An optical device includes a door, a door control unit, a polarized light generation unit and a spectrum response analysis unit. The polarized light generation unit and the spectrum response analysis unit are located at a first side of the door. When the door is opened by the door control unit, a polarized light from the polarized light generation unit is transmitted through the door and externally projected on an under-test object at a second side of the door, so that a scattered light is generated. After the scattered light is returned back and transmitted through the door, the scattered light is projected on the spectrum response analysis unit, so that the spectrum response analysis unit performs a spectrum response analysis. The optical device has enhanced signal-to-noise ratio. Moreover, the optical device is capable of acquiring more explicit and diverse inherent information of the under-test object.