Reply To: RMS spot size interpretation

[Ethan Keeler]

Hi Jurgen,

The reason that the values are somtimes negative is that they are not directly representing the RMS in X or in Y. Instead, they are measuring the transerse aberration in X (TRCX) or Y (TRCY). The ray deviation for this aberration measurement can be positive or negative depending on where the real ray lands on the Image plane.

This Default Merit Function has been set such that the overall Merit Function value, shown at the top of the window, represents the RMS spot (X+Y). This value will always be positive. When you run the optimization, the optimizer looks to move this overall value as close to zero as possible by correcting abberations with the variables you set in your system. In your case where you are simply looking at the values calculated, then you'll want to consider the overall value, and  not the individual values of each TRCX/TRCY operand.

Ethan