A system includes a light emitting unit, a front mirror, a rear mirror, an imaging unit and a processor. The light emitting unit is configured to emit a collimated light beam. The front mirror is configured to reflect part of the collimated light beam to produce and project a front focused ring of structured light to an object to obtain a front reflected ring of light, and configured to allow part of the collimated light beam to pass by. The rear mirror is positioned downstream of a light transmitting path of the front mirror. The rear mirror is configured to reflect at least part of the collimated light beam passing by the front mirror to produce and project a rear focused ring of the structured light to the object to obtain a rear reflected ring of light.