A diffuser sheet which compensates for chromatic dispersion generated in a hologram light guide plate, and an illumination apparatus employing the diffuser sheet are provided. Thus, the helical groove is extended through the whole length of each of the two arc-shaped plates of the rotation bar, so that each of the two brush strips is extended through the whole length of each of the two arc-shaped plates of the rotation bar without interruption, thereby enhancing the cleaning effect of each of the two brush strips. A first electrode of a parallel piezoelectric resonator 12b is connected to a connection point between the series piezoelectric resonator 11 and the output terminal 15b, and a second electrode of the parallel piezoelectric resonator 12b is connected to a first terminal of an inductor 13b. The light rack assembly can be quickly removed from the vehicle, or be mounted such that the assembly can be rotated to adjust the light beams while the vehicle is in motion and rotated to retract the assembly to protect the lights when not in use.