**3.2. Visibility Runway Visual Range (RVR)**

Visibility was defined originally for meteorological needs, as a quantity estimated by a human observer. It defined as (Kruse model) means of the length where an optical signal of 550 nm is reduced to 0.02 of its original value [10]. However, this estimation is influenced by many subjective and physical factors. The essential meteorological quantity, namely the transpar‐ ency of the atmosphere, can be measured objectively and it is called the Runway Visual Range (RVR) or the meteorological optical range [11]. Some values of atmospheric attenuation due to scattering based on visibility are presented in Table (3).


**Table 3.** Variation in atmospheric attenuation due scattering based on visibility (data obtained from [7,12]).

When the length difference between the two optical paths varies, the energy passes through minima and maxima. The visibility *V* is defined by:

$$V = \frac{I\_{\text{Max}} \cdot I\_{\text{min}}}{I\_{\text{Max}} + I\_{\text{min}}} \tag{6}$$

The visibility depends on the degree of coherence of the source, on the length difference between the paths as well as on the location of the detector with respect to the source. The coherence between the various beams arriving at the detector also depends on the crossed media: for example the diffusing medium can reduce the coherence. For links referred to as "in direct sight" links, coherent sources can be used, provided that parasitic reflections do not interfere with the principal beam, inducing modulations of the detected signal [11].

Low visibility will decrease the effectiveness and availability of FSO systems, and it can occur during a specific time period within a year or at specific times of the day. Low visibility means the concentration and size of the particles are higher compared to average visibility. Thus, scattering and attenuation may be caused more in low visibility conditions [13].
