1. A method for generating an imaging signal representative of a three dimensional surface of a target comprising the steps of measuring a fixed number of first intensity values, wherein each of said first intensity values corresponds to one of a plurality of positions on the surface of said target, each of said first intensity values being measured at a first height of said target, storing each of said first intensity values at one of a plurality of addresses within a first memory, each of said plurality of addresses within said first memory corresponding to one of said plurality of positions on the surface of said target, measuring a fixed number of second intensity values, wherein each of said second intensity values corresponds to one of said plurality of positions on the surface of said target, each of said second intensity values being measured at a second height of said target, comparing said second intensity values with said first intensity values, such that said second intensity values and said first intensity values which were measured at the same positions on the surface of the target are compared, overwriting said first intensity values in said first memory with said second intensity values when said second intensity values are greater than said first intensity values.