Why can't a human being see at night?
Cones - to distinguish colors
Sticks - to set the brightness
When the level of light falls, only the rods - 1000 times more sensitive than the cones and the number of 92 to 100 million for a human being (compared to the cat which has around 150 million of them and which is nyctalope) - react. This explains why your vision goes into "black & white" mode. Likewise, the objects appear “blurry” because the transmission of photoreceptors to the optic nerve is less efficient with the rods. Basically, to activate the ability for natural "night vision" and let in residual light, the pupil widens and "activates" the rods. But with a limit that does not allow effective night vision.
What is the Infra-Red?
- 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 night vision and thermal is the infrared 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 ranges of waves that your remote control, your LED lamp, missile guidance, thermal cameras, lasers… and a whole bunch of other applications work!
The electromagnetic spectrum
What is residual light?
Absolutely essential for the operation of your glasses (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) which form a luminous halo over a vast area - residual light is the set of photons that roam 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 (of more or less good quality depending on the " generation ”of the tube which contains the photocathode).
Now that the physical principle which allows "night vision" technology is installed, we will be able to explain how it works!
How does a night vision telescope work?
As seen above, the basic principle (for a passive operation glasses) is to amplify the residual light as much as possible to render an image with the best definition and the best possible brightness. I will tackle only quickly (and in the chapter “infrared torch) the exploitation of infrared in an active way, 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 microchannels, the MCP (which is considered to be a photomultiplier wafer). Built in such a way as 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 enter the microchannels, they strike their walls and cause the emission of other electrons, which, by an amplification effect, will in turn strike the walls of the microchannels, thus creating d '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 returned through a lens will constitute the final image - which you visualize “in green” due to the properties of the phosphor. The lens should allow focus (and possibly 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, this was the solution for a (more or less) optimal contrast to a controlled cost.
The schematic operation of a night vision telescope (at least 2 generation)
So why are there several “qualities” of night vision goggles?
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, what 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 3 generation tube the integration of a finer ion filter - improved SNR and light sensitivity - to the detriment of the service life
The "AUTOGATED" function (from generation 3)
- This function manages in an extremely fast way (of the order of a millisecond) the supply of the tube. As soon as the tube is exposed to an "aggressive" light source, the power supply will be immediately cut off, thus preserving the tube and its lifespan.
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)
- It is the ratio between the voltage of the signal (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 a Generation 1 and Generation 3 hit 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 +)
Still the most commonly used around the world today! Developed during the 60s and exploited during the Vietnam War by the USA, it exploits the first "passive" tube with intensification of light with an S20 photocathode (for a Intensification gain of approximately x1000). The image is clear and offers good contrast in the center of the image, with distortion at the edges and an SNR that generates disturbance - “snow” - on the rendered image. The generation 1 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 generation “1+” tube is nothing more than a generation 1 tube improved to offer better image quality (Armasight Core or Pulsar Edge) with optimized resolution.
- Definition: from 35 to 60 pair of lines per mm
- Average life: about 4000 hours
- Photocathode: S20
- Intensification: around 1000x - requires a high residual light level
- Average price: from 150 to 700 euros - depending on the type of telescope (monocular, binocular, riflescope, with or without magnification, etc.)
The 2 generation (and 2 +)
This second generation introduces the MCP (the micro-channel wafer) and an S25 photocathode, for an intensification gain of up to 20000x, a significant improvement in SNR, resolution (45 pairs of lines per mm minimum) and brightness sensitivity - the addition of an IR torch will no longer be necessary and the level of residual light will have to be much lower for an image rendering superior to generation 1. The phosphor screen can use ( according to its manufacturer) a phosphor which improves the contrast of the green “color” and therefore makes a better level of detail.
The so-called “2+” generation tube (really) optimizes the resolution (with an average of 60 line pairs 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: from 45 to 73 pair of lines per mm
- Average life: about 10000 hours
- Photocathode: S25
- Intensification: approx. 20000x - requires low residual light level
- Average price: from 900 to 2500 euros - depending on the type of telescope (monocular, binocular, riflescope, with or without magnification, etc.)
- FOM (Figure Of Merite): from 810 to 2044 (theoretical - in reality rather 1800 maximum)
Generation 3 (and 3 standardized Omni-VII)
The integration of the photocathode made from gallium arsenide (improves the sensitivity to the far infrared range but is more "fragile" than the S25 type photocathodes) and a coated "second generation" MCP a filtering film (which protects the cathode from ions) - this reduces the number of electrons generated and increases the halo seen around the light 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 details is about 3x better than a generation 2 tube but your eye will not be sensitive to this optimization (or in a reduced way); On the other hand, the exceptional sensitivity to luminosity allows you to use the glasses in very degraded residual light conditions. The “AUTO GATED” feature will preserve the tube from accidental exposure to aggressive and sudden illumination while preserving the rendering of the image - which will be essential for a combat operator who without the AUTO GATED could be dazzled. by sudden 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 which offer a “black and white” or even “bluish” rendering for a better view of the 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: from 57 to 73 pair of lines per mm
- Average life: 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 scope (monocular, binocular, rifle scope, with or without magnification, etc.) and the components used
- FOM (Figure Of Merite): from 1400 to over 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 infra-red spectrum and converts into a digital signal . The digital signal is amplified and then 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 (or even greater) 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 “leisure” use or for securing areas at “low” vigilance levels and in low intensity combat. IT WILL AVOID COMBAT FACING PROFESSIONAL AND EQUIPPED SOLDIERS.
WHAT TO REMEMBER TO CHOOSE YOUR NIGHT VISION GLASSES:
- Simple logic: the investment made 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 "NOCTURNAL 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 called "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 signal amplitude and noise amplitude. If the noise (see definition of “flicker”) is as bright and large as the intensified image, you cannot 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, the better the device performs in a “dark” environment - with 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 details of your surroundings. The resolution of the image intensifier tube is measured in pairs of lines per millimeter (lp / mm) while the resolution of the system is measured in cycles per milliradian. For any night vision system with magnification of 1, the tube resolution will remain constant while the resolution of another telescope 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 “wafer” of micro-channels which multiplies the electrons produced by the photocathode. An MCP is only found in 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 an MCP is a major factor in determining the resolution.
- Figure of Merit (FOM):
If there is one thing to take away from this blog post, this is it! The FOM is determined as follows: resolution (pairs of lines per millimeter) x signal to noise. It is on this criterion that you will determine the "quality" of the tube of your glasses.