Abstract: There are studies that use a combination of daylight and automatic electric lighting systems that allow a noticeable energy reduction in no residential buildings. However, in order to be able to determine the energetic performance of a building, it is necessary to know the natural illumination levels of natural lighting of the location were the maximum benefit is found. Recently, several illuminance studies are being done focusing on the construction field due to the trend of saving energy and the design of sustainable buildings. Nevertheless, studies that match outside daylight measurements with building inside illumination measurement or with building scale models measurement are not found. But currently, it is a priority for engineers, architects and researchers to reduce the number of luminaires and electricity consumption through the suitable and efficient use of sunlight. The aim of this work is to obtain an empirical model that allows determining the illuminances in a projected building, based on a scale model and daylight measurements. This way, it is possible to optimize some building parameters as orientation, numbers and sizes of the windows, etc…, to obtain the best conditions for the maximum use of natural light with the consistently energy saving. To do this, the global illuminance on horizontal surfaces within a room and in its scale model for different distances from the façade windows has been measured with photometric sensors previously calibrated and connected to dataloggers. Also, one photometric sensor is placed outside the model to known the global exterior horizontal illuminance. From these measures a ratio between global horizontal illuminance in the real space and in the scale model has been obtained. This ratio depends on the global horizontal exterior illuminance and the facade distance. For each distance to the facade the relation between the global horizontal illuminance on the real space and the scale model depends linearly on the global horizontal exterior illuminance. From this linearly relation, the slope an intercept parameters and its variation to the face distance has been obtained. It has been observed that not only the slope of each fitted line as well as the intercept have a linear dependence with the façade distance. From this data, a general equation that allows obtaining a global horizontal illuminance in a real enclosure from measurements taken inside a scale model and the global horizontal exterior illuminance has been obtained.