Turn on the optical power meter (OPM) using the power button. Select Wavelength: Use the wavelength selection feature to set the wavelength corresponding to the fiber optic system under test. To augment the absolute power measurements NIST provides nonlinearity, spectral responsivity, and uniformity measurements. We explain the measurement standards, systems, methods, and uncertainties related to. The basic process is straightforward: turn the meter on, set it to the correct wavelength, clean your connectors, plug in, and read the display. This current is fed into a transimpedance amplifier, which outputs a voltage that is proportional to the input current.