To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant. It is a natural phenomenon that occurs for any type of transmission—whether it's electricity or data. This reduction of signal, also called attenuation, is directly related to the length of a cable—the. Insertion Loss (IL) is one of the most fundamental performance indicators in fiber optic networks. The core process is the same across fiber optics, RF electronics, and acoustics: establish a baseline reference without. Whether in telecommunications, data centers, or photonics applications, insertion loss testing ensures systems operate with minimal signal degradation, maintaining reliability and accuracy.