How Nightvision Works

In Generation II and III systems, the objective lens collects light that you cannot see with your naked eye and focuses it on the image intensifier. Inside the image intensifier a photocathode absorbs this light energy and converts it into electrons. These electrons are then drawn toward a phosphor screen but first pass through a microchannel plate that multiplies them thousands of times. When this highly intensified electron image strikes the phosphor screen, it causes the screen to emit light that you can see. Since the phosphor screen emits this light in exactly the same pattern and degrees of intensity as the light that is collected by the objective lens, the bright nighttime image you see in the eyepiece corresponds precisely to the outside scene you are viewing.

How nightvision works


Terminology

Automatic Brightness Control (ABC)

An electronic feature that automatically reduces voltages to the microchannel plate to keep the image intensifier's brightness within optimal limits and protect the tube. The effect of this can be seen when rapidly changing from low-light to high-light conditions; the image gets brighter and then, after a momentary delay, suddenly dims to a constant level.

BlackSpots

These are cosmetic blemishes in the image intensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the performance or reliability of a night vision device and some number of varying size are inherent in the manufacturing processes. Spots due to dirt or debris between the lenses should be removed by careful cleaning if the system is designed for interchanging optics.

Bright-Source Protection (BSP)

An electronic function that reduces the voltage to the photocathode when the night vision device is exposed to bright light sources such as room lights or car lights. BSP protects the image tube from damage and enhances its life; however, it also has the effect of lowering resolution when functioning.

Cycles per Milliradian (cy/mr)

Units used to measure system resolution. A milliradian is the angle created by 1 yard at a distance of 1,000 yards. This means that a device that can detect two 1/2-yard objects separated by 1/2 yard at 1,000 yards has a resolution of 1.0 cy/mr. Do not confuse cy/mr with line pair per millimeter. For example, a system can have a 3X lens attached and increase the system resolution by a factor of 3, yet the image intensifier's resolution (measured in lp/mm) has not increased.

Diopter

The unit of measure used to define eye correction or the refractive power of a lens. Usually adjustments to an optical eyepiece accommodates for differences in individual eyesight. Many military system provide a +2 to -6 diopter range.

Distortion

Three types of distortion are most significant to night vision devices: geometric, "S", and sheer.

Nightvision Explination Defined

Geometric distortion is inherent in all Gen 0 and Gen I image intensifiers and in some Gen II image intensifiers that use electrostatic rather than fiber-optic inversion of the image. Geometric distortion is eliminated in image tubes that use a microchannel plate and fiber optics for image inversion; however, some S-distortion can occur in these tubes.

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, Gen III image tubes manufactured to U.S. military standards since 1988 have nearly no perceptible S-distortion.

Sheer 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 "sheered."

Non-inverting image intensifiers that use microchannel plates and clear glass for the optics are free of distortion. The image intensifier ITT manufactures for its Night Enforcer is distortion free.

Equivalent Background Illumination (EBI)

This is the amount of light 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 the night vision device, the brighter the background illumination. EBI is measured in lumens per square centimeter (Im/cm2) wherein the lower the value the better. The EBI level determines the lowest light level at which you can detect something and, below this light level, objects will be masked by the EBI.

Emission Point

A steady or fluctuating pinpoint of bright light in the image area that does not go away when all light is blocked from the objective lens. The position of an emission point within the field of view will not move. If an emission point disappears or is only faintly visible when viewing under brighter nighttime conditions, it is not indicative of a problem. If the emission point remains bright under all lighting conditions, the system needs to be repaired. Do not confuse an emission point with a point light source in the scene being viewed.

Eye Relief

The distance your eyes must be from the last element of an eyepiece in order to achieve the optimal image area.

Fixed-Pattern Noise (FPN)

A faint hexagonal (honeycomb) pattern throughout the image area that most often occurs under high-light conditions. This pattern is inherent in the structure of the microchannel plate and can be seen in virtually all Gen II and Gen III systems if the light level is high enough.

Footcandle (fc)

A unit of illuminance equal to one lumen per square foot.

Footlambert (fL)

A unit of brightness equal to one footcandle at a distance of one foot.

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 seen in values of tens of thousands. If tube gain is pushed too high, the tube will be "noisier" and the signal-to-noise ratio may go down. U.S. military Gen II and Gen III image tubes operate at gains of between 20,000 and 37,000.

On the other hand, system gain is measured as the light output (fL) divided by the light input (also fL) and is what the user actually sees. System gain is usually seen in the thousands. U.S. military systems operate at 2,000 to 3,000. In any night vision system, the tube gain is reduced by the system's lenses and is affected by the quality of the optics or any filters; therefore, system gain is a more important measurement to the user.

Gallium Arsenide (GaAs)

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

Generation 0

Typically uses an S-1 photocathode with peak response in the blue-green region (with a photosensitivity of 60 uA/lm), electrostatic inversion, and electron acceleration to achieve gain. Consequently, Gen 0 tubes are characterized by the presence of geometric distortion and the need for active infrared illumination.

Generation I

Typically uses an S-20 photocathode (with photosensitivity of 180-200 uA/lm), electrostatic inversion, and electron acceleration to achieve gain. Because of higher photosensitivity, Gen I was the first truly passive image intensifier. Gen I is characterized by geometric distortion, poor performance at low light levels, and blooming.

Nightvision Definition

 

Generation II

Usually an S-25 (extended red) photocathode (with photosensitivity of 240+ uA/lm and a microchannel plate to achieve gain. Can be found with either electrostatic or fiber-optic inversion. Gen II tubes provide satisfactory performance at low light levels and exhibit low distortion.

Night Vision Explained

Generation III

Uses gallium-arsenide for the photocathode and a microchannel plate for gain. The microchannel plate is also coated with an ion barrier film to increase tube life. Produces more than 800 uA/lm in the 450 to 950 nanometer (near-infrared) region of the spectrum. Gen III provides very good to excellent low-light-level performance, long tube life. Recent mil-spec quality tubes have no perceptible distortion.

Line Pairs per Millimeter (lp/mm)

Units used to measure image intensifier resolution. Usually determined from a 1951 Air Force Resolving Power Test Target. The target is a series of different sized patterns composed of three horizontal and three vertical lines. You must be able to distinguish all the horizontal and vertical lines and the spaces between them to qualify for that pattern.

Lumen

The unit denoting the photons perceivable by the human eye in one second.

Microamps per Lumen (uA/lm)

The measure of electrical current (uA) produced by a photocathode when it is exposed to a measured amount of light (lumens).

Microchannel Plate (MCP)

A metal-coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in Gen II and Gen III systems. These devices normally have anywhere from 2 to 6 million holes (or channels) in them. Electrons entering a channel strike the wall and knock off additional electrons which in turn knock off more electrons producing a cascading effect. MCPs eliminate the distortion characteristic of Gen 0 and Gen I systems. The number of holes in an MCP is a major factor in determining resolution. ITT's new MCPs have 6.34 million holes or channels compared to the previous standard of 3.14 million.

Milliamps per Watt (mA/W)

The measure of electrical current (mA) produced by a photocathode when exposed to a specified wavelength of light at a given radiant power (watt). As with microamps per lumen, usually, the higher the value, the better the performance; however, it is important to know where in the spectrum the wavelength falls. Because more night light is present in the near-infrared region than in the visible or ultraviolet regions, a high value in the blue regions would not be a good as a moderate value in the near-infrared region.

Near-Infrared

The shortest wavelengths of the infrared region, nominally 750 to 2,500 nanometers. Gen III operates from around 450 to 950 nanometers.

Photocathode

The input surface of an image intensifier that absorbs light energy and in turn releases electrical energy in the form of an electron image. The type of material used is a distinguishing characteristic of the different generations of image intensifiers.

Photoresponse (PR)

See Photosensitivity.

Photosensitivity

Also called photocathode sensitivity. The ability of the photocathode material to produce an electrical response when subjected to light waves (photons). Usually measured in microamps of current per lumen of light (uA/lm). The higher the value, the better the ability to produce a visible image under darker conditions.

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 milliradian. 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 center of the image and at the periphery of the image. This is especially important for devices selected for photography or video where the entire image resolution is important.

Signal-to-Noise Ratio (SNR)

A measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. Atube'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 intensifiers performance.

Scintillation

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. Sometimes called "video noise." Do not confuse scintillation with emission points.

Spectrum

The range of electromagnetic energy from cosmic rays to extra-low frequency used in submersed submarine communication. Electromagnetic frequency is measured in cycle per second and wavelength in microns or nanometers. The ultraviolet region extends from 100 to 400 nm with the near-ultraviolet nominally 300 to 400 nm. The visible portion of the spectrum extends from 400 to 750nm. The infrared region extends from 750 to 2xlO5 nm with the near-infrared nominally 750 to 2,500 nm.


Evaluation

Characteristics of night vision equipment fall into four major categories that help in evaluation: performance, human factors, suitability to its application, and the overall cost of ownership. If you have any questions, do not hesitate to call your local ITT dealer.


Performance

The very need for a night vision capability necessarily focuses on performance as the most important - Can you see a clear image when it is dark and you cannot see the scene or objects with your unaided eye? Most night vision equipment available today will provide an adequate image under higher night light conditions such as a full moon. Evaluate the following parameters to determine how well a system will perform when you need to see under truly dark conditions such as starlight.

Photosensitivity

The ability of a night vision system to detect light energy and convert it to an electron image is reflected in the image intensifier's photosensitivity. Usually, the higher the value, the better the ability to "see" under darker and darker conditions. However, be aware that at night there is more light energy available in the near-infrared region than in the visible region. Therefore, if a device claims a high photosensitivity, make sure to find out where in the spectrum this is measured. A high photosensitivity in the blue or visible region may not perform as well as another system with a lower overall photosensitivity, but a higher value in the near-infrared region.

Signal-to-Noise Ratio (SNR)

This is probably the single most significant factor in determining a system's ability to see when it gets dark. Be aware that SNR can be computed many ways to get desired results. Be sure to find out how SNR was computed. When measured according to U.S. mil specs, the SNR takes into account the photosensitivity, as well as the efficiency of the phosphor screen in reconverting the electron image to visible light and the "noise" contribution of the microchannel plate. Because the, SNR determines an image intensifier's low-light-resolution, the higher the ratio, the clearer will be the signal compared to the background noise, hence, the better the ability to see under increasingly darker conditions.

Gain

This tends to be a confusing parameter when evaluating night vision devices. The most important gain measurement is the system gain. Very high gain values for an image tube are not especially significant - the U.S. military procures devices with the tube gain ranging from 20,000 to 37,000. Look for the system gain. U.S. military systems operate at 2,000 to 3,000. The higher the value the better the ability of the device to amplify the light it detects.

A word of caution; gain is only part of the story. If a system does not possess a good photosensitivity and SNR, a very high gain value simply means that you will make a poor image brighter, not better. Also, very high gain values could mean the tube is driven very hard and the life of the tube will be reduced. The very best test is field evaluation under very dark conditions.

Resolution

Usually this is measured as tube resolution (lp/mm) or system resolution (cy/mr). The more significant measurement is system resolution as this is what the viewer will actually experience and takes into account the quality of the system's optics. If you are evaluating systems with similar optical quality and filters, the tube resolution is an important criteria. Resolution is often measured at high and low-light conditions. Most systems produce an optimal resolution at some point between very high light and very low light conditions.

As long as resolution is measured the same way using the same magnification and the same conditions (i.e., per U.S. mil specs) the higher the value, the better the ability to present a sharp picture. However, be aware that many devices will produce a sharp image in the center of the viewing area, but less sharp as you look toward the periphery. The lack of a sharp image, except at the center of the viewing area, can be due to the presence of a Gen 0 image tube or to the system's optics. Again, remember that many night vision systems will produce adequate results under higher night-light conditions, but perform poorly under darker conditions.


Human Factors
Here, such issues as weight, size, safe equipment, and the ease of operation should be considered. Remember that the ease of operation should be determined under dark conditions where the user cannot see the device being used. What may appear to be an acceptable level of operating ease under room lights may not be "user friendly" at all when it is dark. Protracted use should also be considered when evaluating weight. What may seem an acceptable weight when using a device for a short time, may not be so when viewing for long periods of time.

Additionally, consider such functions as the on/power switch. Will you need to continually hold down the switch? - even light pressure for one finger for a long time can produce fatigue. Do you need to repeatedly press the switch to recharge the image tube? - such devices usually produce an initially bright image which gradually fades, reducing the ability to see and then shuts off unless you repress the switch. This characteristic could cause you to lose an image at a crucial moment.


Suitability to its Application
Within this category, characteristics such as field of view (FOV), magnification, versatility, weather resistance, and image distortion affect the ability of a night vision device to perform as needed.

Magnification and FOV

Regarding magnification and FOV, consider the distance you will need and the overall area you are observing or searching. For most surveillance or search applications, the higher the magnification or narrower the FOV, the greater the number of times you need to scan an area to avoid missing important objects or events. Usually a 1:1 lens with a 400 FOV provides optimal performance.

For long range observation or weaponsight applications, the amount of magnification needed will vary; however, be sure to consider the other performance characteristics of the device; as the magnification increases, FOV decreases and the F number increases, all reducing the amount of light captured. Consequently, you will need an image tube with excellent performance at very low-light levels and/or high-performance lenses.

Another factor involves the versatility of a device if it is used in situations that may require different magnification. How easily and quickly can the magnification be changed? Is it necessary to open the system to install the optics? In some cases, this may be inescapable, and the susceptibility of internal components to damage should be considered.

Distortion

Gen 0, Gen I, and 25-mm Gen II electrostatically inverted image tubes produce a certain amount of geometric distortion in the image. In Gen III and 18-mm Gen II systems, geometric distortion is eliminated although it is possible to encounter some perceptible "S" and sheer distortion. The degree of any distortion and its interference with the application should be considered. When the application involves photography, video work, or weaponsights, the distortion and peripheral resolution are critical.

Weather Resistance

The ability of a night vision system to operate under adverse environmental conditions is another important factor. Any system built to U.S. mil specs for environmental factors will perform suitably under almost any condition encountered. The major concern is internal fogging that destroys the ability to see an image, hence, the ability to resist humidity and moisture is vital.

In addition, when a night vision system is used on or around rivers or bodies of water, floatability can be a determining factor. ITT's Night Enforcer 150/160 monoculars and 250/260 binoculars will float if dropped into water.


Overall Cost of Ownership
Evaluation factors that impact the actual cost of acquiring a night vision capability are image tube life (referred to as "reliability"), warranty coverage, repair availability, service support, and overall workmanship as an indicator of quality. When evaluating night vision equipment, the initial acquisition cost does not equate to the cost of ownership. How often will you need a new image tube? What is the likelihood for repairs? Are batteries available? What about exposure to bright lights?

All image intensifiers will "wear out" over time due to gases generated within the tube that migrate to the photocathode and slowly kill it. Because of this, characteristics such as reliability, a bright-source protection (BSP) circuit, and the presence or absence of an ion-barrier film on the microchannel plate are important. U.S. mil specs describe procedures for projecting reliability. You should know what the reliability is for the tube you evaluate.

An important factor that can influence reliability is the voltage used to produce gain. If an image tube is "driven" hard to produce high gain, it will accelerate the production of gases and more quickly kill the ability to convert light into electrons.

A final evaluation criteria is to determine whether or not the night vision device incorporates automatic protection for the image intensifier when it is exposed to high-light conditions or bright-light sources. Image tubes manufactured by ITT have a BSP circuit built into the image intensifier. This circuit automatically reduces the voltage to the photocathode when the system is exposed to bright light sources. The BSP feature protects the image tube and enhances its life. If there is doubt, consult the warranty; does it exclude exposure to high light or bright lights?

Note 1: Generation classification:

Some night vision advertising has presented confusing information listing Russian equipment as Gen I, Gen II, and Gen III, when in fact, by worldwide classification it is Gen 0, Gen I, and Gen II, respectively.

Note 2: Reconditioned Generation II:

While the prices of "reconditioned" Gen II systems may be attractive, be aware that the hours of remaining life and photosensitivity performance cannot be restored to Gen II tubes. "Reconditioned" usually means the system has a new or repaired power supply but the photosensitivity will be lower, the SNR will be lower, and the remaining life will be less. Some reconditioned units may be operating at below acceptable minimums and few companies possess the necessary test equipment to evaluate the tube's level of performance.

The U.S. military specifications for Gen II require a reliability of 2,000 hours of operating time (ITT's new Gen II image intensifier has tested to well beyond the military specification). This situation does not pertain to Gen III equipment. Due to the presence of an ion-barrier film in Gen III devices, the gallium-arsenide photocathode is protected from degradation and the life and performance are extended many times longer than Gen II.

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