1. A system for determining a position of a portable device relative to a luminaire, comprising:at least one luminaire, said luminaire comprising at least two light emitters for emitting light, the light comprising a first light signature and a second light signature, wherein the first light signature and second light signature identify the light emitter from which the light originates, and wherein the first light signature and the second light signature are different;
an optical system for guiding the emitted light to create a distribution of the light emitted by the at least two light emitters via the optical system, wherein a ratio between light signatures originating from the at least two light emitters within the distribution of light is unique for every position in a two-dimensional space;
and a portable device comprising:
a sensor for sensing light;
a memory for storing distribution information indicative of the distribution of the light emitted via the optical system; and
a processor coupled to the light sensor for receiving data on the sensed light and coupled to the memory to get access to the information stored in the memory, said processor is arranged to derive the light signatures from the received data on the sensed light; determine contribution information indicative for contributions of light with the derived signatures to the light sensed and determine a position of the portable device relative to the luminaire on the basis of contribution information and the distribution information.