Output window material is selected to match the readout method. Different output window types include borosilicate glass, fibre-optic plate and twisted fibre-optics.
A borosilicate glass window is used for relay lens readout. In this case, the relay lens is focused on the phosphor screen.
Fibre-optic output plate—a standard output window—is ideal for direct coupling to a CCD with fibre-optic plate input window. It consists of millions to hundreds of millions glass fibres bundled in parallel. A fibre-optic plate can transmit an optical image from one surface to another without causing any distortion. Diameter of the glass fibre matches the channel diameter of the MCP.
Twisted fibre optics as output window are used for night time viewing applications. Twisted fibre optics reduce eyepiece length, thereby making the night vision device more compact.
Image intensifier tube-based night vision devices are made in a variety of package configurations (Fig. 3), according to the requirements of specific application scenarios.
These mainly include night vision monoculars, binoculars, weapon sights and goggles.
There are two common modes of operation of an image intensifier tube: gated and photon counting. In the gated mode of operation, the intensified image can be gated to open or close the optical shutter by varying the potential difference between the photocathode and the inside surface of the MCP, thereby either allowing or disallowing the formation of an intensified image.
In gate-on mode, potential of the photocathode is lower than that of the MCP. As a result, photoelectrons are attracted towards the MCP, to be subsequently multiplied and hit the phosphor screen, producing an intensified image.
In gate-off mode, potential of the inside surface of the MCP is less than that of the photocathode so that photo electrons can revert back to the photocathode. Therefore no intensified image is seen on the phosphor screen.
In practice, MCP potential is fixed, and the intensifier tube is turned on by applying a negative polarity pulse of about 200 volts to the photocathode. Gated operation is very effective in analysing high-speed optical phenomenon.
Image intensifier tubes using a three-stage MCP have much higher sensitivity than those employing a single-stage MCP. This is important when it comes to operating at extremely low-light levels. When the light level is as low as 10–4 lux, a three-stage MCP helps in producing an image of acceptable quality. However, when the light level falls below 10-5 lux, incident photons are separated in time and space.
It is no longer possible to capture an image with a gradation. At extremely low-light levels, when only a few light spots appear on the phosphor screen per second, a good-quality image can be obtained by detecting each spot and its position, and integrating these into an image storage unit. Brightness distribution in photon counting mode is given by the difference in the number of photons at each position.
Different generations of image intensifiers
Night vision technology has undergone substantial changes in the last 40 years. These changes have led to gigantic improvements in performance standards of night vision devices. Each substantial change in technology is associated with a generation. We have seen several generations of night vision devices based on image intensifier tube technology. Beginning with generation-0, we are currently on generation-4.
The earliest night vision devices existed during World War II and the 1950s. These devices were based on image conversion rather than image intensification. The night vision devices primarily comprised a photocathode that converted incident photons into electrons. The electrons were accelerated towards an anode by applying a positive potential to the anode. The devices also had an IR source of radiation, called IR illuminator, mounted on the device. The IR illuminator irradiated the target scene with IR radiation. IR radiation reflecting off the target back to the night vision devices was collected by their objective lens and focused on to the photocathode.
Generation-1 night vision devices were an adaptation of generation-0 technology—both used a photocathode and an anode, the former for photon-to-electron conversion and the latter to accelerate photo electrons towards it. A major deviation in generation-1 devices from generation-0 ones was the absence of an IR source that was used in the case of the latter devices to provide scene illumination.
These vision devices were the first to use an MCP for electron multiplication. This led to significant increase in device sensitivity.
Consequent light amplification was of the order of 20,000x. This resulted in much improved performance even in very low-light level ambient conditions of cloudy and moonless nights.
Introduction of the MCP into the intensifier tube also obviated the need to connect the tubes in series, as was done in generation-1 devices. This significantly reduced the size of the night vision devices, and made handheld devices and helmet-mounted devices a reality.