Why can't a human being see at night?
Cones - to distinguish colors
Rods - to set the brightness
When the level of luminosity decreases, only the rods - 1000 times more sensitive than the cones and with a number of 92 to 100 million/per human being (compared to the cat - 150 million approximately and which is nyctalope) - react. This explains why your vision switches to "black & white" mode. Also objects appear "blurred" because the transmission of photoreceptors to the optic nerve is less efficient with rods. Basically, in order to activate the natural "night vision" capacity and let the residual light pass through, the pupil widens and "activates" the rods. But with a limit that does not allow an efficient night vision.
What is the Infra-Red?
- INFRA RED WAVELENGHT RANGES FROM 0,7 to 100 μm
- VISIBLE WAVELENGHT RANGE from 0,38 to 0,7 μm
- GAMMA, X, ULTRAVIOLET and RADIO RAYS, no interest here
What interests us for the technology used in night and thermal vision is the infrared wave range, divided (by the CIE system) into 4 spectral bands:
- Near infrared: from 7μm to 1,6μm
- Mid-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, the missile guidance, the thermal cameras, the lasers...and a lot of other applications work!
The electromagnetic spectrum
What is residual light?
Absolutely essential to the functioning of your optical system (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 wander over the space in which you are (at the speed of light by the way), day and night. It is by amplifying this light (obviously at night for a night vision) with the help of 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 that allows the "night vision" technology is laid, we will be able to explain how it works!
How does a night vision telescope work?
As seen above, the basic principle (for a passively operated scope) is to amplify the residual light as much as possible to give the best possible image definition and brightness. I will only briefly discuss (and in the chapter "infra-red torch") the use of infra-red in an active way, this technology being potentially dangerous in tactical use.
- A lens (at the front of the goggles) captures the residual light and part of the near infrared spectrum and directs it to the electron tube (a photomultiplier).
- When passing through the photomultiplier, the light (photons) strikes a photocathode and generates electrons by photoelectric effect.
- The electrons are projected towards a wafer - polarized by electrodes - of microchannels, the MCP (which is considered as a photomultiplier wafer). The MCP is designed to facilitate the collision (each microchannel is oriented at a more or less important angle - from 5 to 8°) and to reduce the "noise". When the initial electrons penetrate the microchannels, they hit the walls and cause the emission of other electrons, which, by amplification, will in turn hit the walls of the microchannels, 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, when passed through a lens, will constitute the final image - which you will see "in green" because of the properties of phosphorus. The lens will have to allow the focus (and possibly the magnification) for the best possible quality.
- It should be noted that the "green" vision is due to the choice by the manufacturers of a specific phosphor - the human eye being more sensitive to green, it was the solution for a (more or less) optimal contrast at a controlled cost.
The schematic of a night vision goggle operation (2 generation+)
But then why are there different "qualities" of night vision goggles?
As with any human invention, there will be a continual effort to improve the capability of a technology. Through physics, biology or chemistry, through user experience, and simply through the ability to manufacture parts that improve 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
Autogating 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.
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 tubes
The image rendering of the different tube generations (the term "4 generation" is overused and corresponds to the standardized 3 generation Omni-VII)
In 1929 the Hungarian physicist Kálmán Tihanyi established the principle of night vision (for the benefit of the British army). From 1935 a German company (AEG - which still exists today) develops the technology of night vision, in parallel with the USA. During the Second World War, these two countries used the capabilities of night vision in combat, on armored vehicles as well as on small arms. The USA will develop the concept and continue its operational use during the Korean War. The technology used is active - it projects a wide beam of infra-red
Generation 1 (and 1 +)
Still the most commonly used throughout the world today! Developed during the 60 years and used during the Vietnam war by the USA, it uses the first "passive" light intensification tube with a S20 photocathode (for a Intensification gain of approximately x1000). The image is clear and offers a correct contrast in the center of the image, with a distortion on the edges and a SNR that generates disturbances - "snow" - on the image rendering. The 1 generation tubes currently offered by manufacturers are mostly from the stocks of 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 device.
The so-called "1+" generation tube is nothing more than an improved 1 generation tube to offer a better image quality (Core from Armasight or Edge from Pulsar) with an 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.)
Generation 2 (and 2 +)
This second generation introduces the MCP (the microchannel wafer) and a S25 photocathode, for a gain in intensification up to 20000x, a significant improvement in SNR, resolution (45 line pair per mm minimum) and light 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 a superior image rendering than the 1 generation. The phosphor screen can use (depending on its manufacturer) a phosphor that improves the contrast of the green "color" and thus renders 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 Merit): from 810 to 2044 (theoretical - in reality rather 1800 maximum)
Generation 3 (and 3 standardized Omni-VII)
The integration of the gallium arsenide based photocathode (improves the sensitivity in the far infrared range but is more "fragile" than the S25 type photocathodes) and a "second generation" MCP covered with a filtering film (which protects the cathode from the ions) - this reduces the number of electrons generated and increases the halo visualized around the luminous points - allows an increase in the life of the tube (up to 20000 h) and amplification of residual light from 30 to 50000x. The image purity and detail rendering is about 3x superior to an 2 generation tube but your eye will not be sensitive to this optimization (or to a reduced extent); On the other hand the exceptional sensitivity to brightness allows you to use the scope in very degraded residual light conditions. The AUTO GATED function will preserve the tube from accidental exposure to sudden aggressive illumination while preserving the image rendering - which will be essential for a combat operator who, without AUTO GATED, could be dazzled by gunshots, explosions, fires...
The 3 generation tube standardized by the US Omni military standard (level VII) mainly improves the MCP with a thinner filter film than on a conventional 3 generation tube (while retaining the elements of a 3th generation tube). This modification - which brings the life of the tube to about 15000 hours - will drastically increase the definition and the image rendering, the resolution and the contrast level. Generally reserved for military use, with an amplification gain of 80 to 120000x (theoretical - but it's still pretty impressive).
It should be noted that some manufacturers offer phosphor tubes P43 which offers a rendering "black and white" or "bluish" for a better vision of contrast and detail in the image.
It should be noted that depending on the level of US omni standardization (from level II to VII) the filtering film of the MCP will give a more or less clear and detailed image. Some tubes of 3 generation are proposed without any film (filmless). The image rendering is clearly improved but the lifetime 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 Merit): from 1400 to over 2000
FOR WEAPON MOUNTING, YOU SHOULD CHOOSE A SCOPE WITH A TUBE CAPABLE OF WITHSTANDING THE RECOIL OF THE TARGET WEAPON CALIBER - THIS WILL PRESERVE THE LIFE 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 or your webcam: a CCD or CMOS modified to be sensitive not to the visible spectrum but to the infrared spectrum and converted 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 an 1 generation tube, a digital night vision scope can only amplify residual light, without the integration of an MCP. In fact you will need either a significant residual light (full moon...) or IR diodes, or an IR torch. It is essential to note that any infra-red emission is detectable. It's stupid to be a shot by a sniper because of these kinds of mistakes.
The amplification will be identical (or even superior) to a "1+" generation tube (i.e. 1000x) with a better image rendering - notably because of the absence of distortion on the edges.
Its most decisive advantage is that, of course, the constraints linked to the tubes disappear. You can use the scope during the day 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 images or videos, integration of a rangefinder, a barometer ...).
This type of product will be perfect for " recreational " use or for securing areas at low alert 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 user will have to include a device renewal threshold
- Whenever possible try to select a device that is versatile (hand-usable, weapon AND helmet-mounted 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
NIGHT VISION GLOSSARY
- Automatic Brightness Control (ABC):
Automatic brightness control (allows the modulation of the voltage transmitted to the MCP according to the intensity of the residual brightness).
- Auto Gating (ATG):
Allows the control of the voltage transmitted to the photocathode (and to reduce or cut the cycle) during an exposure to aggressive luminosity (night shooting, fire, lightning, public lighting, halo released by urban areas...). This function preserves your vision of details in intense light and secures the photocathode (which could be durably degraded without this function). Useful, even essential, for aircraft pilots - 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. Typically determined from a U.S. Air Force resolution power test target from 1951. The target is a series of differently sized patterns consisting of three horizontal lines and three vertical lines. A user must be able to distinguish all the horizontal and vertical lines and the 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 the signal amplitude and the noise amplitude. If the noise (see definition of "scintillation") 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 works in a "dark" environment - with low residual light.
- μA / lm (Microamperes per 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 in your environment. Image intensifier tube resolution is measured in line pairs per millimeter (lp/mm) while system resolution 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 scope 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 microchannel "wafer" that multiplies the electrons produced by the photocathode. An MCP is found only 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 (line pairs per millimeter) x signal to noise. It is on this criterion that you will determine the "quality" of the tube of your scope.