(Backward) ray tracing, also called Monte Carlo ray tracing, is the second of the two most popular processes used for the calculation of light distribution. Unlike radiosity and photon mapping, however, it does not trace a ray of light from the light source. Instead, the rays start from the eye and are followed backwards to the model and the light sources. If the rays from the eye hit a surface, other rays of light are used to see whether this point reflects light or contains shadows. The result is shown as pixels on a focal plane. The higher the resolution required on the focal plane and the more reflecting surfaces there are, the more rays of light are required for the simulation and the more complex the calculation becomes.
Ray tracing has the advantage of producing exact representations of details and the smallest shadows. Since this method depends on the focal plane, a change of angle and the line of vision requires a new calculation. Scenes with very high contrast ratios are difficult to represent, as the incidental rays of light for calculation start from the observer/camera position and light apertures such as small windows in a large wall can initially be disregarded.