Rotation angle detector for a color wheel and a projecting apparatus using the same

The invention provides a rotation angle detector for detecting a rotation angle of a color wheel. The color wheel has an axial surface, which includes a plurality of differently colored blocks. The rotation angle detector includes an emitter and a receiver. The emitter emits an input signal to the axial surface of the color wheel, which reflects or transmits the input signal and then generates an output signal. The receiver receives the output signal, and the rotation angle of the color wheel may be judged by detecting the variation of the output signal.