Trioptics produces optical measurement systems for R&D and production for every manufacturing step in optics. The company has been involved in optical measurement systems for R&D and production since its founding in 1991. In the beginning, Trioptics focused on combining traditional optical testing with electronic imaging and motorized procedures such as automated focusing. At that time, this was mainly for testing in the R&D lab or for small volume production. In 2001, Trioptics started to enter the high volume mobile phone camera business. With manually loaded, single piece lens testers, customers were measuring the image quality of comparatively low-resolution lenses of CIF or VGA format. The basic principle of measuring the image quality with several electronic cameras simultaneously across the whole lens´ field of view has proven to be very successful and is still key to today´s high speed lens testing.
In the past, optical testing was often done one at a time, yet today companies such as Trioptics are producing systems capable of testing optics in series production. Trioptics added a high-precision lens tray to the formerly single piece tester. The tray can carry as many as 100 lenses or more to be tested, and it simplified the feeding of the measurement system. The measurement cycle time could be continuously reduced to today´s level of about 1 second per lens and measuring the image contrast at up to 25 image field locations.
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Yield is the key to success
The market is very fast and cyclic. At defined release dates, a huge number of lenses is required. The complex lenses with 5 or 6 lens elements are produced very rapidly with micron accuracy. The yield is the key to success, regarding cost and supply capacity. Trioptics´ MTF test equipment is used for the final assessment of the image quality of the lenses. The result is needed to optimize the whole process chain of manufacturing with the goal of improving the yield. High reliability of the measurement equipment and greater certainty in the results are essential for the equipment.
For the mobile phone lens testers, Trioptics is a leader in the market, but the company does see competition from “home-made” test stations in larger organizations that are sometimes outsourced to local manufacturers. These systems are usually lacking continuous development and long-term service and support, according to Dr. Stefan Krey, Managing Director of Trioptics.
Trends in qualifying mobile phone lenses
The race to higher pixel densities has somewhat reached a limit. Today´s systems are capable of measuring the highest resolution lenses. One trend is to use multiple camera systems, further increasing the number of required lenses. This puts higher demand on the throughput of Trioptics´ systems. Traditionally, the field of view is designed to allow the typical “selfie” image. Now for example, there is more diversity in the field of view, and wide angle and tele lenses are combined. Manufacturers also tend to further reduce the f-number of lenses to improve light sensitivity and add a bit of depth to the image (f-number is the “aperture stop” of the lens. The smaller the number, the larger the aperture and light sensitivity).
Dr. Krey noted that they are also seeing some demands in combining different measurement possibilities in one system, for example, adding distortion, relative illumination and/or veiling glare measurement capabilities. This reduces overall cost and measurement complexity for the manufacturer.
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The described tray loaded systems still fulfill today´s requirements on throughput, however, labor costs are continuously rising in the countries where mobile phone lenses are produced. The run for yield requires continuous monitoring and traceability of the manufactured parts, therefore, Trioptics has developed automatic tray loaders and sorting machines to sort the samples into different quality classes.
Trioptics´ measurement approach is used by mobile phone manufacturers, allowing them to scale measurement instruments according to the market demands in terms of throughput, resolution and accuracy requirements. The benefit to the customer is that they can easily reconfigure and upgrade the hardware to the demands of new (next generation) lens designs. Over the years, Trioptics´ software has developed from a quite simple MTF measurement tool to a complex analysis tool, not only supplying image quality data but also opto-mechanical parameters like back focal length, field curvature or depth of focus, which are important for the later assembly into the camera module. MTF stands for modulation transfer function and it is an aid to evaluating how well optical systems form images. MTF can be calculated from the lens design data, giving optical systems designers reliable predictions of system performance.
Reducing costs with a smaller footprint
Customers would like to have as many analysis functions as possible in one machine. This reduces acquisition and maintenance costs, has a smaller footprint and less learning effort. Therefore, it is Trioptics´ goal to implement multiple measurement functions as long as it is not counterproductive. The manufacturing process of a mobile phone lens is a complex chain of production steps. It contributes to the added value only by improving yield. The more you can learn from the measured values about your production process the better you can control the process.
Trioptics´ systems are not only used by lens manufacturers, but also for receiving inspection by the integrators who assemble the camera modules. They are often using second or third source lens suppliers and lens quality can differ in various aspects but have to meet all the same quality requirements. Therefore, it is important to measure all specified parameters, such as relative illumination, veiling glare, back focal length and many more.
Tomorrow´s tools and technologies
The manufacturing processes are reaching the limits of what is mechanically possible with today´s manufacturing and assembly machines of the highest precision. It is obvious that current lens design and manufacturing capabilities cannot keep up with the demand to make phones thinner and thinner. Further shrinking of the lens will automatically put enormous pressure on all mechanical tolerances and manufacturing precision. Future manufacturing tools will require more sensors, more testing and active feedback loops to better control the production. Process feedback has to be given across machines, so inter-process communication and industrial internet functions become very important.
Mobile phone cameras have already surpassed the quality of digital compact cameras. In the future the focus will be on the improvement of image quality in low light or other difficult lighting situations, and adding realistic impressions of depth and bokeh (the aesthetic quality of the blur produced in the out-of-focus parts of an image) that people are used to from using digital SLR cameras.
Written by Anne Fischer, Managing Editor, Novus Light Technologies Today