Procedures are outlined for characterizing the performance of radiometers, with special attention to photometers. Quantitative methods are given for evaluating the most important errors. These include: systematic calibration errors; errors caused by non-standard spectral response; - non-standard spatial evaluation of - irradiance (illuminance), - spherical irradiance (illuminance), - cylindrical irradiance (illuminance) or - radiance (luminance); - non-linearity; - fatigue; - temperature coefficient. Ways of judging the significance of less important aspects are suggested. These include: - effect of modulated radiation; - influence of polarization; - non-uniform response; - detector instability; - zero drift. The report deals with general problems of evaluation of photometric and radiometric instruments. Numerical requirements for individual types of instruments in later reports are not included. The report includes tables of the relative spectral distributions of five special sources that can be used to estimate the quality of the V(lambda) correction of photometer heads.