Stefan M. B. Baumer

Appearance characterization by a scatterometer employing a hemispherical screen

Characterization of optical appearance by measurement of the hemispherical scattering distribution using a concave projection screen and a camera is investigated. Secondary intensities by repeated internal screen reflections can be measured separately and compensated for. The concept is coupled to functional properties of product surfaces and we use it in an industrial environment. Only little less accurate than a photogoniometer, the hardware is much cheaper, contains no moving parts and is up to 1000 times faster.

Ultra-fast MTF Test for High-Volume production of CMOS Imaging Cameras

During the last years compact CMOS imaging cameras have grown into high volume applications such as mobile phones, PDAs, etc. In order to insure a constant quality of the lenses of the cameras, MTF is used as a figure of merit. MTF is a polychromatic, objective test for imaging lens quality including diffraction effects, system aberrations and surface defects as well. The draw back of MTF testing is that the proper measurement of the lens MTF is quite cumbersome and time consuming. In the current investigation we designed, produced and tested a new semi-automated MTF set up that is able to measure the polychromatic lens system MTF at 6 or more field points at best focus in less than 6 seconds. The computed MTF is a real diffraction MTF derived from a line spread function (not merely a contrast measurement). This enables lens manufacturers to perform 100% MTF testing even in high volume applications. Using statistic tools to analyze the data also gives possibility to find even small systematic errors in the production like shift or tilt of lenses and lens elements. Using this as feedback the quality of the product can be increased. The system is very compact and can be put easily in an assembly line. Besides design and test of the MTF set up correlation experiments between several testers have been carried out. A correlation of better than 6% points for all tested systems at all fields has been achieved.

Integral optical system design of injection-molded optics

Injection molded optics are frequently applied in many high volume applications. Bar code scanners, CD / DVD systems, CMOS cameras are a few examples. In all of these applications cost effective and fast design cycles are essential. At Philips High Tech Plastics we developed a design system that touches on all different aspects of the system design. Starting with traditional lens design (sequential ray tracing) and tolernacing we transport the initial design into mechanical solid modeling. During mechanical modeling, tolerances, injection molding design rules and integration of mechanical features, reference marks, etc. are incorporated as well. Here the full advantage of injection molding can be utilized. After the opto - mechanical modeling the system is ported back to non - sequential ray tracing for ghost - and stray light analysis. Finally extended tolerancing is performed in order to come to a robust high volume product. If necessary all or several steps in this design process are repeated in order to arrive at the final design. As an additional requirement the metrology possibilities for the design are checked in at an early stage. This integral system approach to optical design, including optical modeling (sequential and non-sequential) combined with mechanical solid modeling is presented using some recent examples.

Temperature-compensated plastic lens for visible light

In many consumer and professional plastic lenses have potential applications because of cost, weight and aspherical shape. However they suffer from a big disadvantage: large amount of focal shift as function of temperature. In particular for bar code scanners, focal shifts due to temperature changes have huge impacts on the function of the scanner. One possibility to improve the temperature behavior of such lenses is to turn them into so- called hybrids: a combination of a refractive and a diffractive surface. This way a temperature compensation can be achieved that reaches the level of glass lenses. In this paper design and manufacturing considerations for such a lens will be given. This includes proper material choice and mechanical design. The lens is temperature compensated over a range from -230 degrees-+60 degrees C. Operating at 650 nm and having a focal length of 4 mm, makes it extra difficult to produce such a lens with sufficient image quality and diffraction efficiency. Results from the design will be compared with measured values from an injection molded sample of the designed lens. Quality parameter such as wavefront quality, focal shift with temperature and diffraction efficiency will be given.

Metrology of injection-molded optical components with multiple optical functions

A well-known advantage of injection molded plastic optical components is the possibility of integration of an optical function and a mechanical mount. The optical part can be positioned accurately with respect to well-defined references of the mechanical mount. The optical function does not have to be restricted to one optical surface. In principle any combination of lenses, mirrors and/or beam splitters is possible. The metrology of these combined optical functions is often not trivial. Commercial available measuring equipment in general has difficulties when the different optical functions are tightly toleranced with respect to each other and when less common types of optical surfaces are involved. In this paper three examples of multi function optical components are presented. One of these examples, a double mirror, is elaborated in detail in terms of metrology. The orientation of both mirrors with respect to the mechanical references is tightly toleranced. The same holds for the orientation of the mirrors with respect to each other. The shape of one of the mirrors is so accurate that the reflected wave front is diffraction limited. The other mirror is an off axis paraboloid. The specially developed measurement tool, based on the autocollimator principle, the obtainable measurement accuracies and the calibration procedure will be described. Also the product accuracies realized with injection molding of this component in mass production will be presented.

Minideflectometer for measuring optical finish quality

The principle of scanning deflectometry is presented as a simple method for measuring optical figure finish quality of odd shaped high-gloss surfaces.

Design of a compact 3D laser scanner

A design study for a compact 3D scanner, called Coplan, is presented. The Coplan is intended to be used for high speed, in-line coplanarity and shape measurement of electronic components, like Ball Grid Arrays and Surface Mount Devices. The scanner should have a scan length of at least 2 inches and a resolution of 5 micrometers in all 3 dimensions. First an analysis of two different scan schemes is made: a so-called pre-objective scheme using an F-(theta) scan lens and a post- objective scheme using a so-called banana field flattener, consisting of a convex, cylindrical hyperbolic mirror and a concave, cylindrical parabolic mirror. Secondly, an analysis of height resolution requirements for triangulation and confocal depth sensing has been made. It is concluded that for both methods of depth sensing a synchronous scheme with a 50-60 degrees detection angle in cross scan direction is required. It is shown that a post-objective scheme consisting of a banana mirror system combined with triangulation height detection offers the best solution for the optical requirements.

Integrating functions in polymer optical components

One of the big advantages of polymer optics is the possibility of integrating several functions into one component. These functions can range from mechanical reference datums all the way to micro channels in “lab-on-chip” type of applications. In this paper an overview on several designand manufacturing principles of such integrated components will be given. Furthermore the next steps in the Design – Build – Test cycle will be discussed as well: mold manufacturing, molding and finally metrology.

Optical figure testing by scanning deflectometry

Scanning Deflectometry is a powerful method to measure optical figure quality of various optical components and systems in a simple way. This principle uses detection of slope deviations rather than optical path length variations. As an example, the design of a basic deflectometer for testing flat mirrors is presented.