Let There Be-LIGHT is a Mac OSX application that permits natural and artificial lighting calculation with a color radiosity method applied to a finite elements plan. LTB-LIGHT is able to share information with other CADs through DXF files. Light sources are totally managed inside the main program; it is possible to import, show, modify and use the IESNA LM-63 photometric data files (ASCII text file commonly used by North American lighting fixture manufacturers) EULUMDAT (european standard) and even DXF files (the program can convert a drawing in a photometry!); photometric data are modelled through 36 "C" (azimuth) curves and 181 "gamma" (zenith) angles, while lamp color is controlled by its color temperature or RGB color. LTB-LIGHT has a series of powerful tools that permit to optimize Your projects: a 'CIE module' to precalculate a rectangular, cutted-corner or L-shape rooms and a 'Road tool' to optimize a road installation as regards installed power, fixture choice, pole distance and height.