Abstract

In meteorology, daytime atmospheric visibility distance (or visibility) is defined as the greatest distance at which a large dark object against the light sky at the horizon can be seen and clearly recognized by an unaided eye. For determination of maximum visible distance at nighttime, usually, a known, preferably unfocused, moderately intense light source is used in which, at the maximum distance it can be recognized. These atmospheric visibility distance definitions were developed based on human observation through the atmosphere. Presence of dust, fog, haze, pollution, or smoke, at the atmosphere reduces the meteorological visibility. Visibility degradation is one of manifestations of the atmospheric pollutions and airborne particles, which is mainly due to absorption and scattering effects of aerosols in the atmosphere. In this regard, the local air quality can also is reflected by the atmospheric visibility distance. Measurement of atmospheric visibility distance is an important issue in the transportation. Low visibility of atmosphere is mainly a problem of traffic safety. Therefore, existence a reliable atmospheric visibility, at roads for driving, at airports for takeoff and landing of airplanes, at ports for movement of ships, and so on, is necessary. Based on the mentioned definitions, it seems that, the measurement of atmospheric visibility distance is affected by many factors such as the size and shape of the target, the air light intensities of the observing area, the observer’s angle to the target and height, the light intensities for night targets, and so on. In addition, human factors also affected the measurements because of the requirement that the visibility targets be both detected and recognized by the naked eye. Many instrumentation approaches for measuring atmospheric visibility distance have been developed. Transmissometers and scatter meters are two types of instruments are used for determination of the atmospheric visibility distance. A transmissometer operates by sending a narrow collimated laser beam through the atmosphere. It extrapolates the attenuation of the laser beam at a known path length in order to estimate the distance for which the emitted light is attenuated by 95%. A scatter meter assesses the dispersion of a light beam at a particular scattering angle. In this work, we introduce a new, originally an optical method, based on the measurement of the optical visibility or contrast of image of a periodic pattern that captured by a telescope equipped with a digital camera through the atmosphere. In comparing to other methods of the measurement of atmospheric visibility distance, this method is less affected by the setup and instruments factors. In this paper we have presented a new method for measuring atmospheric visibility distance by imaging from a reflective sinusoidal linear grating having variable spatial period. In the experiment a sinusoidal grating with variable period of