Abstract: A method to manufacture a hairspring of a timepiece movement, which includes: producing a malleable elongated element in a form of under a fiber or ribbon form, from a first heated material capable of guiding light; conforming the malleable elongated element in order to achieve into a spiral form; and handling processing the spiral form thus created in order to obtain a hairspring for providing both a mechanical oscillating function in a balance wheel and a light guiding guide lighting function arranged for in situ adjusting of a mechanical performance of said hairspring. The conforming includes coiling the malleable elongated element around a rotating mobile conformation tool, and receiving the malleable elongated element in a guiding channel within a guiding mechanism and guiding the received malleable elongated element via mobile equipment turning on an inner periphery of the guiding mechanism.