1. A method of calculating coordinate data of an object, comprising the steps of: providing a light projecting device comprising a mirror surface configured to generate a reflection of an object; providing an image sensor for capturing an image of the object and an image of the reflection, wherein a height of the image of the reflection is less than that of the image of the object; determining an exposed portion of an edge of the image of the object within an overlapped image formed by the image of the object and the image of the reflection when the overlapped image is detected; and providing a light filter device covering a portion of a light receiving surface of the image sensor and configured to block light of a first frequency spectrum, wherein the light projecting device comprises a light filter member disposed on the mirror surface and configured to let the light projecting device reflect light of the first frequency spectrum.