# Night Version Sensors Sensors that aid aircrews in low light operations continuously improve as new technology is developed. However, aircrews should always develop a secondary scan, especially in critical situations, to decrease the risk of adverse sensor failures. *** ## Sensor Types The two types of aviation night vision devices used by Army Aviation are image intensifier systems and thermal imaging systems. Image intensifier systems amplify both visible and near infrared (IR) light energy (figure 4-8). They greatly improve night vision, but require some degree of light to function. Their performance is degraded during low ambient light and adverse weather conditions.
### Electromagnetic Spectrum Just as a radio must be tuned to receive specific frequencies, NVG, thermal sensors, and human eyes are each sensitive to a specific wavelength or frequency range of the electromagnetic (EM) spectrum. The optical band covered by visible light is a relatively small portion of the entire EM spectrum. NVG and thermal sensors such as FLIR operate at different but complementing regions of the EM spectrum. Energy in the visual and short-wave IR portion of the EM spectrum are normally referred to as light, while energy in the mid and far IR bands are normally referred to as thermal or heat energy. This is because the light energy is primarily reflected from the observed object while the thermal energy is primarily emitted by the object. ### Image Intensifiers An image intensifier is an electronic device that amplifies light energy. The Army only uses one image intensifier system for flight operations—the Aviator Night Vision Imaging System (ANVIS).
*** ## Aviator Night Vision Imaging System The ANVIS is a helmet-mounted or hand-held passive binocular that provides the capability for pilots to fly in terrain flight modes at night. The ANVIS is used with the HGU-56/P helmet and the AH-64 Apache Integrated Helmet and Display Sighting System (IHADSS) when equipped with an ANVIS helmet mount. This system amplifies ambient light so the viewed scene becomes clearly visible to the operator. ### System Performance Visual acuity for aviators can be defined as the ability to resolve details in the visual scene. Visual acuity is measured according to what the unaided viewer can see at specific distances. Normal daytime visual acuity with the unaided human eye is 20/20. This means that the viewer can distinguish at a distance of 20 feet what a normal observer can distinguish at 20 feet. A visual acuity of 20/40 means that the viewer can distinguish at 20 feet what a normal observer can distinguish at 40 feet, indicating that individual’s eyesight is not as clear as standard daylight vision. The visual acuity of ANVIS is directly related to the light level available to the image intensifier. As available light decreases, ANVIS visual acuity declines with it. Visual acuity maximizes at 100 percent illumination (full moon conditions), degrading slowly until available illumination reaches approximately 50 percent, or ½ moon. As illumination drops below 50 percent, visual acuity degrades more rapidly, causing the halo effect around light sources to be more pronounced. As illumination levels drop below 30 percent, scintillation (sparkling) becomes apparent. Visual acuity is at its lowest at 0 percent illumination (starlight conditions), with a fully overcast sky, and no cultural (manmade) sources of illumination. Generally speaking, visual acuity at 100 percent illumination and a high-contrast environment is approximately 20/25 as described above. This means that the aviator's visual acuity is slightly lower than 'normal' daytime vision. Under these conditions, aviators should be able to perceive small obstacles, such as wires. It should be stressed that acuity for ANVIS operators is below daytime acuity, even under optimum conditions. Under the lowest light conditions (0 percent illumination, with overcast skies and no cultural lighting), ANVIS visual acuity is approximately 20/70. Under these conditions, aviators may not be able to perceive large objects, such as standard power poles. Low-contrast environments (such as snow-covered territory, sandy deserts, large bodies of water, or grassy hills) degrade visibility, making it difficult to distinguish features of the terrain. In these environments, ANVIS visual acuity can deteriorate to 20/200 or worse. ### System Function The image intensifier is the heart of the ANVIS. It is designed around the principle that it is impossible to amplify light. Light entering the intensifier is converted to electricity. The electrical energy is then amplified and reconverted into light. The image intensifier is composed of four components: photocathode, micro-channel plate, phosphor screen, and fiber-optic inverter. System function is fully explained in TM 11-5855-313-10 (ANVIS), but generally works as follows.
Light energy, consisting of photons, enters the objective lens and is inverted and focused onto a photocathode that is responsive to both visible and near IR radiation. * Photons striking the photocathode are converted to a proportionate number of electrons. Electrons are accelerated away from the photocathode to the microchannel plate (MCP) via an electrical field produced by the power supply. * The MCP is a thin wafer of tiny glass tubes that are tilted at a slight angle. Electrons enter these tubes and strike the walls. This reentry reaction exponentially increases the number of electrons created.
* These increased numbers of electrons are then accelerated to the phosphor screen (figure 4-12). The phosphor screen creates a lighted image proportional to the number and velocity of the electrons striking it.
* The image is then passed through a fiber-optic inverter to rotate the image 180 degrees to correct the image for the viewer. The image is focused onto the viewer's eye through an eyepiece lens ### **Power Supply** The power supply provides automatic brightness control that adjusts MCP voltage to maintain image brightness within set limits. Bright source protection reduces voltage to the photocathode when exposed to bright light, protecting the image intensifier. ### **Minus Blue Filter** The objective lens has a "minus-blue" coating to reduce interference from instrument panels and cockpit lights, ensuring clear viewing outside the aircraft. ### **Halo** The halo is a hazy ring around light sources in the NVG, primarily caused by electron scatter between the photocathode and the micro-channel plate. Moisture in the air may intensify this effect.
### **System Counterweights** ANVIS often uses a counterweight system to offset the downward pull of the device, keeping the helmet properly positioned on the wearer's head.
### **Heads-up Display Systems** Aircraft systems provide heads-up displays superimposed on the ANVIS image, enhancing situational awareness. Proper adjustment of brightness is essential for clear visibility in different light conditions. *** ## Weather The presence of thin clouds that progress into thicker ones can hide terrain features and creates a severe hazard for NVG operations. Low clouds lying upon and between hills present a particularly dangerous situation due to the inability of the aircrew to distinguish between the clouds and the terrain. The aircrew must be alert for a gradual reduction in light level and notice the obstruction of the moon and the stars. The less visible the moon and stars are, the heavier the cloud coverage. If the NVG image becomes grainy and begins to scintillate (sparkle), this is an indication that weather may be causing a low ambient light condition. Since NVG can 'see' through a limited amount of obscurant, IIMC is a real possibility under these conditions. Aircrew members must be prepared to respond accordingly *** ## Visibility Restrictions Visibility restrictions, caused by fog, rain, dust, haze, or smoke, reduce the effectiveness of the ANVIS, especially at low altitudes during terrain flight. Conditions indicating restricted visibility include: * Loss of celestial lights. * Loss of ground lights. * Reduced ambient light levels. * Reduced visual acuity. * Increased halo effect. * Scintillation. *** ## **Weapons Effects** Forward firing ordnance immediately affect the ANVIS image due to bright source protection activation. Effects of ordnance from other sources vary, with secondary explosions and fires causing loss of detail. Smoke amplifies these effects, potentially disorienting the aviator. *** ## **Thermal Systems** Thermal imaging systems detect temperature differences to create images. Performance is not affected by ambient illumination but can be degraded by atmospheric conditions to a lesser extent. *** ### **Principle of Operation** Thermal sensors convert thermal energy into electrical signals, which are processed and converted into images visible to aircrew. Thermal energy passes through a filtered window, is focused by an IR telescope, scanned, and transmitted to a sensor array, creating a video image.
Currently, Army attack and reconnaissance helicopters utilize the FLIR for target acquisitions during day and night operations. The AH-64D/E modernized pilot night vision system (MPNVS) and modernized target acquisition device system (MTADS) are passive systems which sense and display various levels of IR energy radiating from objects. This allows operators to view objects regardless of visible light levels required for unaided and aided operations. The effectiveness of the FLIR depends on the difference in detected IR radiation between an object and its background. Effectiveness also depends on atmospheric considerations, specifically, the degree of obscuration present between the system and the object. FLIR is most effective when a large difference in IR radiation exists between an object and its background and when obscuration is minimal. In the AH-64, MTADS is the primary flight sensor for the Co-Pilot Gunner (CPG) (front seat) and is also used for targeting. The MTADS is used as a backup to the MPNVS if the pilot's (back seat) MPNVS fails and the pilot still wants to fly, that aviator will take the MTADS from the CPG. However, this is not always the case; often the pilot passes control of the aircraft to the CPG, while that person figures out what went wrong with the MPNVS. At which time the CPG would use the MTADS to fly the aircraft. Aviators should consult the appropriate aircraft operator's manual for specific operating instructions ### Minimum Resolvable Temperature A thermal system's ability to detect temperature differences is referred to as the minimum resolvable temperature (MRT). MRT is the lowest equivalent thermal difference between an object and its background that can be resolved, or seen, by an observer through a thermal system. Thermal sensors "see" objects in the field of view by detecting small differences in the object's temperature in relation to the background. If the thermal sensor is not sensitive enough, objects are not easily detected. Visual discrimination of objects in the thermal scene is made possible when objects have a different radiated temperature (energy) than their immediate backgrounds. If there were no difference between the radiated temperature of an object and its background, the object would have no contrast and would not be distinguishable from its background. It would, in effect, be invisible. *** ## Forward Looking Infrared Optimization Detailed procedures for optimizing FLIR are found in appropriate operator manuals. Generally, FLIR optimization involves adjusting level and gain settings to produce the most detailed image. Proper adjustment ensures the highest resolution picture for the operating environment. Considerations for FLIR optimization include: * Allowing FLIR to cool to proper operating temperatures. * Selecting a scene rich in detail. * Selecting the desired polarity. * Adjusting only one control at a time. To optimize FLIR, the aviator fully decreases level and gain controls, then gradually increases level until the display begins to brighten. Gain control is adjusted until variations in shading appear, then fine adjustments are made to complete the process. Minor adjustments may be needed based on changes in atmospheric conditions and scene content. ### **Atmospheric Effects** The atmosphere significantly affects the performance of thermal sensors by absorbing or scattering thermal energy, reducing the energy reaching the sensor. The impact of these mechanisms varies based on the sensor's operating range.
For example, FLIR visibility through diesel fog or oil smoke is very good. However, phosphorous smoke from a White Phosphorous (WP) marking round or flares significantly hinders thermal transmission. Therefore, WP markers are easily seen by a FLIR. There are also special obscurants designed to block thermal signatures and serve as thermal camouflage. **Atmospheric Absorption** Atmospheric absorption, primarily due to water in the atmosphere, hinders IR energy from reaching the sensor. The quality of the thermal image is heavily influenced by humidity. Dry winter days are transparent to far IR energy, while wet tropical atmospheres act as barriers to sensor operation. ### **Scattering** Scattering, the second factor affecting thermal performance, occurs when light and thermal energy encounter particles in the atmosphere. Large particles scatter energy by reflection, affecting thermal sensors differently than human vision and NVGs. Thermal systems can penetrate conditions like smoke or haze more effectively. ### **Sand and Dust** Sand or dust particles affect FLIR performance differently than precipitation. While they may reduce navigation information, targeting capability may remain unaffected, especially if particle size is small. However, dust during takeoff or landing poses significant challenges and requires caution. ### **Lights** Lights visible to the naked eye at night are typically not visible through FLIR. Crewmembers should use unaided vision to compensate. ### **IR Crossover** IR crossover occurs when soil, water, and concrete thermal radiation levels are nearly equal. Objects in the environment heat and cool at different rates, influencing crossover timing. Modern thermal sensors mitigate the impact of IR crossover by detecting small temperature differences effectively.
### **Depth Perception and Distance Estimation** The FLIR system affects depth perception and distance estimation by eliminating peripheral vision cues and presenting a two-dimensional image. Flight information is overlaid on the FLIR image to aid in accurate depth perception and distance estimation. ### **Color Discrimination** FLIR systems lack color discrimination due to their reliance on IR energy rather than visible light. FLIR displays are monochromatic, with shading used to represent different energy levels. However, bright lights may still be distinguishable by the naked eye. ### **Parallax Effect** In an MPNVS, the parallax effect arises from the distance between the FLIR sensor and the HDU. As the FLIR sensor is positioned separately from the aviator's eye, objects may appear differently in the FLIR image compared to the unaided eye's view. This effect is influenced by the turret offset angle and the proximity of obstacles to the aircraft. Understanding this effect is crucial for accurate interpretation of the FLIR scene.
### **Binocular Rivalry** Binocular rivalry occurs when the MPNVS-aided eye competes with the unaided eye, causing shifts in attention between desired and undesired visual reference points. Factors influencing this rivalry include HDU and ambient scene luminance, scene complexity, and eye dominance. Aviators must manage both eyes' direction while absorbing information from the unaided eye, requiring concentration and planning to minimize rivalry effects. ### **Scanning Techniques** During FLIR night flights, proper scanning techniques are crucial to compensate for the limited FOV and loss of peripheral cues. Aviators must continually scan left and right while referencing flight symbology. Close, midrange, and far cues aid in obstacle detection and route evaluation. Changing FLIR polarity and incorporating the unaided eye into the scanning pattern enhances obstacle recognition. ### **Spatial Disorientation** Large bank angles or rapid attitude changes can induce spatial disorientation, especially when using MPNVS. Slow, purposeful head movements and positive aircraft control help prevent disorientation. Crew coordination and familiarization with recovery procedures are essential for regaining orientation in case of disorientation events. ### **Airspeed and Ground-Speed Limitations** Aviators must understand the relationship between FLIR visual range, atmospheric conditions, and airspeed to avoid overflying FLIR capabilities. Weather conditions affect FLIR performance, with poor visibility indicating potential IMC. Reductions in ground speed may be necessary in poor atmospheric conditions to maintain safe operations. ### **Target Detection** FLIR facilitates target detection at night, but target identification can be challenging. The copilot gunner's ability to optimize and operate FLIR influences target detection capability. Features like black/white-hot settings enhance target detection and recognition, with white-hot providing positive contrast for detecting warm targets against cooler backgrounds.
### Weapons During rocket firing, the motor burn from the rocket illuminates the cockpit area letting the aviator see some sparkling effect to the front of the aircraft. Other than this momentary distraction to the unaided eye, the crew should not experience any adverse effects. When the aircrew member fires the 30-millimeter cannon, the muzzle blast may distract the unaided eye if the gun is fired off axis. Crew coordination and communication can minimize this temporary distraction. While firing Hellfire missiles, the crew experiences a temporary illumination of the cockpit area similar to rocket firings. This temporary distraction from the flight motor of the missile does not affect either crewmember's aided eye, which is already adapted to photopic vision. *** ## Sources of Thermal Energy There are different sources of thermal heat that require consideration by mission planners. Those listed below should be considered during overall mission planning in regards to both protection of organic equipment and attack elements. ### Solar Radiation Those objects that do not have their own power source obtain most of their thermal energy from the sun. Factors that affect available sunlight impact the thermal scene. The two most prominent factors that dictate the amount of solar radiation available on a clear day are time of day and time of year. The higher the sun is in the sky, the more intense the available solar radiation. Solar elevation coupled with how much of the object's surface is exposed to the sun influences the amount of solar radiation absorbed (figure 4-20).
### **Fuel Combustion or Frictional Heat** Thermal energy sources such as fuel combustion and frictional heat contribute to the thermal signature of objects. Engine compartments, exhausts, and moving parts generate hot spots visible on FLIR displays. Frictional heat, like that from wheels or tank tracks, may produce less intense hot spots and can be obscured by mud or snow. These localized hot spots aid in detection but can hinder identification efforts, highlighting the importance of thermal training aids for vehicle recognition. ### **Heat Transmission** Objects' appearance on FLIR displays depends on exposure to various thermal energy sources and their reflective, absorptive, transmissive, and emissive properties. Reflectance refers to an object's tendency to bounce energy, while absorptance determines its capacity to retain energy. Transmittance indicates an object's ability to transfer heat to other objects, while emissivity affects how quickly an object's temperature changes to match its surroundings.
### Reflectance The tendency of energy to bounce (reflect) off of an object is called reflectance (figure 4-22). Objects with a high reflectance (such as a container with a shiny surface) tend to heat more slowly in sunlight.
### Absorptance The tendency of an object to retain energy is called absorptance (figure 4-23). Objects with a high absorptance (such as an engine block) hold more of the energy that contacts the object. The ability of an object to store heat is also called thermal mass. Thermal mass is determined by the material's temperature, mass and type. The greater the amount of heat stored, the hotter the object appears in the thermal sensor. A larger object or mass (density X volume) can also store more heat. Some materials store heat very well (steel), whereas others store heat very poorly (air). High thermal mass means high heat storage, but it also means it takes more heat to change the temperature of an object. While the amount of heat an object holds is significant, temperature differences cause differences in the FLIR image, not heat differences. For instance, a large rock has a higher heat value than a smaller rock of the same temperature and composition. Except for size, the FLIR image of the two rocks would appear identical, even though the larger rock has a higher thermal mass. However, the larger rock takes longer to radiate its excess heat than the smaller rock and maintains a higher temperature for a longer period of time.
### Trasmittance Objects with a high transmittance value (such as the metal skin of a truck) will transfer heat energy to other objects in contact with them, retaining only a portion of that energy internally. (figure 4-24) These objects are more likely to display the same temperature to thermal sensor as other objects to which they are connected (like the hood of a car).
### Emissivity The temperature of an object tends to become the same as its environment. If an object has a higher temperature than its surroundings, it becomes cooler. Conversely, if the object is cooler than its surroundings, it absorbs energy and become warmer. Whether the object is increasing or decreasing in temperature, it is always emitting some amount of radiant energy. Different materials transfer energy to balance their temperature at different rates. This capacity is known as the emissivity of an object (figure 4-25). Rocks, for example have a higher emissivity than metal objects, so the majority of their thermal signature results from self-emission instead of thermal reflectance. Very hot objects emit (radiate) high amounts of thermal energy, and are easily visible using thermal sensors. The sun is a good example of a thermal radiator. Others include the hot metal of a jet engine tailpipe, aerodynamically heated surfaces, motor vehicles, personnel, and terrain.
### **Thermal Inertia** Thermal inertia is determined by the combination of thermal mass and thermal resistance, providing insight into how quickly objects change temperature. Thermal mass signifies the amount of heat stored, while thermal resistance governs the rate of heat flow. Objects with high thermal mass and resistance change temperature slowly and possess high thermal inertia. Examples include large, dense objects such as rocks, armored targets, and bodies of water. Conversely, lightweight objects like grass, leaves, bushes, and the surface layer of the ground heat and cool rapidly, characterized by low thermal inertia. ### **The Thermal Scene** The temperature exhibited by each object in the thermal scene is influenced by various factors. This culmination results in the specific temperature detected by the sensor at any given moment. The thermal scene encompasses a collection of objects, including natural elements like the earth and sky. The visibility of each object within the scene depends on its temperature contrast with the background and surrounding objects. ### **NVG/Thermal Operational Integration** Night vision goggles (NVG) and thermal sensors operate in different segments of the electromagnetic spectrum, responding to distinct principles of radiated energy and occupying different wavelengths. This discrepancy can lead to varying performance under specific environmental conditions. Understanding these differences is crucial for effective mission planning and adaptation during dynamic changes in mission events and environmental conditions. Aviators should be well-versed in the capabilities, limitations, and tactical advantages of each sensor type. Atmospheric factors such as absorption, scattering, particulate size, and concentration influence the performance of both NVG and thermal sensors. Predicting performance requires consideration of factors such as absolute humidity, illumination level, moon angle, and weather conditions. In adverse weather, experience with sensor response to atmospheric conditions guides decision-making regarding mission continuation or abort. Therefore, a comprehensive understanding of sensor behavior under different conditions is essential for mission success. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://229.gitbook.io/229-hub/knowledge/army-aviation-fundamentals/rotary-wing-night-flight/night-version-sensors.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.