مؤسسه ژئوفیزیک دانشگاه تهران فیزیک زمین و فضا 2538-371X 42 2 2016 08 22 Measurement of the atmospheric visibility distance by imaging a linear grating with sinusoidal amplitude and having variable spatial period through the atmosphere اندازه گیری عمق دید جوی با استفاده از تصویربرداری از میان جو از یک توری خطی بازتابی با تابع برتابندگی سینوسی و گام کاهشی در امتداد بردار شبکه آن 449 458 58553 10.22059/jesphys.2016.58553 FA سیف اله رسولی عضو هیئت علمی دانشگاه تحصیلات تکمیلی علوم پایه زنجان الهه حریری دانشگاه زنجان سیامک خادمی عضو هیئت علمی دانشگاه زنجان Journal Article 2015 04 14 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. <br />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. <br />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 𝟕mm to 𝟏𝟏cm is printed on an area 𝟏 m 𝟑m and pasted on a suitable wooden frame. The frame is installed at 𝟑m height from the ground surface and 𝟒𝟕𝟓m distance from an imagining system consisting a telescope and a CCD camera. The telescope is a Newtonian telescope. The CCD is installed at the focal plane of the telescope. Images of the grating are recorded through the atmosphere by the CCD at different days and different day times. Local visibilities of the grating images are measured and cut-off frequency of the patterns is determined. From the cut-off frequency of the image patterns the atmospheric visibility distance is determined for the recording time. Our results for the visibility distance at different times are comparable with the weather report from the Zanjan airport station that was used another method. در این مقاله روشی جدید برای تعیین عمق دید جوی، با استفاده از تصویربرداری از یک توری خطیِ انعکاسی با تابع برتابندگی سینوسی که گام آن متغیر است ارائه می شود. اندازه گیری عمق دید در ساعت مشخصی از روزهای مختلف ماه های آذر و دی ماه سال 1391 که عمق دید متفاوتی داشتند انجام شد. یک توری خطی سینوسی با گام متغیر در محدوده 7 تا 110 میلیمتر در مساحت 1 متر در 3 متر روی مقوای سفید رنگ و مات چاپ شد و توری روی قاب چوبی مناسبی تعبیه و بطور عمودی در ارتفاع 3 متری از سطح زمین نصب شد. از این توری از فاصله 475 متری توسط یک تلسکوپ که در کانون آن یک عدد دوربینCCD قرار داده شده بود تصویربرداری شد. تصاویر ثبت شده با استفاده از نرم افزار MATLAB تحلیل شد و نمایانی موضعی تصاویر توری تعیین شدند. گرد و غبار موجود در جو باعث از بین رفتن تفکیک گام های کوچکتر توری می شود. با تعیین کوچکترین گام قابل مشاهده در تصاویر، حد تفکیک زاویه ای از فاصله 475 متری تعیین شد. با مقایسه کوچکترین زاویه دید بدست آمده با کوچکترین زاویه مربوط به اندازه گیری عمق دید توسط چشم غیرمسلح و فرض خطی بودن حجم آلاینده ها نسبت به طول مسیر دید، عمق دید بدست ‌آمد. مقادیر عمق دید بدست آمده از این روش با عمق دید گزارش شده توسط ایستگاه هواشناسی فرودگاه زنجان که در مجاورت محل اندازه گیری قرار دارد، مقایسه شدند که مطابقت خوبی مابین آنها مشاهده می شود. https://jesphys.ut.ac.ir/article_58553_3cbe0de07413a9a7be8de28aa57bf46d.pdf