PROCESS CONTROL & LENS INSPECTION OF FREEFORM LENSES

Transcription

1 [ A&R Optical Machinery, Inc. Technology Series: III October 2013 [ PROCESS CONTROL & LENS INSPECTION OF FREEFORM LENSES EXECUTIVE SUMMARY: Digital surfacing has been a tremendous breakthrough for the ophthalmic industry by providing the means to surface any individualized design directly on the back side of the lens. However, this high-tech and delicate process requires frequent maintenance and control to keep the quality constant. Besides, the quality of the final product is expected to be higher. Finally, the responsibility of the design replication has been transferred from the blank manufacturer to the lab. For these reasons, the lab needs to rethink the inspection. Among the available solutions, the optical mapping systems appear to be the most appropriate for the lab. During the last 10 years, two generations of mapping systems have been released on the market. The first generation uses a Single Mapping technology, while the second generation uses a Dual Mapping technology. And these two generations have very different performances. The Dual Lensmapper, a second generation mapper, has been chosen as the reference tool for labs in terms of FF process control and FF lens inspection by the top players in the market. And by using the same accurate tool, partners like the lab, the machine vendor and the design supplier work better together.

2 DIGITAL SURFACING BRINGS A NEED FOR INSPECTION: INTRODUCTION: For 25 years A&R has been providing solutions to the ophthalmic industry to improve quality control and production process. Partnerships have been established with top players and many small companies all around the world. We have subsidiaries or offices on four continents, in five countries. 25 years ago, A&R introduced the Focovision to bring a radical change to the inspection of conventional lenses. Over a 1000 units have been sold. Since then, it has been considered as the reference unit in the industry. With the advent of FF lenses, the R X labs have a new need for inspection. We believe that the Dual Lensmapper is the new industrial inspection unit that brings a response to this need. During this presentation, we will first wonder what FF changed in term of inspection and what kind of solution is needed. Optical mapping will appear to be the most appropriate and we will see together the difference between the first generation and the second generation of mappers. Finally, we will see how a second generation mapper, the A&R Dual Lensmapper, can be of great help every day in the Freeform lab. Highly accurate but delicate process Digital surfacing has been a tremendous breakthrough for the ophthalmic industry by providing the means to surface any individualized design directly on the back side of the lens. The change in technology is huge. The difference in accuracy is impressive. As an example, the hard lap tool was chosen based on a 0.125D pitch while the accuracy of the freeform process reaches 0.01D. The difference is a factor of 10! However, this high-tech process requires frequent maintenance and control to keep the quality constant. For example, a defective temperature regulation system can lead to thermal expansion which creates power errors in the end. This effect needs to be detected and adjusted. Consequently, power deviations can have different causes, linked to a multitude of parameters. These deviations need to be detected, identified and corrected. Superior optical acuity This highly accurate process brings value to the final product by providing superior visual acuity, but only if two conditions are satisfied: it has to be made from a high class design that takes individual parameters into account and this design needs to be replicated accurately on the lens. Transferred responsibility The design replication is then a key to get the superior product, but who s responsible for this part? Before, at the time of traditional surfacing, the design replication was under the responsibility of the blank manufacturers and was a mass production process. The design was on the front side, the back side being simple (sphere or torus). Now, with FF, the design is now on the back side and it is applied to the lens by surfacing. It moved from mass production to R X, each lens being individual. The responsibility has then been transferred to the lab. In other words: - Before, the surfacing was simple and forgiving regarding errors. The lab s responsibility was to make sure of the use of the good tool. - Now, the surfacing is more complex. The lab s responsibility is to make sure the design is replicated correctly. What FF changes in term of inspection For these three reasons (delicate process, higher expected quality and transferred responsibility) the lab needs to rethink the inspection. Available solutions So, how can the labs control this design replication then? Let s consider the three following solutions: the lensmeter, the XYZ table and the Optical Mappers. One common solution could be the lensmeter. It is affordable and very common. However, process drift can cause power deviations in the areas not inspected by lensmeters. The quality department cannot rely on one or two points to check if the design was replicated correctly. Another solution could be the XYZ table. This solution has been used by the mold manufacturers for process development and process control. It has the advantage to isolate the generating results by inspecting the back surface. However, its cost is high by its price (price > 100k ) and the maintenance it requires (engineering team, cooling system, ). It is also a long measurement process (>30min). And it is destructive. There is another type of inspection unit that appears to be more adequate for the lab: the Optical Mappers. It has a medium price (20-40k ). The whole lens is inspected to control the design replication. Besides, it only takes a few second to make a measurement. Finally, the measurement is not destructive (no contact). Let s summarize the comparison. First, since one point is not enough to control the design replication, the lensmeter does not provide a solution for freeform. Second, the XYZ table is not compliant with the lab s need, especially in term of cost. Finally, the optical mapping system turns out to be a solution that is compliant with the lab s need. However, there are two generations of optical mappers. The first generation uses a Single Mapping technology, while the second generation uses a Dual Mapping technology. And these two generations have very different performances.

3 FIRST GENERATION OF OPTICAL MAPPERS: SECOND GENERATION OF OPTICAL MAPPERS: A few years ago, a second generation of mappers was introduced on the market. This 2nd generation is based on a new principle: the Dual Mapping technology. The Dual Mapping technology And this significant comparison results in a significant Error Map. Let s have a look at the comparison with the reference system. It shows a significant error map for the 2G mapper revealing the center defect, while the 1G mapper showed a very different result. Let s examine these optical mappers by starting with the first generation. The principle The single mapping technology creates an optical map for the entire lens. It is a multi-point measurement, not a one point measurement like the lensmeter. The principle is to compare the optical map to the theoretical map, as defined by the progressive theoretical file. As a consequence, this technology will provide a point-by-point comparison between the target lens and the manufactured product. The limitations This generation of instruments has been on the market for 10 years. However, it suffers from serious limitations, the first one being the poor resolution. The resolution A comparison with a reference system has been realized by one of our customers. It shows the measured map from a reference system and the measured map of a first generation mapper. Look at the center of the map. The lack of detection of this important defect is clearly evidenced. Actually the 1G mapper missed it due to its poor resolution. The design deviations are not significant. Let s have a look at the comparison between an Error Map provided by the reference system and the 1G mapper. The center defect is still missing. Besides, the Error Map does not seem consistent with the reference system. The deviation is actually not significant. To explain why, I first have to make a technical comment. It is important to understand that the power map depends on the point of view of the camera. If the target is calculated using a different point of view than the measurement, the results are different and the comparison is not significant. Since single mapping devices have no means to determine the exact point of view, the target cannot be calculated in the real measurement conditions. The consequence is a poor accuracy on the target and a poor matching between the measured map and the theoretical map, leading to inaccurate error map. First generation of mappers - summary As you have seen, optical mappers have many advantages over the other available solutions like the lensmeter and the XYZ table. They provide multi-points measurements and provide the user with a comparison between the lens and the target. However, 1G mappers suffer from serious limitations. The resolution is poor and the design deviations are not significant. This technology combines the use of light reflection off the front surface and light that is transmitted through the lens: measurements by reflection and transmission. This new technology brings two advancements. The first one is an increase of the resolution. The higher resolution The optical measurement is based on an innovative method that provides a local and accurate measurement. With a higher resolution, the 2G mapper can evidence process defects the 1G mapper didn t show, as we can see on this picture for the center defect. Significant design deviations The second enhancement is a significant comparison between the lens and the target. This is possible because the Dual Mapping uses the same point of view for the calculation of the target than for the measurement. You remember, using a different point of view in the calculation of the target leads to a comparison which is not significant. However, if the same point of view is used, the comparison is significant. This is achieved with the reflection technology. This technology is used to localize the lens in the 3D space. It is then possible to know which point of view was used for the measurement and to take it into account. The theoretical map is calculated using the exact measurement conditions. 1G mappers vs 2G mappers Let s now summarize the comparison between the two generations. Both the 1G and 2G mappers do not require any lens preparations like blackening. The 2G is easier to handle since the lens markings are recognized automatically by the system. The generating, turning defects, and center defects are evidenced by the 2G mappers while they do not appear on the 1G maps. Finally, the 2G mappers provide a significant error map while the 1G does not. From this comparison, you can clearly see the higher performance of the 2G mapper over the first one.

4 THE A&R DUAL LENSMAPPER The A&R Dual Lensmapper is the only second generation mapper and is a very useful tool in the lab. Now I would like to show you 3 applications of the Dual Lensmapper: first process control, second lens inspection, and finally measurement of edged lenses. Process control With the Dual Lensmapper, the lab can have a direct feedback on the process quality. Typical process defects can be detected before the lens quality is affected. These defects appear to have typical signatures on the Error Map, as the one shown on the picture. They can be detected, identified, and corrected. For example, rings are often characteristics of a polishing problem. The problem might be for example a bad flow at the polishing machine. The solution would then be to re-orientate the flow so it cools down the lens correctly during the polishing process. Here are other examples of defect signatures on the Error Map. Lens inspection Controlling the process is crucial to maintain the production quality. But to guarantee the quality of the final product, we need to rethink the inspection process. Indeed, a Freeform lens is more than a SPH and an ADD, it is made of an optimized design that brings high visual comfort to the wearer. In other words, the lens quality depends on more than 2 points. The Go/NoGo feature of the Dual Lensmapper is an objective and easy way to inspect a freeform lens. Different criteria are displayed. They correspond to different areas of the lens. For each criteria, a power error in Diopters is displayed and compared with a tolerance. In the end, an objective decision can be made regarding the lens quality. Note that the tolerances are configurable and can be adapted to the lab s needs. These significant design deviations can be recorded. Further treatment can thus lead to statistical analysis where the evolution of each design deviation is described. Edge lenses measurement Even with a reliable process control and a Freeform inspection, final products are sometimes returned by the optician. The Dual Lensmapper can also be of a great help to analyze these returned lenses, since it can measure edged lenses. It is then possible to determine if the design was replicated correctly or if the visual discomfort of the wearer is more related to the mounting. CONCLUSION Let s now review together the important points of this discussion. First, we have seen that the change provided by digital surfacing have many implications in term of inspection: first, the process is highly accurate but also delicate and needs to be monitored, second the quality of the final product is expected to be higher and finally the lab is now responsible for the design replication. For these three reasons, the lab needs to rethink the inspection. Then, available solutions to this new need were examined and the optical mappers turned out to be the most appropriate to fulfill the lab s needs. However, there are two generations of mappers and they have very different performances. We have then compared the two generations. The second one has better performances thanks to its higher resolution and is the only one to provide significant power deviations. This gain in performance makes the difference while detecting process defects like generating defects or center defects. The Dual Lensmapper is the only second generation mapper. It is used intensively in labs today for process control, freeform lens inspection or the analysis of returned lenses. The Dual Lensmapper has been chosen as the reference tools for labs in terms of FF process control and FF lens inspection by the top players in the market. By using the same tool, partners like the lab, the machine vendor and the design supplier work better together. We invite you to follow these companies in making the best choice for the inspection of your freeform lenses.

5 A&R Technical Advisory Board Dr. Christian Laurent Automation & Robotics Director Research and Development After receiving his PhD in the fields of Instrumentation and Measurement at the University of Liege in Belgium, Dr. Christian Laurent has been focusing on the conception and development of various proprietary inspection and accurate measurement devices based on optics and artificial vision. Dr. Laurent is has been director of research and development for automation and robotics at A&R since Thomas Zangerle Automation & Robotics Research and Development, Product Leader Holding master s degrees in Physics Engineering and General Management, Thomas Zangerle is a product leader in research and development at A&R. Zangerle is currently leading the development of optical mapping instruments and innovative mathematical models for optical power measurement. Ing. Benoit Heins Automation & Robotics, Product Manager With over 20 years of experience in the ophthalmic industry, Benoit Heins specializes in finding solutions to maintain customers equipment in high levels of productivity. Heins graduated from The Higher Education Institution of the Province of Liege in Belgium and is currently a product manager at A&R. John T. Fried Automation & Robotics, Inc. President, CEO Specializing in company leadership in ophthalmic laboratory equipment and instruments, John Fried has been in the optical industry for over 40 years. Fried is now the President and CEO of automated equipment for ophthalmic lens production, measurement, packaging, and logistics within the North American Market for A&R. A&R Optical Machinery, Inc arom@ar.be