Image intensifier

The idea of an image tube was first proposed by G. Holst and H. De Boer in 1928, in the Netherlands [1], but early attempts to create one were not successful.

Subsequent development of this technology led directly to the first Generation 0 image intensifiers which were used by the military during World War II to allow vision at night with infrared lighting for both shooting and personal night vision.

The first military night vision device was introduced by the German army[citation needed] as early as 1939, developed since 1935.

Early night vision devices based on these technologies were used by both sides in World War II.

Unlike later technologies, early Generation 0 night vision devices were unable to significantly amplify the available ambient light and so, to be useful, required an infrared source.

This UV range is termed "solar blind" because it is shorter than the wavelengths of sunlight that typically penetrate the Earth's atmosphere.

Discovered in 1953 by Taft and Apker [2], solar blind photocathodes were initially developed using cesium telluride.

With the discovery of more effective photocathode materials, which increased in both sensitivity and quantum efficiency, it became possible to achieve significant levels of gain over Generation 0 devices.

The additional sensitivity made these tubes usable with limited light, such as moonlight, while still being suitable for use with low-level infrared illumination.

These experiments worked far better than expected and night vision devices based on these tubes were able to pick up faint starlight and produce a usable image.

Known as "cascade" tubes, they provided the capability to produce the first truly passive night vision scopes.

With the advent of fiber optic bundles in the 1960s, it was possible to connect smaller tubes together, which allowed for the first true Starlight scopes to be developed in 1964.

This scheme has not been used in rifle scopes, but it has been used successfully in lab applications where larger image intensifier assemblies are acceptable.

[1] Second generation image intensifiers use the same multialkali photocathode that the first generation tubes used, however by using thicker layers of the same materials, the S25 photocathode was developed, which provides extended red response and reduced blue response, making it more suitable for military applications.

The micro-channel plate is a thin glass wafer with a Nichrome electrode on either side across which a large potential difference of up to 1,000 volts is applied.

The wafer is manufactured from many thousands of individual hollow glass fibers, aligned at a "bias" angle to the axis of the tube.

Secondly, the photocathode exhibits negative electron affinity (NEA), which provides photoelectrons that are excited to the conduction band a free ride to the vacuum band as the Cesium Oxide layer at the edge of the photocathode causes sufficient band-bending.

The high sensitivity of this photocathode, greater than 900 μA/lm, allows more effective low light response, though this was offset by the thin film, which typically blocked up to 50% of electrons.

With sensitivities of the photocathodes approaching 700 μA/lm and extended frequency response to 950 nm, this technology continued to be developed outside of the U.S., notably by Photonis and now forms the basis for most non-US manufactured high-end night vision equipment.

To overcome the ion-poisoning problems, they improved scrubbing techniques during manufacture of the MCP ( the primary source of positive ions in a wafer tube ) and implemented autogating, discovering that a sufficient period of autogating would cause positive ions to be ejected from the photocathode before they could cause photocathode poisoning.

Generation 3 Thin Film technology is presently the standard for most image intensifiers used by the US military.

In 2014, French image tube manufacturer PHOTONIS released the first global, open, performance specification; "4G".

Electronic Gating (or 'gating') is a means by which an image intensifier tube may be switched ON and OFF in a controlled manner.

This makes gated image intensifier tubes ideal candidates for use in research environments where very short duration events must be photographed.

In such an instance, the gating operation may be synchronized to the start of an event using 'gating electronics', e.g. high-speed digital delay generators.

GPANV devices can allow a user to see objects of interest that are obscured behind vegetation, foliage, and/or mist.

Auto-gated tubes rapidly shut off current to the photocathode and microchannel plate at a high frequency that is imperceptible to the user.

This is a measure of how many lines of varying intensity (light to dark) can be resolved within a millimeter of screen area.

This creates ambiguity in the marketing of night vision devices as the difference between the two measurements is effectively pi or approximately 3.142x.