A method and apparatus for utilizing the benefits of encoded signal transmission and reception to enhance the performance of medical testing devices adapted to evoke and measure biological response signals such as auditory evoked potentials, and the auditory brainstem response signals in particular. The exposure apparatus includes a light source generating illumination light and a projection optical system. The first rotatable part has axially oriented slots that extend up to a free end face of the first rotatable part and that separate coupling segments of the first rotatable part from each other. The ribs are shaped to maintain the perforations open during stacking and shipping.