Selecting the correct optical material for designing precision medical optics is a critical decision for optical engineers and designers. Determining whether to use glass or polymer material can have a significant impact on imaging performance and manufacturing costs. Precision glass molding (PGM) or moldable glass optics is a hybrid variation; leveraging polymer molding type manufacturing with raw glass material.
The technique has been available for over 30 years and is finding more and more applications in high precision optical systems. With a wider selection of moldable glass types, better optical characteristics, ability to create aspheric and complex geometries and advances in tooling have enabled production of precision molded glass optics to address a wider range of applications. When is glass molding the optimal choice for your medical optical system? What parameters drive the selection process?
Understand the Product’s End Use
Are you designing a single use endoscope or a robotic endoscopy system with sophisticated imaging requirements? Will the device be used in a surgical or clinical environment? What are the forecasted annual product volumes? Will the device need to withstand various sterilization procedures for multiple use? In all cases, the optical design meets a set of determined performance requirements and constraints driven by the end use of the product.
“There are many different characteristics to consider in terms of the life expectancy and survivability of each component in optical sub-assemblies and systems used in external as well as invasive use medical products,” explains John Deegan, Director of Technology, Research and Development at Rochester Precision Optics in Rochester NY. “It is wise to think in terms of whether the end product will be disposable or reusable and how we treat them along with the materials we’re selecting.”
Plastic or polymer optics are common in high volume, weight conscious, cost sensitive products requiring high performance with unique capabilities. Glass optics typically provide a better surface finish, are more durable, and can withstand harsher use conditions. Many medical products requiring high end imaging – such as in Ophthalmology and Robotic Endoscopy procedures – specify glass optics for these reasons.
“Material choice has cascading implications across the entire system, impacting design, performance, tooling investments and end product stability,” explains Wallace Latimer, VP US Sales and Marketing at Fisba USA. “Multi-Use vs single-Use coupled with performance expectations of the users need to be balanced. Aspherical vs Spherical, sealing, resolution, survivability, DOV offset repeatability/performance, illumination approach, these are just a few topics that can influence material selection.”
Design & Manufacturing
The majority of optical technology used in medical applications convey critical imaging information. Overall, your optical system must deliver the best transmission characteristics and imaging quality. Minimally invasive medical devices are continually shrinking in size creating optical design and component integration challenges. This is a good example of how aspherical molded glass optics can reduce lens elements, weight and size of a device.
Tara Holloway, with Rochester Precision Optics adds that “molded aspheric lenses offer an alternative design approach that nets significant return, especially in multi-element lens assemblies. A single molded glass asphere can replace multiple elements; reducing cost, weight and overall footprint of a lens assembly. If the performance requirements and environmental conditions are amenable, using a molded polymer asphere will further reduce the cost and weight.”
Additional decision drivers include: Integration of mounting features or multiple optical surfaces—one of the primary advantages of plastic injection molded optics—is also possible with lenses formed from PGM, unlike ground and polished lenses. Other shapes and features are commonly molded in glass such as lens arrays, stepped features, diffractive surfaces, and noncylindrical outer surfaces. Glass can also be molded directly into metal housings with PGM.
Optical Component Cost
Cost is always a prime consideration for optical component and sub-system selection. Tooling costs not-withstanding, the cost of individual plastic optics can be low. Tooling costs for injection-molded plastic and glass molded optics vary greatly, with more complicated optics requiring more expensive tooling. The cost gap between PGM and plastic optics has narrowed with the difference being in the processing and not the raw material.
When it comes to cost, Wallace believes that, “Having a very low-cost solution will certainly gain attention and fit to a subset of application needs, however, it is not a one size fits all. Asking challenging questions about the entire uses case and customer experience is necessary to define the right strategy. Driving your decision primarily on the perceived cost implication of either approach (Glass or Plastic) can lead you down an unrealistic path.”
Optical engineers and designers have a choice when determining whether to design their optical system with glass or plastic. Addressing questions about the intended products application and end use will determine the optimal balance between material selection, design, manufacturing and cost.