## Optical Power In order to measure optical loss, you can use two units, namely, dBm and dB. While dBm is the actual power level represented in milliwatts, dB (decibel) is the difference between the powers. **Figure 4 – How to Measure Optical Power** [](https://www.cisco.com/c/dam/en/us/support/docs/optical/synchronous-digital-hierarchy-sdh/29000-db-290008.gif "db_290008.gif") If the optical input power is P1 (dBm) and the optical output power is P2 (dBm), the power loss is P1 - P2 dB. In order to see how much power is lost between input and output, refer to the dB value in this power conversion table: |dB|Power Out as a % of Power In|% of Power lost|Remarks| |---|---|---|---| |1|79%|21%|-| |2|63%|37%|-| |3|50%|50%|1/2 the power| |4|40%|60%|-| |5|32%|68%|-| |6|25%|75%|1/4 the power| |7|20%|80%|1/5 the power| |8|16%|84%|1/6 the power| |9|12%|88%|1/8 the power| |10|10%|90%|1/10 the power| |11|8%|92%|1/12 the power| |12|6.3%|93.7%|1/16 the power| |13|5%|95%|1/20 the power| |14|4%|96%|1/25 the power| |15|3.2%|96.8%|1/30 the power| For example, when direct line (LD) optical input into the fiber is 0dBm and output power is -15dBm, optical loss for the fiber is calculated as: > **Input Output Optical Loss** > 0dBm - (-15dBm) =15dB In the power conversion table, 15dB for optical loss equals 96.8 percent of lost optical power. Therefore, only 3.2 percent of optical power remains when it travels through the fiber.