Indirect light is produced as a result of light reflecting off a surface. The reflectance of the surface and the degree of diffusion which is often assumed, determines the calculated, reflected indirect light. To create an accurate impression of the room, designers need to calculate as many interreflections as possible to achieve a representational light distribution in the room. It was not until the 1990s that progress in hardware allowed such a complex calculation. The calculation of indirect light is also known as "global illumination".