Shortwave reflected terrestrial irradiance is routinely measured at satellite by wide field-of-view radiometers on board earth radiation budget experiments. Although these irradiances are then corrected for time-dependent variations in mean filter-dome transmissivity, asymmetries in the amount of degradation over the filter dome hemisphere have the potential of producing large biases in the estimated irradiance field when positively or negatively correlated with inhomogeneities in the incident radiance field. The event of this asymmetry, caused by the combination of instrument configuration and contamination, direct and diffuse solar radiation and contact with exospheric constituents, is obtained empirically for the radiometer on board the NIMBUS 7 spacecraft during the first data year. Further analysis assesses the impact of this phenomenon on earth radiation budget parameter measurements.