Lambert cosine distribution for 2D systems.
For a 3D system the probability that the angle to the normal is theta is proportional to cos(theta), see note.
When the lambert distribution for secondaries is called for in the CPO2D program, the non-meridional mode is automatically activated at the time that that each secondary is created.
In the CPO2D programs only a cut through the system is shown on the screen. Therefore a ray that appears on the screen represents a set of rays that have the same distance from the axis but are rotated around the axis. So a ray that appears on the screen emitted at an angle theta to the normal has a distance from the axis proportional to sin(theta). When it is rotated around the axis it traces out a circle of radius proportional to sin(theta). So in a Lambert distribution the effective probability is proportional to sin(theta)*cos(theta), which is the distribution used in the CPO2D programs for cathode rays and secondary electrons.