Fiber optic signal loss, also known as attenuation, occurs when optical signals weaken as they travel through the fiber. 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. The estimate, called a "loss budget" is calculated using typical component losses for. Fiber optic loss is one of the most fundamental parameters in optical network engineering, yet it is often misunderstood as a purely theoretical value used only during design calculations. Contractors often install, terminate, and certify cabling without knowing the client's specific requirements.