Converting power meter specifications into applicable values can sometimes be a challenge. Absolute power accuracy is typically expressed as a percentage value and represents the maximum allowable deviation between the measured power and the actual (true) power of an optical signal. It tells you how accurately the power meter can determine the real optical power.

For example, a 2.7% absolute accuracy (the specifed value of a santec OPM-200) implies that at the reference conditions (-10dBm in this case) the accuracy is ±0.11dB. But what does this really mean? And where did the 0.11dB come from?

First, the absolute accuracy of 0.11dB indicates that if you were to input a true power of exactly -10dBm (according to NIST or another national laboratory standard) then the OPM-200 will correctly deduce and show that this power is between -9.89dBm and -10.11dBm.

To transfer between the absolute accuracy in percent and the dB value use the calculator below.