of this pair have unequal sizes not only a first zero which corresponds to the desired focusing may be produced in the curve representing the variation of the focusing error signal as a function of the focusing error but also a second zero may be produced which does not correspond to the desired focusing then there is a risk that the focus servo system of the apparatus adjusts the focus of the scanning beam above or below the information plane 2 according to the invention the detector geometry shown in perspective in fig2 and in a plan view in fig4 is used in the composite detector 10 of fig4 the angle u03c6 is equal or substantially equal to zero in other words the separating strips 22 and 23 are parallel to each other and to the separating strip 24 and the connection line cl a less stringent requirement is then to be imposed on the distance y t between the points m and 0 moreover the apparatus is then less sensitive to a tilt of the composite detector 10 about the y axis in fig4 the reference symbol w denotes the overall width of the composite detector 10 and s denotes the nominal distance between the radiation spots v.