Two-axis angular displacement errors such as pitch and yaw of a flat mirror target can be detected by the laser autocollimation method [1]. This optical angle-detective method based on optical lever is easily satisfied the key requirements for applying to the ultra-precision profile measurement such as flatness measurement of large silicon wafers [2-3], or measuring a dynamic angle motion errors such as precision stage movements, since the system has advantage of its high sensitivity and quick response. However, roll-directional angle error can't be detected by simply applying this method. In this paper, we describe simultaneous measurement about three-axis angle detection. This method improves the laser autocollimation method from two-axis measurement to threeaxis measurement by using a diffraction grating as the target mirror. Two prototypes (Prototype I, Prototype II) of the three-axis angle sensor were designed and fabricated. In Prototype I, the three-axis angular measurement of the target was carried out by sensing the displacements of the 0th-order and ±1st-order diffraction beam spots on the focal plane of the objective lenses. The 3-axis angle measurement methods were categorized into three types according to the behavior of the spots on the focal plane and each of them has been evaluated by the experiment.
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