In my thesis I compared two computer tools for calculation of illuminance in rooms. These
two programs are Velux Daylight Visualizer 2 and DIALux 4.10 and are free to download.
We found out that there are some differences between the results that they give at calculating
the interior illuminance on horizontal plane and with it related daylight factor. The theme of this thesis includes determining how the large deviations occur between them, and why.With comparative analysis I verified calculations of both programs and I presented them. On an assumed room with typical realistic parameters and same distribution of windows I verified horizontal work plane illuminance and day light factor in the room. Calculations were made
for different types of reference skies. I presented these results for referential days and hours in a year to get the data about 15th percentile. I found that both programs calculate illuminance in the room correctly, when comparing results within the programs, the level of illuminance
varies. It showed that the outer horizontal illuminance taken into account by the programs differ by almost 54%. This is the main reason for different absolute values of illuminance, which may be given by the program.