The proper calibration of an optical power meter requires the verification of two critical parameters: absolute accuracy at a given reference power over a wavelength range and linearity at a given wavelength over a power range. We discuss the key elements involved in determining the uncertainty in the absolute accuracy and the non-linearity of the power response under what are termed "reference conditions of calibration". We also discuss the underlying causes of erroneous measurements resulting from each of the three subsystems constituting a power meter: detector element, collecting optics and electronics.