Image intensifiers were primarily developed for night time viewing and surveillance under moonlight or starlight conditions. They are capable of detecting and amplifying low-lightlevel images to appear as contrast sharp images. Over a period of time, image intensifiers have started covering a wide range of applications to include military, industrial products, inspection and scientific research, especially when combined with chargecoupled device (CCD) or Intensified CCD (ICCD) cameras.

The science and technology behind image intensifiers

In a typical image intensifier there are three components and processes which include a photocathode which converts light into photoelectrons, a microchannel plate (MCP) that multiplies electrons, and a phosphor screen that reconverts electrons into light. These are arranged in an vacuum tube.

Types. Types of image intensifiers are often broadly classified by “generation”. The first generation refers to image intensifiers that do not use an MCP and where the gain is usually no greater than 100 times. The second generation image intensifiers use MCPs for electron multiplication. MCPs using a single-stage MCP have a gain of about 10000, while those using a 3-stage MCP offer a much higher gain of more than 10 million.

Photocathode material. A variety of photocathodes materials are currently in use. Of these, photocathodes made of semiconductor crystals such as Gallium Arsenide (GaAs) and GsAs Phosphide (GsAsP) are called “third generation”. These photocathodes offer extremely high sensitivity.

Conversion into photoelectrons. The number of photoelectrons emitted at the first stage is directly proportional to the intensity of the input light. These electrons are then accelerated by a voltage applied between the photocathode and the MCP surface and thereby enter individual channels of the MCP. As each channel of the MCP operates as an independent electron multiplier, the input electrons striking on the channel wall produce secondary electrons. This process is repeated several times by the potential gradient (a potential gradient is the local rate of change of the potential with respect to displacement i.e. gradient) across the both ends of the MCP and a large number of electrons are in this way released from the output end of the MCP. The electrons multiplied by the MCP are further accelerated by the voltage between the MCP output surface (MCP-out) and the phosphor screen, and strike the photocathode which emits light according to the amount of electrons. Through this process, an input optical image is intensified about 10,000 times (in the case of a single stage MCP) and appears as the output image on the phosphor screen. Additional MCPs will add to the amplification.

Gating Process. An image intensifier can be gated to open or close the optical shutter by varying the potential between the photocathode and the MCP-in. by this potential difference towards the MCP and multiplied there. The gate function is very effective when analysing high-speed optical phenomenon. Gated image intensifiers and Intense CCDs having a gate function are capable of capturing instantaneous images of high-speed optical phenomenon while excluding extraneous signals.

Single stage and three stage image intensifiers and EM-CCD (Electron-Multiplying Gain-CCD) Cameras. Image intensifiers and EM-CCD cameras using a single stage MCP have been used in low-light-level imaging. However, these imaging devices cannot capture a clear image when the light level is lower than 10-5 lx (Lux). Image intensifiers using a 3-stage MCP are ideal for photon (a photon is the smallest quantum of electromagnetic radiation which is always in motion at constant speed in vacuum) counting. Image intensifiers with a 3-stage MCP can be considered high-sensitivity image intensifiers. However, these have two operation modes, one of which is completely different from normal image intensifier operation. At light levels down to about 10-4 lx, these 3-stage MCP image intensifiers operate in the same way as normal image intensifiers by applying a low voltage to the MCP. This operation mode allows the 3-stage MCP to provide a lower gain of 102 to 104 and is called “analogue mode”. On the other hand, when the light intensity becomes so low (below 10-5 lx) that the photocathode emits very few photoelectrons and obtaining a continuous image is then no longer possible. In such cases, by applying about 2.4 kV to the 3-stage MCP to increase the gain to about 106, light spots (single photon spots) with approximately a 60 µm diameter corresponding to individual photoelectrons will appear on the output phosphor screen. This operation is known as photon counting mode. Such an approach can be used in a 3-stage MCP for use in a wide spectrum of applications from extremely low light levels to light levels having motion images.

Hamamatsu Photonics’ Filmless MCP. Hamamatsu Photonics is a leading company of light technology and products and have designed filmless MCP. In conventional image intensifiers having a crystalline photocathode, a thin film is deposited over the surface of the MCP to prevention feedback. The company claims that their improved fabrication method successfully eliminates the thin film which eliminates the loss of electrons passing through the MCP and therefore improves the signal-to-noise ratio by more than 20 per cent as compared to filmed image intensifiers with longer life. Combining their filmless MCP fabrication technology with the highsensitivity GaAs and GaAsP photocathode will produce better results like clear, sharp images can be obtained with no chicken wire and images without distortion can be obtained at the periphery.

Some common terms

Photocathode Sensitivity Luminous Sensitivity. The output current from the photocathode per the input luminous flux from a standard tungsten lamp usually expressed in µA/lm (microamperes per lumen). Simply it is a guideline for sensitivity.

Radiant Sensitivity. The output current from the photocathode per the input radiant power at a given wavelength, usually expressed in amperes per watt.

Quantum Efficiency (QE). The number of photoelectrons emitted from the photocathode divided by the number of input photons, generally expressed in percentage.

Luminous Emittance. This is the luminous flux density emitted from a phosphor screen and is usually expressed in lumens per square meter.

Gain. Gain is applied to photocathode spectral response range. ‘Luminous emittance gain’ is used for image intensifiers having sensitivity in the visible region. ‘Radiant emittance gain’ and ‘photon gain’ are used for image intensifiers intended to detect invisible light or monochromatic light. ‘Luminous Gain’ is the ratio of the phosphor screen luminous emittance to the illuminance incident on the photocathode etc.

EBI (Equivalent Background Input). This indicates the input illuminance required to produce a luminous emittance from the phosphor screen, equal to that obtained when the input illuminance on the photocathode is zero.

Dark Count. The dark count is usually expressed as the number of bright spots per square cm on the photocathode measured for a period of one second. Cooling the photocathode is very effective in reducing the dark count.

Sensor fusion

Sensor fusion combines the positive qualities of the image intensifier and thermal imaging technologies into one device. Such a combination enables a user to view the image in much greater part of the light spectrum which spans from visible to near-infrared to long-wave infrared. The combination enables the viewer to see the image in the visible and thermal spectrum. The sensor fusion technology has led to the development of new night-vision technologies and devices like the enhanced night-vision goggle (ENVG) that combines a thermal imager with an image intensifier. The image intensifier works like a standard NVG in this system but image from the thermal sensor is seen through a video display. Both the inputs are then optically overlaid to provide a fused image. Developments are on to combine the video output of a thermal imager directly with the video output of an electronic output image intensifier. These new devices would then display a complete digitally fused image through HMD (headmounted/helmet display) in a device known as the digitally enhanced night-vision goggle (ENVG-D).

Combination of ENVG III and Family of Weapons Sights-Individual (FWS-I) technology

The FWS-I, when mounted on a soldier’s weapon, will transmit its sight picture through radio to the ENVG III, which is mounted on a soldier’s helmet. The FWS-I is designed for the M4 and M16 rifles. The ENVG will combine thermal imaging with image intensification technology. A variety of modes will allow soldiers to see in their goggles only the image from the ENVG III itself, or only the image from the FWS-I, or a combination of the two. Using a ‘picture-in-picture’ mode, the image from the FWS-I is displayed at the bottom right of the image that is coming from the goggle. This combines the rapid target acquisition technology and can effectively be used for surveillance, aiming weapons during daylight, darkness, adverse weather and dirty battlefield conditions. This system is being introduced to the US Army during 2019.