The Materials and Geometry Format (referred to henceforth as MGF) is a description language for 3-dimensional environments expressly suited to visible light simulation and rendering. The materials are physically-based and rely on standard and well-accepted definitions of color, reflectance and transmittance for good accuracy and reproducibility. The geometry is based on boundary representation using simple geometric primitives such as polygons, spheres and cones. The file format itself is terse but human-readable ASCII text.