A light emitting device includes a body, and a lampshade, a heat dissipation device and an adjusting plate are disposed on the body. The illumination device comprises a reflector cup, a blazed diffraction grating formed on the inner surface of the reflector cup, and a plurality of light emitting sources positioned linearly in a focal plane of the reflector cup. The test pattern areas are distributed across the entire printing width, and each of the test pattern areas partially overlaps at least one other test pattern area in the cross-process direction. In other embodiments, the capacitive structures may be implemented using fusible elements to isolate the capacitive structures in case of shorts.