a laser diode applies a laser beam through a grating , a beam splitter , and an objective to a compact disc . a laser beam reflected by the compact disc is applied through the objective and the beam splitter to a photodiode . the objective has a large numerical aperture of 0 . 6 in order to be able of play back a digital video disc that has a thin substrate and contains information recorded at a high density . to prevent the photodiode from suffering aberrations caused due to the large numerical aperture of the objective , the photodiode has a relatively small photodetector unit having a normalized detector size ranging from 3 μm to 16 μm , for detecting only returning light up to a numerical aperture of 0 . 3 and not detecting light of greater numerical aperture values .