This paper presents a methodology for detailed daylighting numerical simulation of an office space with an advanced window system and calculation of dimming levels for electric lights to provide the additional illuminance required so as to minimise electric energy consumption for lighting. The particular system considered is a double-glazed window with motorized highly reflective blinds between the two glazings installed in an outdoor test-room and operated by a custom-built computerised building automation system. The daylight transmittance equations for the window system were determined from experimental measurements. A major requirement for on-line control is the automatic determination of the blind tilt angle required in order to reduce glare and simultaneously allow the maximum possible amount of daylight into the room, Numerical simulations of blind control and light dimming are presented for a typical office. (C) 2002 Published by Elsevier Science Ltd.