Models for diffuse illuminance and irradiance and for the luminous efficacy of diffuse solar radiation are developed, specifically for clear skies, and also for all sky types. All luminous efficacy models reported in the present work have the distinctive feature of being obtained from empirical models developed for diffuse illuminance and irradiance, looking in this way, apparently for the first time, for a mathematical coherence between the luminous efficacy and the two magnitudes that define it. Statistical assessment of all the models shows that those for all sky types developed from two independent variables, solar elevation and brightness index, predict the illuminance for clear skies better than the specific clear skies models developed with solar elevation as the only independent variable.