The impact of aerosols on the optical properties of our atmosphere must be taken into account when using either daylight or irradiance models. A sudden increase in stratospheric aerosols following a large volcanic eruption will largely determine the respective proportions of direct and diffuse radiation impinging on the earth's surface, without really affecting the total global radiation. The negative effect of urban aerosols on direct luminous efficacy is shown to oppose a net increase in diffuse luminous efficacy, again without much effect on the global luminous efficacy. Using airport visibility data as model input to predict direct luminous efficacy brings a spectacular improvement to model precision, shown here by comparing the predictions of different models to an independent set of data.