Nightvision Technology (NVD)
Soon after my interests in lasers began and not very long following the construction of my first laser in 1990, several articles had been published in one of the former Australian Electronics magazine, 'Electronics Australia' on night vision devices and in particular that of image intensifier tubes which are at the heart of night vision devices.
Oatley Electronics had begun to stock Image converter tubes, in particular the RCA 6032 / 6032A, Phillips XX1080 tubes and later some surplus German tank viewers.
In 1991, I constucted my first night viewer device. My very first viewer used the RCA 6032 tube, was constructed from two lengths of PVC tube glued side-by-side, one housed the image converter tube and the other, the large size power supply unit. A camera lens was used for the objective and a simple eye loupe for the eyepiece.
Overall, the performance of the viewer was not spectacular, however it was provided a fundamental insight into the technology and paved the way for better units as time when on. My first device was dismantled and parts reused for other projects. The tube on the left in image below is of my first Image converter tube, the RCA 6032.
The RCA 6032 tube is an infrared image converter tube, converting infrared (invisible) light to a visible image via a phosphor covered glass screen. A high (+15kV) voltage is applied to the Anode, and a grid voltage around 1-3kV to focus the electrons (electrostatic focusing).
Such tubes are classed as 'Generation-0' Technology.
Download the RCA 6032 Datasheet
On 3rd August 1992, I had purchased my first GEN-1 Tube, using the earlier variant of the EEV-P8073 tube, this used a smaller custom built high-voltage power supply (located in the shorter tube) and powered by a 9-volt battery.
This unit uses a simple double-convex lens for the eyepiece and a Minolta camera lens as the objective, with the advantage of an aperture to control incoming light levels.
Overall performance from this tube was significant, with clear, less distorted images due to the use of a superior photocathod and the use of fibre-optic input and output windows. I still have this unit in my collection and works to this day.
The EEV-P8073 are a compact and ruggered image Intensifier tube, featuring 25mm input, S25 photocathode and a P25 phosphor output fibre-optic windows. The tubes offer a gain of 100 Lux, a resolution of 50 line-pairs/mm and around 7% distortion.
In 1993, Oatley electronics was sourcing some limited quantities of the three-tube cascade that uses the EEV-P8073 25mm tubes. The early tubes did not have the intergrated power supply, therefore a special custom power supply had to be contstructed to allow the tube to work.
Once completed, this this nighviewer delivered very impressive performance, to this day I still have the working device, and is the unit I am holding in the first image above. I had also built a second unit that was subsequently sold.
In 1998, I had some other GEN-0 tubes removed from German tank viewers. These tubes had the integrated optics, as such I built the unit with an integrated IR illumination source. Performance from this unit was not great.
At the heart of the a night vision system is the image converter or image intensifier tube which is an electro-static vacuum tube (electron tube) that converts incoming photons into electrons via a sensitive coating known as a photocathode, an electric field then accelerates the electrons towards a phosphor screen to view through an eyepiece.
The photocathode is coated with a light-sensitive material sensitive to the infrared spectrum, while the screen (at positive high-voltage potential) is coated with a phosphor that fluoresces when hit by electrons, typically with a green emission (similar to a cathode Ray Tube).
The major difference with an image intensifier tube (vs a CRT), is that the electrons are not scanned, but optically focused with an objective lens onto the photocathode.
Image converter / intensifier tubes are classed based on their major technological developments and improvements known as 'generations' or GEN-No, where 'No' denotes the generation number.
Early development of an image converter tube began in the late 1920s, however it was not until the mid-1930s and through the second world war where the demand for night vision technology pushed development further, beginning with GEN-0.
Generation 0 was first inverting image converter tube was developed by RCA and used during world war II. GEN-0 tubes utilised a spherical photocathode and screen and an electrostatic inverter to focus the image from the photocathode onto the screen, and required active Infrared illumination, which is also a downside. Unlike later technology, GEN-0 technology does not amplify the incoming signal (light).
Generation-0 (GEN-0), Image Converter Tube.
GEN-0 was first inverting image converter tube was developed by RCA and used during world war II. GEN-0 tubes utilised a spherical photocathode and screen and an electrostatic inverter to focus the image from the photocathode onto the screen, and required active Infrared illumination, which is also a downside. Unlike later technology, GEN-0 technology does not amplify the incoming signal (light).
Developed from 1936, GEN-1 technology offered significant amplification with the use of more effective photocathode materials. S-11 photocathodes offered an improved sensitivity of around 80 uA/lm.
In 1956, S-20 photocathode had been discovered, offering sensitivity upto 200 uA/lm.
A further development of the GEN-1 tube was the use of fibre-optic plates for the input and output windows., allowing for flat-front surfaces which also had the advantage of being able to cascade tubes together. The 1960s saw the development of the cascaded (passive) image intensifier tubes, these made it possible for true passive nigh vision scopes, most common is the AV/PVS-2 rifle scope.
Generation-1 (GEN-1), Image Intensifier Tube.
Developed from 1936, GEN-1 technology offered significant amplification with the use of more effective photocathode materials. S-11 photocathodes offered an improved sensitivity of around 80 uA/lm.
In 1956, S-20 photocathode had been discovered, offering sensitivity upto 200 uA/lm.
A further development of the GEN-1 tube was the use of fibre-optic plates for the input and output windows., allowing for flat-front surfaces which also had the advantage of being able to cascade tubes together.
The 1960s saw the development of the cascaded (passive) image intensifier tubes, these made it possible for true passive nigh vision scopes, most common is the AV/PVS-2 rifle scope.
Generation-2 (GEN-2), Image Intensifier Tube.
The next major development and GEN-2 technology came in the late 1960s with the introduction of the Micro-Channel Plate (MCP). The Micro-Channel Plate is a device that allows for amplification (or gain) to occur through secondary emissions that occur within the individual channels that make up the plate. Typically, a large potential voltage (>1000V) is placed across the MCP.
Generation-3 (GEN-3), Image Intensifier Tube.
GEN-3 development began around the mid-1970s and has continued to be improved to this current day.
The major differentiation between GEN-2 and GEN-3 is the use of gallium arsenide (GaAs) photocathode, which also has a greater response to the near-infrared spectrum between 800nm - 900nm.
A major problem that GEN-3 tubes suffer, is the degradation of the photocathode by positive ion poisoning which led to the premature failure of the photocathode.
To mitigate ion poisoning, a thin ion film (known as an ion barrier) was applied to the MCP, leading to greater tube life, however the ion barrier also reduced the sensitivity by attenuating electrons.
Development of the technology has continued to progress, advancing techniques in attempts to remove the requirement of the ion barrier, increased resolution and signal-to-noise ratio (SNR).
The current standard, the ion film continues to be used, however additional development include thin-film technology, reducing the thickness of the ion barrier as well as reduction of voltages required to operate the tube.
Modern Image intensifier tubes have additional protection features and extending an Image Intensifier tube's lifetime:
Automatic Brightness Control (ABC): Reduces the voltage across the MCP to keep the tube's output brightness within limits.
Bright Source Protection (BSP): Reduces the voltage to the photocathode with changing soure intensity.
Auto Gating: Auto gating is a feature where the voltage to the tube is turned on and off at high frequency in a controlled manner. An advantage of a gated tube is the apparent increase in resolution in higher-contrast conditions.
A standard measures for the quality of a GEN-3 image intensifier is its Figure of Merit, which is a numerical value that multiplies the resolution of the tube by the signal-to-noise ratio:
Resolution in lp/mm x SNR = FOM.
In the mid 2000s, I had found a seller in the UK whom was supplying the more recent EEV-P8079HP 3-Stage Image Intesifier tubes. These tubes also have the intergrated high-voltage power supply.
Download the EEV-P8079HP Datasheet: Datasheet
Someday I envisage to build a scope utilising this tube. The eyepiece seen in the photo is the actual eyepiece used with the original night vision scope.
In late 2007, I was fortunate to be able to acquire a GEN-2 image intensifier tube. The original unit was made by Optex which was used with the canon VL mount camcorders.
The tube is a Philips (Photonis) XX1665/P, however I have not found any data on this tube. The desire would be to repurpose the tube into a GEN-2 Scope.
- Flavio Spedalieri -
Written: 16 November 2020
Updated: 1st August 2021
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