A design tool for interior daylighting description of luminous enviroment with image synthesis

As part of visualization of interior luminous environment in daylighting a programme (NATUREL) has been developed at CSTB. It allows to take into account complex shapes of building, furniture and apertures. These, located on facade and/or on roofing, are characterized with various parameters : area, thickness of wall (roof), luminous transmission, outdoor surroundings ... All surfaces (indoor and outdoor) of the scene are described in term of plane facets and are supposed to have perfectly diffuse reflection, which is advised for interior lighting. The software includes three main steps. Calculation of the direct component of illuminances (light arising from exterior : sun + sky + ground + buildings). This calculation is made at points which are determined to take into account the luminous gradients on the surfaces. These gradients are due to sunspot and to geometric interaction between apertures and furniture (umbra and penumbra zones). Calculation of interreflections. Interreflections are estimated by using the radiosity method. Visualization on graphic colour VDU. Two stages are necessary : a graphical calculation (projection along a given direction), and a colorimetric calculation. In a first time this method has been applied to "grey" surfaces. Then the spectral properties have been taken into account for the incident light, transmission of glazing materials and reflection of various surfaces.