Visibility refers to the distance at which an object can be distinguished from its background based on contrast. The human eye needs at least a 5% contrast ratio- meaning 5% of the reflected light must be seen- to differentiate objects. Visibility is expressed as a distance; for example, if visibility is 500 meters, objects within 500 meters are visible, while those beyond are not. This concept directly affects FSO performance, as clear visibility is essential for effective optical signal transmission.

Visibility and Free Space Optics Link Planning

Visibility is typically measured either informally by human observation or scientifically with instruments at weather stations. The aviation industry has collected nearly a decade of global visibility data at over 2,700 airports, providing valuable insight for FSO planning.

Commercial FSO systems use infrared light, which penetrates the atmosphere better than visible light—allowing FSO links to operate even beyond typical visual visibility limits. While a 5% contrast (or 13 dB attenuation) is the limit for human sight, FSO photodiodes can detect signals 30 dB lower (at around 1/1000 of the power), making them more resilient to poor visibility.

Visibility accounts for all major atmospheric conditions like fog, rain, snow, and smog, making it a practical, all-in-one metric for assessing FSO link viability. Though there are no global FSO standards, vendors have developed planning tools using this data to predict link performance by location.

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