All of these Intensifiers have useful properties for scientific imaging or spectroscopy and are therefore offered by Andor. Gen III filmless photocathodes are a more recent development which Andor brought to the market. Gen III intensifiers are now replacing Gen II intensifiers for most military purposes and use a semiconductor photocathode. Gen II intensifiers use Bi or Multi Alkali Photocathodes. Gen I intensifiers used a different construction which didn’t use a MCP and are no longer in regular use. The early intensifiers were classified as Gen II intensifiers. There are a number of intensifiers routinely used in the scientific applications and they have been classified in relation to the Military classifications that originally developed them. The spectral response of an ICCD camera is primarily determined by the photocathode material used in the Image Intensifier. To achieve good sharp images the phosphor voltage must be typically 4kV -8kV depending on the phosphor and tube type. To set the gain of the MCP, a stable voltage must be applied typically in the range of 600 to 900 volts. To achieve fast Gating a high voltage pulser must be used which can pulse 200V pulses with sub-nanosecond rise and fall times. Specialist power supplies are needed to operate the Image intensifier. What are the disadvantages of lens coupled systems?ĭisadvantages of lens coupled systems are larger physical size, lower coupling efficiencies and increased scatter.
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With a suitably high quality image intensifier, the lens coupled arrangement can also produce a better quality image as the fiber-to-fiber variations and blemishes are removed from the system. This has the advantage of allowing the image intensifier to be removed, thus enabling the CCD to be used alone for unintensified applications. An alternative coupling method is to use a lens between the output of the image intensifier and the CCD – a ‘lens-coupled ICCD’. The high efficiency fiber optic coupling means that the image intensifier can be operated at lower gains, and this in turn results in better dynamic range performance from the image intensifier (better than 15 bit). Fiber coupled systems are physically compact with low optical distortion levels. The output of the image intensifier is coupled to the CCD typically by a fiber optic coupler: see diagram below. The degree of electron multiplication depends on the gain voltage applied across the MCP which can be controlled in the camera. Gains in excess of 10,000 can readily be achieved. These electrons in turn undergo acceleration which results in a cloud of electrons exiting the MCP. When the photoelectron has sufficient energy, it dislodges secondary electrons from the channel walls. The MCP is a thin disc (about 1mm thick) which is a honeycomb of glass channels typically 6-10 µm, each with a resistive coating.Ī high potential is applied across the MCP, enabling the photoelectron to accelerate down one of the channels in the disc. When a photon of the image strikes the photocathode, a photoelectron is emitted, which is then drawn towards the MCP by an electric field. The photocathode is coated on the inside surface of the input window and it captures the incident image: see the diagram on the right. The Image intensifier tube is an evacuated tube which comprises the Photocathode, Microchannel plate (MCP) and a Phosphor screen, and the properties of these determine the performance of the device. Image intensifiers were initially developed for night vision applications by the Military but increasingly their development is being driven by scientific applications.
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The gating and amplification occurs in the image intensifier tube. Intensified CCDs are also cameras which can exploit gain to overcome the read noise limit but also have the added feature of being able to achieve very fast gate times. Indeed Andor was the first company to offer a fully integrated ICCD which included a high performance delay generator, a high voltage gating unit and camera unit all built into an ICCD camera. An Introduction to Gated Intensified Cameras (ICCD cameras)Īndor first introduced an Intensified CCD camera into its range in 1995.