The Default Merit Function (DMF) Tool, under Tools > Default Merit Function in the Merit Function Editor's menu, builds merit functions that are evaluated at the current image surface (i.e. the last surface in the Lens Data Editor). For example, the RMS spot size DMF evaluates the spot size at the last surface and the RMS wavefront DMF at the exit pupil of the current system with the last surface being the image. 

The IMSF optimization operand allows the default merit function to be calculated at any surface in your system. In the included sample file, let's suppose that we want to optimize for minimum RMS spot at surface 3# while at the same time achieve best collimation at the image surface (#6). This can be done using the default merit function tool and the IMSF optimization operand. The variables in this system are the radii of curvature of surfaces #2 and #5 and the conic of surface #2.



If you are not familiar with optimizing with the default merit function, please refer to the following knowledge base article.

http://www.zemax.com/kb/articles/20/1/Designing-A-Singlet-in-ZEMAX

To learn about the IMSF operand, open the included sample file and follow the steps below. The file can be downloaded from the last page of this article.

1) Open the Merit Function Editor and build the default RMS spot radius merit function.



Insert an IMSF operand before the DMFS and specify surface #3 as the surface on which the default RMS spot radius merit function is calculated at.



Insert a blank line after the last operand (BLNK as operand #10)

Insert a new RMS angular radius default merit function starting from line 10.



Insert another IMSF operand before the second DMFS and set the surface parameter to 6 the image surface.



This merit function will optimize for minimum RMS spot size at surface #3 and best collimation, i.e. minimum angular spot radius, at surface #6 (image). 

Click on Tools> Optimization> Optimization and press the Automatic button. You should get the following result.




ZEMAX has optimized at two surfaces, with two different criteria, simultaneously. With multiple IMSF operands in the merit function, you can optimize at several intermediate surfaces at the same time with the same merit function, which can contain different criteria for each surface.

There are a few things to be careful of when changing the image surface value.

First, if the field type is either real or paraxial image height, the field type is changed to angle or object height for infinite or finite conjugate systems, respectively. The angles and heights used correspond to the primary wavelength chief ray angles and heights as computed for the unaltered system.

Secondly, imagine a system where you set the image surface to be prior to the currently defined stop surface. ZEMAX moves the stop surface to a (possibly virtual) dummy space prior to the existing surface 1. Unless the system aperture is object space numerical aperture or cone angle, the system aperture is changed to entrance pupil diameter, and the aperture value is set equal to the original paraxial entrance pupil diameter computed for the original stop position. Note this assumption might not be valid for systems that require ray aiming.

An alternative method for computing system performance on multiple surfaces is to use multiple configurations. In the MC editor, click on Tools >  Make Conjugate. This tool lets you re-define the object, stop and image surfaces, plus make any desired changes to system aperture, field definitions, use of ray-aiming etc that you need.

This article has demonstrated the use of IMSF operand for intermediate surface optimization. The default merit function is evaluated at an intermediate surface the same way OPD or Ray fan is computed at an intermediate surface. Care must be taken when using IMSF operand, just like when evaluating OPD or Ray Fan at intermediate surfaces.