Using vendor-generated stock lenses in a design is usually more cost-effective than using custom lenses. That is why OpticStudio has the Stock Lens Matching tool, which will swap out lenses in a design for the nearest equivalents in the selected vendor catalogs.

Since most stock lenses are plano-curved or equi-curved, this tool works far better in systems with modest field angles and narrow wavelength ranges. It is also due to this reason that any asymmetric bi-curved lenses should be replaced with two plano-curved lenses when using this tool.

As is shown in this blog post, when the lens is swapped, the system may need to be refocused and some merit function constraints may need to be relaxed or removed - particularly ray height operands.

Stock Lens Matching example

Consider the system shown below. This is the design of a 5x beam expander that has approximately λ/10 wavefront error.

In the initial sample file, the design was optimized using one radius of curvature of each lens (making the lenses plano-curved) and by adding a REAY operand that targets the marginal ray to land at a radial height of 12.5 mm (giving a 5x magnification). The lens-to-lens distance is fixed at 200 mm to yield a reasonably compact design. If you simply run the Stock Lens Matching tool with the design as currently set up, the performance will degrade substantially, and you will be disappointed!

To prepare the system for stock lens matching a couple of changes should be made:

1. The lens-to-lens spacing (thickness of surface 3 made a variable so that as OpticStudio swaps the lenses out for catalog ones, it can refocus the gap to compensate the small changes of effective focal length:
   
   
2. The merit function constraint REAY has been modified to allow a small range of allowable magnification:
   
   

Now, Stock Lens Matching tool can be run with the default settings:

OpticStudio will find the closest equivalents for each lens individually and find the best combination of those chosen lenses.

Since "Save Best Combination" was selected, the optimal system will be saved to the same directory as the current lens file and is identified by the ‘_SLM’ suffix on the filename. In this case the OPD is only very slightly worse with stock lenses used.

The magnification has also changed slightly (the marginal ray height is 12.4950 mm instead of 12.5, so the magnification is 4.9980 instead of exactly 5).

This decrease in performance is expected! Assuming your starting position is well optimized, this tool will always make performance worse. This is because the design will now be constrained to using only catalog values of parameters, rather than the absolute optimum. Your starting design should therefore be well within the required specification before running the tool.

However, when used effectively, the Stock Lens Matching offers the potential to make great cost savings in the finished design by eliminating the need for custom manufacture.

To access the samples files shown in this post, please view the full article here.

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Author:
Mark Nicholson