Lahoux Optics Glossary
The most important terms of night vision and thermal imaging technology.
Auto-Gating is the electronic solution that reduces the duty cycle of the cathode voltage by rapidly switching the voltage on and off. It is a proprietary means to preserve the high MTF (Modulation Transfer Function) and resolution that is obtained at the full operating voltage while reducing the strain of high illumination levels on the image intensifier tube. This is done by reducing the average current coming from the photocathode.
Auto-Gating constantly operates to improve the quality of the image, not only during day-night-day transition, but also under dynamic lighting conditions such as military operations in urban terrain which define many of today's missions.
Auto-Gating is standard on XR5TM and optional on XD-4TM Image Intensifier Tubes.
Black Spots - Blemishes
Cosmetic blemishes in the image intensifier or dirt/debris between the lenses. Black spots in the image intensifier do not affect the performance or reliability of the device and are inherent in the manufacturing processes
When referring to an image intensification tube, brightness gain is the ratio of the brightness of the output in units of lm/cm2
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, BSP may have the effect of lowering resolution when it is functioning.
These are signal- induced blemishes in the image area caused by a flaw in the film on the MCP. A bright spot is small, non- uniform, bright area that may flicker or appear constant. Bright spots usually go away when the light is blocked out. Not all bright spots make the ANVIS unserviceable. A test can be performed as follows: Place a cupped hand over the lens to block out all light. Make sure any bright spot is not simply a bright area in the viewed scene. If the bright spot remains, an emission point exists and needs to be checked.
The act of making rays of light travel in parallel lines. Also the process of aligning the various internal optical axes of a system with each other.
The shifting of an observer's eyes inward to view a nearby object i.e. crossing the eyes.
An irregular pattern of dark lines in the image area or in parts of the image area. Under the worse condition, these lines will form hexagonal or square-wave shaped lines.
Three types of distortion are most significant to night vision devices: geometric, "S" and sheer.
- Geometric Distortion - is inherent in all Gen 0 (infrared Tubes ie. B-20) and Gen I image intensifiers and in some Gen II image intensifiers that use electrostatic rather than fibre-optic inversion of the image. Geometric distortion is eliminated in image tubes that use a microchannel plate and fibre-optics for image inversion, however, some S-distortion can occur in these tubes.
- S-Distortion - results from the twisting operation in manufacturing fibre-optic inserters ). Usually S-Distortion is very small and is difficult to detect with the unaided eye.
- Sheer Distortion - can occur in any image tube that uses fibre-optic bundles for the phosphor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area as through the line were sheered.
Also known as 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
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.
Fiber Optic Inverter
A bundle of microscopic light transmitting fibers twisted 180 degrees.
The width or spatial angle of the outside scene that can be viewed through the intensifier tubes measured laterally and vertically. Typical NVGs have a 40° FOV. There are NVGs in development that attempt to increase this FOV significantly in an effort to enhance pilot performance. An example of a wide FOV NVG would be the Panoramic NVGs'.
FOM – Figure of Merit
The FOM is an indication of performance, formed into a value, to determine the performance of an image intensifier tube. It is calculated as follows: SN/R x lp/mm. (SN/R = signal to noise ratio (the sensitivity) times lp/mm = line pairs/mm (the resolution)).
Since the production of tubes is very difficult, they all differ from each other. This means the FOM value varies from tube to tube. Therefore, every night vision device from Lahoux Optics comes with a data sheet with the individual values of the assembled tube.
The FOM value varies roughly between 1000 and 2500. Above 1500 the FOM is very good. Above 1800 the tubes are normally reserved for government use. However, Lahoux Optics can supply tubes to the private sector between 1800 and 2000 FOM. Image intensifier tubes with a higher FOM than 2000 are exclusively for government application.
Since the FOM is a culmination of sensitivity and resolution a tube with the same FOM can have very different performance. One can have a high resolution and lower SN/R while the other can have a lower resolution and higher SN/R. Therefore the FOM is a standardised indication of performance.
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.
Also called brightness gain or luminance gain. This is the number of times an image intensifier tube amplifies light input. It is usually measured as tube gain and system gain. 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 of the Gen III photocathode.
Generations of Image Intensifiers
Image intensifier tubes are classed by generation (GEN) numbers assigned by the US Army's Night Vision Laboratory. Each GEN denotes a new contract or manufacturing process.
Note: Generations do not define the performance specs of the image intensifier.
- Generation I
It started with electrostatically focused Generation I tubes featuring high image resolution, a wide dynamic range and low noise.
- Generation II
Introduced the Micro Channel Plate for much higher gain in the 1980’s. The original image resolution was less than that of the first generation intensifiers but the gain was much higher.
- Generation III
In the late 1980’s an Image Intensifier with a GaAs photocathode was developed showing an enhanced sensitivity in the Near-Infrared. In the late 1990’s Gen III tubes with greatly improved performance appeared on the market. These types are called Gen III Omni III and Gen III Omni IV.
PHOTONIS introduced the new European Standard for low light-level imaging in 1996. This new performance family brings superior performance which exceeds the performance of GEN III image intensifiers.
See Fixed-Pattern Noise.
This condition may exist when there is a difference in brightness between the two image intensifier assemblies within the same binocular.
This problem is more easily detected in high- light conditions. Image distortion is evidenced by vertical objects, such as trees or poles appearing to wave or bend when the user moves his head vertically or horizontally when looking through the goggles. Ground surfaces in the direction of hover may appear to swell or sink. Distortion does not change during life of an image intensifier. Limits on allowable distortion are an important part of performance specifications since excess distortion can interfere with viewing the image and thus with the operator’s ability to perform necessary flight maneuvers.
Line Pairs per Millimetre (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. The lines and spacing between lines in each of the different patterns differ in width; the narrower the width, the greater the resolution is needed to distinguish the lines in a given pattern. Human test subjects must be able to clearly distinguish all the horizontal and vertical lines of a particular pattern in order for an image intensifier to achieve the resolution represented by that pattern.
The unit denoting the photons (light) perceivable by the human eye in one second.
A unit measurement of illumination. The illuminance produced on a surface that is on meter square, from a uniform point source of one-candela intensity, or one lumen per square meter.
The magnifying power of the lens. Four power (4X) indicates that the image will appear four times larger than if viewed with a 1X lens.
Microamps per lumen
The measure of electrical current (A) 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. PHOTONIS SAS produces MCP's with 11 million pores and is constantly developping and improving the performance of the MCP for enhanced image intensifier tube performance. Electrons entering the channel strike a wall and knock off additional electrons which in turn knock off more electrons, producing a cascading effect. MCPs eliminate the distortion characteristic of GEN I systems. The number of holes in an MCP is a major factor in determining resolution.
Term for Military Specification. It is the minimum acceptable requirements for products procured by the US Department of Defence. Use of the term MILSPEC indicates that the product meets applicable military specifications.
The shortest wavelengths of the infrared region, normally 750 to 2,500 nanometers (nm). GEN II operates from around 440 to 950 nanometers.
Output Brightness Variation
This condition is evidenced by areas of varying brightness in or across the image area. The lower contrasts do not exhibit distinct lines of demarcation nor do they degrade image quality. This condition should not be confused with shading.
The phosphor screen converts electrons into photons. A very thin layer of phosphor is applied to the output fiber optic system, and emits light when struck by electrons. See also 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 generations of image intensifiers.
Also called photocathode sensitivity or photoresponse. 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 (mA/lm).
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 millimetre (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 millimetre (lp/mm).
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.
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.
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.
Equal to tube gain minus losses induced by system components such as lenses, beam splitters and filters.