Here at Mathews Optical we encounter a variety of methods for specifying aspheric lenses, from materials to dimensions and form accuracy. While there are standards in place for drawings which most everyone adheres to, many designers do not understand the manufacturing aspects of aspherics as it relates to the previously mentioned points. 

A. Material (Glass):  

A huge and diverse selection of glass is available to the designer. What we are seeing is a definite trend toward using higher index glasses. While this is a huge benefit to design work, many of the higher index glasses are very difficult to obtain a suitable grind prior to polishing, thereby lengthening the polishing cycle and creating more distortion on the surface. While the use of resin bond diamond wheels can greatly reduce sub-surface damage, they are not practical because of excessive wheel wear. Mathews Optical uses only metal bond wheels, thereby keeping the grinding process more cost-effective. However, metal bond wheels can create more sub-surface damage on the softer high-index glasses. A very effective method we have established is the “ working factor” method for determining the degree of difficulty for a particular glass type. Ohara Glass incorporates two very useful factors in specifying the physical parameters of these glasses, and through the benefit of cross-referencing, glass equivalents from other manufacturers can be used. These two factors are  Knoop hardness and abrasion resistance. By simply dividing the Knoop hardness by the abrasion resistance, a factor is obtained. After over twenty years of  using this method we have determined that if the factor is greater than six, the glass becomes more difficult to work. For example, S-LAM2 has a factor of 3.5 while S-LAH53 has a factor of 8.2. Therefore, if a client wishes to use a higher index glass in their application, S-LAM2 would be a more cost-effective choice if slightly stronger curvatures can be tolerated. Anything above a 6.0 becomes suspect. This is the “red flag” that tells us if it may be less costly and improve delivery if another glass is recommended. 

B. Design Changes to Reduce Cost and Improve Delivery Time: 

Occasionally a design comes across where the aspheric surface is designed on the concave side of the part and the opposite side is a convex sphere. As a concave aspheric is far more difficult and costly to process, especially if the curve is steep, we make the following suggestion: establish the aspheric concave side as the sphere and design all of the aspheric correction on the convex side. If this makes the curvatures more pronounced, then a higher index glass is suggested as previously mentioned in A. We have found that 70% of the time the design can be changed in this manner. The only two reasons that this typically cannot be done is, either the required correction simply cannot be accomplished in this manner or too much prior design work has already been done to the system to justify a full systems design change. The cost-savings can be substantial.  

Considering reverse-curvature surfaces (surfaces where the curve reverses itself), these are perhaps the most difficult, and therefore costly, to process accurately. To eliminate or reduce the severity of the inflection this recommendation sometimes coupled with a higher index glass is an effective option.   

C. Surface Tolerances: 

For us here at Mathews Optical, the preferred method of specifying the form accuracy of an aspheric is peak-to-valley in microns or fringes for power and irregularity. The drawing standards that are in place are quite effective in expressing this.  
 

Robert M. Mathews

R. Mathews Optical Works, Inc.

Poulsbo, WA