1. A method for defining the position of a light-emitting object comprising the steps of: splitting the beam of light from the light emitting object to form at least two beams of light; focusing the two beams of light onto an array of light sensitive elements each light sensitive element generating an electrical signal related the amount of light focused thereon; generating a first beam signal from a first subset of the light sensitive elements; generating a second beam signal from a second subset of the light sensitive elements; and combining the first and second beam signals to form a composite signal representing the position of the light-emitting object.