For singlemode fiber, the loss is about 0. 5 dB per km for 1310 nm sources, 0. 5 dB/km at either wavelength for outside plant max per EIA/TIA 568)This roughly translates into a loss of 0. 1. 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. The acceptable dB loss for single mode fiber can vary depending on several factors, including the specific application, the length of the fiber, the quality of the components used, and the overall design of the network. You can either compare this loss value to the application requirement or calculate the expected loss based on how many connectors and splices are in the link along with the length of. ity check. This type of testing is the most accurate testing available and is the most accurate characterization of the fiber optic system's apability.