Calendar

International Optical Design Conference
June 14-16, 2010
Jackson Hole, WY
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OPTATEC
June 15-18, 2010
Frankfurt, Germany
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SPIE Optics and Photonics
August 1-5, 2010
San Diego, California
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Introduction to CODE V
September 13-17, 2010
Pasadena, California
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Advanced Topics in CODE V
September 20-24, 2010
Pasadena, California
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Introduction to CODE V
November 2010
Munich, Germany
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Advanced Topics in CODE V
November 2010
Munich, Germany
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For a complete list of CODE V events worldwide, visit our Web site:

• USA
• Asia
• Europe

Quick Tip—Modeling Fresnel Lenses in CODE V

Fresnel lenses are commonly used in illumination applications. Some classic examples are the condenser of an overhead projector, where the Fresnel lens is used to image the lamp onto the projection lens, and a lighthouse lamp or stage light where a somewhat collimated beam is delivered. Fresnel lenses are less common in imaging systems, unless the image quality requirements are fairly modest and stray light is not of great concern.

Fresnel Lens Example as Used in Stage Light

There are two methods for modeling a Fresnel lens in CODE V:

• Infinitesimal-step model
• Finite-step model

The infinitesimal-step model is useful when:

• You want to do a quick investigation
• The step height or zone widths on the Fresnel lens surface are small enough to be insignificant.
• The Fresnel lens is curved (curved "substrate," as it is sometimes called.)  Most Fresnel lenses are flat, such as those used in an overhead projector.

To define an infinitesimal-step Fresnel surface:

1. In the GUI, select the Thin Fresnel surface type in the Surface Properties window (Lens menu). Alternatively, on the command line, enter the ASP command to define the surface as aspheric.
   
   

   
   

2. Set the Fresnel Surface Curvature (CUF command) to a non-zero value.  For a flat Fresnel substrate, use a small non-zero value (e.g., 1E-10).

While the sag of the Fresnel surface is defined by the Fresnel Surface Curvature (CUF), the power of the surface is computed from the surface radius/curvature (RDY/CUY) and its aspheric coefficients (K, A, B, C, D, etc.).  The ray tracing process uses only the Fresnel Surface Curvature to compute where a ray intersects the surface, and then uses the computed surface slope based on the surface curvature and aspheric coefficients to determine the angle of incidence and direction of the refracted ray.  Physically speaking, this independence of sag and slope is like having facets on the surface, except the facets are infinitesimally small.  No accounting is made for the depth of the facets.

 Lens Drawing of Thin Fresnel Example

CODE V has three surface types to represent Fresnel lens surfaces with finite steps:

• Fresnel Planar Substrate (SPS FRS): This is a flat-substrate Fresnel surface with a fixed step depth, or Zone Sag (SCO ZONESAG).  The surface shape is the same as the aspheric surface (ASP), except a step occurs every time the sag reaches a multiple of the fixed step depth.  The depth of the first step (closest to the center) can be made different from all the remaining steps, if desired. The vertical step surfaces or "back cuts," which are effectively cylinders concentric to the Z axis, are not modeled.  Typically if you change an infinitesimal-step Fresnel surface to this finite step form, you need to adjust (optimize) the aspheric coefficients slightly.

Lens Drawing of Fresnel Planar Substrate Example

• Fresnel Parallel Back Cut (SPS CN1):  This is a finite-step Fresnel surface with cylindrical power in the Y meridian.  The substrate can be flat, cylindrical (curved in the X meridian), or conical (curved in the X meridian, but radius depends linearly on Y).  The back-cuts are planes, all parallel to the XZ plane, and are included as part of the surface, so that stray light paths can be investigated.

• Fresnel Non-Parallel Back Cut (SPS CN2): This is similar to the SPS CN1, except the back cuts are conical, with the axis of the cone parallel to the Y axis.

The SPS CN1 and CN2 surfaces are commonly used for beacons, airport lighting, lights at the top of a ship's mast, etc., like the one pictured below, which would have parallel back cuts (CN1): 

Airport Runway Beacon

Tips on ray tracing and drawing Fresnel Planar Substrate surfaces: Because of the fine facets on the Fresnel surface (SPS FRS), be sure to increase the number of rays when tracing systems with these surface types (e.g., RIM; NRD 50) and increase the resolution factor (LRS) when drawing the lens (Display > View Lens menu, Lens Drawing tab).

Updates to the Customer Support Web Site

The March 2010 edition of the CODE V Enews, which features  the new Glass Expert Macro (simulates the process an experience optical engineer would use in selecting glass) and new user-defined surface (UDS) forms of the Forbes polynomial aspheres, has been added under CODE V Support > CODE V Enews & Tips.

2010 Regional CODE V User Group Meetings – Limited Space Still Available

Limited space is still available at this year’s regional CODE V User Group Meetings.  Following are the meeting dates and locations.

Topics will include:

• New Features in CODE V 10.2
• Beam Synthesis Propagation (BSP) Case Study
• Glass Expert Macro
• Tips from Tech Support
• ORA Engineering: Designing with Aspheres
• Approaching the 2010 IODC Design Problem in CODE V
• CODE V Future Plans
• Open Forum

If you haven't already registered for one of these events, there's still time.  We hope to see you there.