Why does not a human being see the night?
Cones - to distinguish colors
Sticks - to set the brightness
When the level of brightness drops, only sticks - 1000 times more sensitive than cones and 92 to 100 million for a human being (in comparison with the cat that has 150 million around and is nyctalope) - react. Which explains why your vision changes to "black & white" mode. In the same way objects appear "fuzzy" because the transmission of the photoreceptors towards the optic nerve is less efficient with the rods. Basically, to activate the ability to natural "night vision" and pass the residual light, the pupil widens and "activates" the rods. But with a limit that does not allow effective night vision.
Infra-Red what is it?
- THE INFRA-RED WAVE RANGE EXTENDS FROM 0,7 TO 100 μm
- VISIBLE WAVE RANGE ranges from 0,38 to 0,7 μm
- GOING ON GAMMA, X, ULTRAVIOLET and RADIO RAYS, no interest here
What interests us for the technology used in the night vision and thermal is the infra-red wave range, subdivided (by the CIE system) into 4 spectral bands:
- Near infrared: from 7μm to 1,6μm
- The average infrared: from 1,6 μm to 4 μm
- Thermal infrared: from 4 μm to 15 μm
- Far infrared: from 15 μm to 100 μm
It is thanks to these different wave ranges that your remote control, your LED lamp, missile guidance, thermal imaging cameras, lasers ... and a whole bunch of other applications work!
The electromagnetic spectrum
What is residual light?
Absolutely essential to the operation of your telescope (without residual light - and therefore without photons, no night vision possible), emitted by the sun, the moon, the stars - and all the light sources found in urban areas (public lighting , vehicle headlights, illuminated signs) that form a luminous halo over a vast area - the residual light is the set of photons wandering on the space in which you are (at the speed of light of elsewhere), day and night. It is by amplifying this light (at night obviously for night vision) using a photocathode and a phosphorescent screen that we will restore an image (more or less good quality depending on the " generation "of the tube which contains the photocathode).
Now that the physical principle which allows the technology "night vision" is posed, we will be able to explain how it works!
A night vision telescope how does it work?
As seen above, the basic principle (for a passive operation bezel) is to amplify maximum residual light to make an image with the best definition and brightness possible. I will only briefly discuss (and in the chapter "infra-red torch") the active exploitation of infra-red, this technology being potentially a danger in tactical use.
- A lens (at the front of the telescope) captures the residual light and a part of the spectrum of the near infra-red and directs them to the electron tube (a photomultiplier).
- Passing through the photomultiplier light (photons) strikes a photocathode and thus generate electrons by photoelectric effect.
- The electrons are projected towards a wafer - polarized by electrodes - of micro-channels, the MCP (which is considered as a photomultiplier wafer). Built to facilitate the collision (each micro-channel is oriented at a more or less important angle - from 5 to 8 °) and to reduce the "noise". When the initial electrons penetrate the micro-channels, they come to hit the walls and cause the emission of other electrons, which, by amplification effect, will in turn go to hit the walls of the micro-channels, thus creating other electrons.
- The electrons (now numbering several thousand) will pass through a phosphorescent screen. Thanks to the kinetic energy acquired, the electrons (which have retained the structure of the initial photons - which will allow the restitution of the image) will excite the phosphorus atoms ... which will release photons. This light restored through a lens will constitute the final image - that you visualize "in green" from the properties of phosphorus. The lens should allow the focus (and possibly the magnification) for the best possible quality.
- It should be noted that the vision "in green" is due to the choice by the manufacturer of a specific phosphorus - the human eye being more sensitive to green, it was the solution for a contrast (more or less) optimal to a cost under control.
The schematic operation of a night vision telescope (at least 2 generation)
But then why are there several "qualities" of night vision glasses?
As with any human invention, we will continually seek to improve the capacity of a technology. Via physics, biology or chemistry, via the user-reported experience, and simply by a piece-making capability that improves with the advent of related technologies.
In the case of night vision, which mainly allowed the improvement is:
- Improvement of the photocathode and its sensitivity (through the 2 and 3 tube generations)
- S1, S20, S25 photocathode and Gallium arsenide photocathode (GaAs) photocathodes are used to improve sensitivity in the spectral range of visible and near infra-red.
- Inserting the micro-channel slab (from generation 2)
- This will make it possible to generate a much larger quantity of electrons (in comparison with the 1 generation) and therefore an improvement in the amplification and the quality of the image rendering.
- On an 3 generation tube, an ion filter film is affixed (to protect the cathode from exposure to an unwanted light source). This reduces the number of electrons generated and increases the visible halo on the light spots. On the contrary, the film significantly improves the life of the tube
- On an OMNI-V-VII generation tube 3 the integration of a finer ion filter - improvement of the SNR and light sensitivity - at the expense of the service life
- The "AUTOGATED" function (from generation 3)
- This function manages extremely fast (of the order of one millisecond) the supply of the tube. As soon as the tube is exposed to an "aggressive" light source, the power supply will be cut off immediately, thus preserving the tube and its life.
- The resolution (defined by the measurement in line pair per mm)
- In summary - and very succinctly - it's improving your visualization of the fineness of the details
- Improvement of the SNR (Signal Noise Radio)
- This is the ratio between the signal voltage (the electrical signal of your tube) and that of the noise it generates. Basically the "snow" (scintillation) that appears in the image. The difference between an 1 generation tube and 3 is obvious.
The different generations of tube
The image rendering of the different generations of tube (the term "4 generation" is overused and corresponds to the standardized 3 generation Omni-VII)
The 0 generation
In 1929 the Hungarian physicist Kálmán Tihanyi poses the principle of night vision (for the benefit of the British army). From 1935 a German firm (AEG - which still exists today) develops night vision technology, in parallel with the USA. During the second world war, these two countries will use the night vision capabilities in combat, on armored vehicles as well as on small arms. The US will develop the concept and continue its operational use during the Korean War. The technology used is active - it projects a wide infra-red beam
The 1 generation (and 1 +)
Always the most commonly used around the world today! Developed during the 60 years and exploited during the Vietnam war by the USA, it exploited the first tube "passive" with intensification of light with a photocathode S20 (for a Intensification gain of approximately x1000). The image is clear and has a good contrast in the center of the image, with edge distortion and an SNR that causes disturbances - "snow" - on the image rendering. The 1 generation tubes currently offered by manufacturers are mostly from stocks in the former Soviet Union - which is rather positive. The lifetime of this tube will be around 4000 hours (plus or minus) of active use et it will only be possible to operate with a high level of residual light (visible moon), except when using an IR torch in conjunction with the telescope.
The so-called "1 +" generation tube is nothing more than an improved 1 generation tube for better image quality (Armasight Core or Pulsar Edge) with optimized resolution.
- Definition: 35 to 60 line pair per mm
- Average life time: about 4000 hours
- Photocathode: S20
- Intensification: about 1000x - requires a high level of residual light
- Average price: from 150 to 700 euros - depending on the type of glasses (monocular, binocular, riflescope, with or without magnification ...)
The 2 generation (and 2 +)
This second generation introduces the MCP (the micro-channel slab) and a S25 photocathode, for an intensification gain up to 20000x, a significant improvement of the SNR, the resolution (45 line pair per mm minimum) and sensitivity to brightness - the addition of an IR torch will no longer be necessary and the residual light level will have to be much lower for an image rendering superior to the 1 generation. The phosphor screen can use (depending on its manufacturer) a phosphor that enhances the contrast of the green "color" and therefore makes a better level of detail.
The so-called generation tube "2 +" optimizes (really) the resolution (with an average of 60 line pair per mm), the SNR gains up to 10 points compared to an 2 generation tube and sensitivity changes to 400-800 μA / lm (for 500-600 μA / lm sensitivity for 2 generation and its S25 photocathode). 2 + generation tube with quality components is significantly closer to 3 generation tubes.
- Definition: 45 to 73 line pair per mm
- Average life time: about 10000 hours
- Photocathode: S25
- Intensification: approximately 20000x - requires a low residual light level
- Average price: from 900 to 2500 euros - depending on the type of glasses (monocular, binocular, riflescope, with or without magnification ...)
- FOM (Figure Of Merite): from 810 to 2044 (theoretical - actually rather 1800 maximum)
Generation 3 (and 3 standardized Omni-VII)
Integration of gallium arsenide-based photocathode (improves sensitivity to far infrared range but is more "fragile" than S25 type photocathodes) and "second generation" MCP overlay a filter film (which protects the cathode of the ions) - this reduces the number of electrons generated and increases the visualized halo around the luminous points - allows an increase in the life of the tube (up to 20000 h) and a amplification of residual light from 30 to 50000x. The purity of the image and the rendering of the details is about 3x superior to an 2 generation tube but your eye will not be sensitive to this optimization (or in a reduced way); On the contrary, the exceptional sensitivity to brightness allows you to use the telescope in very degraded residual light conditions. The "AUTO GATED" function will preserve the tube from accidental exposure to aggressive, sudden illumination while preserving image rendering - which will be essential for a combat operator who, without the AUTO GATED, may be dazzled by kick starts, explosions, fires ...
3 generation tube standardized by US Omni Military Standard (Level VII) primarily improves MCP with a thinner filter film than on a conventional 3 generation tube (while retaining the elements of a tube of 3i generation). This change - which reduces tube life to about 15000 hours - will drastically increase picture definition and rendering, resolution and contrast level. Usually reserved for military use, with an amplification gain from 80 to 120000x (theoretical - but it's still really impressive).
It should be noted that some manufacturers offer P43 phosphor tubes that offer a "black and white" or "bluish" rendering for a better view of contrasts and details in the image.
It should be noted that, depending on the US standardization level omni (from level II to VII), the filter film of the MCP will make a more or less clear and detailed image. Some 3 generation tubes are offered without any film (filmless). The rendering of the image is significantly improved but the life of the tube is obviously shortened.
- Definition: 57 to 73 line pair per mm
- Average life time: 20000 to 15000 hours
- Photocathode: gallium arsenide
- Intensification: from 30 to 120000x (very theoretical) - requires a very low residual light level
- Average price: from 2300 to 6000 euros - depending on the type of telescope (monocular, binocular, riflescope, with or without magnification ...) and used components
- FOM (Figure Of Merite): from 1400 to greater than 2000
FOR ASSEMBLY ON ARM, IT WILL BE NECESSARY TO MAKE A CHOICE OF A BEZEL THAT BRINGS A TUBE CAPABLE OF RESISTING THE REAR OF THE CALIBER OF THE DESTINATION WEAPON - THIS IN ORDER TO PRESERVE THE LIFETIME OF THE TUBE AND THE IMAGE RENDERING. IF IN DOUBT CONTACT US.
The special case of digital night vision
A technology identical to that used in your camera, your digital surveillance cameras, your webcam or your digital camera: a CCD or CMOS modified to be sensitive not to the visible spectrum but to the spectrum of infra-red and converts into a digital signal . The digital signal is amplified and transmitted to the LCD screen where you view the image. The lack of a phosphor screen will remove the black and green rendering to render a black and white image.
Like a 1 generation tube, a digital night vision goggle can only amplify residual light without the integration of a PCM. In fact you will need either a substantial residual light (full moon ...) or (like a security camera for example) IR diodes, or an IR torch. It is essential to note that any infra-red emission is detectable. It's stupid to be a sniper shot because of these kinds of mistakes.
The amplification will be identical (see above) to a tube of generation "1 +" (ie 1000x) with a better image rendering - in particular by the absence of distortion on the edges of this one.
Its most decisive advantage is that obviously the constraints related to the tubes disappear. You can use the telescope without any risk, neither for your eyes nor for the device. It will also be much easier to exploit all the advantages of a digital camera (recording of images or videos, integration of a rangefinder, a barometer ...).
This type of product will be perfect for use of "leisure" or zone security in "low" level of vigilance and low intensity combat. IT WILL AVOID COMBAT FACING PROFESSIONAL AND EQUIPPED SOLDIERS.
WHAT YOU NEED TO CHOOSE YOUR NIGHT VISION GLASSES:
- Simple logic: the investment must be related to the mission (s) to come
- Each tube has a shelf life - so a professional use will have to include a device renewal threshold
- Whenever possible try to select a telescope that is versatile (hand-usable, which mounts on a helmet AND on a weapon for example) - except for very specific uses (sniper ...)
- Determine the overall quality of a telescope thanks to its FOM (Figure Of Merite) - refer to the glossary below to understand the formula
GLOSSARY "NIGHT VISION"
- Automatic Brightness Control (ABC):
Automatic brightness control (allows modulation of the voltage transmitted on the MCP depending on the intensity of the residual brightness).
- Auto Gating (ATG):
Allows control of the voltage transmitted to the photocathode (and reduce or cut off the cycle) during exposure to aggressive brightness (night shooting, fire, lightning, public lighting, halo cleared by areas urban ...). This function preserves your vision of details in intense light and secures the photocathode (which could be permanently degraded without this function). Useful, even essential, for pilots of aircraft - especially at low altitude - special forces and interventions in urban areas.
- lp / mm (pairs of lines per millimeter):
Unit used to measure the resolution of the image intensifier. Usually determined from an 1951 US Air Force resolution test target. The target is a series of patterns of different sizes composed of three horizontal lines and three vertical lines. A user must be able to distinguish all horizontal and vertical lines and spaces between them.
Random and shiny effect in the whole area of the image. Scintillation, sometimes referred to as "video noise," is a normal feature of micro-channel plate intensifiers and is more pronounced in low-light conditions.
- Signal to noise ratio (SNR):
Ratio between the amplitude of the signal and the amplitude of the noise. If the noise (see definition of "scintillation") is as bright and tall as the image intensified, you can not see the image. The signal-to-noise ratio changes with the light level because the noise remains constant but the signal increases (higher light levels). The higher the SNR ratio, the better the device operates in a "dark" environment - with a low residual light.
- μA / lm (Microamperes by Lumen):
Measurement of the electric current (μA) produced by a photocathode when exposed to a measured amount of light (lumens).
The ability of an image intensifier or night vision system to distinguish the details of your environment. The resolution of the image intensifier tube is measured in line pairs per millimeter (lp / mm) while the resolution of the system is measured in cycles per milliradian. For any night vision system with 1 magnification, the resolution of the tube will remain constant while the resolution of another bezel can be affected by changing the focus and magnification of the eyepiece and adding magnification filters or "relay" lenses. Often, the resolution in the same night vision device is very different when measured at the center of the image and at the periphery of the image. This is especially important for cameras selected for photography or video where the resolution of the entire image is important.
- MCP (Microchannel Plate):
The famous "galette" of micro-channels that multiplies the electrons produced by the photocathode. A MCP is only found in the Gen 2 and Gen 3 systems. MCPs eliminate the distortion characteristics of Gen 0 and Gen 1 systems. The number of "holes" (micro-channels) in a PCM is a major factor in determining the resolution.
- Figure of Merit (FOM):
If there is one thing to remember from this blog post is this one! The FOM is determined as follows: resolution (line pairs per millimeter) x signal on noise. It is on this criterion that you will determine the "quality" of the tube of your telescope.