Abstract: To reduce energy costs in buildings where there is an input of natural light, a method of maintaining an ambient light intensity in a building area at a predetermined level is proposed. It comprises obtaining an ambient light intensity level for the building area; comparing the ambient level to the predetermined level of light intensity; if the ambient level differs from the predetermined level, calculating an artificial lighting input to be generated in the building area to attain the predetermined level. It can further comprise generating the artificial lighting input in the building area and carrying out the steps of obtaining, comparing and calculating a second time to determine a quality of the calculating and modify the generating.