Research Alliance in co-operation with the Night Vision and Electronic Sensors Directorate has developed a dual-band infrared (IR) focal plane array based on Qwip technology and tested this as a possible method of detecting buried land mines. The spectra of the detectors had peak response near 9.2 micrometres (blue channel) and at 10.5 micrometres (red channel), values chosen to take advantage of spectral features associated with disturbed soils. The two channels' images were combined into single fused images.

A true-colour system would have significant operational advantages. For example, since pattern recognition is easier with a colour image than with a monochrome image, a colour NV system would be useful for surveillance applications.

Several methods of producing colour NV images have been tried, but most have disadvantages. One technique uses two synchronised fast-spinning colour filter wheels in front of and behind the image intensifier tube. The front filter controls the colour of light being admitted into the tube, while the rear filter produces the appropriate colour of output light. Howler, since there are three primary colours, if each colour is given an equal time slice, it is available for only 33 per cent of the time, effectively reducing the incoming signal by 66 per cent.

Another approach is similar to the process used to create a colour image on a CRT tube, using coloured fibre-optic cables or coloured micro-lenses to dedicate individual pixels in the image to a single primary colour. This approach is widely used in consumer hardware, i.e. digital cameras, which have patterned colour filters mounted ahead of the pixel detectors. Here the penalty is one of resolution, since several pixels must be teamed to create a single 'pixel' of coloured output.

Multicolour FPAs would provide a large reduction in system complexity, eliminating complex optics and critical alignments.

Various techniques have been demonstrated for creating multi-colour arrays, while others have been proposed. The most straightforward approach uses stacked detector layers with different spectral responses--demonstrated with mercury cadmium telluride and Qwip (Quantum Well Infrared Photoconductor) detector arrays.

These have proved limited in response wavelength and/or performance, particularly at the very long wavelength infrared portion of the spectrum, says DRS Sensors and Targeting Systems. The company has developed blocked impurity band (Bib) detectors that are better at VLWlR, but notes that spectral response is not easily tailored to the needs of a two-colour FPA.

DRS Technologies has fabricated diffractive microlenses on the backside (illuminated-side) of thinned specially-designed Bib detector arrays. A linear diffractive grating superposed on a refractive microlens for each pixel focuses the light onto a spot much smaller than the pixel. This allows sub-pixel-sized detector elements to be spaced so as to intercept the diffracted light at two or more wavelengths.

Other schemes have been devised, but all involve some form of image degradation compared with that of an equivalent monochrome device.

ITT has developed and patented a colour NV system which uses three image intensifier tubes, each one associated with a different primary colour. A dichroic frequency splitter positioned ahead of these tubes divides the incoming light into the primary colours and distributes these to the tubes. Each tube amplifies the light of a single primary colour, and the outputs of the three tubes are fused or combined in the dichroic output reflector, creating a single full-colour image that is viewable via an ocular or eyepiece.

Another organisation involved in colour night vision is the US company Canvs. Potential military applications for the company's colour night vision technology include close quarters combat, operation in urban terrain, surveillance, physical security and facial recognition in low light environments.

At the request of various government agencies Canvs has severely limited the availability of this technology to commercial markets. Like the other advances, colour NV is likely to be supplied only to the Nato and a group of traditional US allies such as Australia, Egypt, Israel, Japan and South Korea. How soon it will be copied and freely manufactured around the world remains to be seen. Five years ago, who would have predicted that the US would be able to maintain its current near-monopoly in Gen III tubes?