The knowledge of luminous flux and illuminance phenomena is necessary for proper and qualitative dimensioning and arrangements of openings and thus for comfortable and efficient living environment. In this paper the concept of luminous efficacy of the solar energy as the basis for the control algorithm is taken into account. Luminous efficacy K is defined as the ratio of luminous flux to radiant flux, K=Φ_V/Φ_e. The idea was to design a flexible dynamic building envelope, which will be able to adjust itself to the changeable environmental conditions regarding the inner illuminance requirements. Changes of the opening's geometry are implemented by moving the roller blind. The roller blind is used to control the inside illumination of human-built environment in real time conditions. We realized the concept of the control with fuzzy elements in the control algorithm for automatically adaptable shadow device on the window. For this purpose a model of a real physical system, a test chamber with an opening equipped with movable roller blind on the south side was build. The fuzzy control system manages the roller blind movement, which enables the optimal use of the available solar energy in order to assure the desired internal illumination.