Metrology Featured Article

Nobody can afford to dissatisfy their customers

Closing the capability gap of standard metrology tools

It is paradoxical that a high precision industry that uses measurement units such as nanometers, angstroms, and fractions of waves of light to describe surfaces should be willing to settle for uncertainty and low levels of accuracy in many of the parts it produces. And yet today, most optical shops can only measure 60-70% of their optics over a full aperture and the optics industry has tolerated this level of uncertainty for many years. There is no other high-tech industry that would accept this incomplete level of workpiece characterization. The lack of comprehensive, full-aperture metrology leads to unpredictability in manufacturing, lengthy cycle times, poor yields and can ultimately cost tens of thousands of dollars when shipments fail to meet customer specifications. And as we struggle in uncertain economic times, no one can afford to dissatisfy their customers, or even turn down the high margin orders because of a lack of confidence in manufacturing ability.

Many people in the optics industry just accept 60-70% because they believe there is no alternative. But that is simply no longer the case. Today's computer power has enabled powerful stitching processes that give 100% coverage for every optic they measure. It also gives the optics buyers the ability to have 100% certainty of what they are buying.

Optics buyers are catching on - they are no longer willing to settle for "best effort” quotes. They are using sophisticated tools to measure the parts they receive and are no longer willing to rely on the manufacturers' claims of quality.

The age-old adage among optics manufacturers is that "if you can't measure it, you can't make it.” Sophisticated manufacturers have long recognized the need for improved metrology. When one visits optics manufacturing companies, it is not unusual to see a variety of interferometers (typically with 4” or 6” aperture sizes) being used in production or final quality control. Interestingly, and no matter how many interferometers are purchased, most optics shops can only see a portion of the optics they manufacture. From these subaperture measurements they gauge the quality of the entire surface. This, however, can be quite misleading, and surfaces that look good in the subaperture will very often fail over the full surface specification.

The capability gap of standard metrology tools is considerable and in fact, even many optics less than 50 mm in diameter can not be measured with a standard 4” interferometer. Larger interferometers and transmission spheres capable of covering these surfaces are available only at great expense by custom order, with long lead times or by self-manufacture. The problem is compounded for aspheric surfaces, since a standard method for asphere metrology does not exist.

SSI® Metrology can eradicate these roadblocks. SSI systems mathematically "stitch” together subaperture measurements to generate high-resolution, high-precision, full aperture surface measurements. The flexibility of SSI systems makes them efficient in-process metrology tools, well suited to final quality control.

Stitching offers a flexible and cost-effective alternative to traditional metrology approaches. Stitching has four major advantages over standard interferometry: 1.) a larger field of view—you can see the entire surface; 2.) measure higher lateral spatial frequencies—you can see a better picture of the surface; 3.) improved accuracy—you can feel confident in the quality of your results because the unit automatically calibrates and subtracts errors from the reference optics; and 4.) measure aspheres without null lenses. [To read how stitching achieves these advantages, and see the data behind the story, click here.]

The optics industry has become resigned to unique, dedicated metrology set-ups being used for every product, particularly when manufacturing high precision flats, spheres and aspheres. With today's lower volumes and higher-precision applications, optics shops do not have the time or money to invest in expensive, unique, one-off metrology systems for each product. They also cannot afford the loss in productivity, cycle time and failed products that are the result of the lack of easy-to-use, full aperture metrology. And even more importantly, they cannot afford to lose orders and ultimately their customers when they miss delivery dates or fail to meet specifications. Subaperture stitching metrology systems, particularly when combined with deterministic finishing tools give manufacturers the confidence so that they can ensure that every part is delivered to specification, on time and every time. Subaperture stitching interferometry is a universal measurement tool that has the potential to close the gap in precision optical manufacturing.