The difference between the virtual light source positions of beams in parallel and perpendicular directions to the junction is the astigmatism. It is the one of the simplest method for detecting the virtual light source position to measure the beam diameter at the point closest to the laser end and to find the position where the beam diameter is minimized while moving the laser diode.
The astigmatism of a laser diode produces large effects on its imaging performance. Therefore, it is particularly an important item for readout operation of optical discs with high recording density.