It is of interest to construct windows that can spread the transmitted light in a specified manner. The Kirchhoff approximation in the geometrical optical limit in combination with a chosen general form of the window surfaces yields the profile for the window surfaces, letting us specify how the light should be spread.The probability distribution function for the slopes of a window surface consisting of joined line segments was implemented in the simulation software Maxwell1D. Simulations show the feasibility of such windows. However they do not respond well when subject to light incident with another angle of incidence than the angle in mind.By using Snell's law to compensate for using a simpler system, the time needed for the simulations can be greatly reduced while simultaneously obtaining a higher accuracy in the results.