Using the reference power level, it's time to calculate loss! Subtract the measured power reading from the initial reference power level (set in Step 2). The result is the total loss across the fiber link, typically displayed in decibels (dB). 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. Modern power meters are designed to operate across a wide range of wavelengths. Optical power loss (attenuation) refers to the reduction of signal strength as light propagates through fiber. Measured in decibels (dB), loss degrades signal quality, limits distance, increases bit-error rate, and escalates infrastructure cost. We also call this fiber loss "light attenuation".