A tool includes a driving member with a protrusion. A circumferential closure is provided for constricting the inner layer of the envelope and thereby adjusting a circumference of the inner layer of the envelope. The interference light is incident on an image sensor only when an inclination angle of a diffraction grating plate and a wavelength of the interference light satisfy a predetermined condition. 0 mm, and a surface hardness of the intermediate layer is higher than the central point hardness of the center by 20 to 50, and the hardness difference between the cover hardness and the surface hardness of the intermediate layer is within the range of 0 to 12. The at least one substrate has a cavity formed in an exterior surface of the at least one substrate and extends radially inward. The magnitude of the second tracking error signal is then subtracted from the magnitude of the first tracking error signal to obtain a difference value.