There are several factors to consider regarding signal attenuation in any atmospheric optical transmission link.

Signal loss in atmospheric transmission systems using LEDs or laser diodes (LDs) is primarily caused by absorption by the transmission medium—the atmosphere. As a light beam passes through the air, a certain degree of molecular absorption inevitably occurs. Furthermore, the atmosphere absorbs light at specific wavelengths so strongly that those wavelengths are entirely unsuitable for signal transmission. Wavelength bands where attenuation due to atmospheric absorption remains within acceptable limits are known as "atmospheric windows." Data regarding these windows is readily available in technical literature; consequently, all LED and LD-based systems must operate at wavelengths falling within these windows.

Absorption by Airborne Particles

Airborne particles, such as dust and smoke, constitute another factor contributing to the absorption of optical signals. The atmosphere invariably contains some concentration of these particles; levels can be particularly high near bodies of water. In such environments, optical equipment is typically mounted at a greater height above the ground to optimize transmission performance.

Absorption and Scattering by Fog

Fog is another significant cause of infrared absorption and also induces both forward and backward scattering of light. Therefore, in regions prone to fog, the operating schedule of optical transceiver equipment must be aligned with local weather conditions, as the system cannot function reliably during foggy periods.

Impact of Atmospheric Turbulence