How accurate are fiber optic power meters?
All optical power meters which are calibrated to NIST (the US standards body) or any national standards lab will measure optical power to an uncertainty of about +/- 0.2 dB or 5%. Therefore, since every power meter has an uncertainty of +/- 0.2 dB, any two meters can differ by 0.4 dB in the worst case (one at +0.2 dB and one at -0.2 dB) even if both are within their specification! More information on calibration uncertainty.
