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Illumination is a critical factor when it comes to NVD operations emanating from both natural and artificial sources. Illumination is measured by the value of lux (Lumen / m 2). Natural sources of illumination at night include the Moon (lunar), Night sky illumination, Solar influence and terrain. Artificial Illumination is generally considered to be man made influences like IR / white illumination from searchlights, towns / cities and from vehicles to name a few. The major source of natural illumination comes from the moon, reflecting about 7% of the sun's radiation. To gain a full understanding of the amount of illumination it is essential to understand the characteristics of the moon.
The relationship between illuminance and luminance.
Illuminance is the amount of light coming from a light fixture that lands on a surface. It is measured in Footcandles (Lux in the metric system). A typical office has an illuminance of between 30 to 50 footcandles (300 to 500 lux) on desktops. Horizontal illuminance describes the amount of light landing on a horizontal surface, such a desk, and vertical illuminance describes the illuminance landing on a vertical surface, such as a wall or a face.
Luminance describes the amount of light leaving a surface in a particular direction, and can be thought of as the measured brightness of a surface as seen by the eye. Luminance is expressed in Candelas per square foot, or more commonly, Candelas per square meter (Cd/m²). A typical computer monitor has a Luminance of about 100 Cd/m². These two diagrams illustrate the difference between illuminance and luminance.
FIGURE 1: Illuminance vs luminance
Phases of the Moon
From any location on Earth, the moon appears to be a circular disk, which at any specific time, is illuminated to some degree by direct sunlight. Like the Earth, the moon is a sphere that is always half illuminated by the Sun but as the moon orbits the Earth we get to see more or less of the illuminated half. During each lunar orbit (lunar month), we see the moons appearance change. The cycle is a continuous process divided into eight distinct and traditionally recognised stages called phases. The phases designate both the degree to which the moon is illuminated and the geometric appearance of the illuminated part. These phases of the moon and the sequence of their occurrence are shown in Figure 2.
New Moon - The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).
Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Figure 2: Phases of the moon
Following the waning crescent is the New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon's "age". Each complete cycle of phases is called a "lunation". Because the cycle of the phases is shorter than most calendar months, the phase of the Moon at the very beginning of the month usually repeats at the very end of the month. When there are two Full Moons in a month (which occurs, on average, every 2.7 years), the second one is called a "Blue Moon".
Although Full Moon occurs each month at a specific date and time, the Moon's disk may appear to be full for several nights in a row if it is clear. This is because the percentage of the Moon's disk that appears illuminated changes very slowly around the time of Full Moon (also around New Moon, but the Moon is not visible at all then). The Moon may appear 100% illuminated only on the night closest to the time of exact Full Moon, but on the night before and night after will appear 97-99% illuminated; most people would not notice the difference. Even two days from Full Moon the Moon's disk is 93-97% illuminated.
New Moon, First Quarter, Full Moon, and Last Quarter phases are considered to be primary phases and their dates and times are published in almanacs and on calendars. The two crescent and two gibbous phases are intermediate phases, each of which lasts for about a week between the primary phases, during which time the exact fraction of the Moon's disk that is illuminated gradually changes.
The phases of the Moon are related to (actually, caused by) the relative positions of the Moon and Sun in the sky. For example, New Moon occurs when the Sun and Moon are quite close together in the sky. Full Moon occurs when the Sun and Moon are at nearly opposite positions in the sky - which is why a Full Moon rises about the time of sunset, and sets about the time of sunrise, for most places on Earth. First and Last Quarters occur when the Sun and Moon are about 90 degrees apart in the sky. In fact, the two "half Moon" phases are called First Quarter and Last Quarter because they occur when the Moon is, respectively, one - and three - quarters of the way around the sky (i.e., along its orbit) from New Moon.
Natural Sources of Illumination - Lunar Illumination
The amount of light provided by the moon is highly variable and is influenced by four factors: (1) lunar cycle, (2) moon angle, (3) lunar albedo (reflectivity), and (4) variation in earth-moon distance.
Lunar Cycle. The primary lunar illumination factor is lunar cycle (new, full, quarter, etc.). These phases of the moon will each provide different levels of illumination (Figure 3). A lunar month is about 29.5 days. There will be many times as a new night systems player that you will find yourself "chasing the moon." This term is used to describe the attempt by a prudent training program to introduce ANVIS flying under optimal conditions. Therefore, introductory flight operations will be driven by high lunar illumination. Moon phases are influenced by the time of year and global position (Iat, long).
Approx. Levels of Illuminance
(Lux = lm m-2)
1 - 1.3 x 105
Full Daylight (Not Direct Sunlight)
1 - 2 x 104
Very Dark Day
Moonless, Clear Night Sky
Moonless, Overcast Night Sky
FIGURE 3: Lunar Cycle Illumination Values
Moon Angle. Moon angle, or altitude in relation to the horizon, is the second most significant factor which affects lunar illumination. The moon is at its brightest when it is directly overhead and provides less illumination as it rises or sets (Figure 4). Many people will look at a low angle full moon and assume high illumination. However a quarter moon high overhead can actually be brighter. Moon altitude is also important because of the phenomenon known as terrain shadowing which will be discussed later.
Lunar Albedo. A difference in the albedo (reflectance) of the illuminated portions of the moon surface during the lunar cycle is the third factor. For example, the moon is about 20% brighter during the first quarter (waxing) than it is during the third quarter (waning) due to differences in the lunar surface.
Earth-Moon Distance. The final and least significant factor is the variation in the earth-moon distance due to the elliptical nature of the lunar orbit around the earth. The changes in illumination resulting from this 26% change in distance are deemed insignificant for NVG purposes.
FIGURE 4: Moon Angle v. Illumination.
Light Level Planning Calendar. Because of the wide and rapid changes in lunar illumination a computer program was developed to help night systems squadrons properly plan their Sorties. This information is available in the Light Level Planning Calendar that is available for MS-DOS computers. The computer program provides a global prediction of sun and moon position for most areas of the world. The information is available in various formats through a number of selectable menu options. The two most useful options are the Light Level Planning Calendar and the Sun and Moon Position Chart. An example of a Light Level Planning Calendar is shown in Figures 5 and 6 below.
FIGURE 5. Light Level Planning Calendar for Oakey 4-17 October 1995
Date: The local date that the visible times begin.
Begin: The local time that the Moon rises above the minimum height and the Sun is below the "Set Value".
End: The local time that the Moon goes below the minimum height and the Sun is above the "Rise Value".
Start: The altitude of the Moon at the Begin time.
High: The highest altitude of the Moon during the visible time span.
End: The altitude of the Moon at the End time.
Best View: The time that the Moon's view is the best.
Degree at Best: The altitude of the Moon at it's best view.
Phase: This is actually percent illumination of the Moon's face.
Age: The number of days since the previous new moon.
FIGURE 6. Light Level Planning Calendar for Maroochydore Oct-Nov 2008
Night Sky Illumination. Moonless nights also have significant useable light for ANVIS operations. This is because of the large near IR composition of night sky illumination that matches the peak sensitivity of the ANVIS ( Figure 7). It is possible to fly effectively with ANVIS under these conditions with a good training program and proper flight planning. On a moonless night, about forty percent of the light is provided by electrons from atoms and molecules in the upper atmosphere known as airglow. Starlight is the other significant light source and provides about .00022 lux (about 1/10 the level of a quarter moon).
FIGURE 7: Moonless Night Spectral Composition
Other possible light sources include auroras: (luminous rays, ribbons, and arc patterns caused by charged particles),Gegenscheins (diffuse, faint light caused by sunlight reflecting off of air particles), zodiac lights (faint elliptical disks around the sun caused by reflection off particulate matter), and noctilucent clouds (usually coloured, thin clouds with unknown origin).
Solar Influence. The sun can provide adequate light for ANVIS operations at nautical twilight (7-12 degrees below the horizon). Civil twilight (0-6 degrees below) is too bright and astronomical twilight (13-18 degrees below) is too dark for NVG operations (considering only the contribution of the sun). A sun that is well below the horizon can continue to be a significant nuisance if flying toward it, especially in mountainous terrain because of potential activation of the ANVIS gain circuitry. Figure 6 diagrammatically shows the various types of twilight.
FIGURE 8: Solar Illumination
Artificial Illumination. Lights from cities, vehicles, weapons, flares, etc., can all provide illumination. This light can be helpful or a tremendous hazard. There is a potential for the ANVIS gain control system to be activated causing the NVD to "bloom" or "shutdown."
NVG Terrain Considerations.
Terrain considerations must be understood when discussing ANVIS performance. There are three primary terrain factors that need to be examined for proper NVG Sortie planning: (1) terrain reflectivity (albedo), (2) terrain contrast, and (3) terrain shadowing.
Terrain Reflectivity. Terrain reflectivity (albedo) will greatly influence luminance, as surfaces such as snow will reflect more light than surfaces like asphalt or dark rock. Our ability to see terrain features with ANVIS is solely a function of the amount of light reflected by the terrain. Specific examples of reflectivity values are given in Figure 9.
Dark - Ploughed
Light - Ploughed
Figure 9: Terrain Reflectivity (Albedo) Values
Terrain Contrast. Terrain contrast is a measure of the difference between the reflectivity of two or more surfaces. Examples are given in Figure 10. The greater the difference in contrast, the easier it is to see terrain or objects. ANVIS contrast improves with illuminance up to about full moon and degrades as light levels decrease below quarter moon. Therefore, terrain such as water and desert, which usually have very little contrast, can be troublesome to fly over in low light conditions. This problem is compounded by a lack of terrain features or texture. The comfort level flying over different terrain is also a function of terrain texture. Texture is important as it provides recognition and depth perception cues. Forests provide a lot of texture, whereas deserts, even though they have higher light reflectivity values, have poor contrast and little texture. This is why flying over forests can be easier, even though they do not reflect as much light as the desert or snow. However, in conditions of low illumination, this advantage is usually lost as the contrast between the trees is similar and they therefore blend together.
Asphalt / Grass
Asphalt / Snow
Dirt / Grass
Grass / Leaf
Sand / Leaf
Figure 10: Sample Contrast Values
Terrain Shadowing. Shadows form at night just as they do during the day. Anything blocking moonlight will create a shadow. This can include terrain, cultural objects and even an aircraft. One big difference between day and night shadows is the amount of energy present inside the shadow. During the day we can see pretty well even when in a shadow because there is a lot of energy within the shadow. However, there is considerably less energy available at night, and much less energy inside a night shadow. Since NVD's need some illumination to function, it follows that they will not be as useful inside shadows, or for viewing from outside to inside shadowed areas. Consequently, detail, and even large objects, can be lost in shadowed areas (this will be more fully discussed later). At this point it is important to note that shadows can be helpful as well as a hindrance. When flying into shadowed areas, the aircrew may depend on help from sensors that do not need illumination to function, such as the searchlights and the RADALT.
Atmospheric Impact on NVD Performance. The atmosphere is the most important environmental factor controlling the performance of the ANVIS. The atmosphere can attenuate light and thermal energy, reducing the level of energy reaching the ANVIS. This attenuation can occur by refraction, absorption or scattering. The impact of refraction is almost negligible so absorption and scattering are the only attenuating mechanisms addressed here. The impact of these mechanisms on the NVD image varies based upon their spectral sensitivities. ANVIS operates by intensifying light energy between 625-960 nanometres. Any attenuation, either before or after it strikes the terrain, will effectively reduce the useable light available to the NVD and thus affect the resulting image.
Scattering. Scattering is second to absorption in the impact of atmospheric effects on NVD performance. As light and thermal energy travels through the atmosphere, it can strike a particle thereby changing the energy's path. This scattering of energy causes attenuation of the signal. Scattering is generally less significant than absorption when considering NVD performance. It is sometimes difficult to differentiate between the effects of aerosol scattering and absorption for wavelengths less than five microns. There are two types of scattering, molecular and aerosol.
Clouds. There is a great variety of particle sizes within individual as well as multiple cloud formations. Therefore, it is difficult to predict how much they will attenuate ANVIS performance. The problem is exacerbated by the fact that water in low level clouds is found in the gaseous, liquid, and sometimes even solid forms. This means a varied and unpredictable effect on NVDS, so you must be attuned to noting degradations in each.
ANVIS Effects. Water vapour exists at all temperatures. Because the amount of water vapour a cloud formation can hold increases with temperature, summer clouds generally have higher liquid water content than winter clouds. These liquid water particles are normally between 0.5-80 microns in diameter, are generally opaque to visible and near IR light, therefore they reflect these wavelengths. For this reason, thick, dense clouds can be easily seen with NVGS, especially when silhouetted against the night sky. This also means that thick clouds can reduce the amount of illumination that strikes the ground, thereby reducing the available luminance to the ANVIS. Thin and wispy clouds have greater space between particles so pass more of the near IR radiation without scattering. Near IR wavelengths have a greater chance of passing through these clouds without being scattered than do the shorter visible wavelengths. For this reason, it is possible for thin, wispy clouds to be seen by the naked eye (because the visible light is reflected) but be invisible when viewed through the ANVIS. This potential "invisibility" is possible given three conditions:
The clouds are thin and wispy, at least on the edges.
The clouds are low level and set in against the terrain, vice being silhouetted against the night sky.
The ambient illumination is either very high or very low, degrading NVG performance.
The presence of thin clouds that progress to thicker ones can result in hiding terrain features. This can obviously create 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. In that regard, a common question occurs; if the cloud is invisible, why can't the aircrew see the terrain behind it?" The answer is predicably complex. First, the cloud reduces visual and near IR contrasts and detail. This produces a false perception of distance, resulting in aircrew either not seeing the terrain, or thinking it is much farther away than it actually is. Second, the cloud may get progressively thicker, allowing the pilot to progress through the cloud without initially perceiving a "cloud wall." If a cloud is detected, the perception may be that it is off at a distance. Clouds reduce illumination to an extent dependent on the amount of cloud coverage and density, or thickness, of the clouds. For example, a thick, overcast layer of clouds will reduce the ambient light to a much greater degree than will a thin, broken layer of clouds. The aircrew must be alert for a gradual reduction in light level and/or notice the obstruction of the moon and the stars. The less visible the moon and stars, the heavier the cloud coverage. If the ANVIS image becomes grainy and begins to scintillate (sparkle), this is an indication that weather may be causing a low ambient light condition. Also, shadows caused by broken or scattered cloud layers blocking the moon illumination can be seen on the terrain. Although clouds can decrease illumination and resulting luminance from the moon and stars, they can, especially if they are low and broken to overcast, reflect enough cultural lighting to help offset the loss of lunar illumination. Obviously, this will only occur in and around areas with significant cultural lighting and is only helpful if it is clear beneath the overcast.
Fog. The effects of fog are similar to those of clouds. Generally, fog is distinguishable from clouds only in regard to distance from the ground. Particle size varies from 2 to 20 microns, which is very similar to a cloud. Typically, fog has fewer particles and a smaller range of particle sizes than clouds. Fall is the most likely season, and early morning the most likely time to encounter fog. Urban areas tend to have less fog (probably due to urban heat islands) than rural areas, and mountainous areas tend to have more fog than sites nearer sea level. Chances of fog increase as temperatures decrease and the dew point spread approaches zero.
ANVIS Effects. Since fog tends to stay close to the ground it is more a navigation hazard to rotary wing aircraft than to fixed wing aircraft which, in general, will be flying at higher altitudes. Fog, however, can mask or partially mask ridgelines and other navigational features making it more difficult to navigate or use funnelling techniques in the target area. One way to note an increase in the moisture content of the air while utilizing ANVIS is to observe a decrease in the intensity of ground lights. This is especially obvious if you are flying at an altitude high enough to use as a comparison ground lights that might be out of the area of increased moisture content. Also, the halo effect noted around lights when viewed directly with NVD will tend to get larger and more diffuse in an area of increased moisture. The enhanced contrast in an area illuminated by ground lights will also be lessened or absent
Rain. Like clouds, the performance of ANVIS in rain is difficult to predict as droplet size and densities are variable. All previous discussions on water vapour, clouds, fog, absorption and scattering are applicable here.
ANVIS Effects. Due to small droplet size and low density, light rains or mists cannot be readily seen with ANVIS. However, contrast, distance estimation and depth perception will be affected due to light scattering and the resulting reduction in light level. Heavier rains will be more discernible due to luminance blocking and more obvious sign such as rain on the windscreen.
Humidity. Atmospheric water vapour (humidity) is the most influential absorbing gas, and certainly the most variable. Local humidity conditions can easily double the water vapour content in a matter of hours, such as seen with a changing weather front. When considering performance, absolute humidity is more relevant than relative humidity. Absolute humidity, also known as vapour concentration or vapour density, is the amount of the mass of water vapour present in a given volume of air and is expressed as grams of water per cubic meter of air (g/m3). Relative humidity is a ratio that expresses the amount of moisture in the air compared to the maximum amount that could be held at a particular temperature. This is the common measurement used by meteorologists when discussing humidity and is expressed as a percentage.
ANVIS Effects. Humidity effects on ANVIS performance vary with particle size and density. Visible and near IR wavelengths tend to pass more readily through an area of high absolute humidity as long as the particle sizes are small as found in light rain or fog
Snow. Snow occurs in a wide range of particle sizes and shapes. Snow crystals, while small in size, are generally large in comparison to the wavelength of visible, near and far IR radiation and will easily block or scatter those wavelengths. However, snow will not normally degrade thermal signatures as much as fog and rain due to its lower particle density.
ANVIS Effects. As with other forms of moisture, the density of the flakes will determine how much illumination and luminance is blocked and thus how much degradation occurs to the NVD image. Snow can occasionally be of help, in that it can reflect available light and thus enhance luminance when on the ground. Snow covering trees and rocks with some large rocks protruding through can add a degree of depth perception to an otherwise benign scene. Also, snow can add a slightly different texture that can aid in contrast discrimination. Due to the excellent reflectivity of snow, less illumination is required to give the same luminance for the subject without snow. That means the ANVIS can see the area as well with less moon illumination. This must be balanced against a lot of snow covering a large area, which will tend to act like a large, flat, featureless, bright picture from the NVD. Also of consideration is landing in snow. Helicopter operations can create a condition referred to as "White out" which effectively blocks the NVG image.
Sand, Dust and Other Obscurants. Again, the affect here is due to particle size and density. Examples include such things as smoke from fires or other sources or sand storms.
ANVIS Effects. The effect of blowing sand or dust is similar to that created by snow except that the particulate material is far less reflective and much larger. This condition is significant since dust particulate completely block the near infra-red light from striking and reflecting from the terrain. Since there is less luminance, the scene is darker. As with snow, during vertical operations or with strong winds, there can be an almost total block of IR radiation resulting in a "brown out". Weather impacts in the right terrain would put large amounts of sand or dust into the air. The impact of such obscurants on NVD performance is similar to those mentioned above and depends on particle size and density. ANVIS visibility "inside" these obscurants would be poor and should be balanced against the overall size of the cloud and its position relative to where you are and where you need to see.
Winds. Winds may affect the area of interest for NVD operations by increasing the density of particles in the air as well as the area of coverage. The resulting image would be degraded in the same way that dust affects the image.
Lightning. Lightning will temporarily increase illumination. Looking directly at it will cause NVD' s to shut down briefly. However, when not looking directly at it, lightning will briefly illuminate the area giving you an enhanced NVD image albeit for a short duration. The brief duration and enhanced image clarity can create the impression that objects are much closer than they really are. Whether it's an aid or a hindrance depends on the frequency of lightning and its source relative to your flight path.
Two other considerations that will affect NVGs during operations are those of shadow and moisture.
Even when an NVG crew takes into account and ensures that all conditions are met, there remain many variables that can adversely affect the safe and effective use of NVG (e.g. flying towards a low angle moon, flying in a shadowed area, flying near extensive cultural lighting and flying over low contrast terrain). It is important to understand these limitations when considering the capabilities of NVG.
A shadow is usually treated as an obstacle by an NVG crew unless they can illuminate the area in shadow. Never be tempted to simply fly into a shadow area as there is a good possibility that obstacles may be hidden from detection.
Shadowing. Moonlight creates shadows during night-time just as sunlight creates shadows during daytime. However, night-time shadows contain very little energy for the NVG to use in forming an image. Consequently, image quality within a shadow will be degraded relative to that obtained outside the shadowed area. Shadows can be beneficial or can be a disadvantage to operations depending on the situation.
Benefits of Shadows. Shadows alert crews to subtle terrain features that may not otherwise be noted due to the reduced resolution in the NVG image. This may be particularly important in areas where there is little contrast differentiation, such as flat featureless deserts, where large dry expanses and high sand dunes may go unnoticed in the absence of contrast. The contrast provided by shadows helps make the NVG scene appear more natural.
Moisture. Different types of moisture will have varying effects and it is important to understand these effects and how they apply to NVG operations.
It is important to know when and where fog may form in the flying area. Typically, coastal, low-lying river, and mountainous areas are most susceptible.
Light rain or mist may not be observed with NVG but will affect contrast, distance estimation, and depth perception. Heavy rain is more easily perceived due to large droplet size and energy attenuation.
Snow occurs in a wide range of particle sizes, shapes, and densities. As with clouds, rain, and fog, the denser the airborne snow, the greater the effect on NVG performance. On the ground, snow has mixed effect depending on terrain type and the illumination level. In mountainous terrain, snow may add contrast, especially if trees and rocks protrude through the snow. In flatter terrain, snow may cover high contrast areas, reducing them to areas of low contrast.
On low illumination nights, snow may reflect the available energy more effectively than the terrain it covers and thus increase the level of illumination.
All atmospheric conditions reduce the illumination level to some degree and recognition can be difficult. Thus, a thorough weather briefing, familiarity with local weather patterns and an understanding of the effects on NVG performance are important for successful NVG flight.