Figure 4. Defining the anterior sector (AS). An orthographic projection of an anterior lens quadrant is generated. A circle (1) of diameter D is fitted by eye to the arc of the quadrant. A square (2) with sides equal to D is fitted around the circle. The intercept of diagonals (3 and 4) defines the center of the circle. The fiduciary nucleus, d, in the constellation a, b, c, d is identified in the projection. The distance between d and the center of the circle is measured and used as the radius of a second, smaller circle (5), concentric with the first. An anterior sector (yellow) is drawn in the smaller circle, with a central angle (θ) of 60°.