Optronic Sensors Night Vision Technologies (Part 3 of 6)

By Dr Anil Kumar Maini and Nakul Maini

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Image intensifier tube

An image intensifier tube amplifies low-light images to levels that can be seen with the human eye or detected by a digital image sensor. An image intensifier tube collects the existing ambient light originating from natural sources such as starlight or moonlight, or from artificial sources such as streetlights or IR illuminators, through the objective lens of the night vision device.

Low-level light consisting of photons enters the night vision device through the input window and strikes the photocathode. Inside of the input window is generally coated with a thin layer of light-sensitive material. This layer acts as the photocathode, which is protected from any damage from oxidation by operating the image intensifier tube under vacuum of the order of 10–9 to 10–10 torr. Photo electrons released by the photocathode are accelerated and focused by a high magnitude electric field towards the microchannel plate (MCP). The MCP has millions of small channels and the electrons entering these channels are both accelerated by another high magnitude electric field within the MCP and multiplied by secondary emission resulting from electrons bouncing off the inner walls of these channels.

For each electron entering the MCP, approximately 1000 electrons are generated and, subsequently, accelerated from the output of the MCP by a third electrical field towards the phosphor screen. The phosphor screen, which is a thin light-emitting layer deposited on the inside of the output window of the image intensifier tube, converts the impinging electrons back to photons. For every photon entering the input window of the intensifier tube, tens of thousands of photons come out of the output window after emission from the phosphor screen.

This photon multiplication takes place due to electron acceleration in the region between the input window and the photocathode, electron acceleration and secondary emission within the MCP channels, and electron acceleration in the region between the MCP and the phosphor screen. This multi-stage process produces an intensified or amplified image of the object that is much brighter than the original image.

Construction

Constituent parts of an image intensifier tube include input window, photocathode, MCP, phosphor screen, output window and power supply. Fig. 1 gives an overview of the constructional features of an image intensifier tube.

Construction of an image intensifier tube
Fig. 1: Construction of an image intensifier tube

Input window material is selected according to required sensitivity at shorter wavelengths. Common materials used for input window are synthetic silica (transmitting wavelength of 160nm or longer), fibre-optic plate (with transmission wavelength of 350nm or longer), magnesium fluoride (with transmission wavelength of 115nm or longer) and borosilicate glass (with transmission wavelength of 300nm or longer).

The photocathode that converts photons into photo electrons, the MCP that multiplies the photo-generated electrons, the phosphor screen that reconverts the electrons back into photons and the power supply that produces the electric field responsible for acceleration of electrons in different regions are all arranged in close proximity in an evacuated ceramic case.

Efficiency with which the photocathode converts photons into electrons, also known as photocathode radiant sensitivity or quantum efficiency, depends on wavelength. A number of photocathode materials is in use. Of these, gallium arsenide and gallium arsenide phosphide crystals offer extremely high sensitivity. Photo electrons are accelerated by an electric field produced by a high voltage applied between the photocathode and the MCP input surface.

The MCP is a thin glass disk, about 0.5mm thick, consisting of an array of millions of tilted glass channels, each five to six microns in diameter, bundled in parallel (Fig. 2). A single-stage MCP used in second-generation intensifier tubes provides electron multiplication of about 103. Two- and three-stage MCPs produce a gain of 105 and greater than 106. First-generation tubes did not use an MCP.

Microchannel plate
Fig. 2: Microchannel plate

The number of stages to be used in the MCP depends on the required value of gain. Strip current that flows through the MCP decides the dynamic range or linearity of the image intensifier tube. A low-resistance MCP causing a large strip current to flow through the MCP is desirable for achieving high linearity.

The phosphor screen reconverts the impinging electrons back to photons. Commonly-used phosphor types include P24, P43, P46 and P47. Phosphor screens are characterised by peak emission wavelength, decay time, power efficiency and emission colour.

Phosphor screen decay time is one of the most important parameters to be considered while selecting a suitable phosphor type. The ideal phosphor type is one whose decay time matches the readout method and whose spectral emission matches the readout sensitivity. When used with a linear image sensor or high-speed CCD, a short decay time is recommended for the phosphor screen, to avoid the appearance of an after-image in the next frame. On the other hand, a short decay time that minimises flicker is recommended for night time viewing and surveillance applications.

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