Hier ist ein Überblick über die Leseeinrichtung mit den Sensoren A,B,C,D und E,F:
http://www.andrew.cmu.edu/user/cchao...heCDsystem.htm

Fig. 2-5 If the disk is too close, quadrants A and C will get more light than quadrants B and D [18]

Fig. 2-6 If the disk is too far away, quadrants B and D will get more light than quadrants A and C [18]

When focusing is done, the optical head has to be kept on track by the tracking servo control system, ...





Der CD code und die Reflexion sind auch hier beschrieben:
http://www.laesieworks.com/digicom/Storage_CD.html

The typically used laser for CD players is an AlGaAs laser diode with a wavelength in air of 780 nm (± 10 nm).
The wavelength inside the polycarbonate is a factor of n=1.55 smaller (about 500 nm). The pit is fabricated so that it is a quarter of a wavelength high.
The idea here is that light striking the land travels 1/4 + 1/4 = 1/2 of a wavelength further than light striking the top of the pit. The light reflected from the land is then delayed by 1/2 a wavelength, and so is exactly out of phase with the light reflected from the pit. These two waves will interfere destructively, so effectively no light is reflected.