We often see customers ask for our help on understanding wavefront calculation in OpticStudio, for example, why wavefront error displayed in Wavefront Map differs from what I get when adding optical path lengths manually. To understand wavefront calculation in OpticStudio, there are two important things user needs to be aware of. 

First, by default the Reference OPD is set to “Exit Pupil” in System Explorer/Advanced section. This means the wavefront is calculated at the Exit Pupil, not the Image plane as some may assume. You can find this Reference OPD setting explained in the help file at The Setup Tab > System Group (the Setup Tab) > System Explorer > Advanced Options (System Explorer) > Reference OPD.  

When OPD Reference is set to Exit Pupil, OpticStudio first propagates rays all the way to the image surface, then back propagates to the Exit Pupil, and the OPD or Wavefront Error is captured at the Exit Pupil instead of the Image Plane. Users can change the OPD reference from Exit Pupil to Absolute or Absolute 2 which uses Image Plane as the OPD reference location. That said, changing this setting will have an impact on the calculated wavefront error. We recommend reading through the above help documentation carefully before making any changes. For most focal systems, Exit Pupil should still be used as the OPD reference. 

Second, the Wavefront map displayed is not the raw accumulated optical path length, but rather it’s the wavefront error, or the optical path length difference between a pupil ray and the chief ray. OpticStudio uses the chief ray path length as the OPD reference. For focal system, a reference sphere at the Exit Pupil is created as the reference. Imagine your system outputs a perfect spherical wavefront, meaning converging rays come to a perfect point focus, you will observe a flat wavefront map of 0 wavefront error, due to the spherical reference used. For Afocal system, OpticStudio uses a planar reference. User can switch between spherical and planar OPD reference by turning on and off the Afocal Image Space box in System Explorer.