First of all we want to clarify what night vision is...

There are basically 3 options for night vision:

1.The possibility of digital image amplification using so-called CCD/CMOS   sensors. Whereby the processor processing results in a delayed display of the output image. 

2.The possibility of amplifying weak residual or infrared light (NIR) through an image intensifier tube (IIT's).

3.And the ability to see heat signatures. In this way, the infrared radiation in the MWIR / LWIR range is recorded and converted into an image, that allowes you to see temperature differences in the environment.

On this page it is primarily about the "classic" night vision devices with image intensifier tubes (IIT's), so let's go on...   .



- How do NVD's work?

Night visions let you see in the dark! So there is a small amount of light (photons) which went trough the lens of the objective. After the photons are entering the lens there will got trough a photocatode in the image intensifier tube (IIT) which converts the photons into electrons. After that the electrons went trough the MCP ( Micro channel plate). This Part of the night vision tube does what the name is saying... it multiplies the electrons what results in a much stronger electron-signal what amplifying the image what we can see. After the huge amount of electrons went into  the phosphor screen at the end of the image intensifier tube (IIT) there will be converted into photons! After all this processes we are able to see the typical green or white image with our eyes when we look into a night vision device.

                                                                                                                                                                                                                             

-What tubes are we using?




Photonis ECHO   (Commercial grade)

Res: min. 57 lp/mm

SNR: min. 24Sens: N/A FOM:  min. 1600  –  max 2000  Photonis Echo tubes with wide spectrum bandwidth. They see what others do not see.  Echo tubes allow for more spots, but at a great value and performance. -But even with some spots there will be no really disadvantages for the normal end-user, especially in motion spots are not really disturbing.



Photonis ECHO+ (Commercial grade)

Res: min. 57 lp/mm

SNR: min. 26   Sens: N/A   FOM:  min. 2000   Photonis Echo+ tubes with wide spectrum bandwidth. They see what others do not see. Echo+ tubes allow for more spots, but at a great value and performance. Compared to regular Echo tubes these have a min 2000 FOM.

 

Photonis 4G (Milspec grade)

Mil spec grade tubes from Photonis N/A   FOM:  min. 1800   Photonis 4G tubes with wide spectrum bandwidth and a better performance in lower / no light conditions. They see what others do not see. 4G tubes are allowed to have less spots than ECHO or ECHO+ tubes, but for their performance there are more expensive Compared to regular Echo tubes.



                                                                                                                                                                    

                                                                                                                                                                                                                             

-Green vs. White Phosphor


One of the most essential parts of an IIT is its phosphor screen. It is what you see and what converts the electrons back to photons.

Typically, the phosphor is deposited on the back of the fiber optic and then covered with a layer of aluminum. The aluminum layer increases the light efficiency up to almost 100% by reflecting the light and reducing the electrical charge retained.

There is a multitude of phosphor types that are usually referred with with a P-number i.e. P-22. Four parameters are the most critical ones for evaluating a phosphor screens performance and suitability: Conversion factor, light spectrum, decay time and resolution.

Conversion factor
Refers to conversion rate of electrons vs photons. So how many photons are emitted by one electron hitting the screen.
A high conversion rate does not necessarily mean the eye will perceive a bright image as that depends entirely on the emitted light’s wave lengths.

Light spectrum
The emitted light spectrum of the screen i.e. 475nm-600nm for P43.

Decay time
The time the screen needs to go back to a low brightness after being activated. It has to be multiple times shorter than the eyes temporal inertia. It is comparable to the refresh rate of a TV screen or a light bulb.

Resolution
This is depending on the size of the average size of particles that are the screen. Bigger particles would be more efficient but would eventually create a literally grainy image. Normally this is kept at about twice the resolution of the final system.

As a fifth indicator serves the luminous efficiency. This is not a fixed number but rather describes how well a phosphor screen produces light within the visible light spectrum or in other words how well the light output of the screen fits the to the human eye.



 


 

P43 (yellow-green)

Pro:
·          High luminous efficiency, almost the same as the human eye for the color green
·          Human eye is most sensitive to contrast with green colors
·          Low decay time (1.2 ms to 10%)
·          More yellow-green image compared to older P22 (is not produced anymore),and less eye fatigue in comparison
·           Much higher conversion rate and light emissions compared to P45

Con:
·          Faster eye fatigue compared to P45
·          Narrower bandwidth which uses basically just one type of receptors (cones) in the eye where P45 uses most of the                   bandwidth

 

P45 (white)

Pro:
·          White light feels more natural in low light environments and therefor causing less eye fatigue and having a noticeably               perceived brighter image
·          As recently introduced into (broader) production, most of these tubes use state of the art  technology
·          Low decay time (1.5ms to 10%)
·           White light allows to use the rod cells in the eye which are most sensitive in low light  which  can  create a brighter                   perceived image

Con:

·          Lower luminous efficiency than P43/P22 which can produce a darker perceived image
·          Hardly any eye pieces match the bandwidth of P45 which can cause distortion on the image (chromatic aberration) and             hard to design for the wide bandwidth


 

Summary

There are strong arguments between green and white phosphors and both have their up and downsides. A noticeable trend from green to white is visible in the market, especially the US market and since P45 tubes are now certified for aviation this will increase.
As the broader introduction of P45 tubes is fairly recent to this date, the performance and technology is absolute state of the art like filmless or 4G IITs which enhances the performance compared to older P43 or P22 (P22 older green phosphor) tubes.
As everyone is different and their eyes are different, you cannot say that P45 or P43 is better as it rather depends on the users and what they can work better with. Our recommendation is to try both in a comparable environment and decide afterwards.

                                                                                                                                                                                                                             

-Spots

Why are people worried about spots?  Of corse it is nicer to have a spotless tube... but spots are cosmetic blemishes in the image intensifier tube which are normal and come from the manufacuring process. We are testing our tubes and document every tube which we are building in our devices. Spots aren`t effect the operation or the reliability of the night vision device. And even in a dynamic scenario where the most people use the devices there will be no distrubing you as a end user. 


                                                                                                                                                                                                                          

-The Generations


Generation 0

Early snipers used image converters (sniper-scopes) that required an additional infrared light source to illuminate the scenery. Known as Gen 0, these image converters evolved from RCA's image converter tube developed in the mid-1930s for use in televisions. The Gen 0 image converter used an S-1 photo-cathode, a high-voltage electron acceleration electrostatic field and a phosphor screen. The S-1 cathode (AgOCs) did not have as much quantum efficiency as the cathodes used today, it heavily relied on the IR light used to illuminate the surrounding.   

The process by which the image was intensified was quite simple in this generation. It was more a transformation then an intensification, transforming the invisible IR light in to visible light. The reflected IR illuminator light entered the tube and the photo-cathode converted the light to electrons. Electronic elements focused these electrons through a cone-shaped anode and accelerated them using high voltage so they hit the phosphor screen with greater energy, creating a visible image. This did not produce much gain and caused distortion in the image. Also, tube life was not very good by today's standards.



Generation 1

The starlight scope, developed during the early 1960s and used during the Vietnam War, was made using Gen 1 image-intensifier tubes. In this scope, three image-intensifier tubes were connected in series, making the unit larger and heavier than today's night-vision goggles. This early generation produced a clear center image with a distorted periphery. The use of multiple tubes connected in series allowed for much greater overall light gain as the output of the first tube was amplified by the second and the second by the third. Due to the simple power supply design, the image was subject to instances of blooming momentary image washout due to an overload in the intensifier tube caused by bright light sources.

The primary difference between Gen 1 and Gen 0 was the more sophisticated chemical process employed to create the photo-cathode. The S-20 cathode, a multi-alkali antimonide process, enhanced the sensitivity as well as the spectral response. However, Gen 1 did have some of the same drawbacks of image distortion and decreased tube life as seen with Gen 0. Tubes built with Gen 0 and Gen 1 technologies are commonly found in many of today's civilian low budget night vision viewers.



Generation 2

In the late 1960s, Gen 2 technology brought a breakthrough in night vision with the development of the microchannel plate (MCP). Additionally, the photocathode was further refined to the S-25 cathode and produced much higher photo response.

It was the introduction of the MCP that made Gen 2 unique. The MCP begins with two dissimilar pieces of glass. A large tube of solid glass (core) is placed within a tubular sleeve of glass (clad). The two glasses are then heated together and stretched to form a very small diameter glass fiber. The fibers are ultimately compressed together to form a bundle of glass fibers called a boule. The boule is then sliced at an angle to obtain thin discs. Further chemical processing removes only the core glass, thus creating the channels within the MCP. During the tube operation, the electrons travel into the channels and, as they strike the channel walls, they produce secondary electron emissions which create several hundred electrons, multiplying with each strike.

The other advancement with Gen 2 was the reduction in overall size and weight of both the tube module and the power supply. It allowed for compact head mounted night vision devices.


Generation 3

Developed in the mid-1970s and placed into production during the 1980s, Gen 3 was an advance in photocathode technology. Gen 3 tubes use gallium arsenide (GaAs) photocathodes. This increases the tube's sensitivity dramatically and particularly in the near-IR. The increased sensitivity improved system performance under low-light conditions.

However, the highly reactive GaAs photocathode could be easily degraded by the inherent chemical interactions that take place within a tube under normal operation. Most of the chemical reactions take place within the MCP due to the electron interactions with the walls of the MCP channels. Thus, to overcome the degrading effects of the photocathode, a thin metal-oxide coating was added to the input side of the MCP. This coating, more commonly known as an ion barrier film, not only prevented premature degradation of the photocathode but also enhanced the tube life by many times that of the Gen 2 tubes. Today's Gen 3 tubes have a lifetime of at least 15.000h.

Gen 2 and Gen 3 tube manufacturers have made continuous improvements through the years to increase the signal-to-noise ratio within each respective technology. Continuous improvements have been made within MCP manufacturing so as to improve the overall resolution also.

There has been considerable effort expended in developing a Gen 3 tube without the ion barrier film. The effort proved successful, but the manufacturing costs were to high compare to the performance improvements. For a brief period of time, the Gen 3 tube without the ion barrier film was termed Gen 4. This terminology, however, was rescinded shortly after it was announced, though some resellers of night-vision tubes still use the nomenclature. Today L3 is the only company manufacturing these Filmless tubes. L3 calls them infinity tubes.

 

Continuous developments

One area that has contributed to the improvements is the advancement of the miniature high-voltage power supply. Early developments with the power supply included protection circuits to automatically control the output brightness of the tube under changing input light conditions. These effects, known as automatic brightness control (ABC) and bright source protection (BSP), were directed at protecting both the image tube from highlight exposure and the user's eyes from excessive brightness. The ABC automatically reduces voltage to the microchannel plate to keep the image intensifier output brightness within optimal limits and to protect the tube. This effect can be seen during rapid changes from low-light to high-light conditions when the image gets brighter and then quickly returns to a consistent level. The BSP reduces voltage to the photocathode rather than the microchannel plate. The BSP protects the image tube from damage and enhances its lifetime. The scene resolution can be degraded under high-light conditions. Advancements in miniaturized power supplies include the addition of Autogating circuits. These circuits control the way the photocathode is operated under changing input light conditions. Autogating allows the image tube to be used under higher input lighting with much less degradation of the image quality. Autogating turns off the photocathode voltage for brief periods of time, the effect is not visible to the human eye. The cathode voltage is constantly oscillating, but the image appears as if it were continuous. The Autogating circuit reduces the time the voltage is on during each oscillation but keeps the peak voltage level up. By controlling the application of voltage in this manner, the resolution quality remains high. In effect, the Autogating feature tricks the device into thinking it is always in a low-light environment, which is the optimal environment for maximum efficiency and clarity for the image-intensifier tube. While the most obvious effect of Autogating for the user may be improved resolution in high-light conditions, its original purpose was to help extend the lifetime of the tube, a benefit which is most realized with thin-film or filmless tubes. As opposed to the gradual lifetime decay seen in tubes without an ion barrier film, the Gen 3 tube with gating and the use of a thin film improves tube life and performance far more than any other image-intensifier technology. Typical reliability is well in excess of 15,000 hours without noticeable degradation. This change in durability is a significant accomplishment when considering the much shorter lifetimes of Gen 0, Gen 1 and Gen 2 tubes.

                                                                                                                                                                                                                             

-What is state of the Art?

The development of Gen 3 IITs started end of the 1970 and the first contract for Gen 3 IITs (the OMNI I contract) was signed in 1981.
There are several differences between the Gen 3 and Gen 2+ technology. Main difference is that Gen 3 uses GaAs photocathodes instead of the multialkali photocathodes used in Gen 2+. Their conversion-rate is much higher then any of the Gen 2+ tubes. But they have one drawback, due to the nature of GaAs, they deteriorate much faster and have a lifespan of only about 200h. So they added a thin oxidation layer on to it, limiting its conversion-rate but extending its lifespan to more then 15.000h. This film layer usually cuts the conversion-rate in half. On a side note, the conversion rate is also given in most datasheets and is called the Luminous Sensitivity measured at withe light (2856°K) and the unit is uA/lm. (uA being the electrons and lm the light, lumens, so how many electrons do you get per Lumen) We will go deeper in to the numbers and how to evaluate a datasheet in another Sundays post.

The very first Gen 3 IIT was developed in Germany mid 70's. The US was quick to acquire the technology and further develop it. ITT and Litton became the leading manufacturers. Then in Russia, Katod started manufacturing Gen 3 IITs. Today in Germany, the country where it all started, Harder Digital is producing tubes with great performance.

 

Then there is Photonis   www.photonis.com  , perfecting the Gen 2+ multialkali technology with astonishing results. Tubes where one is unable to tell a difference compared to a good gen 3 tube. They do have some very distinct advantages, such as excellent highlight performance, a very small halo and shock resistant up to 500G. These tubes are widely used in Europe, both military and civilian applications. The XD-4 and XR5 being mostly used by military and the Supergen line is the civilian version, tubes that did not meet one or more of the military requirements.  Mythical Gen 4, yes they do exist, there was a short period where they where officially called Gen 4. Today they are called infinity tubes, manufactured by L3. In the late 90's Litton managed to alter the GaAs photocathode in a way so that one didn't need a oxidation film to protect it from deteriorating to fast. This meant a much better light to electrons conversion rate resulting in outstanding lowlight performance. However, they where expensive to manufacture and very shock sensitive, but they are still even today seen as the nonplus ultra of all IITs. 

                                                                                                                                                                                                                             

-Glossary of terms

What does Automatic Brightness Control (ABC) mean
This allows the tube to protect itself from overexposure. It keeps the image within a certain brightness range with upper limit, meaning there is a limit to how bright a tube can get to prevent damage.

Autogated PSU
This is nothing else then PWM but for image intensifier tubes. Offering the tube better protection and preventing resolution loss in high light condition.

Black Spots
Almost every tube has them, bigger or smaller. These are dust-particles trapped in the tube during the manufacturing process and are almost impossible to avoid.

Bright Spots
This is a defective MCP, meaning electrons are being released even if they shouldn't. This is a common phenomenon in well used older tubes. Their visibility can vary depending on the light exposure conditions.

Biocular
Using a single image intensifier tube with both eyes.

Binocular
Viewing a scene through two image intensifiers, i.e. one channel per eye.

Blooming
Loss in visibility due to a bright light source. Common in Gen 0 and Gen 1.

Chicken Wire
A pattern of dark thin lines in the field of view either throughout the image area or in parts of the image area. Under the worst-case condition, these lines will form hexagonal or square wave-shape lines. This is due to the structural bundles of with either the MCP or the fiber optic.

Diopter
The unit of measure used to define eye correction or the refractive power of a lens. Usually, adjustments to an optical eyepiece accommodate for differences in individual eyesight.

Distortion
There are two types of distortion found in night vision systems. One type is caused by the design of the optics, or image intensifier tube, and is classical optical distortion. The other type is associated with manufacturing flaws in the fiber optics used in the image intensifier     tube. Optical Distortion: Optical distortion occurs when the design of the optics causes straight lines at the edge of the field of view to curve inward or outward. This curving of straight lines at the edge will cause a square grid pattern to start to look like a pincushion or barrel. This distortion is the same for all systems with the same model number. Good optical design normally makes this distortion so low that the typical user will not see the curving of the lines.  

Fiber Optics Distortions: Two types of fiber optics distortions are most significant to night vision devices: S-distortion and shear distortion:  

S-Distortion: Results from the twisting operation in manufacturing fiber-optic inverters. Usually S- distortion is very small and is difficult to detect with the unaided eye.  

Shear Distortion: Can occur in any image tube that uses fiber-optic bundles for the phosphor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area, as though the line were sheared.

Equivalent Background Illumination (EBI)
This is the amount you see in an image tube that is turned on but there is no light at all on the photocathode; it is affected by temperature where the warmer night-vision device, the brighter the background illumination. EBI is measured in lumens per square centimetre (lm/cm2) wherein the lower the value the better. The EBI level determines the lowest light level at which you can detect something. Below this light level, objects will be masked by the EBI.

Eye Relief
The distance a person’s eyes must be from the last element of an eyepiece to achieve the optimal image area.

Field-of-View
The angle of the visible area you see through your night vision device.

Figure of Merit (FOM)
Image Intensification tube specification designation, calculated on line pair per mm x signal to noise. (FOM=lp/mm * SNR)

Gain
Also called brightness gain or luminance gain. This is the number of times a night vision device amplifies light input. It is usually measured as tube gain and system gain. Tube gain is measured as the light output (in fL) divided by the light input (in fc). This figure is usually expressed in values of tens of thousands. If tube gain is pushed too high, the tube will be "noisier" and the signal-to- noise ration many go down. Military Gen 3 image tubes operate at gains of between 20,000 and 45,000.

Gallium Arsenide (GaAs)
The semiconductor material used in manufacturing the Gen 3 photocathode. GaAs photocathodes have a very high photosensitivity in the spectral region of about 450 to 950 nanometers (visible and near-infrared region).

Highlight Shutoff
An image intensifier protection feature builds in to the housing, incorporating a sensor, microprocessor and circuit breaker. This feature will turn the system off during periods of extreme bright light conditions.

Interpupillary Adjustment
The distance between the user’s eyes (pupils) and the adjustment of binocular optics to adjust for differences in individuals. Improperly adjusted binoculars will display a scene that appears egg- shaped or as a reclining figure-8 and can cause headache and eyestrain.

Interpupillary Distance
The distance between the users pupils (eyeball centers). The 95th percentile of US military personnel falls within the 55 to 72mm range of IPD.

IR (Infrared)
Area outside the visible spectrum that cannot be seen by the human eye (between 700 nanometers and 1 millimeter). The visible spectrum is between 400 and 700 nanometers. Image intensifiers operate in the near infrared spectrum and thermal devices operate in the far infrared spectrum.

Lp/mm (Line Pairs per Millimeter)
Units used to measure image intensifier resolution. Typically, the higher the line pair, the better the image resolution. Generation 3 tubes generally have a range of 64 - 72 lp/mm, although line pair measurement does not indicate the generation of the tube. Some Generation 2+ tubes measure 28-38 lp/mm, (Gen 2+ at 54-59 lp/mm typically), while a Generation 1+ tube may have measure at 40 lp/mm.

MCP (Microchannel Plate)
A metal-coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in Gen 2 or Gen 3 systems. MCPs eliminate the distortion characteristic of Gen 0 and Gen 1 systems. The number of holes (channels) in an MCP is a major factor in determining resolution. Modern MCPs have about 10 million holes or channels compared to the previous standard of 3.14 million.

Photocathode
The input surface of an image intensifier tube that absorbs light energy (photons) and in turn releases electrical energy (electrons) in the form of an image. The type of material used is a distinguishing characteristic of the different generations.

Photocathode Sensitivity
Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The measuring units of photocathode sensitivity are micro- amps/lumen (mA/lm) or microamperes per lumen. This criterion specifies the number of electrons released by the Photocathode (PC). PC response is always measured in isolation with no amplification stage or ion barrier (film). Therefore, tube data sheets (which always carry this raw figure) do not reflect the fact that over 50% of those electrons are lost in the ion barrier. While for most latest 3rd generation image intensifiers the photo response is in the 1800 mA/lm (2000 mA/lm for the latest Omni VI Pinnacle tubes), the actual number is more like 900 mA/lm.

Resolution?
The ability of an image intensifier or night vision system to distinguish between objects close together. Image intensifier resolution is measured in line pairs per millimeter (lp/mm) while system resolution is measured in cycles per miliradian. For any particular night vision system, the image intensifier resolution will remain constant while the system resolution can be affected by altering the objective or eyepiece optics by adding magnification or relay lenses. Often the resolution in the same night vision device is very different when measured at the centre of the image and at the periphery of the image. This is especially important for devices selected for photograph or video where the entire image resolution is important. Measured in line pairs per millimeter (lp/mm).

Reticle (Reticle Pattern)
An adjustable aiming point or pattern (i.e. crosshair) located within an optical weapon sight.

Signal-to-Noise Ratio (SNR)
A measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. A tube’s SNR determines the low light resolution of the image tube; therefore, the higher the SNR, the better the ability of the tube to resolve objects with good contrast under low-light conditions. Because SNR is directly related to the photocathode’s sensitivity and also accounts for phosphor efficiency and MCP operating voltage, it is the best single indicator of an image intensifier’s performance.

Scintillation
Also known as electronic noise. A faint, random, sparkling effect throughout the image area. Scintillation is a normal characteristic of Microchannel plate image intensifiers and is more pronounced under low-light-level conditions.

Screen
The image tube output that produces the viewable image. Phosphor (P) is used on the inside surface of the screen to produce the glow, thus producing the picture. Different phosphors are used in image intensifier tubes, depending on manufacturer and tube generation. P-45 phosphor is used in the systems offered in this catalogue.

Stereoscopic Night Vision 
When two views or photographs are taken through one device. One view/photograph represents the left eye, and the other the right eye.  

When the two photographs are viewed in a stereoscopic apparatus, they combine to create a single image with depth and relief. Sometimes this gives two perspectives. However, it is usually not an issue because the object of focus is far enough away for the perspectives to blend into one.

Variable Gain Control
Allows the user to manually adjust the gain control (basically like a dim control) in varying light conditions. This feature sets the PVS-14 apart from other popular monoculars that do not offer this feature.