Compared to generation-2, generation-3 night vision devices have two distinctive changes: use of gallium arsenide photocathode and ion barrier coating on the MCP. Due to its higher quantum efficiency or radiant sensitivity, and its spectral response extending to near IR region, gallium arsenide photocathode enables target detection at longer ranges and in darker conditions.
Light amplification factor increased from 20,000 times in generation-2 devices to a maximum of 50,000 times in generation-3 devices. The ion barrier film improved the tube life from 2000 hours in generation-2 devices to 10,000 in generation-3 devices—even though it was at the cost of slight reduction in radiant sensitivity due to fewer photo electrons being able to reach the MCP.
Generation-3+ devices offer improved performance specifications over generation-3 devices. Two important features associated with generation-3+ night vision devices are an automatic gated power supply system and a thinned ion barrier layer. Absence of the ion barrier layer or its thinning improves luminous sensitivity, even though it is at the cost of a slight reduction in the life of the tube.
Another common term encountered while discussing night vision technology is omnibus, or OMNI. This refers to the multi-year/multi-product contracts of the US Army for the procurement of night vision devices from Exelis (formerly ITT Night Vision). Under these contracts, the company delivers generation-3 devices with increasingly higher performance. Current contract is for OMNI-VIII.
As of now, there is nothing like generation-4 night vision technology. Generation-4 night vision devices were initially conceived to use filmless and gated technology. The proposal was to remove the ion barrier film from the MCP, which was introduced in generation-3 devices. Removal of film was aimed at reducing background noise and enhance signal-to-noise ratio. It would also allow more electrons to reach the MCP, so that images were significantly less distorted and brighter.
Introduction of automatic gated power supply for the photocathode enabled the devices to adapt instantaneously to light level fluctuations from low-light level to high-light level, or from high-light level to low-light level. Removal of the ion barrier was also intended to reduce the halo effect seen around bright spots or light sources. While device performance improved, absence of the ion barrier film led to increased tube failure rates. As a consequence of this, the idea of film removal was abandoned in favour of a thinned film, giving birth to generation-3+ described in the previous section.
Intensified CCD, or ICCD, successfully exploits the optical amplification provided by an image intensifier to overcome the limitations of the basic CCD sensor. Two important features of an ICCD are high optical gain and gateable operation. Both attributes are characteristic of the image intensifier tube.
Though image intensifiers were initially developed for the military and law enforcement agencies for a range of deployment scenarios in surveillance, targeting and navigation, development of ICCD technology and devices extended their usage to many a scientific application in spectroscopy, scientific and industrial imaging, and medical diagnostics. In fact, development of image intensifier tubes is increasingly being driven by scientific applications.
An ICCD primarily comprises an image intensifier tube whose light output is coupled with a CCD sensor. Output of the image intensifier is coupled with the CCD, typically by a fibre-optic coupler (Fig. 4). A fibre-coupled system is physically compact with low optical distortion levels. High efficiency fibre-optic coupling also allows the image intensifier tube to operate at lower gains, which, in turn, results in better dynamic range performance.
A lens-coupled ICCD uses a lens between the output of the image intensifier and the CCD (Fig. 5). Lens coupling offers flexibility of using the ICCD sensor in non-intensified mode by allowing the image intensifier to be removed.
Disadvantages of lens-coupled ICCDs are larger physical size, lower coupling efficiencies and increased scatter. Power supply is another important constituent part of the ICCD sensor. The power supply section generates DC voltage (typically 600 to 900 volts) for the MCP to achieve desired gain, DC voltage (typically 4kV to 8kV) for the phosphor screen and voltage pulses (typically 200 volts) for gated operation of the photocathode.
The gating pulse width and rise/fall time depends on desired gating parameters. Gating pulse width of less than a nanosecond and rise/fall time of a small fraction of a nanosecond are achievable.
Important characteristic features of an ICCD include spectral response, spatial resolution, gating time and repetition rate, noise, sensitivity, dynamic range and frame rate. The spectral response of an ICCD camera is primarily determined by the input window, photocathode materials and photocathode size used in the image intensifier tube.
The noise and, hence, sensitivity of the ICCD is also governed by the image intensifier. A noise component called dark current component, also known as effective background illumination (EBI), originates from thermally-generated charge in the photocathode. Dark current is generally not an issue when using short gate times.