What is a 'typical' lens design process?

Published by Optics for Hire.

OFH works on many different types of programs, each with unique goals and deliverables but as an illustration here is a breakdown of a lens design project from 2012.

Our client produced a very low cost diagnostic microscope to view saliva with 30x magnification. They came to OFH to help design a new product with higher magnification.

Production cost and volume dictated that injection molded plastics would be used for manufacturing of the device and lens elements. Having 50x magnification was the most important new specification but before starting design, we worked with the client to define all specifications. To us this means requirements that a.) don’t break the laws of physics, b.) could be produced using existing manufacturing technology at a low cost and c.) described a product would meet market demands..

The initial specifications included:

  • Visual Field of view 30°-40°

  • Object circle diameter .6mm-3.6mm;

  • Focal length of the eyepiece – 5mm-6mm;

  • Diameter of the eyepiece (mechanical) – 25mm-35mm;

  • Length of the eyepiece – 20mm-30mm;

  • Eye relief – 4mm-5mm; (defined as distance between viewer pupil and last element)

  • Pupil diameter – 2mm-3mm. (defined as visual location plane of user eye)

  • Depth of field – +/- 0.025 mm (theoretical value, actual value determined in design)

  • Fixed Focus

  • Resolution- TBD testing required

  • Distortion- TBD testing required

Together we also discussed the expected costs for prototyping and production and how those costs might be impacted by the desired specifications.

The client was not able to specify two key performance requirements - resolution and distortion , so we began the program with testing of a product the client felt had ‘good enough’ performance and then in our lab measured that product’s resolution and distortion .

The project task list looked like this:

  • Testing of existing products,

  • Optical design of the eyepiece using Zemax,

  • Tolerance study

  • Opto mechanical design of the eyepiece,

  • Development of manufacturing documentation

  • Prototyping and then transfer to manufacturing

Task 1 Testing

In our lab we tested the sample lens. The slide below shows a measurements of resolution using different green and white light.

Measurements of resolution using different green and white light.

Measurements of resolution using different green and white light.

Since we knew the client felt what ever the sample lens produced was good enough, we now had the full requirements

Task 2 Design of lens

Using Zemax we modeled different optical concepts for a new eye piece. We also looked at how image quality would be improved if a diffractive optical elements (DOE) was used in the layout.

Improving image quality with a diffractive optical elements (DOE)

Improving image quality with a diffractive optical elements (DOE)

Task 4 opto mechanical design

The tolernacing of the elements and the opto-mechanics go hand in hand, there are trade offs that can provide looser ( less assembly cost) opto mechanical tolerances and tighter (more expensive) lens element tolerances and vice-versa. At this point in the product we know the basic design of the elements are are presenting two different opto mechanical layouts. One uses gluing, the other springs. This is a the stage of the project where the input of the lens manufacturer becomes more important. Their feedback on preferred assembly methods will determine the design path. Below are two of the three concepts OFH presented.

lens assembly
From the elements design to to two different opto mechanical layouts: glue or springs

From the elements design to to two different opto mechanical layouts: glue or springs

Task 5 Documentation

After discussions with manufacturers, a design approach was selected and OFH prepared design documentation suitable for prototyping. One if the lens element drawings is shown below. Note this is not the full drawing

A design approach was selected and we prepared design documentation suitable for prototyping

A design approach was selected and we prepared design documentation suitable for prototyping

Task 6 Prototyping and transfer to manufacturing

At this point the project moved from ‘typical’ to ‘atypical.’ The client decided they no longer had the funds needed to prototype with diamond turning or for the expenses related to making a custom tool for molding. The market opportunity they saw for the new 50x lens, did not turn out to be as large as they expected and would not support the additional cost required to move from a 30x lens to 50x lens and so the project was shelved..

This was frustrating for everyone involved. We’d love to see every design we work on go to production and reach the market, but it doesn’t always happen that way. Corporate priorities change, managers and internal cheerleaders take new jobs. The needs of our client’s and at the customer’s of our clients are never static. It’s part of the life of being in the business of contract optical engineering.