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- How to Design Progressive Lenses
How to Design Progressive Lenses
- By Mark Nicholson
- Published 31 July 2006
- System Modeling
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Analyzing the Surface
The Shaded Model of the lens shows that the surface sag is very complex:
Note that the lens tends to "run away" at the edges where there are no rays to provide control. This is typical of free-form design: either a ray or some other form of constraint needs to be applied over the whole surface to prevent unrealistic sags from being produced.
Now with such a complex surface, simple fans like ray-fan and OPD plots are not enough to describe the performance of the free-form optic. For this optic, we use Analysis -> Miscellaneous -> Power Field Map. Set to show contours at an interval of 0.25 diopters, we can see the spherical and cylindrical power added by this surface over the whole field of view:
Now this plot uses different definitions of power and EFL than the strictly paraxial concepts used elsewhere in ZEMAX. This feature computes optical power or focal length as a function of field coordinate. The power or focal length is determined for the optical system as a whole up to and including refraction from any surface. The method used is to trace a ring of real rays around the entrance pupil at each point in the field. The ray data are used to determine the focal length for each field position. This focal length can then be used to compute the optical power in units of diopters (inverse meters). In the general case, the focal length is a function of orientation in the entrance pupil. By tracing a ring of rays, the average, maximum, and minimum optical power or focal length around the pupil can be determined. From this data, several types of optical power can be computed. The feature can display:
Note that the lens tends to "run away" at the edges where there are no rays to provide control. This is typical of free-form design: either a ray or some other form of constraint needs to be applied over the whole surface to prevent unrealistic sags from being produced.
Now with such a complex surface, simple fans like ray-fan and OPD plots are not enough to describe the performance of the free-form optic. For this optic, we use Analysis -> Miscellaneous -> Power Field Map. Set to show contours at an interval of 0.25 diopters, we can see the spherical and cylindrical power added by this surface over the whole field of view:
Now this plot uses different definitions of power and EFL than the strictly paraxial concepts used elsewhere in ZEMAX. This feature computes optical power or focal length as a function of field coordinate. The power or focal length is determined for the optical system as a whole up to and including refraction from any surface. The method used is to trace a ring of real rays around the entrance pupil at each point in the field. The ray data are used to determine the focal length for each field position. This focal length can then be used to compute the optical power in units of diopters (inverse meters). In the general case, the focal length is a function of orientation in the entrance pupil. By tracing a ring of rays, the average, maximum, and minimum optical power or focal length around the pupil can be determined. From this data, several types of optical power can be computed. The feature can display:
- spherical power,
- cylinder power
- maximum and minimum power
- tangential and sagittal power
- x or y direction optical power
in diopters. Additionally it can display the same data as effective focal length (EFL) in lens units.
These plots are extremely useful in understanding how power is distributed over a freeform surface.
In addition the POWF optimization operand allows direct optimization of any of the terms computed by the Power Field Map at any point. This is vital when a known desired power map is required on a surface.