Patent ID: 8253094

Claim:
An aperture unit, comprising: a cylindrical member defining a cylindrical chamber therein, a light input end portion of the cylindrical member being transparent, and an opposite light output end portion of the cylindrical member being transparent; a magnetic fluid received in the cylindrical chamber, the magnetic fluid comprising a transparent solvent, a surfactant, and a plurality of magnetic nano-particles dispersed in the solvent, each of the magnetic nano-particles enveloped by the surfactant; a heater embedded in the cylindrical member for enhancing the diffusion speed of the nano-particles such that the nano-particles are dispersed in the solvent in an initial state; and a magnetic field generator positioned on one side of the cylindrical chamber and configured for generating a magnetic field applied to the chamber and attracting and clustering the nano-particles such that an annular nano-particle opaque layer is formed on an inner surface of the cylindrical member in a second state.