Office supplies and people may shade illuminance sensors installed on the table, thus the lighting dimming control system is not able to act correctly. Based on the illuminance ratio model, an index of illuminance-variation transfer ratio and an approach using sensors installed on wall instead of table are proposed. The point of this approach is to choose an appropriate illuminance-variation transfer ratio between vertical illuminance on wall and horizontal illuminance on table. The effects of the location of sensor and controlled lighting on the illuminance-variation transfer ratio are investigated by simulations and experiments. Two constraint conditions are presented for determining the location and number of sensors in an office or reading room. As an example, the proposed approach was applied to a lighting dimming control system in classroom, which confirmed the validity of the proposed approach.