How an Aspheric Lens is Made

Edmund Optics Singapore manufacturing plant operates 24 hours a day and produces thousands of precision aspheric lenses each month. Watch as we follow a precision CNC polished asphere through the entire manufacturing process. The lens we will be following is a 532-nanometer laser grade aspheric lens. First, raw glass material is pressed into the approximate shape of the lens. This minimizes time spent removing material later in the process. The first of several grinding steps is curve generation: a rough grinding process that produces the general spherical curvature of the lens. A rotating abrasive cup tool is used to mechanically remove material and form the best fit spherical radius on both sides of the lens. The radius of curvature is checked and controlled using a spherometer. The spherical or planar reverse side of the lens is polished before moving on. To prepare for computer numerical controlled, or CNC grinding, the spherical part must be attached to a metal holder in a process known as blocking. The subaperture asphere grinding tool containing small pieces of diamond is used to remove material and begin forming the aspheric surface. In-process metrology is used to monitor the center thickness and aspheric surface profile between grinding steps. The in-process data is fed back to the CNC grinder so that it can self-correct between each step and produce the desired lens shape. Each grinding step utilizes progressively finer diamond pieces. The next step in the process is CNC polishing. Asphere polishing pads and slurry are used to remove subsurface damage and convert the ground surface to a polished one. In-process metrology is used to make small corrections between CNC polishing steps. Final aspheric surface corrections are made using Magneto Rheological Finishing, or MRF, to reach diffraction-limited performance, or a Strehl ratio greater than 0.8. MRF is a deterministic fine finishing process using a precise interferometrically documented subaperture tool to correct surface errors by selectively removing material under a controlled and predictable process. The asphere continues this process until it achieves an RMS aspheric surface tolerance of 140th of a wave. Edmund Optics uses a variety of metrology techniques to verify aspheric surface tolerances including non-contact 3D profilometer, computer-generated holograms, and stitching interferometry. In this case, stitching interferometry is used. Next, the lens is centered by grinding its outer diameter so that the mechanical and optical axes are aligned. The lens is then cleaned using ultrasonic cleaning. This asphere is intended to be used with 532-nanometer lasers and so must be coated with a 532-nanometer anti-reflective V coating to maximize transmission. the asphere is inspected to guarantee that it meets all other performance requirements including surface quality. The manufacturing process is now complete, and the lens is ready to be shipped to the customer. To learn more about Edmund optics manufacturing capabilities go to edmundmanufacturing.com.