**1.4. Important definitions**

After illustrating the basic concepts of FSO, we return to the important definitions related to the laser power reduction due to atmospheric channel effects phenomena. These definitions are considered as the core principle of FSO transmission channel turbulence namely atmos‐ phere, aerosol, absorption, scattering, and radiance etc. Absorption and scattering are related to the loss and redirection of the transmitted energy. The majority of these definitions will be discussed in detail in the case study of this chapter (section 4).

**An atmosphere** is a layer of gases surrounding a planet or other material body material of sufficient mass that is held in place by the gravity of the body. An atmosphere is more likely to be retained if the gravity is high and the atmosphere's temperature is low. Earth atmospheric, which is mostly nitrogen, also contains oxygen used by most organism for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis, also protects living organisms from genetic damage by solar ultraviolet radiation. Another definition of an atmosphere is the envelope of gases surrounding the earth or another planet.

**An aerosol** is defined as a colloidal system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air. This term describes an aero-solution, clouds of microscopic particles in air. According to the literature, the size range of aerosol particles to be only from 0.1 to 1 μm another authors indicate that the size of aerosol is between 0.01 and 10 μm in radius. Another definition of aerosol is extremely-fine liquid droplets or solid particles that remain suspended in air as **fog** or **smoke**.

**Fog** is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface that obscures or restricts visibility (to a greater extent than mist; strictly, reducing visibility to below 1 km).

**Smoke** is a visible suspension of carbon or other particles in air, typically one emitted from a burning substance.

**Haze** is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky.

**Dust** is a fine powder made up of very small pieces of earth or sand.

**Absorption** of the light is the decrease in intensity of optical radiation (light) as it passes through a material medium owing to its interaction with the medium. In the process of absorption, the energy of the light is converted to different forms of internal energy of the medium; it may be completely or partially re-emitted by the medium at frequencies other than the frequency of the absorbed radiation.

**Light scattering** is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media. Deviations from the law of reflection due to irregularities on a surface are also usually considered to be a form of scattering. When these are considered to be random and dense enough that their individual effects average out, this kind of scattered reflection is commonly referred to as diffuse reflection. Scattering has different types as Rayleigh, Mie, Tyndall, Brillion, and Raman Scattering.

**Radiance** erasures of the quantity of radiation that passes through or is emitted from a surface and falls within a given solid angle in a specified direction. Radiance is also used to quantify emission of neutering and other particles.

Radiance (in Watts): total amount of energy that flows the light source.

**Attenuation** is the gradual loss in intensity of any kind of flux through a medium. Attenuation affects the propagation of waves and signals transmission media.

**Scintillation** is a flash of light produced in a transparent material by the passage of a particle (an electron, an alpha particle an ion, or a high-energy photon).

The process of scintillation is one of luminescence whereby light of a characteristic spectrum is emitted following the absorption of radiation. The emitted radiation is usually less energetic than that absorbed. Scintillation is an inherent molecular property in conjugated and aromatic organic molecules and arises from their electronic structures. Scintillation also occurs in many inorganic materials, including salts, gases, and liquids.
