Calendar

SPIE Photonics West 2008
Booth 1501
Jan. 19-24, 2008
San Jose, CA
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Introduction to CODE V
March 31-April 4, 2008
Pasadena, California
Click here for details and to enroll

Advanced Topics in CODE V April 7-11, 2008
Pasadena, California
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Quick Tip –Variable Stop Surface Location

For complex lenses with internal stops, the best position for the stop may not always be obvious.  To help determine where the stop should go, it is possible to let the stop "float" inside the lens during optimization, and later designate or insert the stop surface, after the optimization converges.

The technique used to float the stop is to have a dummy stop surface (surface with air on both sides) defined as the first surface in your lens.  This dummy stop surface — it’s really the entrance pupil — should have a variable thickness which will typically be negative; this is the distance to surface 2, the first real optical surface.  A good starting location for the dummy stop surface might be the current location of the entrance pupil, which can be obtained using the ENP macro database item, or from the first order data listing (Display > List Lens Data > First Order Data, or the FIR command).

For example, let’s use the wide angle sample lens (WIDEANG.LEN) supplied with CODE V (File > New > CODE V Sample Lens…, or at the command prompt: RES CV_LENS:WIDEANG).  We’ll insert a dummy stop surface at the entrance pupil location by defining its thickness using the macro database item ENP.  To do this in the GUI (the LDM):

1. Right click on surface 1
2. Select Insert, and hit Enter
3. Right click on the new surface 1, and select "Set Stop Surface"
4. Enter "=-(ENP)" as the surface thickness:

Or, at the command prompt: INS S1 0 –(ENP); STO

A lens drawing of the lens with the inserted Stop is shown below.  To get a better looking drawing, we’ve extended the entering rays further to the left.  (In Display > View Lens, on the Ray Properties tab, check the Extend rays on lens entry checkbox, and enter a value of about -150; the equivalent command in VIE is ERI -150.)  Note the pupil aberration at the current internal stop location — the chief rays don’t all cross the axis at the location of the original internal stop.

Now you're ready to optimize the lens.  During optimization, you must include a negative center thickness constraint on the stop surface (CT S1 < 0) in order to override the minimum air thickness general constraint (MNA) on the stop surface.  This allows the stop surface (now the entrance pupil) to maintain a negative thickness so that it can float inside the lens.

A drawback of this technique is that the spherical aberration of the pupil is not included in the optimization.  After optimizing, determine which surface is appropriate for the real stop location by drawing the lens, or tracing chief rays (RSI R1 Fj) in order to determine where the chief rays cross the optical axis.  Set the stop to this surface (or at an inserted surface), recalculate your vignetting factors (SET VIG), and re-optimize to account for the pupil aberration.

Updates to ORA's Customer Service Web Site

The following additions have been recently made to the CODE V Support section of www.oraservice.com:

• The December '07 issue of CODE V Enews, which featured a design study of  CD/DVD Pickup Units, is available under CODE V Enews & Tips.
  
  
• A new FAQ on modeling beam splitters in CODE V has been added under Tech Support FAQs > Lens Modeling FAQs.